Search Results (87)

The advancement of computational capabilities has allowed for more efficient airfoil analysis and design. Consequently, it has become possible to expand the design space and explore new geometries and configurations. However, the current state of development does not yet support a fully automated optimization process. Instead, the newly introduced capabilities have effectively transferred the previously trial-and-error-based approach used in geometry design to the formulation of the optimization problem. The goal of this work is to study the formulation of an optimization problem and propose a new methodology that better portrays the aircraft’s requirements for airfoil performance. The new objective function, added to an existing tool, estimates the main performance parameters of an aircraft for the Air Cargo Challenge (ACC) 2022 competition using a method that extrapolates the characteristics of the airfoil into the aircraft’s performance. In addition, the traditional relative aerodynamic property improvements, in this work, are coupled with the performance results to smooth the polar curve of the resulting airfoil. The optimization algorithm is based on the free-gradient technique Particle Swarm Optimization (PSO), using the B-spline parametrization and a coupled viscous/inviscid interaction method as the flow solver. Full article

This study investigates the aerodynamic performance of a symmetric NACA 0018 airfoil under harmonic pitching motions at low Reynolds numbers, a regime characterized by the presence of laminar separation bubbles and their impact on aerodynamic forces. The analysis encompasses oscillation frequencies of 1 Hz, 2 Hz, and 13.3 Hz, with amplitudes of 4° and 8°, along with steady-state simulations conducted for angles of attack up to 20° to validate the numerical model. The results reveal that the γ-Re_θ turbulence model provides improved predictions of aerodynamic forces at higher Reynolds numbers but struggles at lower Reynolds numbers, where laminar flow effects dominate. The inclusion of the 13.3 Hz frequency, relevant to Darrieus vertical-axis wind turbines, demonstrates the effectiveness of the model in capturing dynamic hysteresis loops and reduced oscillations, in contrast to the k-ω SST model. Comparisons with XFOIL further highlight the challenges in accurately modeling laminar-to-turbulent transitions and dynamic flow phenomena. These findings offer valuable insights into the aerodynamic behavior of thick airfoils under low Reynolds number conditions and contribute to the advancement of turbulence modeling, particularly in applications involving vertical-axis wind turbines. Full article

Turbulent flows over a semi-circular cylinder (a limiting case of a thick airfoil with a chord equal to the diameter base) are investigated using high-fidelity large-eddy simulations at a diameter-based Reynolds number, Re = 130,000, Mach number, M = 0.05, and a zero angle of attack. The aerodynamic flow control system, designed with two trapped vortex cells, achieves a complete non-separated flow over the bluff body, except for low-scale turbulence effects, reaching approximately 80% of the theoretical lift coefficient limit ($2\pi$ for the half-circular airfoil). Viscous effects are analyzed using the conventional Reynolds-averaged Navier–Stokes approach for a broad range of Reynolds numbers, 75,000 ≤ Re ≤ 1,000,000. Numerical results demonstrate that the aerodynamic properties of the implemented concept are independent of the Reynolds number within this interval, highlighting its significant potential for further development. Full article

This report uses computational fluid dynamics (CFDs) to investigate the aerodynamics of a rubber O-ring, with a focus on assessing the influence of fluid velocity and surface topology whilst providing a detailed methodology that promotes correct procedures. A steady state scenario was set up, modelling laminar airflow across two O-rings with 5 μm and 100 μm surface finishes, respectively. Analysis showed that increasing the fluid velocity from 0.01 m/s to 2 m/s significantly translates the separation points downstream, consolidating wake regions behind the airfoil. The CFD simulations also infer that as the fluid velocity increases, the frictional drag coefficients decrease from 3.13 to 0.11, and the pressure drag coefficients increase from 0.55 to 0.6, implying that the recirculation of flowlines behind the O-ring becomes the most hindering factor on aerodynamics. Conversely, variations in surface roughness showed negligible effects on the flow field. This insensitivity is attributed to the low Reynolds number (Re) used in all simulations, where a roughness of 5 μm or 100 μm remains well within the laminar sublayer, therefore minimising their impact on boundary layer disruption and flow separation. Full article

Unmanned aerial vehicles (UAVs) have emerged as practical and potentially advantageous tools for scientific investigation and reconnaissance of planetary surfaces, such as Mars. Their ability to traverse difficult terrain and provide high-resolution imagery has revolutionized the concept of exploration. However, operating drones in the Martian environment presents fundamental challenges due to the harsh conditions and the different atmosphere. Aerodynamic challenges include low chord-based Reynolds number flows and the presence of dust particles, which can accumulate on the airfoil surface. This paper investigates the accumulation of dust on cambered plates with 6% and 1% camber, suitable for the type of flow studied. The analysis is conducted for Reynolds numbers of around 20,000 as a result of dimension restrictions, assuming a wind speed ranging from 12 to 14 m/s. Computational simulations are performed using a 2D C-type mesh in ANSYS Fluent, employing the $\gamma$-Re SST turbulence model. Dust particle modeling is achieved through the Discrete Phase Model (DPM), with one-way coupling between phases. The accumulation of particles is monitored over a 6-month period with monthly intervals, and the airfoil is set at a 0° angle of attack. A deposition model, developed using user-defined functions in Fluent, considers particle–airfoil interaction and forces acting on particles. Results indicate a decrease in airfoil performance for negative angles of attack due to geometric changes, particularly due to accumulation on the bottom side near the tip. The discussion includes potential model enhancements and future research directions arising from the assumptions made in this study. Full article

The dynamic mode decomposition serves as a useful tool for the coherent structure extraction of the complex flow fields with characteristic frequency identification, but the phase information of the flow modes is paid less attention to. In this study, phase information around the locally flexible membrane airfoil is quantitatively studied using dynamic mode decomposition (DMD) to unveil the physical mechanism of the lift improvement of the membrane airfoil. The flow over the airfoil at a low Reynolds number (Re = 5500) is computed parametrically across a range of angles of attack (AOA = 4°–14°) and membrane lengths (LM = 0.55c–0.70c) using a verified fluid–structure coupling framework. The lift enhancement is analyzed by the dynamic coherent patterns of the membrane airfoil flow fields, which are quantified by the DMD modal phase propagation. A downstream propagation pressure speed (DPP) on the upper surface is defined to quantify the propagation speed of the lagged maximal pressure in the flow separation zone. It is found that a faster DPP speed can induce more vortices. The correlation coefficient between the DPP speed and lift enhancement is above 0.85 at most cases, indicating the significant contribution of vortex evolution to aerodynamic performance. The DPP speed greatly impacts the retention time of dominant vortices on the upper surface, resulting in the lift enhancement. Full article

Passive flow control around airfoils, wind turbines, and submarines to enhance their aerodynamic properties is the subject of interest in several studies. Previous research provides different solutions, from basic changes in surface roughness and simple geometries to complex shapes and mechanical solutions. This article presents experimental research using the Particle Image Velocimetry (PIV) method on a NACA 0012 airfoil at a Reynolds number of 66,400. Initially, the airfoil was tested for three different angles of attack: 13°, 15°, 17°, and 19°. These tests revealed that angles of attack above 15° significantly increase boundary layer detachment, as shown in the normalized streamwise velocity fields $U_x$. In the second stage of the research, a different-shaped microcylinder with a characteristic dimension (d/c) of 0.01 was added to the leading edge of the airfoil at a high angle of attack of 17°. Unlike traditional vortex generators placed at the rear of the airfoil, this configuration aimed to reduce boundary layer detachment. The experiment demonstrated that the microcylinder effectively reduced boundary layer detachment at this angle of attack. Full article

The flow characteristics of a 30P30N three-element high-lift airfoil at low Reynolds numbers $\left[\mathrm{O}\left({10}^4\right)\right]$ are examined through three-dimensional simulations using a high-order spectral element method. This study primarily investigates the flow structures of the slat cove and Görtler vortices formed on the upper surface of the main airfoil. Spanwise instability grows exponentially in the slat cove with a constant wavelength, corresponding to that of the subsequently formed Görtler vortices. Görtler number calculations show that curvature-induced centrifugal instability at the slat cusp leads to the subsequent formation of Görtler vortices. Proper orthogonal decomposition (POD) is used to analyze the development of flow structures in the slat cove in different time ranges. At early time, the flow in the slat cove is dominated by shear layers that evolve into spanwise perturbations. These perturbations further evolve into distinct bell-shaped structures close to the slat cusp and are advected to the upper surface of the main airfoil, leading to the formation of Görtler vortices. Full article

The prediction of separated flows at low Reynolds numbers is crucial for several applications in aerospace and energy fields. Reynolds-averaged Navier–Stokes (RANS) equations are widely used but their accuracy is limited in the presence of transition or separation. In this work, two different strategies for improving RANS simulations by means of field inversion are discussed. Both strategies require solving an optimization problem to identify a correction field by minimizing the error on some measurable data. The obtained correction field is exploited with two alternative strategies. The first strategy aims to the identification of a relation that allows to express the local correction field as a function of some local flow features. However, this regression can be difficult or even impossible because the relation between the assumed input variables and the local correction could not be a function. For this reason, an alternative is proposed: a U-Net model is trained on the original and corrected RANS results. In this way, it is possible to perform a prediction with the original RANS model and then correct it by means of the U-Net. The methodologies are evaluated and compared on the flow around the NACA0021 and the SD7003 airfoils. Full article

The multiple additional sampling point method has become popular for use in Efficient Global Optimization (EGO) to obtain aerodynamically shaped designs in recent years. It is a challenging task to study the influence of adding multi-sampling points, especially when multi-objective and multi-fidelity requirements are applied in the EGO process, because its factors have not been revealed yet in the research. In this study, the addition of two (multi-) sampling points (2-MAs) and four (multi-) sampling points (4-MAs) in each iteration are used to study the proposed techniques and compare them against results obtained from a single additional sampling point (1-SA); this is the approach that is conventionally used for updating the hybrid surrogate model. The multi-fidelity multi-objective method is included in EGO. The performance of the system, the computational convergence rate, and the model accuracy of the hybrid surrogate are the main elements for comparison. Each technique is verified by mathematical test functions and is applied to the airfoil design. Class Shape Function Transformation is used to create the airfoil shapes. The design objectives are to minimize drag and to maximize lift at designated conditions for a Reynolds number of one million. Computational Fluid Dynamics is used for ensuring high fidelity, whereas the panel method is employed when ensuring low fidelity. The Kriging method and the Radial Basis Function were utilized to construct high-fidelity and low-fidelity functions, respectively. The Genetic Algorithm was employed to maximize the Expected Hypervolume Improvement. Similar results were observed from the proposed techniques with a slight reduction in drag and a significant rise in lift compared to the initial design. Among the different techniques, the 4-MAs were found to converge at the greatest rate, with the best accuracy. Moreover, all multiple additional sampling point techniques are shown to improve the model accuracy of the hybrid surrogate and increase the diversity of the data compared to the single additional point technique. Hence, the addition of four sampling points can enhance the overall performance of multi-fidelity, multi-objective EGO and can be utilized in highly sophisticated aerodynamic design problems. Full article

In 2021, the Ingenuity helicopter performed the inaugural flight on Mars, heralding a new epoch of exploration. However, the aerodynamics on Mars present unique challenges not found on Earth, such as low chord-based Reynolds number flows, which pose significant hurdles for future missions. The Ingenuity’s design incorporated a Reynolds number of approximately 20,000, dictated by the rotor’s dimensions. This paper investigates the implications of flows at a Reynolds number of 50,000, conducting a comparative analysis with those at 20,000 Re. The objective is to evaluate the feasibility of using larger rotor dimensions or extended airfoil chord lengths. An increase in the Reynolds number alters the size and position of Laminar Separation Bubbles (LSBs) on the airfoil, significantly impacting performance. This study leverages previous research on the structure and dynamics of LSBs to examine the flow around a cambered plate with 6% camber and 1% thickness in Martian conditions. This paper details the methods and mesh used for analysis, assesses airfoil performance, and provides a thorough explanation of the results obtained. Full article

The effects of leading edge radius on the static and dynamic stall characteristics of rotor airfoils are investigated. Initially, a parametric airfoil (PARFOIL) method is employed to generate four morphed airfoils with different leading edge radii based on a NACA 0012 airfoil. Subsequently, the Reynolds-averaged Navier–Stokes (RANS) method is employed to simulate the aerodynamic characteristics of static airfoils, while the improved delayed detached-eddy simulation (IDDES) method is employed for pitching airfoils. The effectiveness and accuracy of the computational fluid dynamics (CFD) methods are demonstrated through favorable agreement between the numerical and experimental results. Finally, both the static and dynamic aerodynamic characteristics are simulated and analyzed for the airfoils with varying leading edge radii. Comparative analyses indicate that at low Mach numbers, the high adverse pressure gradient near the leading edge is the primary cause of leading edge separation and stall. A larger leading edge radius helps to reduce the suction pressure peak and adverse pressure gradients, thus delaying the leading edge separation and stall of airfoil. At high Mach numbers, the leading edge separation and stall are mainly induced by the shock wave. Variations in leading edge radius have minimal impacts on the high adverse pressure gradient induced by the shock wave, thus making the stall characteristics of airfoils almost unaffected at high Mach numbers. Full article

For noise reduction at a low-to-moderate Reynolds number, airfoil trailing-edge tonal noise has multiple prominent tones. Among these tones, secondary tones are greatly influenced by external disturbances such as oscillations commonly in the environment. In previous experiments, the spatial movement of sources was found to be related to an inherent high-frequency oscillation. Therefore, the spatial influence of external low-frequency oscillations was investigated in this study. By using tripping tapes to construct different symmetry source regions on the pressure side with side secondary tones, a transient spatial analysis of an NACA0012 airfoil at 2 degrees was performed by microphone arrays when a 10 Hz pressure oscillation was significant at 24 m/s. Temporally, this 10 Hz periodic strength change became more intense at a broader frequency bandwidth for a longer source region. Furthermore, a substantial time delay, significantly larger than the sound propagating time difference between microphones, was observed exclusively along the spanwise direction. This delay led to a periodic directivity pattern, particularly when two 0.2 m source regions were separated by a 0.2 m or 0.4 m tripping region. This low-frequency oscillation introduces an asymmetric transient switching pattern for symmetric spanwise source regions. Consequently, the response of airfoils to external oscillations in field tests should be considered. Full article

Among the critical factors contributing to the decline in the aerodynamic performance of near-space aircraft under low Reynolds number conditions, a significant one lies in the occurrence of laminar separation bubbles forming on the wings. Within the scope of this investigation, the primary research methodology adopted involves utilizing an unsteady numerical simulation technique rooted in a spring-smoothed dynamic grid system. This study meticulously examines the aerodynamic attributes and flow patterns exhibited by an airfoil undergoing partial oscillation, thereby elucidating the underlying mechanisms through which such oscillations lead to enhanced lift and diminished drag forces. The outcomes of this research reveal that the imposition of partial oscillation engenders a noteworthy augmentation of 4.9% in the lift coefficient of the airfoil, concurrent with a substantial diminution of 15.3% in its drag coefficient when juxtaposed against the non-deforming counterpart. The oscillation frequency exerts a profound influence on both the onset location of transition and the extent of the laminar separation bubble’s development. As the oscillation frequency escalates, it follows an initial ascending trend in the lift coefficient of the airfoil, followed by a subsequent decline, whereas the drag coefficient exhibits an initial decrement prior to a rising tendency, thus indicating the existence of an optimal frequency point where the airfoil achieves its most favorable aerodynamic characteristics. It is observed that the flow control effects are optimally pronounced when the region subjected to partial oscillation is proximate to the airfoil’s leading edge or situated precisely at the centroid of the laminar separation bubble. Full article

This study presents an extensive collection of data on the aerodynamic behavior at a low Reynolds number and geometric coefficients for 2900 airfoils obtained through the class shape transformation (CST) method. By employing a verified OpenFOAM-based CFD simulation framework, lift and drag coefficients were determined at a Reynolds number of 10⁵. Considering the limited availability of data on low Reynolds number airfoils, this dataset is invaluable for a wide range of applications, including unmanned aerial vehicles (UAVs) and wind turbines. Additionally, the study offers a method for automating CFD simulations that could be applied to obtain aerodynamic coefficients at higher Reynolds numbers. The breadth of this dataset also supports the enhancement and creation of machine learning (ML) models, further advancing research into the aerodynamics of airfoils and lifting surfaces. Full article