Airfoil Thickness Research Articles

Effect of Reynolds number and airfoil thickness on the leading-edge suction in unsteady flows

Effect of Reynolds number and airfoil thickness on the leading-edge suction in unsteady flows

Law of incipient separation for turbulent flows over airfoils as inferred by Reynolds-averaged Navier–Stokes

Lu, Aithal, and Ferrante [“Law of incipient separation over curved ramps as inferred by Reynolds-averaged Navier–Stokes,” AIAA J. 59, 196–214 (2021)] discovered a law that predicts the incipience of flow separation over curved ramps by knowing only a few geometrical parameters of the ramp and the Reynolds number of the flow. In that spirit, we have searched for a similar law for airfoils by performing simulations of incompressible turbulent flows over 32 NACA (National Advisory Committee for Aeronautics) airfoils using Reynolds-averaged Navier–Stokes (RANS) equations. First, we have carried out verification and validation of RANS against the experimental measurements by A. J. Wadcock (“Investigation of low-speed turbulent separated flow around airfoils,” NASA Contractor Report No. 177450, 1987), which show the accuracy of the RANS prediction at small angles of attack when flow separation begins to occur on the upper side of the airfoil. Then, we have investigated the effects of the angle of attack, airfoil thickness, and camber on the incipience of flow separation for the Reynolds number based on airfoil chord Rec∈[1.64×106,6×106]. From the analysis of the RANS results, we have determined a law for predicting the incipience of turbulent flow separation over airfoils that relies only on airfoil's newly defined characteristic slope, thickness, camber, and Rec.

Parametric Study on Thermo-Hydraulic Performance of NACA Airfoil Fin PCHEs Channels

In this work, a discontinuous airfoil fin printed circuit heat exchanger (PCHE) was used as a recuperator in a micro gas turbine system. The effects of the airfoil fin geometry parameters (arc height, maximum arc height position, and airfoil thickness) and the airfoil fin arrangements (horizontal and vertical spacings) on the PCHE channel’s thermo-hydraulic performance were extensively examined by a numerical parametric study. The flow features, local heat transfer coefficient, and wall shear stress were examined in detail to obtain an enhanced heat transfer mechanism for a better PCHE design. The results show that the heat transfer and flow resistance were mainly increased at the airfoil leading edge owing to a flow jet, whereas the airfoil trailing edge had little effect on the thermo-hydraulic performance. The airfoil thickness was the most significant while the arc height and the vertical spacing were moderately significant to the performance. Moreover, only the airfoil thickness had a significant effect on the PCHE compactness. Based on a comprehensive investigation, two solutions NACA-6230 and -3220 were selected owing to their better thermal performance and smaller pressure drop, respectively, with horizontal spacings of 2 mm and vertical spacings of 2 or 3 mm.

Numerical Study of Bio-Inspired Corrugated Airfoil Geometry in a Forward Flight at a Low Reynolds Number

In this study, the effects of variations in the parametric geometry on the aerodynamic efficiency and longitudinal static stability of a bio-inspired airfoil were assessed using the computational method at a low Reynolds number of 80000. The investigation aims to recognize the influence of corrugations on aerodynamic forces and moments and compare them with a non-corrugated profile having similar geometry without corrugations. Three different airfoils were chosen, the first triangular peaked corrugated is inspired from the mid-section of a dragonfly wing, the second modified simplified corrugated is a different form of the dragonfly wing section, which was modified to match the maximum thickness of the first airfoil, and the third is a non-corrugated Hybrid airfoil obtained by joining the peaks of the second airfoil. These three models were fabricated using an additive manufacturing process to undertake the experimental work in a low subsonic wind tunnel to find aerodynamic characteristics. ANSYS FLUENT solver was applied to unravel the steady, laminar, incompressible, two-dimensional, RANS equations. The tests were performed for 4 to +20 degrees angle of attack at a Reynolds number of 80,000. The result revealed that the Hybrid airfoil is suitable only for up to a 4-degree angle of attack. The modified simple corrugated airfoil produced significant aerodynamic performance at high angles of attack than the other two tested airfoils. The flow field study also showed the same results. Results are validated with experimental work and also with existing literature.

Adjoint-based unsteady shape optimization to suppress transonic buffet

The traditional aerodynamic shape optimization mainly aims at the enhancement of lift-drag ratio in the steady state. In this study, the onset of transonic buffet is improved by the unsteady aerodynamic shape optimization. To achieve this goal, the variance of lift coefficient in a period is adopted as the optimization objective. In addition, the gradients of the objective function with respect to shape parameters are efficiently calculated by the unsteady discrete adjoint method. The proposed method is verified by two optimization examples of NACA0012 and OAT15A. Not only the onsets of transonic buffet are increased by 0.9∘ and 1.7∘ respectively, but also the aerodynamic performance is improved. According to the shape change of optimization examples, we summarize three airfoil geometric characteristics: the position of maximum thickness of the upper airfoil, the thickness near the leading edge, and the thickness near the trailing edge. The influence of the geometric characteristics on the flow stability has been studied.

Aerodynamic and Acoustic Simulations of Thick Flatback Airfoils Employing High Order DES Methods

AbstractThe results of high fidelity aerodynamic and acoustic computations of thick flatback airfoils are reported in the present paper. The studies are conducted on a flatback airfoil having a relative thickness of 30% with the blunt trailing edge thickness of 10% relative to chord. Delayed Detached‐Eddy Simulation (DDES) approaches in combination with high order (5th) flux discretization WENO (Weighted Essentially Non‐Oscillatory) and Riemann solver are employed. Two variants of the DES length scale calculation methods are compared. The results are validated against experimental data with good accuracy. The studies provide guideline on the mesh and turbulence modeling selection for flatback airfoil simulations. The results indicate that the wake breakdown is strongly influenced by the spanwise resolution of the mesh, which directly contributes to the prediction accuracy especially for drag force and noise emission. The Reynolds normal stress and the Reynolds stress component have the largest contributions on the mixing process, while the contribution of the component is minimal. Proper orthogonal decomposition is further performed to gain deeper insights into the wake characteristics.

Wind tunnel benchmark tests of airfoils

This paper describes a benchmark of four airfoils in the Poul la Cour Tunnel (PLCT). The wind tunnel, the corrections used and the method of making adapters for the airfoils are also described. Very good agreement was in general observed between the measurements in PLCT and in other high quality wind tunnels. Some deviations were seen, but they were mainly attributed to the differences in separation on the airfoil. Apart from the benchmarking, this paper also highlights the challenges in testing airfoils in general such as obtaining 2D flow on thick airfoils that inherently shows separated flow and how to make adapters for airfoils tested in other wind tunnels.

On the combined use of Vortex Generators and Gurney Flaps for turbine airfoils

While Vortex Generators (VGs) and Gurney Flaps (GFs) are commonly used for airfoil flow control, studies of a combination of the two devices are rare. The present investigation aims at examining the combined effect of VGs and GFs on a 20% thick airfoil. To this end, a wind tunnel investigation coupled with a computational study was performed. The present paper presents force, pressure and Stereo Particle Image Velocimetry measurements, along with Unsteady Reynolds Averaged Navier Stokes simulations.

Effect of design parameters on the performance of helical Darrieus hydrokinetic turbines

This study aims to explore the effect of various design parameters, such as maximum airfoil thickness, maximum camber in the blade profile, maximum camber position, helical pitch, blade pitch angle, solidity, and number of blades, on the performance of helical Darrieus hydrokinetic turbines (HDHKTs). An experimental and numerical performance analysis was conducted, corresponding to free-stream velocity of 0.5 m/s. The performance increased with increasing airfoil thickness, up to a maximum of 21%. Effects of the maximum camber and position of the maximum camber on the performance of the turbine were insignificant. The analysis was extended to different helical pitch angles on the NACA 4421 profile. The performance increased with increasing helical pitch angle; however, the operating range of tip speed ratio decreased. The blade pitch angle had an insignificant effect on the turbine performance. Furthermore, the HDHKT of the NACA 4421 blade profile consist of two-, three-, four-, five-, and six-blade turbine performances was examined for different solidity values. For solidity values of 0.10, 0.15, and 0.20, the three-blade HDHKT performed the best. For solidity values greater than 0.20, the four-blade HDHKT performed the best. The four-blade HDHKT had the highest coefficient of power among all the HDHKTs.

Optimum performance of a horizontal axis tidal current turbine: A numerical parametric study and experimental validation

One of the most essential parameters contributing to the performance of hydrokinetic tidal current turbine is its geometry. In this study, the effect of curvature, thickness, and blade pitch angle (BPA0) with a glimpse at the turbine performance considering the probability of fatigue loading presence are investigated. Six different airfoils based on possessing the acceptable lift to drag ratio were selected and used to design turbine blades using the blade element momentum theory. Computational fluid dynamic was employed for simulation the performance of turbines at diverse tip speed ratios (TSR)s and BPA0s so that the condition for obtaining the maximum performance was determined. An experimental setup was fabricated to validate the computational results. The question is: To what extent this maximum performance condition is possible to be implemented in practice? To answer this question, a parameter called superiority of maximum power coefficient (SCPmax) was introduced to assess the usefulness of employment of each airfoil based on maximum power coefficient (CPmax), thrust coefficient (CTh), and the percentage of airfoil thickness (%t) and exhibit the utility of maximum performance against the probability of fatigue hazard. Results indicated that at higher values of BPA0s, by increasing the thickness, the maximum amount of torque enhances. Furthermore, the augmentation in the TSR leads to increase in CPmax at the lower value of BPA0 so that the difference between the lowest (for NACA 65(3)-618) and highest (for NACA 4412) CPmax is around 66%. In contrast, the maximum value of SCPmax belonged to the NACA65(3)-618 due to lower CTh and more %t, while the minimum amount of SCPmax appertained to the NACA2410 which has the minimum %t.

A Study of the Effects of Tornado Translation on Wind Loading Using a Potential Flow Model

This paper investigates the effects of tornado translation on pressure and overall force experienced by an airfoil subjected to tornado loading and presents a framework to reproduce the flow conditions and effects of a moving tornado. A thin symmetrical airfoil was used to explore the effects of tornado translation on a body. A panel method was used to compute the flow around an airfoil and an idealised tornado is represented using a moving vortex via unsteady potential flow. Analysis showed that the maximum overall pressure at a point was found to increase by up to 20% when the normalised translating velocity was 10% of the tangential velocity, but increases up to 60% when the normalised translating velocity is 30% of the tangential velocity. Investigation on the impact of varying airfoil thickness (Case 2) revealed that the location of the tornado has significant effect on the overall lift force. However, the overall lift force appeared to be largely insensitive to the tornado translation velocity due gross changes in pressure on either side of the airfoil cancelling each other out. Further comparison with varying airfoil sizes and distance to tornado translating path (Case 3) showed that the relative inflow and outflow angle is the primary factor affecting the lift on the airfoil. Additionally, the maximum forces on a body subjected to a moving tornado can be predicted using uniform flow providing that the appropriate range of inflow angles are known. Based on the analysis on the database of National Oceanic and Atmospheric Administration (NOAA), the normalised translation speed of the recorded tornadoes across the EF scales, appears to vary from 0.25 to 0.37, with an average of 0.32 (∼18.8 m/s). Finally, the framework using uniform flow to reproduce the flow conditions which are comparable to those generated by a translating vortex simulator is proposed and discussed in detail.

Experimental study of the effect of a slat on the aerodynamic performance of a thick base airfoil

Abstract. There is continuous effort to try and improve the aerodynamic performance of wind turbine blades. This experimental study focusses on the addition of a passive slat on a thick airfoil typically used in the inboard part of commercial wind turbine blades. Nine different slat configurations are considered, with both a clean and tripped main airfoil. The results are compared with the performance of the airfoil without slat, as well as the airfoil equipped with vortex generators. It is found that, when the airfoil is clean, the increase in lift-to-drag ratio due to the presence of a slat is larger than when vortex generators are used. This is also true for the tripped airfoil but only at small angles of attack. As expected, in all configurations, the presence of the slat delays flow separation and stall. Finally, for a clean airfoil and small angles of attack, the slat decreases the lift-to-drag ratio of the main airfoil only. By contrast, as the angle of attack increases, it seems that the slat changes the flow field around the main airfoil in such a way that its lift-to-drag ratio becomes larger than for the airfoil without slat. These effects are less pronounced when the airfoil is tripped. This work helps to better understand the role of a slat in improving the aerodynamics of blade sections. It can also be used to validate simulation tools in the field.

Dynamic stall at high Reynolds numbers induced by ramp-type pitching motions

The transient pressure field around a moderately thick airfoil is studied as it undergoes ramp-type pitching motions at high Reynolds numbers and low Mach numbers. A unique set of laboratory experiments were performed in a high-pressure wind tunnel to investigate dynamic stall at chord Reynolds numbers in the range of $0.5\times 10^6\leq Re _c\leq 5.5\times 10^6$ in the absence of compressibility effects. In addition to variations of mean angle and amplitude, pitching manoeuvres at reduced frequencies in the range of $0.01\leq k\leq 0.40$ were studied by means of surface-pressure measurements. Independently of the parameter variations, all test cases exhibit a nearly identical stall behaviour characterized by a gradual trailing-edge stall, in which the dynamic stall vortex forms approximately at mid-chord. The location of the pitching window with respect to the Reynolds-number-dependent static stall angle is found to define the temporal development of the stall process. The time until stall onset is characterized by a power law, where a small excess of the static stall angle results in a drastically prolonged stall delay. The reduced frequency exhibits a decrease in impact on the stall development in the case of angle-limited pitching manoeuvres. Beyond a critical reduced frequency, both load magnitudes and vortex evolution become reduced frequency independent and instead depend on the geometry of the motion and the convective time scale, respectively. Overall, the characteristics of vortex evolution induced by dynamic stall show remarkable similarities to the framework of optimal vortex formation reported in Gharib et al. (J. Fluid Mech., vol. 360, 1998, pp. 121–140). The data from this study are publicly available at https://doi.org/10.34770/b3vq-sw14.

Multi-objective optimal design of NACA airfoil fin PCHE recuperator for micro-gas turbine systems

• An AF-PCHE was first employed as a micro gas turbine recuperator. • Heat transfer, pressure drop and compactness are multi-objective studied. • A comprehensive optimal solution is obtained for recuperator design. In this study, an airfoil fin printed circuit heat exchanger (PCHE) was first employed as a micro gas turbine recuperator for extended-range electric vehicles. Optimal heat transfer rate, pressure drop, and compactness are important requirements for a PCHE to serve as a recuperator; therefore, in this study, a multi-objective optimal design was proposed using the aforementioned three targets. The considered factors included three airfoil fin structure parameters (arc height, maximum arc height position, and airfoil thickness), two airfoil fin arrangement parameters (horizontal and vertical spacings), and the velocity inlet condition, for both cold and hot sides. A series of 53 numerical tests were designed using the central composite surface method. And the numerical work was performed by the realizable k - ε model with the maximum validation error is less than ± 5%. Moreover, regression formulas for the three targets were established based on the results of the 53 numerical tests, using the high-determine factors. The maximum arc height position was found to be an insignificant factor; thus, it was neglected in the regression formulas. The Pareto fronts for the cold and hot sides were obtained based on nondominated sorting genetic algorithm II; decision-making could be achieved on the Pareto front, according to the design requirements. A comprehensive optimal solution was recommended for the recuperator PCHE with an overall heat transfer rate of 181.29 W/m 2 /K, a total pressure drop of 26.63 kPa/m, and compactness of 8.5 kw/m 2 .

Comparative Study of Lifting Surface and CFD Methods in the Simulation of Morphing Process of Folding Wing

A folding wing aircraft can autonomously change its configuration in flight to respond to different flying environments. Hinge moment during morphing process is an important basis for driving mechanism design and structural strength check, and its calculation depends on the simulation of the morphing process. Existing studies mostly use the simplified lifting surface method for aerodynamic modeling. In this paper, the CFD-based simulation method is studied, and the results are compared with those by the lifting surface method. First, the unsteady aerodynamic modeling method of the folding wing based on the CFD method is studied, an effective description method is given to define the wall mesh motion caused by wing folding and aileron deflection, and an unfolding–refolding strategy is proposed to improve the quality of the internal mesh in the case of large folding angles. Then, the CFD aerodynamic model is coupled through a developed coupling calculation program with the flexible multibody structure model for the simulation of a flight-morphing process. The comparison with the results of the lifting surface method shows that hinge moments obtained according to the two aerodynamic models are markedly different. Analysis of the difference shows that airfoil thickness considerably affects aerodynamic loading distributions and hinge moments of folding wing aircraft. The lifting surface method ignores airfoil thickness, which will cause large simulation errors in hinge moments.

Effect of the J-Shaped Wind Turbine Airfoil Opening Ratio and Thickness on the Performance of Symmetrical Airfoils

Abstract In this work, the effect of the inner opening ratio on the J-shaped airfoils aerodynamic performance was studied and documented for symmetrical airfoils. Three different airfoil thicknesses were investigated: small (NACA0008), medium (NACA0015), and large (NACA0024). For each airfoil thickness, effects of three inner opening ratios were analyzed: one-third, one-half, and two-thirds. The performance of each opening ratio was compared with the performance of the solid airfoil “zero opening ratio” for different angles of attack between 5 deg and 20 deg. All designs were simulated using the computational fluid dynamics (CFD) technology against experimental results for solid NACA4412 airfoil in the University of Wisconsin-Milwaukee (UWM) wind tunnel facility and other published experimental data. It was found that large eddy simulation yields accurate solutions with a smaller number of mesh cells compared to the k–ω turbulence model but with much longer computational time. The lift-to-drag ratio for all studied airfoils has a maximum value for solid airfoils compared to those equipped with openings. For airfoils equipped with 00.00% opening ratio “solid,” NACA0015 airfoil has the maximum lift-to-drag ratio. Furthermore, it was found that NACA0008 equipped with a 33.33% opening ratio has the best performance of all studied J-shaped airfoils.

Study on k−ω−Shear Stress Transport Corrections Applied to Airfoil Leading-Edge Roughness Under RANS Framework

Abstract A computational fluid dynamics study is carried out to model the effects of distributed roughness at the airfoil leading-edge using the equivalent sand grain approach and Reynolds-averaged Navier–Stokes equations. The turbulence model k−ω−shear stress transport (SST) is selected to emulate a fully turbulent flow. Three k and ω boundary conditions are studied to model roughness effects. One refers to Wilcox's boundary condition and the other two refer to Aupoix's boundary conditions. Besides, Hellsten's correction is used to ensure Wilcox's boundary condition compatibility with the shear stress transport limiter. After validating the implementation of these boundary conditions, they are applied to three different airfoils. One of them is a thick airfoil with industrial relevance. For this airfoil, Wilcox's boundary condition significantly underestimates the roughness impact on aerodynamic coefficients. The pressure gradient simplification in Wilcox's boundary condition formulation is the driving factor behind this effect. The pressure gradient effect on Aupoix's boundary condition is minimal.

Implementation of genetic algorithms for supersonic airfoil optimization

This paper describes the exploitation of Genetic Algorithms for the selection and optimization of supersonic airfoils. The main objective of the optimization is to ensure the possibility to reach the maximum efficiency at a given lift coefficient. The surrogate model presented in this paper implements the shock-expansion theory, and the optimization problem is constrained with respect to three design variables, since Bézier curves are used for the parameterization of the Diamond-Shaped and the 40-elements Double Circular Arc airfoil geometries. The airfoils were optimized for three different Mach numbers and two different lift coefficients. A thickness-constrained optimization has been run to evaluate the possibility to obtain a specific lift coefficient at a given Mach number and to understand how this parameter influences the optimized airfoil shape performance in terms of efficiency and the range of feasible angles-of-attack. An off-design evaluation is also presented, allowing for a comparison of the optimized geometries in terms of versatility. High fidelity models were validated with RAE 2822 airfoil using STAR CCM+, and they were compared to the surrogated model to ensure higher quality in the results. In conclusion, the Genetics Algorithms optimization coupled with the shock-expansion theory model results to be a fast and valuable solution to select and optimize airfoil solutions. The application of this approach has shown that a 20% reduction in airfoil thickness leads to a 25% efficiency improvement, while widening the range of viable angles of attack.

An improved LB method for predicting dynamic characteristics of Vertical Axis Wind Turbine airfoils

One of the important challenges for Vertical Axis Wind Turbine (VAWT) is to fully understand its dynamic characteristics in different operating conditions. Meanwhile, it is necessary to seek a fast and accurate method to evaluate the dynamic characteristic of VAWT. In this study, we improve the LB model by considering the operating principle of VAWT to study the dynamic characteristics of the dedicated and commonly used VAWT airfoils in different operating conditions. The results show that the improved LB model is suitable for simulating the dynamic characteristic of VAWT with a thick airfoil. Although the asymmetric airfoil shows the higher lift coefficient, their dynamic characteristic appears huge fluctuation as the increases of tip speed ratio. Moreover, at a low tip speed ratio, the advantages of the asymmetric airfoil are not obvious. While the dynamic characteristic of the symmetric airfoil is relatively stable with the variation of tip speed ratio.

Induced stalled flow due to roughness sensitivity for thick airfoils in modern wind turbines

The mid-span region of wind turbine blades can be thickened to fulfil the structural requirements of the blade. Hence, thick airfoils, that were designed to operate at the root region of the blade, are moved to the mid-span region. This could not imply remarkable variations of the blade performance once its surface is smooth. However, the sensitivity of thick airfoils to roughness could cause significant aerodynamic impacts such as flow separation. This research aims to quantify the impact of the blade thickness, under smooth and rough conditions, in the annual energy production and the fatigue loads of the blade. Ten blade designs, linearly interpolated in thickness, are studied employing aero-elastic computations. The results reveal that the thickest blade increases the annual energy production by 5% with respect to the thinnest blade under rough conditions. Whereas this increase is less than 1% under smooth conditions. The loss of annual energy production varies with the blade thickness linearly for thin blades while it varies exponentially for thick blades up to 22%. Fatigue loads assessment confirmed a reduction of the damage equivalent load under smooth conditions, whereas the thickest blade increased it 28% under rough conditions.