Search Results (17)

A fully passive oscillating foil turbine on a swinging arm in a tandem configuration consisting of two NACA 0015 foils at both ends of its arm and operating in an incompressible flow at a Reynolds number of $3.9\times {10}^6$ is investigated with numerical simulations. The turbine is free to oscillate passively in response to hydrodynamic forces and structural reactions from springs and dampers. The passive motion of the tandem turbine arises from a transfer of energy from the flow, and this motion is solved using a fluid-structure algorithm coupling the Newtonian dynamics of the system with two-dimensional, unsteady, and Reynolds-averaged Navier–Stokes equations. The performance metrics, i.e., the efficiency and power coefficient, of the proposed turbine concept are explored with a momentum gradient ascent algorithm, which uses the near-optimal configuration of an equivalent single-foil concept from a previous study as a starting point. These starting configurations consist of tandem foils operating either under coupled flutter or stall flutter instabilities. The use of gears to adjust the equilibrium position of the pitching motion is also considered, resulting in a total of four baseline configurations. The best configuration found with the gradient ascent algorithm presents an efficiency value near $75\%$ and a power coefficient of $1.46$, showing the great potential of fully passive oscillating foil turbines operating in a tandem configuration and providing valuable insight for further development of this technology through three-dimensional simulations and prototype testing. Full article

The aerodynamic optimization of the airfoil of vertical-axis wind turbines (VAWTs) is limited by the time-consuming nature of computational fluid dynamics (CFD), resulting in difficulty in the efficient implementation of multi-parameter optimization. In response to this challenge, this study constructed a collaborative optimization framework based on the Kriging surrogate model and the multi-island genetic algorithm (MIGA). Based on the NACA 0015 airfoil, 13 geometric variables (including 12 Bernstein polynomial coefficients and 1 installation angle) were defined through the Classification and Shape Transformation (CST) parameterization method. Through sensitivity analysis, seven key parameters were screened as design variables. Seventy training samples and ten validation samples were generated via Latin hypercube sampling to construct a high-precision Kriging surrogate model (R² = 0.91368). The optimized results show that the power coefficient of the new airfoil increases by 14.2% under the condition of the tip velocity ratio (TSR > 1.5), and the average efficiency of the entire working condition increases by 9.8%. The drag reduction mechanism is revealed through pressure cloud maps and velocity field analysis. The area of the high-pressure zone at the leading edge decreases by 23%, and the flow separation phenomenon at the trailing edge is significantly weakened. This research provides an engineering solution that takes into account both computational efficiency and optimization accuracy for the VAWT airfoil design. Full article

A NACA 0015 airfoil is connected to a swinging arm by springs and dampers and is let loose in an incompressible and viscous flow at a Reynolds number of $3.9\times {10}^6$. The foil operates in a power-extracting regime and is free to pitch about a pivot that is itself swinging on a circular path; this contraption is called a fully passive oscillating-foil turbine on a swinging arm. This study explores the potential of four different foil configurations: with the swinging arm being either upstream or downstream of its pivot, and with or without the use of gears to control the equilibrium position of the foil with respect to the flow. The results show that the swinging arm concept offers similar performances, i.e., efficiency and power coefficient, as the railed turbine. Indeed, with arm lengths from 3 to 10 chords, efficiency values near $55\%$ and power coefficients reaching $1.57$ are obtained. Both the railed and the swinging arm turbines can operate under either a stall-flutter or a coupled-flutter instability. However, it is found that the geared models are the only ones suited when the driving mechanism is the coupled-flutter instability while both geared and gearless configurations are effective under the stall-flutter instability. Full article

This study investigated the effectiveness of a dielectric barrier discharge (DBD) plasma actuator operating in burst-in-burst (BIB) mode for flow separation control on a NACA 0015 airfoil. Time-resolved particle image velocimetry measurements were conducted at a Reynolds number of 66,000 and 13° angle of attack. Various BIB signal configurations were tested, with actuation periods of 70 ms and 150 ms, non-actuation periods ranging from 5 ms to 50 ms, and burst frequencies of 300 Hz and 600 Hz. Proper orthogonal decomposition was applied to analyze the flow field dynamics. The results showed that BIB actuation maintained flow attachment with reduced power consumption compared with continuous burst actuation. However, the effectiveness was highly sensitive to the BIB parameters, with some configurations failing to achieve consistent reattachment and becoming unstable. This study reveals complex interactions between actuation vortices and separation processes, highlighting both the potential and challenges of intermittent plasma actuation for efficient flow control. Full article

Small vertical axis wind turbines (VAWTs) are often considered suitable for use in urban areas due to their compact design. However, they are also well known to offer poor performance at low wind speeds, which is a common situation in such environments. An optimised 3D J-shaped VAWT was designed from standard NACA 0015 blades and analysed numerically through computational fluid dynamics (CFD). A finite element analysis (FEA) was also carried out to ensure the model’s structural integrity. Optimal results were obtained with aluminium alloy hollow blades and stainless-steel struts with X-shaped beams, with internal ribs. Numerical results showed that the J-shaped VAWT achieved an 18.34% higher moment coefficient compared to a NACA 0015-based VAWT, indicating better self-starting abilities. Full article

In the past decade, there has been renewed interest in wave energy harvesting utilizing oscillating water columns (OWC), one of the most well-studied wave energy harnessing technologies. In the OWC, pneumatic power from ocean waves is converted to mechanical energy by Wells turbines. It should be noted, however, that such turbines tend to perform poorly, have a limited operating range, and have low efficiency. In the present study, we incorporate a rectangular Gurney flap (GF) at the trailing edge (TE) of a Wells turbine consisting of hybrid airfoil (NACA 0015 and NACA 0025) blades with variable chord distribution along the span. This passive flow control mechanism was adopted to achieve increased power production by the Wells turbine. This study aimed to determine the aerodynamic performance of the variable chord turbine with GF compared to a turbine with a constant chord. By using ANSYS™ CFX, the three-dimensional, steady-state, incompressible Reynolds averaged Navier–Stokes (RANS) equations coupled with the k-ω SST turbulence model are solved. The performance was evaluated through the use of non-dimensional coefficients of torque, pressure drop, and efficiency. In addition, the numerical accuracy was achieved through a grid independence study. There was a good agreement between the computed results and the available experimental and numerical data. The GF increased the torque coefficient by 18.6% and 47.3% but with the expense of peak efficiency of 8.5% and 7.4% for the baseline and the hybrid turbine, respectively. Additionally, the hybrid turbine with GF delayed the onset of the stall by ~3° angle of attack (AOA). Full article

Refinement of the performance of a fully constrained oscillating-foil turbine is carried out via the addition of passive double Gurney flaps. Flaps ranging from $h_{\mathrm{GF}}=0.005c$ to $0.075c$ are added at the trailing edge of the NACA 0015 blade of turbines operating in high-efficiency regimes without leading-edge vortex shedding (LEVS). Performance improvements are determined using 2D numerical simulations with an unsteady Reynolds-averaged Navier–Stokes (URANS) approach. Based on a recent study of the double Gurney flaps on stationary foils, instantaneous power-extraction coefficients are analyzed and modifications of the foil’s kinematics are tested in order to fully benefit from the Gurney flaps’ performance improvements. Modifications to the pivot point location of the foil, to the pitch-heave phase, and to the pitching amplitude of the turbine are considered. Improvements are found for all turbine cases studied, including some of the previously optimal cases reported in the literature. The double Gurney flaps, being a simple and passive device, offer great practical application potential. They represent an efficient refinement to already robust and high-performance oscillating-foil turbines operating without the perceived benefit of leading-edge vortex shedding, an essential characteristic for actual, finite-span applications. Full article

Low-fidelity methods such as the Blade Element Momentum Theory frequently provide rotor aerodynamic performances. However, these methods must be coupled to databases or correlations to compute heat transfer. The literature lacks correlations to compute the average heat transfer around airfoil. The present study develops correlations for an average heat transfer over smooth and rough airfoil. The correlation coefficients were obtained from a CFD database using RANS equations and the Spalart–Allmaras turbulent model. This work studies the NACA 0009, NACA 0012, and NACA 0015 with and without the leading roughness representative of a small ice accretion. The numerical results are validated against lift and drag coefficients from the literature. The heat transfer at the stagnation point compares well with the experimental results. The database indicates a negligible dependency on airfoil thickness. The work presents two correlations from the database analysis: one for the smooth airfoils and one for the rough airfoils. For the zero lift coefficient, the average Nusselt number is maximum. This increases with $R{e}^{0.636}$ for the smooth surface and with $R{e}^{0.85}$ for the rough surface. As the lift increases, the average Nusselt is reduced by values proportional to the square of the lift coefficient for the smooth surface, while it is reduced by values proportional to $Re$ and the square of the lift coefficient for the rough surface. Full article

The primary objective of this investigation was to explore the aerodynamic impact of adding trailing edge serrations to a Wells turbine. The baseline turbine consists of eight NACA 0015 blades. The blade chord length was 0.125 m and the span was 0.100 m. Two modified serrated blade configurations were studied: (1) full-span, and (2) partial-span covering 0.288c of the trailing edge. The numerical simulations were carried out by solving the three-dimensional, incompressible steady-state Reynolds Averaged Navier-Stokes (RANS) equations using the k-ω SST turbulence model in ANSYS™ (CFX). The aerodynamic performance of the modified Wells turbine was compared to the baseline by calculating non-dimensional parameters (i.e., torque coefficient, pressure drop coefficient, and turbine efficiency). A comparison of the streamlines was performed to analyze the flow topology around the turbine blades for a flow coefficient range of 0.075 ≤ ϕ ≤ 0.275, representing an angle of attack range of 4.29° ≤ α ≤ 15.3°. The trailing edge serrations generated a substantial change in surface pressure and effectively reduced the separated flow region, thus improving efficiency in most cases. As a result, there was a modest peak efficiency increase of 1.51% and 1.22%, for the partial- and full-span trailing edge serrations, respectively. Full article

Lift force is an important parameter for the performance evaluation of an H-Darrieus wind turbine. The rotational direction of the streamlined force is effective on the performance of the wind turbine. In order to analyze the flow characteristics around the turbine blades in real-time, a numerical analysis using three-dimensional unsteady Reynold-averaged Navier–Stokes equations has been introduced. Experimental data were obtained from a field test facility constructed on an island in South Korea and was introduced to compare the numerical simulation results with measured data. The optimum tip speed ratio (TSR) was investigated via a multi-variable optimization approach and was determined to be 3.5 for the NACA 0015 blade profile. The turbine displays better performance with the maximum power coefficient at the optimum TSR. It is due to the delay in the flow separation from the blade surface and the relatively lower strength of the tip vortices. Furthermore, the ratio between lift and drag forces is also the highest at the optimum TSR, as most of the aerodynamic force is directly converted into lift force. For one rotation of the turbine blade at the optimum TSR, the first quarter of motion produces the highest lift as the static pressure difference is maximum at the leading edge, which helps to generate maximum lift. At a TSR less than the optimum TSR, small-lift generation is dominant, whereas at a higher TSR, large drag production is observed. Both of these lead to lower performance of the turbine. Apart from the TSR, the optimum wind angle of attack is also investigated, and the results are prepared against each TSR. Full article

A series of wind tunnel tests were carried out to determine the effect of shark scale-based vortex generators (SSVG) on a NACA 0015 symmetrical airfoil’s aerodynamic characteristics. Three different sets of SSVG with varying geometrical parameters, such as chord length, amplitude, and wavelength, were designed and fabricated using 3D printing. The SSVG models were blended to the baseline NACA 0015 symmetrical airfoil. The wind tunnel experiments were performed over the test airfoil mounted with different sets of SSVG at various angles of attack, ranging from 0° to 24° in increments of 3°, and operating in the range of Re = 2 × 10⁵. The results revealed that the SSVG blended test airfoil reduced the drag and increased the maximum coefficient of lift (C_Lmax), thereby enhancing the overall aerodynamic performance. The SSVG offered noteworthy aerodynamic benefits by effectively altering the flow and causing significant spanwise variation in the flow properties. Additionally, attempts were made to identify the optimum chordwise location to blend the SSVG for effective use. Full article

The hydrodynamic effect exerted by a nozzle placed in front of a KP505 propeller on the propulsive performances is studied by using extensive numerical simulations. The influence of a NACA 0015 nozzle with a chord length of 0.3 of the propeller diameter, D, mounted at 0.2 D in front of the propeller plane is studied for a various range of relevant nozzle diameters and different angles of attack. A detached eddy simulation (DES)-based hybrid technique implemented on the ISIS-CFD finite volume solver of the Numeca’s Fine^TM/Marine environment is proposed to fit the purpose. Systematically conducted simulations have proven that the net thrust reflecting the overall drag, which includes the nozzle, depends on the duct size. The duct presence determines two regions of the inflow into the propeller. One is the inner region of the nozzle where the high-speed flow exists because of the contraction of the duct. The other is the outer region of the nozzle where the flow decelerates due to the duct wake. Lower- and higher-pressure coefficients on the suction and pressure sides, cover a significantly wider area than those of the case without the nozzle, leading therefore to greater thrust and torque. The existence of a critical attack angle for which the magnitude of the relative axial force becomes maximum for the smallest nozzle diameter has been noticed. Full article

A closed-loop control algorithm for the reduction of turbulent flow separation over NACA 0015 airfoil equipped with leading-edge synthetic jet actuators (SJAs) is presented. A system identification approach based on Nonlinear Auto-Regressive Moving Average with eXogenous inputs (NARMAX) technique was used to predict nonlinear dynamics of the fluid flow and for the design of the controller system. Numerical simulations based on URANS equations are performed at Reynolds number of 10⁶ for various airfoil incidences with and without closed-loop control. The NARMAX model for flow over an airfoil is based on the static pressure data, and the synthetic jet actuator is developed using an incompressible flow model. The corresponding NARMAX identification model developed for the pressure data is nonlinear; therefore, the describing function technique is used to linearize the system within its frequency range. Low-pass filtering is used to obtain quasi-linear state values, which assist in the application of linear control techniques. The reference signal signifies the condition of a fully re-attached flow, and it is determined based on the linearization of the original signal during open-loop control. The controller design follows the standard proportional-integral (PI) technique for the single-input single-output system. The resulting closed-loop response tracks the reference value and leads to significant improvements in the transient response over the open-loop system. The NARMAX controller enhances the lift coefficient from 0.787 for the uncontrolled case to 1.315 for the controlled case with an increase of 67.1%. Full article

Although improving the hydrodynamic performance is a key objective in the design of ocean-powered devices, there are some factors that affect the efficiency of the device during its operation. In this study, the impacts of a wide range of surface roughness as a tribological parameter on stream flow around a hydro turbine and its power loss are studied. A comprehensive program of 3D Computational Fluid Dynamics (CFD) modeling, as well as an expansive range of experiments were carried out on a Darrieus Hydro (DH) turbine in order to measure reduction in hydrodynamic performance due to surface roughness. The results show that surface roughness of turbine blades plays an important role in the hydrodynamics of the flow around the turbine. The surface roughness increases turbulence and decreases the active fluid energy that is required for rotating the turbine, thereby reducing the performance of the turbine. The extent of the negative impact of surface roughness on the drag coefficient, pressure coefficient, torque, and output power is evaluated. It is shown that the drag coefficient of a turbine with roughness height of 1000 μm is about 20% higher than a smooth blade (zero roughness height) and the maximum percentage of reduction of output power could be up to 27% (numerically) and 22% (experimentally). Full article

This paper numerically investigates the effects of airfoil leading edge radius on the aerodynamic characteristics of H-rotor Darrieus vertical axis wind turbine (VAWT). 10 modified airfoils are generated by changing the leading edge radius of the base NACA 0015 airfoil from 1%c to 9%c, respectively. A 2D unsteady Computational Fluid Dynamics (CFD) model is established and validated with the previously published experimental data. The power, torque, and flow field characteristics of the rotors are analyzed. The results indicate that the maximum and minimum power coefficient at the optimum tip speed ratio (TSR) are obtained for the LE-5%c and LE-1%c model, respectively. The best aerodynamic characteristics are determined by the LE-5%c model below the optimum TSR and the LE-3%c model beyond the optimum TSR. The torque characteristics and pressure distribution for the single blades with different airfoil leading edge radius show an obvious difference in the upwind region and a very small difference in the downwind region. Moreover, the airfoil leading edge radius influences the strength, region, and diffusion rate of the vortices, being the main reason for the observed differences in instantaneous torque coefficient and power coefficient. The vortices of the LE-1%c model are stronger, larger, and diffuse slower than those of the LE-2%c and LE-5%c model at the optimum TSR. Full article