Advances in Riblets Design

Abstract

Continuous evolution in nature has created optimum solutions for creature survival that have inspired many innovative engineering designs. Riblet geometries, passive flow control devices, have been studied, which were inspired by the skin of fast-swimming sharks. Turbulent boundary layer research reveals the positive effect of riblets in reducing drag by manipulating turbulent structures. Reducing drag is an important topic for the aviation industry, as it directly relates to fuel savings and reductions in carbon footprints. Aircraft noise represents another significant area of concern. When riblet designs modify turbulent structures, they can also impact pressure sources within the boundary layer, consequently influencing the generation of self-noise. Earlier research studies have demonstrated the favorable outcomes of riblet configurations on the variations in wall pressure, resulting in reduced levels of noise propagation. The current review paper is mainly devoted to the application of riblets in the aviation industry, focusing on studies that are performed in wind tunnels, flight tests, and using numerical techniques. Proving the desired performance of micro-grooves, their method of fabrication and implementation on aircraft surfaces are important topics that are also discussed. In addition, the effect of durability on the performance and required maintenance intervals was previously investigated and is also presented. Finally, recommendations for future activities in the relevant fields of study are provided.

1. Introduction

From billions of years ago until now, continuous evolution in nature has provided high-performance, innovative solutions for the survival of creatures. During the history of science, different research studies have been conducted by imitating nature to make improvements in engineering decisions. The imitation of birds in realizing the long-held dream of humankind for flight is a well-known example of this argument. The emergence of flight was not a conventional replication of the structure of bird wings in detail but was imitated from other naturally evolved methods of birds flying. As an example, there are differences in methods for short- or long-range flying birds [1,2,3,4,5]. As nature has its own solutions for adapting to the environment, in today’s eco-friendly optimum engineering designs, imitating these naturally evolved and “optimum solutions” would potentially accelerate technology development and reduce engineering research costs [2,3,5,6].

On this basis, one of the researchers’ greatest challenges is finding the cause-and-effect relationship in nature to benefit from natural phenomena in the design of optimized devices. This would be more challenging when optimum results by nature are extracted after a multi-objective evolution, which is improved based on the creature’s habitat for different natural reasons [7]. The theory of silent flight is an example investigated by researchers to mimic the noise reduction abilities of owls’ wings [8,9,10,11,12,13]. Reducing far-field noise is an important parameter that evolved over the body and wing structure of this species. It is revealed that having large wings with low aspect ratios, serrations in the leading and trailing position of the wings, wavelet-like surfaces, and grooves over the feathers on lower parts of the wings would have a positive effect on reducing radiated noise. The optimum design of shark skin that has contact with water is another example that has evolved by nature. As a multi-objective evolutionary process, the anti-fouling and hydrophobic ability also evolved besides its reduction in drag. The idea of using shark skin structure on the surface that is in contact with the fluid flow is introduced as a solution to reduce frictional drag [14,15,16,17,18]. In this regard, many innovative surfaces replicated from shark skin made their way to the industry, facilitating improvements in engineering designs, especially in friction drag [19].

In aerodynamics, drag is a directional force that acts against movements between fluid and solid body. It always resists movements by producing two main types of resisting forces, named form drag, also known as pressure drag and skin friction drag. Form drag is the opposite force generated by air molecules when flowing past the solid body and pushing it harder against forward rather than backward motion [20]. Skin friction drag, or viscous drag, is the resisting force generated through the moving of fluid flow over the object’s surface. Flow viscosity and velocity are the main factors that act on the value of skin friction drag. In addition to the above-mentioned fluid property factors, roughness and surface area exposed to the flow would affect the friction drag value.

Riblets can be considered bio-inspired replications of fast-swimming sharks that acceptably mitigate friction or viscous drag through the surface [14,15,21,22]. These aligned to the flow micro-size geometries can potentially lift high energetic, turbulent vortices from the surface to the riblet tips, leading to less interaction between the surface and turbulent vortices, hence resulting in friction drag reduction [2,23,24,25,26]. Since riblet performance shows the possibility of drag reduction, several numerical and experimental activities have been conducted to investigate different sizes and configurations over these geometries. There is also a wide range of analyses, including simple extrusions as well as complex 3D geometries, combined with various other methods for reducing drag [26,27,28,29,30].

Moreover, riblets show promise for adaptation to various flow control applications for drag reduction, such as aviation, marine ships, automotive, wind energy, sports, and medical fields [1,2,3,17,31,32,33]. Since the first fundamental studies on riblets by the NASA Langley Research Centre in the 1970s [34], many studies have been conducted that show immense potential for drag reduction and greenhouse reduction by sharkskin-inspired riblets in aviation [18,29,35,36,37,38,39,40]. Szodurch estimated that riblets could lead to a 2% draft reduction by an Airbus 320 if 70% of the aircraft is covered with riblets. In 1987, the racing yacht “Stars and Stripes” with the implementation of riblets from NASA and 3M won the America’s Cup and the gold medal at Olympia [41]. In 2010, the BMW Oracle team won the 33rd America’s Cup with their racing yacht covered with riblets [42], In 2012, riblets found their applications on wind turbine blades [31] and a Formula 1 car by Bionic Surface technology [43]. Moreover, riblets’ antifouling properties have been studied for potential applications on marine ships, medical implants, water treatment systems, and food and beverage industries [44,45,46].

Despite the common underlying physics of riblets in drag reduction and other applications, translating riblet technology to different domains poses challenges related to scale, environmental conditions, and surface materials. As an example, in the field of wind energy, wind turbine blades require considerations of larger scales and dynamic load variations, while underwater vehicles involve hydrodynamic complexities. Ensuring the robustness and durability of riblet structures in diverse conditions, as well as addressing manufacturing and maintenance aspects, are key challenges when applying riblet technology [14].

These surfaces have been widely studied during the past few decades. Experimentally, they have been tested with different methods: evaluated in oil [47], water [7,48,49] or wind tunnels [25,31,50,51,52,53] or viscosity base counter-rotating cylinders, like the Taylor–Couette cell device [54]. Moreover, they have non-dimensional geometrical characteristics that facilitate the comparison between different studies, both in experimental and numerical research activities. Several authors [25,55,56,57,58,59] have conducted their research in a manner that allows for the evaluation of their work alongside other references. In the laboratory environment, riblets have been investigated in open-loop or closed-loop channels. In addition, research activities have been conducted on simple flat plates or standard extrude airfoil sections, with the possibility of testing different crossflows and angles of attack in test plates, allowing variable sweep conditions and different pressure gradients, which form more realistic conditions and mimic real flight.

Moreover, several studies related to aircraft-generated noise and solutions for reducing noise at source have been reported in the literature. The source targeting techniques are introduced to control generated noise by manipulation, altering and inhibiting [60] at the locations where sources of noise are forming. One type of self-noise, which is especially dependent on industrial applications, is the noise generated inside the fully developed turbulent boundary layer over the airfoil trailing edge. To investigate and mitigate the effect of airfoil trailing edge noise, experimental activities have been conducted during the past decades. The use of serrations, porous surfaces and pressure shielding are some of the solutions that work by altering the turbulent boundary layer developed over the trailing edge [60]. As an example, Gonzalez et al. [61] made modifications over the trailing edge to control noise at diffusion airfoils by using grooves on both sides with a detached pattern on the endpoints. As near-wall flow structures are altered by riblet geometries, the effect of these geometries on noise reduction was also studied [25,51].

Many advancements have been achieved in the past decades in riblet technologies for drag and noise reduction on aircraft. This review focuses on analyzing the impact of riblets on drag and noise reduction in various configurations, including flat plates, airfoils, wings, and wing–body setups. The investigation considers different flow properties, such as varying Mach numbers and Reynolds numbers. The review also provides insights into the fabrication methods of riblets and the technologies used for their application. Experimental studies, particularly wind tunnel measurements and flight tests, as well as numerical simulations, are discussed. The behavior of fluid flow within turbulent boundary layers is examined through laboratory and numerical activities, while the overall effectiveness of riblet surfaces is evaluated through in-service assessments for aircraft fuel savings. By reviewing previous research efforts related to riblet applications, several key technology areas have been identified and are discussed in this review.

2. Physics of Riblets

Multiple research studies have been undertaken to examine the effectiveness of various riblet geometries in reducing drag. The effectiveness of drag reduction is closely linked to the flow properties, size and configuration of riblets [23,26,30,62,63]. Some conventional shapes of riblet geometries are illustrated in Figure 1. Bladed [62], triangular [29], semi-circular [26,64], trapezoidal [51,65,66,67] and spaced triangular [68] are some of the most conventional geometries. Riblets drag reduction can be characterized by non-dimensional parameters, namely: (1) non-dimensional spacing, defined as $s^{+}=s·u_{\tau }/\nu$, in which ν is the kinematic viscosity and $u_{\tau }$ is the friction velocity known as a function of viscous shear stress ($u_{\tau }=\sqrt{{\tau }_0/\rho }$), and (2) non-dimensional height of the grooves ($h^{+}$). The non-dimensional parameters are generally used to compare the outcomes of using riblets with available numerical and experimental studies [6,22,50,65]. In addition, by utilizing experimental data from diverse riblet configurations, the square root of non-dimensional riblet cross-section area ($l_g^{+}=\sqrt{A_g^{+}}$) has been introduced [57]. The goal was to determine if it could be feasible to represent drag reduction through a geometric parameter capable of encompassing the combined impact of riblet spacing and shape. This approach was implemented as another effective parameter for the characterization of riblets [69,70].

Several researchers have dedicated their studies to investigating the effects of riblets on both boundary layers and free stream flows. Specifically, they have focused on examining the coherent structures present within the turbulent boundary layer, particularly in areas influenced by the wall. These coherent structures exhibit unique characteristics, including smaller sizes and complex three-dimensional fluctuations, which differ from the overall flow behavior [71]. These property fluctuations in the airflow over the surface of a commercial aircraft at an altitude of 12.2 km, with a Mach number (M) of 0.8 and a Reynolds number (Re) of ${10}^6$ can be in the order of microns [32,50].

The coherent structures are a part of the process of sweep and ejection with momentum exchange inside the turbulent boundary layer [18,26,72]. Momentum exchange in the turbulent boundary layer would be accrued by approaching high-speed flows to the surface (known as sweep) that is associated with positive u-component and negative v-component velocities and moving the low-speed flows from the near wall to toward freestream flow (known as ejection) that is associated with negative u-component and positive v-component velocities; this would result in shear stress effects inside the turbulent boundary layer [18,73]. The process of sweep and ejection causes near-wall longitudinal vortices, which lead to small spanwise motions in the viscous sub-layer and then increased friction drag [73]. Riblets, like small longitudinal fences, can hamper the movements of these vortices in such a way as to control skin friction drag [32,35,74,75,76].

Research studies have been mainly focused on the viscous sublayer, the area with $y^{+}<5$, given that 50% of turbulent energy is generated in the lower region (i.e., the lower 5% area) of the turbulent boundary layer [77]. To prevent the occurrence of sweep and injection between flow layers, riblet geometries with sizes comparable to the viscous sublayer are necessary. This size requirement ensures effective impedance of the flow interactions and promotes desired flow behavior [21]. A classical definition of drag reduction by riblet surfaces is defined by using the concept of surface-wetted area [21,78]. Compensation of the surface wetted area is an important factor in evaluating the drag reduction performance by riblets. In this context, the level of interaction between surface and fluid flow is an indication of the shear stress and friction drag [17]. In the riblet surfaces, turbulent flow is lifted up and interacts with riblet tips that decrease wetted area and fluid shear stress [79]. In comparison to the smooth surface, adjusting the flow vortices above the valleys of riblets results in lower values of cross-stream flow fluctuations and reduces turbulent kinetic energy near the surface [65,79,80]. Importantly, these adjustments also minimize pressure fluctuations in the outer layers of the flow. These alterations have the potential to influence the emitted sound in compressible fluids, especially in aviation-related contexts [51,65].

As the reduction in wetted area is the main concept behind drag reduction, the number of indentations per unit of length should be within an optimum range [21]. The effect of non-dimensional spacing, $s^{+}$, in drag reduction for a triangular riblet geometry with peak sharpness of 60° is illustrated in Figure 2. The figure indicates the limit of drag reduction when riblets spacing tends to be zero. Drag reduction achieves its optimum values at about $s^{+}\approx 15$. Increasing $s^{+}$ further would result in decreased effectiveness, causing an increase in drag more than zero, which is defined in the figure as a rough surface [35].

Research efforts have been dedicated to examining the aerodynamic implications of riblets on curved surfaces, particularly in the context of aircraft aerodynamics. Investigations have specifically targeted 3D applications to account for the complexities arising from variations in flow alignment experienced when airflow interacts with curved surfaces. To address these challenges, researchers have introduced curved riblet configurations featuring diverse riblet shapes, non-dimensional spacing and height. These design considerations aim to optimize drag reduction performance and effectively manage the intricate aerodynamic characteristics associated with curved surfaces [81].

Experimental and numerical studies have been conducted to investigate the impact of riblets on the structure of the boundary layer. To comprehensively understand riblet performance across different scales, a hierarchical analysis approach can be adopted. Starting at the micro-scale, detailed simulations and experiments delve into the fluid dynamics within individual riblet grooves and their interaction with the boundary layer, unveiling insights into local drag reduction mechanisms. Advancing to macro-scale, full-scale computational fluid dynamics simulations can evaluate the overall aerodynamic effect of riblets on complete aircraft systems, considering intricate flow interactions. In the experimental and numerical investigations, both viewpoints are considered separately or in a combined method. Generally speaking, these studies analyze the characteristics of the boundary layer before and after the application of riblets on flat and curved surfaces. The flow properties, including mean and fluctuating terms in the longitudinal, lateral, and vertical directions, are examined. The fluctuating terms not only influence the aerodynamic aspects but also have significant implications for the acoustical properties and generated noise [51,62]. This paper also discusses relevant studies that explore the effects of riblets on the boundary layer and associated flow phenomena.

Acknowledging the intersection of experimental and numerical research in exploring the impact of riblets on boundary layer structure, a comprehensive understanding of riblets’ performance across diverse scales can be achieved through a hierarchical analysis framework. This approach entails a progression from the micro-scale, where detailed simulations and experiments illustrate the fluid dynamics within individual riblet grooves and their interaction with the boundary layer, showing localized drag reduction mechanisms. Advancing to the macro-scale, extensive computational fluid dynamics simulations assess the aerodynamic influence of riblets on aircraft systems, encompassing intricate flow interactions. Broadly, these inquiries dissect the characteristics of the boundary layer in both pre and post-riblet application on both flat and curved surfaces. Flow properties, including mean and fluctuating components along the longitudinal, lateral, and vertical axes, undergo meticulous examination. Significantly, these fluctuating terms extend their influence beyond aerodynamics, affecting acoustical attributes and noise generation [51,62]. Furthermore, this paper delves into pertinent research that probes the ramifications of riblets on the boundary layer and associated flow phenomena.

3. Experimental Investigation of Riblets Performance

This section explores the experimental aspects of characterizing the aerodynamic and aeroacoustic properties of riblet surfaces. It provides a review of experimental tests conducted using various instrumentation technologies, fabrication methods, and testing strategies [25,45,56,66,80,82,83,84]. Over the past decades, researchers have introduced innovative techniques, resulting in new insights into the applicability of riblets in industrial settings [84,85,86]. Considering the focus of this paper on aviation, the discussion centers on two main areas: wind tunnel tests and in-flight experiments. The majority of studies have been conducted in controlled research environments, primarily relying on wind tunnel measurements, while a smaller number of investigations have involved flight tests conducted by renowned institutions or airlines.

3.1. Wind Tunnel Testing

Wind tunnel studies on riblet surfaces primarily focus on evaluating their effectiveness and potential for drag reduction. On this basis, the Reynolds number plays an important role in the measurements; at lower Reynolds numbers, where viscous forces dominate, and the flow near the surface tends to be laminar and smooth. In this regime, riblets have a relatively smaller impact on drag reduction since the flow is already less turbulent. However, as the Reynolds number increases, the flow transitions to a turbulent regime characterized by energetic flow with fluctuations and eddies near the surface. In this turbulent flow regime, riblets can be more effective in reducing drag by interacting with the turbulent flow and minimizing energy losses associated with turbulent boundary layers. In addition, riblets have been observed to reduce the occurrence of flow separation. They help to delay or prevent the detachment of the fluid flow from the surface, resulting in improved aerodynamic performance and reduced drag [56].

Table 1 summarizes experimental activities that are related to 2D riblet geometries tested in the wind tunnel. To measure drag reduction in turbulent flows, sensors have to obtain both the mean values and fluctuating components of the flow. Classification of direct and indirect measuring methods has been introduced by Haritonidis [87]. Direct measurements utilize a flush-mounted floating element that is able to displace tangentially by flow viscous force. Wall shear stress is calculated by the load cells that hold the floating surface. Walsh [23], Gaudet [88], Nieuwstadt et al. [55], Gruneberger [67], Zhao et al. [89], and Han et al. [83] use direct measurement in their studies. Walsh [23] studied several riblet geometries in a closed-loop wind tunnel with a floating element at NASA Langley Research Center. In the optimum case, the author recorded around an 8% reduction in drag for triangular and semi-circular riblets. Nieuwstadt et al. [55] tested triangular riblets in both zero and adverse pressure gradients. They found different accuracies of measured data by the drag balance mechanism, which were between 1–3% for zero and adverse pressure gradients, respectively. Although the direct method determines viscous shear stress without any assumption of the flow properties, measurement uncertainties related to the displacements and the presence of gaps between the test plate and tunnel walls must be considered [50,55].

On the other hand, indirect techniques for shear stress measurements in both momentum balance and correlation methods [87] have been implemented. These methods offer alternative approaches to estimating shear stress, providing additional options for researchers in the field. Due to the complex behavior of coherence structures, indirect methods are more popular for viscous shear stress measurements. In this classification, hot wire anemometry, which is used to measure mean and fluctuating components of the velocity, is a subdivision for momentum balance measurements. Walsh [23], Choi [62], Saravi et al. [90], Sundaram et al. [63], Park et al. [80], Zhang et al. [40], Takahashi et al. [91], Lee et al. [92], and Muhammad et al. [51] investigated the drag reduction effects of riblets by the use of momentum balance on flat plates and airfoils. Sundaram et al. [63] proposed hot wire anemometry over the x/c = 0.964 of the NACA 0012 airfoil tested at four different angles of attack between $\alpha$ = 0$°$−6$°$. The results showed more drag reduction for positive angles of attack rather than zero angles, with more drag reduction obtained for $\alpha =6°$. Takahashi et al. [91] investigated the turbulent scales within the turbulent boundary layer by the use of hot wire to evaluate skin friction drag. Flow properties, such as turbulent scales and intensities, were evaluated by free stream velocity and hot-wire anemometry. Their results indicated the effect of riblets on 8.3% turbulent intensity reduction, increasing about 30% in integral scales, and a 2.8% skin friction drag reduction for the hot-wire position at 67% downstream of the flat plate from the leading edge. Choi [62] used hot film/wire anemometry for mean and turbulence measurements. The hot film sensor head with a 1.5 mm diameter was flush mounted within the valleys of riblets, aiming to record low-frequency response by heat losses. The boundary-type hot wire sensor, used with an accurate positioning traverse mechanism, was used to calculate the friction coefficient inside the viscous sublayer [93]. Saravi et al. [90] tested serrate-semi-circular riblet surfaces by focusing on the interaction between the riblet edges and the turbulent boundary layer. The authors used a hot-wire and a 3D displacement traverse system with a displacement accuracy of 0.01 mm. They investigated the effect of riblets with modified tips in the ejection of turbulent flows. To evaluate the friction coefficient in each Mach number, they proposed multi-curves of friction coefficient based on u/U_e and (yU_e)/v of the Clauser [94] chart to find out local skin friction on both flat and riblet surfaces. By comparing the interpolated values, a 7% reduction in drag was observed.

Pitot measurements by rake or a traverse mechanism constitute another momentum integration method used for the indirect calculation of shear stress, which has been implemented by several research studies [63,80,88,90,91,95,96]. A significant number of research studies were facilitated by using both hot-wire and pitot pressure measurements aiming to validate the quality of gathered data. Sundaram et al. [63] used pitot measurements in the wake location after the NACA 0012 airfoil. They assessed the two-dimensionality of the flow by use of airfoil trailing edge wake analyses at three constant stations (x/c = 2.0, 2.2, 2.5). Results showed that variation of $C_D$ in three different stations have less than $\pm 1\mathrm{\%}$ deviation from each other. Viswanath et al. [97] measured the total skin friction drag of a supercritical ADA-S1 airfoil by determining stagnation pressure positioned 1.2 chords downstream of the trailing edge. Their measurements were performed at Mach numbers range of 0.6–0.76 and angles of attack from −0.5$°$ to 1$°$. The drag coefficient was obtained by use of pitot measurements, as follows:

$$C_D=\frac{2}{\gamma P_0{M}_{\mathrm{\infty }}^{2}c}\int {∆P}_{0}dy$$

(1)

where $P_0$ is the total pressure stagnation, and ${∆P}_0$ is total pressure loss. This formulation is valid for a 10% variation of Mach value on both sides of the airfoil compared to the free-stream Mach number. Results showed a drag reduction in the range of 3–6% for the riblet with h = 0.018 m, while drag was increased for h = 0.033 mm. Takahashi et al. [91] also used hot-wire anemometry and pitot rake to measure the riblet surface drag reduction. They indicated the uncertainty of hot-wire velocity measurements by traversing under a turbulent boundary layer, in which the pitot rake was used to calibrate the measured U-velocity component of the hot-wire. Sasamori et al. [96] carried out boundary layer pitot rake measurements to evaluate 2D momentum integral balance and local skin friction measurements.

In addition, wall pressure fluctuations under the turbulent boundary layer are measured by flush-mounted microphones in both streamwise and spanwise directions. The selection of a proper microphone and method of mounting is critical for optimal accuracy of the measurements. The frequency of data gathering is directly related to the flow regime and the frequency of near-wall pressure fluctuations. In very small sizes, such as those from riblet geometries, and due to the limited space for positioning, pinhole microphones are used. Choi [25] evaluated wall pressure fluctuations by use of pinhole microphones in trapezoidal-bladed riblets (see Figure 1e), with appropriate spacing between the ribs. A similar study was carried out over an airfoil by Muhammad [51] to evaluate the effect of riblet geometries in the near wall pressure fluctuations, especially focused on the trailing edge noise. As the riblets affect the structure of the turbulent boundary layer, a miniature loudspeaker was positioned in the middle of the strip to generate turbulent spots with specified square wave pulse signals in a defined frequency. Based on the instrument’s geometrical limitations, the authors used trapezoidal riblets, which allowed the mounting of pinhole microphones between each protuberance in the longitudinal and lateral directions. In addition, a reference microphone and a loudspeaker were temporarily mounted to evaluate each pinhole microphone one by one. As the microphones were positioned at a distance from the test plate (see Figure 3), the response times of signals were calibrated by use of a reference microphone and loudspeaker [51,62].

In the context of wall pressure fluctuation measurements, the size of the microphone diaphragm must be considered due to the size of the turbulent eddies within the boundary layer [65,98]. Pressure sensors should be small enough to resolve small scales of turbulent structures. As an estimation of the sensor’s size, these should be in the order of a tenth of the viscous lengths, which is roughly estimated by the value of Reynolds number based on the momentum thickness. As an example, increasing the Reynolds number by a factor of ten would change the Kolmogorov scales from millimeters to micrometers [98].

Flow visualization techniques made their way to characterize the effectiveness of riblets by measuring the three-dimensional properties of the flow within the turbulent boundary layer. In recent years, researchers developed several techniques for capturing pictures of turbulent flow structures [48,62,83,92,99]. Liu et al. [100] investigated the effect of riblets in the structure of turbulent boundary layers by using dye and hydrogen bubbles. Results showed a burst rate reduction of about 20–25% for different values of $h^{+}$ and $s^{+}$. Particle Image Velocimetry (PIV), Particle Trace Velocimetry (PTV), and Tomographic Particle Image Velocimetry (TPIV) techniques were used in several test procedures to investigate the boundary layer on a flat plate [38,77] or in the wake region produced by airfoils trailing edge [83,99]. Smoke wire and sheets of laser light over the flat plate riblet surface showed a significant change in turbulent coherence in the riblet surface area. Due to the complex structure of flow vortices, fluctuating terms of the flow, and synchronization between the instantaneous 2D anemometry and captured frames should be considered [62].

Flow visualization can also be carried out on the wake region of airfoil geometries. Lee et al. [99] and Han et al. [83] indicated the effectiveness of triangular micro-riblets on the NACA 0012 by the use of a two-frame PIV film measurement technique and ensemble averaging in the wake region behind the airfoil geometry. Sundaram et al. [53] conducted experimental evaluations on a wing by use of the oil-flow technique and titanium dioxide as a pigment for various angles of attack between 0$°$ and 6$°$. Results demonstrated deviations in streamlines for various incidence angles, indicating the presence and effects of adverse pressure gradients on the trailing edge region [68]. Using synchronized smoke wire, Lee et al. [92] investigated the positive effect of riblets in lifting the flow vortices over semi-circular riblet tips, leading to the reduction in wetted area. The authors reported the effect of riblets on the reduction of velocity fluctuations and turbulent kinetic energy inside the viscous sublayer compared to the flat plate. In order to hold wind tunnel testing that is representative of real turbulent boundary layers, it is essential to obtain fully developed turbulent boundary layers at the upstream location of the test plates or airfoils. Implementation of trip wire [40,54,55] or mounted sandpaper [34,40,90,101] upstream of the test plates is a common approach in these tests. In cases in which pressure gradients are to be present in the tests, pressure tapping is used to control static pressure variations [40].

Table 1. Experimental studies of riblets in the wind tunnel.

Researcher	Laboratory	Riblet Geometry	Material	Instruments	Description
Walsh [23]	NASA Langley, close loop low-speed wind tunnel, Test section: $0.279\times 0.178\times 0.914 \mathrm{m}$	Flat plate, triangular	Aluminum machined plate	Floating balance, hot wire	$s^{+}$ = 12, $h^{+}$ = 12, ${Re}_{\theta }=867-3900$ U = 7.6–43 m/s Drag reduction: 4%
		Flat plate, semi-circular			$s^{+}$ = 16, $h^{+}$ = 8, ${Re}_{\theta }=867-3900$ U = 7.6–43 m/s Drag reduction: 4%
Choi [65]	Applied Fluid Mechanics Division British Maritime Technology Ltd., Open loop wind tunnel, Test section: $4.8\times 2.4\times 15 \mathrm{m}$	Flat plate, bladed	machine cut of soft steel	Surface flush hot film, flow visualization by smoke wire, microphone	${Re}_{\theta }$ = 4600, $h^{+}=12$, $s^{+}=20$, U = 3 m/s
Gaudet [88]	Royal Aerospace, UK Test section: $0.6\times 0.45 \mathrm{m}$	Flat plate, triangular	3M	Floating balance, traverse, pressure probe	h = 50 mic, h/s = 1, Re > $4\times {10}^6$ M = 1.25 Drag reduction: 7%
Nieuwstadt et al. [55]	Technical University of Delft Test section: $0.73\times 0.89\times 5.7 \mathrm{m}$	Flat plate, triangular	Machined and flattened PVC pipe	Drag balance, pitot probe, static pressure tapings	APG and ZPG, h = s = 0.36 mm–0.64 mm, $s^{+}$ = 13, U = 4 m/s–17 m/s, Drag reduction in ZPG: 5% Drag reduction in APG:7%
Park et al. [80]	University of Maryland, open low-speed wind tunnel,	Flat plate, triangular	Fiberglass	Hot wire, sensor probe	$s^{+}\approx 28, h^{+}\approx 14$ U = 1–5.5 m/s, in 1.29 m/s the ${Re}_{\theta }=1200$ Drag reduction: 4%
Viswanath et al. [97]	National Aerospace Laboratories, India	Airfoil, Supercritical airfoil ADA-S1, triangular	3M	Pitot probe, static pressure tapings,	M = 0.6–0.76, Rec = 3 × 106 Riblet heights = 0.033–0.018 mm means $h^{+}=8-15$
Sundaram et al. [63]	National Aerospace Laboratories, India Test section: $1.5\times 1.5 \mathrm{m}$	Airfoil, NACA 0012 Triangular	3M	Hot wire, pitot probe, static pressure tapings, oil-flow visualization	U = 30 m/s, Rec = 1 × 106, $\alpha =0-6°$
Lee et al. [92]	Pohang University of Science and Technology, closed-loop wind tunnel, Test section: $0.72\times 0.6\times 6.2 \mathrm{m}$	Flat late, semi-circular		Hot wire, Olive-oil atomizer and Smoke wire, laser sheet, PIV	U = 3–5 m/s S+ = 25.2, 40.6 ${Re}_{\theta }$ = 2340–4950 h/s = 0.192
Han et al. [83]	Pohang University of Science and Technology, Closed-loop wind tunnel Test section: $0.15\times 0.15\times 1.8 \mathrm{m}$	Airfoil NACA 0012 and Cylinder, Triangular	Polydimethylsiloxane (PDMS) micro-molding technique	Drag balance, PIV	Spacing = 300 $\mathsf{\mu }\mathrm{m}$, height = 180 $\mathsf{\mu }\mathrm{m}$ Airfoil drag reduction: 4.3% in U = 3.3 m/s Cylinder drag reduction: 7.6% in U = 3 m/s
Lee et al. [99]	Pohang University of Science and Technology, closed-loop wind tunnel Test section: $0.15\times 0.15\times 1.8 \mathrm{m}$	Airfoil, NACA 0012, spaced triangular micro-riblet films	Polydimethylsiloxane (PDMS) micro-molding technique	Drag balance, PIV	$Re=1.54\times {10}^4\left(U=3 \mathrm{m}/\mathrm{s}\right),4.62\times {10}^4(U=9 \mathrm{m}/\mathrm{s})$, Airfoil chord = 75 mm, Drag reduction: 6.6% at U = 3 m/s
Saravi et al. [90]	Brunel University, Vertical blower Test section: $0.15\times 0.05 \mathrm{m}$	Flat plate, Serrate-Semi-Circular	Fly-cutting method on aluminum plate	Hot wire, Pitot probe Microphone	$s^{+}$ = 14–19.5, $h^{+}$ = 7.5–10.5, U = 30 m/s Drag reduction: 7%
Zhang et al. [40]	Chinese Academy of Sciences, Test section: $0.5\times 0.5\times 4 \mathrm{m}$	Flat plate, triangular		Hot wire, traverse mechanism	$s^{+}$ = $h^{+}$ = 18.7, U = 8.5 m/s, $R_e$ based on BL thickness = 2130–2101, Drag reduction = 6.6%
Muhammad et al. [51]	Brunel University, open loop wind tunnel, Test section: $0.5\times 0.5 \mathrm{m}$	Flat plate, trapezoidal	3D printing by stereolithography	Hot wire, probe microphone	$s^{+}$ = 27.1,31.9,38.7, $h^{+}$ = 12.2,14.3,17.4 U = 10, 12, 15 m/s,

Table 1. Experimental studies of riblets in the wind tunnel.

Researcher	Laboratory	Riblet Geometry	Material	Instruments	Description
Walsh [23]	NASA Langley, close loop low-speed wind tunnel, Test section: $0.279\times 0.178\times 0.914 \mathrm{m}$	Flat plate, triangular	Aluminum machined plate	Floating balance, hot wire	$s^{+}$ = 12, $h^{+}$ = 12, ${Re}_{\theta }=867-3900$ U = 7.6–43 m/s Drag reduction: 4%
		Flat plate, semi-circular			$s^{+}$ = 16, $h^{+}$ = 8, ${Re}_{\theta }=867-3900$ U = 7.6–43 m/s Drag reduction: 4%
Choi [65]	Applied Fluid Mechanics Division British Maritime Technology Ltd., Open loop wind tunnel, Test section: $4.8\times 2.4\times 15 \mathrm{m}$	Flat plate, bladed	machine cut of soft steel	Surface flush hot film, flow visualization by smoke wire, microphone	${Re}_{\theta }$ = 4600, $h^{+}=12$, $s^{+}=20$, U = 3 m/s
Gaudet [88]	Royal Aerospace, UK Test section: $0.6\times 0.45 \mathrm{m}$	Flat plate, triangular	3M	Floating balance, traverse, pressure probe	h = 50 mic, h/s = 1, Re > $4\times {10}^6$ M = 1.25 Drag reduction: 7%
Nieuwstadt et al. [55]	Technical University of Delft Test section: $0.73\times 0.89\times 5.7 \mathrm{m}$	Flat plate, triangular	Machined and flattened PVC pipe	Drag balance, pitot probe, static pressure tapings	APG and ZPG, h = s = 0.36 mm–0.64 mm, $s^{+}$ = 13, U = 4 m/s–17 m/s, Drag reduction in ZPG: 5% Drag reduction in APG:7%
Park et al. [80]	University of Maryland, open low-speed wind tunnel,	Flat plate, triangular	Fiberglass	Hot wire, sensor probe	$s^{+}\approx 28, h^{+}\approx 14$ U = 1–5.5 m/s, in 1.29 m/s the ${Re}_{\theta }=1200$ Drag reduction: 4%
Viswanath et al. [97]	National Aerospace Laboratories, India	Airfoil, Supercritical airfoil ADA-S1, triangular	3M	Pitot probe, static pressure tapings,	M = 0.6–0.76, Rec = 3 × 106 Riblet heights = 0.033–0.018 mm means $h^{+}=8-15$
Sundaram et al. [63]	National Aerospace Laboratories, India Test section: $1.5\times 1.5 \mathrm{m}$	Airfoil, NACA 0012 Triangular	3M	Hot wire, pitot probe, static pressure tapings, oil-flow visualization	U = 30 m/s, Rec = 1 × 106, $\alpha =0-6°$
Lee et al. [92]	Pohang University of Science and Technology, closed-loop wind tunnel, Test section: $0.72\times 0.6\times 6.2 \mathrm{m}$	Flat late, semi-circular		Hot wire, Olive-oil atomizer and Smoke wire, laser sheet, PIV	U = 3–5 m/s S+ = 25.2, 40.6 ${Re}_{\theta }$ = 2340–4950 h/s = 0.192
Han et al. [83]	Pohang University of Science and Technology, Closed-loop wind tunnel Test section: $0.15\times 0.15\times 1.8 \mathrm{m}$	Airfoil NACA 0012 and Cylinder, Triangular	Polydimethylsiloxane (PDMS) micro-molding technique	Drag balance, PIV	Spacing = 300 $\mathsf{\mu }\mathrm{m}$, height = 180 $\mathsf{\mu }\mathrm{m}$ Airfoil drag reduction: 4.3% in U = 3.3 m/s Cylinder drag reduction: 7.6% in U = 3 m/s
Lee et al. [99]	Pohang University of Science and Technology, closed-loop wind tunnel Test section: $0.15\times 0.15\times 1.8 \mathrm{m}$	Airfoil, NACA 0012, spaced triangular micro-riblet films	Polydimethylsiloxane (PDMS) micro-molding technique	Drag balance, PIV	$Re=1.54\times {10}^4\left(U=3 \mathrm{m}/\mathrm{s}\right),4.62\times {10}^4(U=9 \mathrm{m}/\mathrm{s})$, Airfoil chord = 75 mm, Drag reduction: 6.6% at U = 3 m/s
Saravi et al. [90]	Brunel University, Vertical blower Test section: $0.15\times 0.05 \mathrm{m}$	Flat plate, Serrate-Semi-Circular	Fly-cutting method on aluminum plate	Hot wire, Pitot probe Microphone	$s^{+}$ = 14–19.5, $h^{+}$ = 7.5–10.5, U = 30 m/s Drag reduction: 7%
Zhang et al. [40]	Chinese Academy of Sciences, Test section: $0.5\times 0.5\times 4 \mathrm{m}$	Flat plate, triangular		Hot wire, traverse mechanism	$s^{+}$ = $h^{+}$ = 18.7, U = 8.5 m/s, $R_e$ based on BL thickness = 2130–2101, Drag reduction = 6.6%
Muhammad et al. [51]	Brunel University, open loop wind tunnel, Test section: $0.5\times 0.5 \mathrm{m}$	Flat plate, trapezoidal	3D printing by stereolithography	Hot wire, probe microphone	$s^{+}$ = 27.1,31.9,38.7, $h^{+}$ = 12.2,14.3,17.4 U = 10, 12, 15 m/s,

Choi [62] conducted a study where instantaneous wall skin friction signals were collected using visualization systems. The author established a correlation between the structures of low-speed vortices, characterized by counter-rotating behavior and hairpin loop legs. Through his flow visualization technique, a conceptual definition of burst frequencies in unsteady flow conditions was proposed. This was achieved by observing the deformation of vortex structures, where hairpin loops moved away from the wall due to shear stress and self-induction of velocity fields. Yang et al. [102] investigated turbulent coherent structures using the Time-Resolved Particle Image Velocimetry (TRPIV) technique. Turbulent flow was analyzed using planar TRPIV measurements. The results demonstrated the positive impact of riblets on the energy of spanwise velocity components during sweep and ejection events.

3.2. Flight Tests and In-Service Applications of Riblets

The previous section provided a brief overview of wind tunnel tests and testing techniques specifically focused on flat plates and airfoils. In this process, riblets are also tested during flight, aiming to analyze their performance and find out their real performance compared to the wind tunnel tests and numerical simulations. This is an important step in the process of commercialization when it comes to analyzing complex flow structures such as unsteady flows, different pressure gradients, misaligned flows, and wing–body flow interactions.

It is noted that the overall positive benefit of using riblets in the profit of airlines for one year of in-service operation of a long-range aircraft would be roughly one million dollars per plane [6]. This calculation is based on 3% drag reduction by riblets, which reduces fuel consumption and consequently decreases the amount of carried fuel and allows replacing this load with extra commercial payloads. This is a good indication of riblets’ performance

Over the past few decades, several research activities have focused on commercial aircraft [6,66,103,104,105]. These studies have explored the effectiveness of riblet geometries through various approaches. Some investigations involved applying riblet patches over the aircraft body to measure total pressure, conduct drag balance measurements, and assess surface quality after flight. In contrast, other studies encompassed covering a significant portion of the aircraft with riblet films and evaluating their effectiveness by analyzing the aircraft’s fuel consumption [19,105,106].

Table 2. Application of riblets on the aircraft surface.

Institute/Airliner/Aircraft	Year	Description	Riblet	Drag Reduction
Boeing T-33 trainer [103]	1987	Wing sweep = 15° $Re=1.45~4.43\times {10}^6$, M = 0.35–0.7	3M	Friction drag reduction: 6–7%
Learjet 28/29/NASA [76]	1988	Lightweight plastic film was applied by use of two rakes and two drag balances h = s = 0.03 mm (first case) h = s = 0.075 mm (second case) Re = 1–2.75 × 106, M = 0.3–0.7	3M	Friction drag reduction: 6%
Airbus 300–600 [105] Airbus/Lufthansa/3M	1988	Riblet films with triangular cross sections, patched in 15 different areas Metallic riblet foil	3M	Only the deterioration and erosion were checked
Airbus 320 [105]	1989	Riblet films with triangular cross-sections 70% covering area (600 square meters)	3M	Net drag reduction: 2%
Airbus 340/Airbus/Cathay Pacific Airways [6]	1996	Triangular riblets with s = 0.06 mm 30% covering area	3M	Friction drag reduction: 4%
Aircraft Zivko Edge 450 V2./Red Bull Air Race/ Joanneum Research [85]	2008	Joanneum Research printed microstructures in a very thin adhesive film	Joanneum Research
Airbus 300–600ST BLUG/ Fraunhofer IFAM [66]	2011	Implementation of some patches on the body and wing s = 0.05 mm	Micro-structured paint, trapezoidal	Drag reduction effectiveness was evaluated after a period of in-service operation
Bionic Surface Technologies/ Swiss Air Race Team	2013		Bionic surface film
Airbus 340–300/Lufthansa/ Fraunhofer IFAM/BWM GmbH [85]	2014	Automatic application of microstructures	Riblet coating
Hisho (JAXA research aircraft) [104]	2018	h = 0.1 mm, M = 0.5–0.78	Aircraft paint	Demonstrated in different Mach numbers based on flight condition by graphs in log-low region
Lufthansa/Boeing 747–400	2019	The lower half of fuselage was covered with 500 square meters of riblet films with 100 × 50 cm film patches	BASF
Lufthansa/Boeing 777 F [19]	2021	h = 0.05 mm, 800 square meters of riblet films	BASF	Drag reduction: more than 1%
Swiss Airlines/Boeing 777–300 ER [106]	2021	h = 0.05 mm, 950 square meters of riblet films	BASF	Drag reduction: about 1.1%

provides a summary of the testing of riblets in the aviation industry.

Total pressure measurements have been implemented mostly in research activities by use of boundary layer pitot rakes [76,96,103]. McLean et al. [103] conducted flight tests with 3M plastic films, which were mounted over the wing surface of a T-33 aircraft. The effect of riblet films was investigated using pitot rakes positioned at 83% of the chord. Results indicated a 6% reduction in drag in the optimum condition, based on cruise Reynolds number and non-dimensional spacing between 10% and 15%.

Walsh et al. [76] tested riblet geometries on the Learjet 28/29 twin-engine business jet. The instrumentation was installed in the baggage compartment area, and tests were carried out by the 3M Company, using a $30\times 90 \mathrm{c}\mathrm{m}$ patch of two different sizes of triangular microgrooves. The test areas were equipped with two boundary layer-type total pressure rakes and two drag balance mechanisms to evaluate the drag force in both smooth and ribbed surfaces. To ensure the effectiveness of the process, two test panels were considered, i.e., a smooth panel for reference and another for measuring the flow properties after the riblet patch. Measurements from the boundary layer pitot rakes indicated a reduction in drag of about 6%, while drag balance required some correlations due to the crossflow angles resulting in degradation of the drag value.

Sasamori et al. [96] and Kurita et al. [104] investigated the drag reduction of trapezoidal riblets by use of two pitot pressure rakes on the fuselage of a JAXA research aircraft. The position of the riblet surface and pitot rakes are illustrated in Figure 4. During flight tests, the window was replaced with a flush-mounted metal plate aiming to embed the two pressure rakes. Mean velocity measurements in the $y^{+}$ between 100–10,000 show the increase of the U-velocity based on the direction of pitot rakes.

In-service tests indicate the effectiveness of covered-by-riblet surfaces in reducing fuel consumption. In the Szodruch [105] research study, 70% of the area, or 600 square meters of Airbus 320 aircraft surface, was covered by triangular riblet films with about 80 kg of additional weight. Based on the fuel consumption, the results show a 2% reduction in drag.

During the past years, Lufthansa has produced comprehensive research studies on the effectiveness of riblets besides the possibility of applying them on the aircraft surface [85]. Lufthansa, Bionic Surface Technologies, and Joanneum Research have developed and tested aviation-size trapezoidal riblet films. In 2019, tests were performed in a Boing 747–400 aircraft, with 500 square meters of surface covered by riblets. Lufthansa, Technique and BASF [19] made a successful collaboration for the application of riblet films in the 800 square meters of Cargo Boeing 777 F surface by using trapezoidal riblets with a size of about 0.05 mm in height and spacing. Flight showed an annual saving of around 3700 tons of kerosene for the fleet of ten aircraft, with a reduction of about 11,700 tons of carbon dioxide when using the riblets. The result was more than 1% of drag reduction.

After the successful application of riblets on a Boing 777 F, Swiss International Airlines (SWISS), in collaboration with Lufthansa and BASF, implemented riblet film technologies on the twelve Boing 777–300 ER aircraft by using 950 square meters of riblets-covered surface, which lead to 4800 tons of kerosene annual savings and approximately 1.1% reduction in drag. This reduction in fuel consumption corresponds to a decrease of 15,200 tons of carbon dioxide.

In 1988, Airbus, Lufthansa, and 3M Company conducted tests with 15 samples of riblet films mounted on the body and wing of an Airbus 300–600 [105]. After 18 months and checking the durability of riblet patches, no serious damages to the riblet geometries and no degradation in the properties of the riblet films were identified. Stenzel et al. [66] conducted flight tests to investigate the use of micro-riblet coatings on the fuselage and wing of an Airbus 300–600 ST BELUGA. The method of fabrication is also discussed in the next section. The research included a special method for data gathering, continuously carried out for 12 months, in which the evaluation of riblet effectiveness by replication and testing in the laboratory aimed to evaluate the degradation effects. In the meantime, a collaboration between Lufthansa, Fraunhofer IFAM and BWM GmbH investigated the effectiveness of riblet patches on the wing of an Airbus 340–300 [85]. The main target of this activity was the time of implementation of riblets over the aircraft surface, and no results were found about the effectiveness of their method.

4. Manufacturing and Fabrication of Riblets

During the past few decades, the fabrication of riblet geometries has been implemented through several different methods in order to fulfill the requirements of experimental research studies. An array of research activities introduced different procedures of fabrication as a part of their studies on the effectiveness of riblets [55,65,80,108,109,110]; however, a few studies have been specifically devoted to investigating different methods of industrial fabrication [45,66,84,86,104]. Research works that are developed in academic environments have been mostly conducted with cost-effective fabrication techniques, using small-size flat plates or extruded airfoils, while in commercial applications, some other parameters such as weight, time of implementation, durability and maintenance would be more important [18,56]. An overview of manufacturing methods with their advantages and disadvantages is presented in

Table 3. General strategies in riblets fabrication.

Method	Advantages	Disadvantages
Milling [34,65,90,110,111]	High precision Ability to make different patterns Wide material compatibility Suitable surface finished Suitable for research	Time-consuming Material removal Possibility of misalignment
Grinding [109]	Offering excellent surface-finished Material versatility	Time time-consuming but faster than milling method Limited to specific materials Potential for abrasive wear Limited number of patterns Material removal Possibility of misalignment
Rolling [112,113,114]	Ideal for deformable materials No material removal Uniform riblet patterns Quick application for industrial use	Limited to deformable materials Initial tooling setup Possibility of misalignment
3D printing [33,51]	High customization Complex shape compatibility Suitable for research	Material limitations Variable implementation time and cost
Casting and mold making [89,115,116]	Effective replication Versatile material compatibility	Time-consuming mold creation Possibility of misalignment
Chemical Etching and mold making [83,99,117]	High-precision micro-riblets Repeatability Moderate implementation time	Limited to specific materials Chemical handling Shape limitations
Adhesive films [85,118]	Ease of application Time of application Uniform thickness Suitable for aviation	Difficulties during the application on curved surfaces Possibility of detachment during the flight Limited in riblet configurations Application challenges Environmental impact
Painting [66,67,84,119]	Quick application Cost-effective Suitable for complex shapes In some cases possible to have different height Durability Suitable for aviation	Limited precision Environmental impact

.

From the laboratory tests point of view, riblets have been designed and manufactured in-house with specific strategies to cover the requirements of the test. Milling is a widely used method for producing riblet geometries [25,110]. This approach allows for the creation of various riblet configurations. In a study by Nieuwstadt et al. [55], PVC pipes were employed, and lathe machining was utilized to produce triangular-shaped riblets. Subsequently, the pipes were flattened to create flat riblet plates with equal height and spacing. Husen et al. [110] implemented a CNC five-axis milling micro-machining for producing different sizes and configurations of 2D and 3D riblets. Their method was able to produce modular ribbed plates, which were tested in a wind tunnel. The authors developed an iterative process to finalize the commands and toolpaths by use of CAM design and CNC machining and evaluated the quality of production by laser confocal microscope. Similar to Husen et al. [110], Siegel et al. [111] applied the micro-machining technique by use of a diode-pumped, regenerative-amplified ps-Nd:YVO4-laser source on eight different types of metal.

During the riblets fabrication process, the positioning of instruments should be strategically considered. Choi et al. [65] used machine cuts of soft steel flat plates to produce bladed riblets, and pin-hole probes were mounted for pressure measurements between the ribs. 3D printing methods are another approach enabling the manufacture of riblet plates of different shapes. In a similar investigation to Choi et al. [65], Muhammad et al. [51] used a Stereo Lithography Apparatus (SLA) 3D printing technique to manufacture the riblet test plates, with the possibility of mounting pin-hole probes. Likewise, Wen et al. [33] produced riblets by using multi-materials 3D printing. They designed and fabricated a sheet of flexible shark skin by use of micro-CT imaging from the shark, in which thousands of 3D-printed synthetic scales were mounted in a linear array pattern on the flexible membrane.

Mold-making and casting is another approach applied to replicate shark skin geometries [89,115,116]. A method of vacuum casting was used for bio-replicating different scales of shark skin. Zhao et al. [89] implemented unsaturated polyester resin under vacuum conditions to produce a mold, in which the cavity inside the mold was then poured with silicone rubber in a vacuum and using an oven to produce the film of shark skin. Zhang et al. [116] conducted a synthetic bio-replication from pre-treated shark skin to produce flexible female silicon rubber by soft die formation. They applied a nano-long-chain drag reduction interface by use of a water-borne resin epoxy.

Etching techniques have also been explored for the manufacture of riblet geometries. Generally, the etching process has been used for mold making due to the possibility of making negative volumes [83,99,117]. The fabrication of microstructure triangular riblets by use of anisotropic etching, performed by Han et al. [83] and Lee et al. [99], made a silicon wafer with tetramethylammonium hydroxide (TMAH) as etchant. The authors produced micro-riblets with a polydimethylsiloxane (PDMS), and the micro-molding technique was used by manufacturing silicon wafers with micromachining (photolithography and anisotropic etching). They produced a 180 $\mathsf{\mu }\mathrm{m}$ height and 300 $\mathsf{\mu }\mathrm{m}$ spacing of triangular riblets, in which the mold space was filled by PDMS (poly-dimethyl-siloxane). The method was also used over the curved surfaces. The authors successfully tested and fabricated the surface on a NACA 0012 airfoil and cylinder geometries. This method requires more caution during the implementation process to avoid misalignments between the pattern of the UV mask and the silicone mold, which can reduce the quality of fabrication. Barbier et al. [117] fabricated microstructures by using both the anodization and the etching methods. The fabrication was iteratively controlled by the use of anodization and pore-widening treatments.

The use of microscale features with indentations aligned to the flow can be an effective solution to reduce drag by up to 10% [67]. In this context, a number of techniques are available for laboratory tests. One of the main constraints on the fabrication process is the flow properties passing through the surface. This directly depends on the flight condition with the ability to make riblets in the order of microns to meet the optimum operation of riblet surfaces [18,21]. As an example, for near transonic commercial aircraft, due to the reduction in density and other aerodynamic parameters, micro-scales of height (around $80 \mathsf{\mu }\mathrm{m}$) are required for optimum drag reduction [50]. By focusing on the aviation industry, microfabrication techniques were investigated during the past decades, with successful results [19,66,86,120].

Industrial manufacturing of riblet films was introduced by 3M Company with the development of polyvinylidene fluoride plastic films using conventional treatments [118], such as rolling and casting, with h/s = 1 triangular riblet patterns and with the possibility of implementing curved and riveted surfaces. Several experiments were conducted for 3M products in both laboratory and flight tests (see Table 1). In another research study, Bionic Surface Technology and Joanneum Research [85] printed microstructures using a special process on a very thin adhesive film. BASF, a chemical producer company, worked in collaboration with Lufthansa to develop industrialized versions of adhesive riblet films. The films were tested as patches on the Boeing 747–400. Although adhesive films were easily applied on about 70% of aircraft surfaces, they had a few drawbacks, such as difficulties during the application on curved surfaces and the possibility of detachment during the flight [25]. In the meantime, to overcome the disadvantages of riblet adhesive films, innovative paint applications were introduced by researchers [67,84,104] and the Fraunhofer Institute for Manufacturing Technology (Table 1 and Table 2). The process was applied by a transparent and flexible silicon endless belt. Gruneberger et al. [67] and Stenzel et al. [66] carried out this technique with nanoparticle-reinforced painting of large parts, like aircraft and ship surfaces. The process consists of coating, embossing and partial curing in a single step. A roller-based mechanism of painting is used with continuous UV-curable urethane-acrylates that enable a microstructure surface with proper durability and reasonable costs; the mechanism is illustrated in Figure 5a. This process was applied by a robot using an automatic application of microstructures on the surface of aircraft and was launched as a collaboration between Lufthansa, Airbus, BWM GmbH, and Fraunhofer IFAM in a project named FAMOS and included a multi-functional coating [85]. The automatic application of riblet coating is depicted in Figure 5b.

Lithographic methods can also be used for printing micro-grooves on the surface of flying vehicles [5,45,86]. Bilinsky [86] and Bilinsky et al. [45] applied a photo-curable coating method over a military aircraft. The polymer material was a combination of three important parts containing monomers, oligomers, and photoinitiator that, in exposure to UV light, makes a strong polymer. The main objective of this method was to achieve continuous and contactless production of riblets (Figure 6), which enables the change in height and shape of the riblet structures with optimum implementations over the aircraft surface.

Due to the interaction of airflow over the aircraft body and wing, the optimum selection of riblet height is an important consideration. The lower cost of implementation, quality of micro-fabrication, and increased lifetime were some of the advantages of this method.

5. Computational Efforts

Numerical studies are widely implemented for the investigation of flow properties over riblet surfaces. Although experimental studies offer a good understanding of the flow properties within the boundary layer, numerical methods could be efficient enough to evaluate the behavior of the flow over the riblet surfaces.

On this basis, the flow properties are numerically evaluated by use of Direct Numerical Simulations (DNS), Large Eddy Simulation (LES) and Reynolds Average Navier-Stokes equations (RANS). All flow scales would be resolved by the DNS technique, which requires more computational time and therefore remains unpractical for high Reynolds number flows and complex 3D geometries such as aircraft surfaces.

In the realm of simulating riblet performance, the DNS method has been the primary choice. However, owing to the computational demands associated with DNS, the vast majority of evaluations are carried out using flat plates, particularly at low Reynolds numbers. Additionally, researchers have extended riblet simulations to include airfoil configurations, which may take the form of 2D extruded shapes or intricate 3D wing geometries. These simulations have employed either DNS or LES techniques. To address the computational challenges posed by these methods, the RANS technique has been used as an alternative approach.

Within this context, it is important to note that over the past few decades, numerous flow simulations have been conducted to analyze the performance of riblet geometries. These simulations encompass a wide range of scenarios, starting with basic flat plates and progressing to more complex configurations, including extruded airfoils, wing–body combinations, standard aircraft models, and unmanned aerial vehicles (UAVs). In the following paragraphs, an overview of the research efforts dedicated to the abovementioned categories is presented.

Flat plate simulations: Previous DNS numerical investigations in riblet geometries were only devoted to a simple simulation of a flat plate in low Reynolds numbers [78,79,121,122,123,124]. Choi et al. [78] investigated drag-reducing configurations by use of DNS for triangular shapes in a fully developed turbulent channel at low Reynolds numbers. The authors indicated the effect of small spacing on the riblet tips wetted area due to the restricting position of longitudinal vortices. A virtual origin was defined at the location of maximum turbulent kinetic energy production, at around $y^{+}\approx 13$. By increasing the sharpness, the virtual origin moved up and became closer to the riblet tips. They indicated an upward shift in the logarithmic region, and a 5–6% reduction in friction drag was observed for $s^{+}=20$.

The mechanism of lifting streamwise vortices was also investigated by some researchers. Zhang et al. [125] carried out DNS to investigate turbulent characteristics and drag reduction in a compressive flat plate flow by comparing vorticity fluctuations and streak structures in the riblet plate and the smooth surface. Streaks are elongated regions in a plane parallel to the wall where the instantaneous velocity over there is below the average. The results showed an upward shift in the longitudinal vortices (see Figure 7), resulting in fewer interactions with the wall and suppression of the intensity in Reynolds shear stress. In a similar study, Choi et al. [78] investigated the size of the vortices and riblets in various 20 and 40 non-dimensional spacing, based on the non-dimensional diameter of vortices (which was roughly located at $y^{+}\approx 13)$ to indicate the diameter of streamwise cylindrical vortices.

Besides two-dimensional riblet families, three-dimensional geometries were also investigated by some researchers. Boomsma et al. [121] investigated three-dimensional shark skin replicated geometries in a channel flow by DNS. The authors reported that secondary flow generation within the turbulent boundary layer resulted in an increase in skin friction drag. Ran et al. [38] demonstrated the possibility of drag reduction by riblets of about 10%. The authors studied the effectiveness of triangular riblets with various sizes in a fully developed channel flow. Drag reduction was analyzed by changing spacing and height for different riblet tip angles. They also conducted a comparison with experimental results and demonstrated that at $s^{+}<20,$ the drag reduction is in their optimum values for ${Re}_{\tau }=186$.

Goldstein et al. [123] studied turbulent flow over riblets by directly integrating the equations of motion, modeling the damping effect of crossflow velocity fluctuations and displacement of turbulent quantities by use of LES. Although LES effectively simulated the larger scales of the flow, it still required a substantial amount of computational resources. Martin et al. [32] reported that the average vortex diameter is in the order of 20–45 of non-dimensional diameter, for Reynolds number values of 2500–13,000. Results showed both drag decrease and increase in the wide range of non-dimensional spacing. They applied the theory of cylindrical vortices to investigate the continuous and segmented riblets, by replicating them from shark skin. Impeding cross-flow motions by riblet surfaces has also been investigated by researchers. Choi et al. [78] pointed out that the reduction in the crossflow velocity vector is a result of the reduction in Reynolds shear stress. In a similar study, Tullis et al. [126] carried out time-dependent models of the viscous wall region with $y^{+}\approx 40$. Their studies reveal that lateral displacement of the flow towards the surface would be limited by riblets and, as a consequence, would lower the values of momentum in the riblet valleys.

The effect of applying riblets in the laminar to turbulent transition region was investigated by Klump et al. [127]. The authors investigated the effect of riblets installed on a flat plate with Zero Pressure Gradient (ZPG) by use of the Mono Integrated LES for streamwise aligned riblets amplification of T-S waves in laminar-turbulent transition observed for both K-type and oblique transition. In the process of transition, three main configurations are ᴧ-, hairpin vortex and turbulent breakdown. It resulted in the ᴧ-vortex structures in the K-type transition becoming weakened but not delayed by weak spanwise disturbances, while turbulent breakdown was delayed in the oblique transition.

Since the microstructure geometries over the surface require numerical simulations in high Reynolds numbers, DNS and LES methods are unpractical due to the high computational costs; a number of research studies are implemented by making modifications to turbulent models. This approach would more simply simulate riblet surfaces rather than exactly capturing all the flow scales inside the boundary layer [58,69,70,126,128]. The rough surface was an idea applied by Aupoix et al. [58]. The authors developed a numerical formulation in the turbulent model to sensitize to riblet geometries. The model contains two steps: (1) the first step was mainly developed according to the previous experimental data in triangular and semicircular grooves, with the related shifting of streamwise velocity over the surface; (2) in the second step, turbulent models were modified according to their contribution to the logarithmic region. The study looked at the effectiveness of their solution for Spalart–Allmaras and k-ω models in the domains where experimental data was available, while the lack of experimental data decayed the quality of the results. Moreover, the accuracy of the results was reduced by adding pressure gradients and crossflows.

Airfoil geometry simulations: Zhang et al. [129] employed the LES method to analyze the Eppler E374 airfoil under specific conditions: a free stream Mach number of 0.2, angle of attack of 3° and Reynolds number of $2\times {10}^5$. Triangular riblets were positioned from 30% to 99% of the airfoil’s chord length and the numerical trip was located at 13% of the chord. The airfoil model and computational grid are illustrated in Figure 8. The computational results, both with and without trip, closely matched the experimental data. The authors demonstrated that implementing triangular riblets leads to an increase in mean flow velocity in the streamwise direction and also effectively suppresses the Reynolds stress and pressure power spectrum density over most of the upper surface of the airfoil.

Wing body simulations: Mele et al. [69] investigated two models for estimating riblets performance. The turbulent model and slip length were based on the boundary conditions and introduced according to the k-ω turbulent model. The authors introduced the concept of slip-length as a representation of the tangent to the wall velocity components. Increasing the slip-length value leads to an upward shift of the log-law, indicating changes in the velocity profile near the wall. Mele et al. [70] applied the RANS equations for riblet geometries by considering them as a singular roughness problem and by modifying the Wilcox boundary condition in which the boundary condition for ω in the k-ω turbulence model for rough walls was modeled as a function of non-dimensional riblets cross-section area [57], introducing the $l_g^{+}$ value. They evaluated the effectiveness of micro-grooves on a flat plate, transonic airfoil, and standard NASA Common Research Model (CRM). Results showed the effect of riblets height on the optimum operation of riblet geometries.

The effect of riblet height on a wing body with Natural Laminar Flow (NLF) was also investigated [69]. By imposing a transition line on the wing and setting the turbulence model’s production terms to zero in the laminar zone, the NLF conditions were established. Riblets were then introduced in the turbulent zones of the wing body. The study found that the installation of riblets at various heights resulted in a significant reduction in drag. The drag reduction was particularly prominent during cruise conditions, with higher riblet heights demonstrating better drag reduction performance. Figure 9 depicts that the choice of riblet height plays a crucial role in optimizing the aerodynamic efficiency of NLF wing bodies.

UAVs simulation: the impact of riblets on a fixed wing, low-speed UAV investigated by Cacciatori et al. [37]. The authors utilized RANS and incorporated the riblet geometry as a homogenized boundary condition on a smooth surface at a free stream speed of 22 m/s. They made a virtual shift in wall units on the non-slipping wall parameter, by introducing a dimensionless upward adjustment to the streamwise velocity profile. This shift effectively exploited the presence of riblets through a slip boundary condition.

After testing this approach on both flat plate and NACA 0012 airfoil, the authors demonstrated that applying riblets across the entire surface of the UAV model resulted in a drag reduction of approximately 3%. Alternatively, by applying riblets only to the upper side of the wings, they achieved a drag reduction of approximately 1.7%. This means that by covering just around 29% of the total surface with riblets, one can achieve significant drag reduction, thereby reducing the cost of implementation and maintenance.

In another study carried out by Bliamis et al. [130], research was conducted on the HCUAV RX-1, a medium-altitude, long-endurance, fixed-wing UAV prototype. The researchers implemented a modification in surface roughness similar to the approach used by Mele et al. [70] that considers the riblet surface as a singular roughness problem. In the design conditions, the authors achieved a 5% reduction in drag. However, the reduction in off-design conditions was less pronounced. When considering the primary performance parameters for UAV missions, the results demonstrated a 7% improvement in endurance or a 5% enhancement in payload capacity.

A summary of numerical simulations is tabulated in the

Table 4. Numerical studies of riblets in aviation.

Reference/Year	Geometry	Method	Solver	Result and Discussion
Choi et al. [78]	Flat plate, triangular	DNS	In-house code	Re = 4200, Different triangular geometries $s^{+}\approx 20$ drag reduction: 5–6% $s^{+}\approx 40$ drag increased
Launder et al. [128]	Flat plate, triangular, semi-circular, bladed	RANS	Two-equation eddy viscosity model	10,000 < Re < 15,000 Drag reduction achieved for $h^{+}~10$ Little drag reduction in $h^{+}>15$
Chu et al. [79]	Flat plate, triangular	DNS	In-house code incompressible Newtonian fluid governed by NS	Re = 3500 Drag reduction: 6%
Goldstain et al. [123]	Flat plate, triangular	DNS	In-house spectral method code	Drag reduction: 4%
Klumpp et al. [127]	Flat plate, semi-circular	LES	In-house code
Aupoix et al. [58]	Flat plate, semi-circular, triangular	RANS	Modified k-ω and Spalart–Allmaras ONERA	Adverse pressure gradient Re_channel = 5000, h/s = 0.3 Drag reduction: 9% in APG ($\beta =0.25$)
Boomsma et al. [59]	Flat plate, semi-circular	LES	In-house code	Use of riblet experiments, sweep angle: 0, 10, 15, 20, 30 different s/h values
Martin et al. [32]	Flat plate, semi-circular, Blade, triangular	LES	ANSYS Fluent 14.5	ZPG and mild APG, ($\beta =0.5$), h/s = 0.5
Zhang et al. [129]	Flat plate, bladed, semi-circular, triangle	LES	In-house code	$h^{+}=8$ and $s^{+}=8.3-41.1$ Drag reduction in bladed: 11.6% Drag reduction in semi-circular: 5.6% Drag reduction in triangular: 4.1%
	Airfoil Eppler E374, triangular riblet film, numerical trip	LES		Comparing two grid sizes and Choi DNS data Streamwise riblet, Re = 2800 $s=0.1135\delta$, $h=0.0892\delta$
Ran et al. [38]	Flat plate, triangular	DNS	In-house code incompressible Newtonian fluid governed by NS	M = 0.2, $\alpha =3°$, $Re=2.0\times {10}^5$ Drag reduction of about 9.5% in drag at 45° of sweep angle
Zhang et al. [125]	Flat plate, triangular	DNS	In-house code	${Re}_{\tau }=186$ and ${Re}_{\tau }=547$ Different riblets tip angles: 45°, 60°, 75°, 90°, 105°
Mele [69]	Flat plate, triangular, transonic airfoil, NASA CRM	RANS	FLOWer	$s^{+}$= 30.82 $h^{+}$= 15.41
Sasamori et al. [96]	Flat plate, TRA2022 aircraft, spaced triangular	RANS	FaSTAR	s = 100µm for flat plate $50<s_{opt}<150$ for the aircraft
Cacciatori et al. [37]	UAV, triangular	RANS	OpenFOAM	Cruise speed: 22 m/s Re chord base: $5\times {10}^5$ Drag reduction about 3%
Bliamis et al. [130]	UAV, triangular	RANS	ANSYS Fluent	Cruise speed = 39 m/s Re chord base: $1.9\times {10}^6$ Drag reduction: 5%

.

Conducting comprehensive numerical simulations to determine optimal riblet configurations prior to their implementation is essential. These simulations encompass integrating aerodynamic properties over the aircraft surface. Notably, the optimal shapes and sizes of riblets might differ between commercial aircraft and UAVs due to the distinct operational requirements and aerodynamic characteristics of each aircraft type. This approach ensures that riblet designs are tailored to specific aircraft categories, maximizing their performance benefits and adaptability across diverse applications [96].

6. Crossflows and Riblet Surfaces

The effect of crossflows on the ribbed surfaces has been investigated in a number of studies [26,53,70,74,131]. Supercritical airfoils and swept wings are commonly employed in commercial aircraft to enhance stability and enable high-speed flights. However, one drawback of these design features is the occurrence of crossflows on the wing surface, which contributes to increased drag generation. In this regard, a number of research activities have been devoted to investigating the misalignment between streamlines and riblets. McLean et al. [103] installed 3M riblet films, with an angle of 15° of misalignment to the flight direction, on the wing of a T-33 aircraft with up to a M = 0.7 flight speed. It was shown that the effectiveness of riblet surfaces was reduced by half when compared to those aligned with the flow. In the same study with 3M riblet films, Walsh et al. [26] indicated unchanged characteristics of riblets until 15°, which were reduced to the smooth flat plate properties up to 30°. Gaudet [88] conducted tests at supersonic speed (M = 1.25), with ribbed geometries between 0° to 90°, by measuring drag from the misaligned flow with 5° steps. At 20°, the drag reduction effect of riblet geometries became halved, and at 30°, their effectiveness was lost completely. Hage et al. [74] tested four different geometries in various yaw angles in the range of 0–20°. The authors indicated that increasing yaw angle would have a negligible effect on the lower values of $s^{+}$ when compared to those with higher values. The study concluded that the decay in drag reduction was attributed to sloshing phenomena (the flow between ribbed geometries would move laterally between the rib geometries, which produced a motion normal to the wall).

To explore the complexity of aircraft surfaces leading to crossflows on the wing or wing–body flow interactions, detailed studies were conducted to find optimum riblet families with adjusted geometrical variables. The optimum design of riblet geometries, in terms of direction, configuration, height and spacing, should consider the flow behavior over the solid surfaces. Infinite wings were investigated by considering the effect of yaw angle [53,131]. Sundaram et al. [53] carried out experimental studies with a 25° yaw angle. The authors indicated that the angle of attack and riblet streamwise location would affect the effectiveness of drag reduction. By changing the incidence angle between 0° to 6°, they showed that the maximum drag reduction was continuously reduced from its maximum of about 8% at 0° to 1% at 6°. The rapid fall in riblet effectiveness was recorded for increased angle of attack with significant yaw angles. Zhang et al. [131] numerically studied triangular geometries mounted on four different wings with variations of sweep angle between 0° and 45°. The maximum drag reduction observed was 9.5% at $45°$ of yaw angle. The authors evaluated local streamlines behavior with the possibility of re-laminarization. They indicated that riblets have no effect on the direction of the streamlines for x/c < 0.3, while streamlines would be aligned with the direction of riblets for x/c > 0.3. Suppression of crossflows by riblets continued until the trailing edge region but rose for x/c > 0.99.

7. Pressure Gradients in Riblet Surfaces

Measuring the performance of riblets under the influence of different pressure gradients in flat plates [16,55,59,95,96,101,132] and curved surfaces such as those in aircraft [70,96] has been previously studied. The Clauser parameter ($\beta$) [94] has been defined to evaluate the effect of varying pressure gradients on the surfaces, as defined as follows:

$$\beta =\frac{{\delta }^{\ast }}{{\tau }_w}\frac{dp}{dx}$$

(2)

In which ${\delta }^{\ast }$, ${\tau }_w$ and dp/dx represent boundary layer displacement thickness, wall shear stress, and streamwise pressure gradient, respectively. Zero amount of Clauser represents ZPG, and negative and positive values represent Favorable Pressure Gradient (FPG) and Adverse Pressure Gradient (APG), respectively. Pulvin et al. [101] showed the positive effects of riblets for both FPGs and APGs by testing flat plates in a wind tunnel in which riblets were applied to both sides of the plate. The results showed a reduction in momentum thickness in the grooved surface under positive values of $\beta ,$ but it was reduced by increasing the Reynolds number toward $5\times {10}^5$ and $\beta$ values toward nine. Choi et al. [132] conducted tests with Clauser parameters above 5 with trapezoidal riblets. Their results did not show sensible change compared with a smooth surface. In the meantime, Nieuwstadt et al. [55] indicated the effectiveness of APG on the trapezoidal riblets by concluding that there is almost always a reduction in drag for almost all pressure gradients, except for higher values of the Clauser parameter.

The effect of semi-circular riblets in ZPG and mild APG flows was numerically studied over an airfoil with a high-resolution LES method by Boomsma et al. [59]. Simulations were carried out by changing $s^{+}$ and then comparing the results with Walsh et al. [23] and Bechert et al. [35]. Their simulations indicated only a small improvement in $\beta =0.5$ (see Figure 10), showing a moderate reduction in drag. This result was confirmed by Nieuwstadt et al. [62], which concluded that at mild APGs, the drag reduction increased by about 7%.

Sasamori et al. [96] investigated the relation of riblet drag reduction with $s^{+}$, and different pressure gradients by use of the Clauser parameter in flat plates and the TRA2022 conceptual aircraft. The study indicated the effect of pressure gradient based on the non-dimensional diameter of vortices in the different Clauser values as a function of normalized wall units and the impeding mechanism of longitudinal vortices. The drag reduction effect decreased for higher values of $\beta$ and $s^{+}$, respectively, while it increased for lower values of $\beta$ and higher values of $s^{+}$. Numerical simulations on the surface of the TRA2022 aircraft showed a small portion of the aircraft surface with $\beta \approx 2.0$ in the trailing edge position of the wing (roughly 15% of the total suction surface), as shown in Figure 11, while almost all of the aircraft surfaces had near zero pressure gradient. By comparing the original with the riblet surface, they estimated a total drag reduction of about 2%. As the friction drag was about 50% of the total drag, a 4% reduction in friction drag is roughly the same as the effectiveness of riblets in the flat plate with a ZPG. The study concluded that APG has less effect on drag reduction when all of the aircraft surface is covered by riblets.

8. Fluctuating Terms of the Flow

As the turbulent boundary layer is formed by passing flow over the solid surface, it would contain some unstable, three-dimensional, rotational, dissipative and highly disorganized flow elements in space and time. These structures are characterized by fluctuations of different scales in all directions, even though one can average these fluctuating components. Some research studies have been conducted to find out details about the flow behavior under turbulent boundary layers, especially their fluctuating effects. The aim of this section is to discuss the effect of riblet geometries in the fluctuating terms of the flow.

8.1. Riblet Effects on Velocity Fluctuations

Low-speed streaks are coherent structures that play a significant role within turbulent boundary layers. They are particularly relevant in the transition zone and contribute to the formation of turbulence. These streaks are characterized by regions of reduced velocity compared to the surrounding flow. They are closely associated with the bursting phenomenon, which marks the transition from laminar to turbulent flow. The presence and behavior of low-speed streaks influence the overall turbulence characteristics, including turbulence intensity and flow mixing. Studying the formation and dynamics of these streaks is crucial for a better understanding of turbulent flows and their impact on various flow properties.

The spanwise motion of low-speed streaks under the viscous sublayer would be restricted by using an optimized size and shape of riblet geometries. Optimized riblet geometries could also possibly produce lower levels of velocity perturbations [56,91,92]. This reduction has been studied in both streamwise [56,126] and spanwise [92] directions by considering different boundary layer heights. Several analyses showed that the turbulence intensity would be reduced for a specified amount of non-dimensional height above the riblet geometries. Choi [62] pointed out an increase in the amount of mean velocity over the bladed riblet surface, with a reduction of about 10% in the turbulent intensity compared to the smooth surface. The author reported the reduction in turbulence intensity just for the $y^{+}<70$. The results showed a reduction in maximum turbulence intensity for a smooth surface in $y^{+}=18$ with $u^{\prime }/U_{\mathrm{\infty }}=0.104,$ which was reduced to 0.0934 at $y^{+}=19$. Lee et al. [92] observed lower values of Root Mean Square (RMS) velocity fluctuations in the flat plate, inside semi-circular riblet valleys in drag reduction cases and $y^{+}<30$, while there were slightly lower or similar to smooth surface behavior at $y^{+}>30$. Viswanath et al. [56] indicated a reduction in turbulence intensity between 10–15% for $y^{+}<100$ in the trailing edge position of a NACA 0012 airfoil for 0–4° of inlet flow angels, but such reductions were observed for 5–6° in the $y^{+}<40$. Takahashi et al. [91] investigated the fluctuating terms of U velocity at 2 mm and 20 mm, with a boundary layer height of approximately 10 mm, and observed that turbulent structures would become favorable by reducing skin friction drag. Their measurements inside the boundary layer indicated an 8.3% reduction in turbulence intensity compared to the smooth plate, while it remained nearly unchanged in the outer boundary layer.

The reduction of turbulent intensity also affects the boundary layer transition time. Tullis et al. [126] indicated that the growth rate of the momentum thickness during the non-linear stage of the transition over a smooth surface is greater than over the ribbed surface. Additionally, the turbulence intensity is reduced by riblets, supporting the fact that the transition has been delayed. The evaluation of energy dissipation rate has been investigated by Takahashi et al. [91] and Sundaram et al. [63]. Energy dissipation is related to the small scale of turbulence by assuming a relation between producing and dissipating among large and small-scale turbulence structures. Riblet surfaces showed lower dissipation (about 18%), while large-scale turbulent structures became larger in the streamwise direction. Sundaram et al. [63] observed the reduction in energy levels in a NACA 0012 airfoil for all ranges of incidence angle, with $\beta =0.5-1.06$ at $y^{+}=20$, and low-frequency range (lower than 200 Hz) on the riblet surface. The effect of riblets on Reynolds shear stress profiles has also been studied by Viswanath et al. [56], Suzuki et al. [68] and Walsh [29]. Walsh [29] reported a reduction of about 10% in the $y/\delta \approx 0.1$ for ZPG flows. Viswanath et al. [56] results indicated the relative contribution of sweep (Q4) and ejection (Q2) events investigated by quadrant contribution of $u^{\prime }{v}^{\prime }$. For the riblet case with $\alpha =0°$, and for the duration of sweep and ejection close to the wall, the contribution of mean stress was higher while there was a reduction in ejection. The total amount of shear stress from Q2 and Q4 motions produced a small reduction in cases with riblet geometries.

By implementing drag-reducing riblets, it is possible to reduce velocity fluctuations, which, in turn, suggests the potential for reducing fluctuations in wall pressure. This relationship between the vortical velocity field and wall pressure sources is highly relevant to the phenomenon known as airfoil self-noise, which will be discussed in the following section.

8.2. Effects of Riblets on Wall Pressure Fluctuations

The generation of noise by airfoils interacting with turbulent flows is a significant concern in the field of aviation. Understanding how riblets can affect velocity and pressure fluctuations contributes to the exploration of noise reduction strategies and the development of quieter and more efficient aircraft designs. Wall pressure fluctuations due to the turbulent boundary layer associated with the application of riblets were investigated by a few researchers [51,62]. Choi [62] compared wall pressure fluctuations inside the turbulent boundary layer with and without riblets. He found that the RMS intensity of the wall pressure would be reduced by about 4% on the riblets’ surface, representing a positive effect on the dissipation of turbulent structures [65]. The author used the Variable Interval Time Averaging (VITAL) method to evaluate the near-wall bursts in a three-dimensional view. Results showed that burst signatures of wall pressure fluctuations over the riblet surface would be dissipated rapidly when compared to the smooth surface. Choi shows the results for the trapezoidal bladed surface and the effect of wall pressure spectra over the flat plate. The energy level was decreased near the wall for the low-frequency region (below 20 Hz), while it increased at higher frequencies (between 20 to 100 Hz) and remained unchanged for frequencies above 100 Hz. Choi performed similar analyses with near-wall miniature hot film sensors with a frequency rate lower than 300 Hz. The results showed a similar low-frequency reduction in the wall pressure fluctuations spectrum [62], as illustrated in Figure 12.

Similar to Choi [38], another investigation was conducted by Muhammad et al. [51] to evaluate the energy signatures of the turbulent boundary layer in three different ranges of frequencies. The frequency range $0.2\le f\le 2 \mathrm{kHz}$ was defined as low/mid frequencies, and $2\le f\le 6 \mathrm{kHz}$ was considered the high-frequency range. The large size of turbulent eddies refers to the low-frequency spectrum and considers spectrum decay of about $f^{-1/2}$, while $f^{-5/3}$ corresponds to the mid-range and $f^{-5}$ to the smallest size of turbulent eddies. Figure 13 shows the obtained data in three different axial locations at 10 m/s of freestream speed. Based on the provided information, it seems that riblet surfaces have been observed to exhibit lower values of the wall pressure spectrum compared to smooth surfaces in both low and high-frequency regions. While the wall pressure spectrum shows almost the same decay rate in comparison to the smooth surface, the high-frequency zone represents a higher level of frequency decay in $f^{-5}$. The analyses from Choi [25,62], Muhammad et al. [34] in pressure fluctuations, and Sundaram et al. [39] in velocity fields indicated that riblets have the potential for modifications in low and high frequencies by affecting large- and small-scale turbulent structures. As these structures have a direct relation to sublayer low-speed streaks, force alignment of these vortices by riblet valleys would significantly reduce the signature of turbulent energies [65]. Although a decrease in turbulent structures was observed in the low and high frequencies, the mid-frequency becomes slightly increased.

While a decrease in turbulent structures was observed in the low and high frequencies on riblet surfaces, there appears to be a slight increase in the mid-frequency range. This increase is believed to be a result of modifications to the boundary layer through momentum exchanges between the inner and outer layers, which is facilitated by the riblets. The reduced boundary layer thickness caused by the presence of riblets limits the energy in the mid frequencies compared to smooth surfaces, resulting in a slight increase in the wall pressure spectrum [62].

8.3. Far-Field Noise

Far-field noise generation is dependent on the hydrodynamic field under the developed turbulent boundary layer. The reviewed studies evaluate the near-wall levels of flow perturbations and their relation to the far-field noise [60]. The analytical approaches mainly based on Green’s formulation made their way to a relation between far-field scattered power spectrum and boundary layer frequency spectrum. Green’s formulation, Ffowcs Williams et al. [133], and Howe and Amite [134] models relate wall pressure fluctuations to the acoustical far-field noise. Figure 14 shows the quality of these approaches when compared to experimental data for a control diffusion airfoil with an angle of attack of 8°.

According to this comparison, the low frequencies are in good agreement with the experimental data, while Howe’s model shows higher values when compared to Amiet’s model. At higher frequencies, values from Howe’s model are close to values from Amiet’s model, although Howe’s model does not predict broadband humps while Amirt’s model does. By utilizing these theoretical approaches, Muhammad et al. [51] made a comparison between smooth and riblet airfoils by use of the Amiet [134] formula, according to wall pressure spectra and turbulent lateral coherent length. The calculated far-field noise was reduced in the low-frequency range (150 < f < 600 Hz), while in the mid-frequency (600 < f < 2000 Hz), a similar or slightly increased trailing edge noise was observed, with a positive effect in higher frequencies (2000 < f < 6000 Hz), and appropriate range of the freestream velocity based on the designed riblet geometry.

Although the interaction of turbulent length scales can potentially scatter into broad frequencies with powerful acoustic disturbances propagated to the far field, analyzing the effect of riblets geometries on the far-field noise is an important research area that requires more detailed evaluations, especially when the relations between near-field pressure perturbations are not directly related to the far-field propagated noise. Moreover, due to the number of experimental limitations of the theoretical correlations and models available, the predictions lose accuracy when used to evaluate surfaces that contain riblets.

9. Contamination and Deterioration in Riblets

The long-term performance of riblet designs can be influenced by extended exposure to operational conditions, weather variations, and maintenance schedules. Factors such as UV radiation, temperature fluctuations, and particle interactions can contribute to the degradation of riblet surfaces over time, potentially affecting their effectiveness in drag reduction. Maintenance schedules that account for these factors, aligned with regular aircraft maintenance cycles, are crucial to maintaining riblet performance. In this issue, developing accurate predictive models for riblet degradation and maintenance intervals is a complex endeavor that requires a comprehensive understanding of material behaviors, environmental impacts, and operational requirements.

By applying polymer-based micro-structure riblets on aircraft surfaces, an essential factor would be their durability under the aircraft in-service environment. As the aircraft surface is confronted with external and weather factors, the lifetime of these micro-grooves ideally should be aligned with the maintenance intervals of an aircraft, e.g., depainting and painting cycle every five to seven years. Strong radiation of UV light can potentially affect the effectiveness of riblet geometries or, in the case of adhesive-backed riblet films, can strongly damage the adhesion quality [18]. This degradation would be even more important when the variation of the ambient temperature from flight conditions (around −60 °C) to extreme ground conditions (around 50 °C) is added to the problem.

On the other hand, the impact of high-velocity particles, including dust, water droplets, and ice, could potentially erode or remove riblets. In 1988 [83], Lufthansa conducted long-term studies on the triangular riblets of 3M Company by applying 15 riblet patches that were mounted over the surface of an Airbus 300–600 to analyze the effect of erosion on the geometries. After 18 months of in-service operation of the aircraft, the patches were carefully removed and tested by Airbus industry partners (Aerospatiale, British Aerospace, Deutsche Airbus, and 3M Company). Despite high-risk areas like the fuselage nose and all leading edges, the other parts did not show serious damage. Airbus, in collaboration with Lufthansa and Cathy Pacific Airways, initiated a test to monitor the degradation of the riblets film in commercial flights. For this purpose, patches of 3M riblet films were attached to the Airbus 340 aircraft. The results suggested that the riblet films need to be replaced every 2 to 3 years to maintain the drag reduction capacity [25].

Stenzel et al. [66] conducted in-service investigations for about twelve months by mounting patches of micro-fabricated trapezoidal riblets over the body and door of the Airbus 300–600 ST BELUGA to investigate the durability of the coatings. They used the dentist’s casting compound to make a mold of micro-fabricated patches for a specified period of time. The negative periodic generated mold was scaled up in the order of 100 for testing in the Berlin oil tunnel [47]. Their microscopic investigations revealed that the process of deterioration was non-homogenous. As illustrated in Figure 15, the effect of degradation on the negative mold of micro-riblets shows more degradation on the door patches than on the aircraft body patches. Figure 16 shows the oil tunnel measurements after application and cleaning of the paint bumps, as well as the measured drag after 411 flight hours. It appears that investigating the deterioration of the performance of riblet surfaces would be an essential area, which would also affect the procedures for riblet fabrication and would clarify their maintenance intervals in the procedures for in-service applications.

10. Highlighted Challenges

The previous sections provided a review of research studies that focused on examining the impact of riblet surfaces on flow properties within the boundary layer. These studies have consistently demonstrated the positive effects of riblet geometries in reducing drag. However, there are specific activities that should be pursued to further advance this field, and corresponding recommendations for future studies are outlined.

It is crucial to recognize that while drag reduction is highly advantageous for the aviation industry, it is important to consider other factors, such as the aeroacoustic properties of riblet surfaces and their potential degradation over time. Moreover, as the turbulent spots typically exhibit relatively small levels of fluctuating wall pressure sources, it is recommended to conduct noise studies in a controlled experimental environment, such as an anechoic wind tunnel, to minimize external noise interference.

Although great effort has been made to optimize riblet design for drag reduction, it is equally important to focus on the lifetime and durability of the riblets with consideration of the aircraft maintenance schedule. In addition, to achieve optimum riblet performance, fabrication techniques should be further advanced to address the need for lower cost and versatility, as previous studies reveal that a combination of different configurations and sizes should be applied in dissimilar aircraft surfaces to achieve optimum performance.

Moreover, production uncertainties encompass the challenges that arise due to variations in the manufacturing process. Riblets, being intricate structures, might exhibit variability in terms of geometry, dimensions, or material properties during production. Incorporating these uncertainties into the optimization process ensures that the designed riblet configuration remains effective even when faced with these variations. This enhances the robustness of the design against real-world production conditions.

All in all, in the realm of aircraft design, where multiple goals like drag reduction, noise control, and overall robustness are important factors, an optimization approach stands out as crucial. This strategy involves tackling conflicting objectives simultaneously. Specifically, in the context of managing noise and drag, a multi-objective optimization strategy becomes essential. This involves using techniques such as surrogate modeling and genetic algorithms to strike a balance between noise and drag reduction. However, the complexity increases when we consider two key factors: production uncertainties and riblet degradation over time.

11. Summary

Riblet geometries are introduced as passive flow control surfaces inspired by fast-swimming sharks. Previous studies reveal the positive effect of riblet geometries in drag reduction by modifying the turbulent structures inside the turbulent boundary layer. A variety of numerical and experimental research activities have been conducted to obtain optimum geometries in size and configuration according to flow characteristics. The several wind tunnel tests and in-flight evaluations of riblets are proof of the importance of riblets for the industry. By applying riblets, the flow properties can be improved on the basis of lifting large-scale turbulent vortices and changing the effects of the turbulent flow. Due to the possibility of improving near-wall turbulent structures, riblets consist of an interesting method for controlling related properties such as drag and noise. In a turbulent boundary layer, the streamwise steady and unsteady terms of the flow have been improved by applying optimum designed riblet geometries. In addition, mean values of streamwise velocity inside the boundary layer are increased compared to the smooth surface, and fluctuating terms of the velocity and pressure are reduced, showing a positive influence over the properties of near-wall flows.

Numerical simulations on flat plates, extruded airfoils and wing–body structures have been performed by DNS, LES and RANS. Evaluation of riblet configurations has been conducted by comparing research studies with previous activities. Besides DNS and LES, which are both time-consuming methods in the scale of aircraft geometry simulations, numerical turbulent models have been modified according to experimental results. However, it has been illustrated that some errors are inserted when geometry or flow properties deviate from the basics of developed correlations, revealing that these methods require further development.