Leading Edge Of Airfoil Research Articles

Flapping-wing energy harvesting experiences with modified time variations at high re-number

AbstractWind-energy converters with rotating blades are most common for energy harvesting. In special cases a system of flapping wings may be of interest where the wings are oscillating in combined pitching and flapping (plunging) modes respectively. Then the ratio of pitching and plunging amplitudes is of main concern: If the amplitude of the pitching motion exceeds the amplitude of the induced angle of the plunging motion, energy transfer from the oncoming flow is present. To accomplish maximum energy transfers and maximum efficiencies a considerable number of parameters have to be varied. Most of the published investigations assume harmonic time variations of both pitching and plunging motion. In the present paper the time variation will be modified such that the stroke reversal times are favorably be influenced, i.e., the fraction of the period where the airfoil changes the angle of attack from its maximum to its minimum or vice versa. In the present paper this variation is represented by an analytical function. Further, the phase between pitch and plunge motion and the time dependent deformation of the airfoil-leading edge will be investigated. It will be shown that the different variations and modifications will lead to considerable increases of power transfer and efficiency.

A new approach to account for turbulence distortion effect on Amiet's leading-edge noise prediction

A turbulent inflow distorts as it approaches an airfoil's leading edge (LE), which is an important phenomenon influencing the airfoil LE noise. This mechanism is not yet fully understood and is not accounted for in Amiet's theory for LE noise. Thus, the main contribution of this paper is to discuss a possible new approach to account for turbulence distortion in Amiet's LE noise model. The analysis is based on wind tunnel experiments performed for four NACA airfoil geometries. Hot-wire anemometry and a microphone array were used to evaluate the turbulence in the LE region and the radiated noise, respectively. Turbulent inflow was generated with a rod and a grid. Based on the measured turbulence data, a formulation that describes the turbulence length scale near the airfoil LE is introduced, which yields more accurate noise predictions when used in combination with Amiet's model. The proposed approach to estimate LE noise consists of using an appropriate turbulence spectrum formulation for the distorted turbulence and the proposed expression to determine the turbulence length scale near the airfoil LE, yielding a maximum discrepancy between measured and predicted LE noise of 4dB for a rod Strouhal number from 0.3 to 3.

Stall characteristics of wavy leading-edge airfoil in subsonic and transonic airflows

Based on the bioinspired wavy leading-edge, the stall characteristics of the NACA0012 airfoil are optimized. In this paper, the semicircle plus line segment is used to obtain the wavy leading edge. The aerodynamic forces of the airfoil are measured by a high-precision balance, and the detailed flow features of the airfoil are obtained by the oil flow tests. Then, combined with numerical simulation, the optimization mechanism is obtained. The operating conditions are as follows: Mach number ranging from 0.4 to 0.8, and angle of attack ranging from −4° to 25°. The results show that in high speed airflows, compared with the basic airfoil, the lift coefficient of the wavy leading-edge airfoil does not decrease sharply with the increase of the angle of attack, and the drag coefficient of the wavy leading-edge airfoil is similar to the basic airfoil; among the three types of airfoils studied, the larger wavy leading-edge feature size has better aerodynamic characteristics; combined with the numerical simulation results, it can be seen that the stall mechanism of airfoils varies at different Mach numbers. The wavy leading-edge generate streamwise vortex. The streamwise vortices increase energy transport in the boundary layer. Therefore, the separation zone moves toward the trailing edge of the airfoil, and the stall characteristics of the airfoil are optimized.

The dynamics of supersonic flow past a new cusped leading edge airfoil

The dynamics of supersonic flow past a new cusped leading edge airfoil

Inflow turbulence distortion for airfoil leading-edge noise prediction for large turbulence length scales for zero-mean loading.

Turbulence distortion due to airfoil finite thickness is an important but not fully understood phenomenon that affects the airfoil radiated noise, resulting in inaccurate noise predictions. This study discusses the turbulence distortion in the leading edge (LE) region of an airfoil aiming to obtain more accurate LE noise predictions. Wind tunnel experiments were performed for National Advisory Committee for Aeronautics (NACA) 0008 and NACA 0012 airfoils at zero angle of attack subjected to large turbulence length scales (between 10 and 43 times the airfoil LE radius) generated by a grid and a rod. Hot-wire and surface pressure measurements were performed in the LE region. Results show that the root mean square of the velocity fluctuations urms and the turbulence integral length scale Λf at the stagnation line decrease considerably as the LE is approached. Rod-airfoil radiated noise was measured and compared with Amiet's model. The predicted noise overestimates the LE noise for high frequencies. However, the prediction agrees well with measurements when the turbulence spectrum based on the rapid distortion theory is used in Amiet's model, with as inputs the urms and Λf values measured close to the LE. This work's main contribution is to demonstrate that more accurate noise predictions are obtained when the inputs to the model consider the turbulence distortion effects.

Influence of blade maximum thickness on airfoil performance with varied leading edge erosion rate

In this study, the effect of different blade shapes on the aerodynamic performance of NACA series airfoils under a wide variety of working conditions at Reynolds number Re = 700,000 was explored. The SST k-ω turbulence model was used for the CFD method to investigate the effect of leading edge erosion on the aerodynamic performance of NACA series airfoils. The results indicated that an optimal thickness range of the blade exists considering the leading edge erosion. If the airfoil was too thin or too thick, the leading edge erosion increased, and the aerodynamic performance was significantly reduced. However, the aerodynamic performance will not decrease greatly when the thickness was moderate, even if the leading edge erosion increased. For the same airfoil, the less the leading edge erosion, the better its aerodynamic performance will be. When the erosion degree (depth) of the leading edge airfoil was small, the variation of the erosion size would significantly affect the aerodynamic performance of the airfoil. When the erosion size of the leading edge increases to a certain extent, the performance of the blade decreases greatly compared with the original cases, but the sensitivity of the erosion blade performance to the change of the leading edge erosion size decreases.

Numerical Investigation of Flow Past Bio-Inspired Wavy Leading-Edge Cylinders

A numerical investigation is proposed to explore the flow past a novel wavy circular cylinder as a passive flow control, whose shape is determined by a sinusoidal function applied to its leading edge line, similar to studies with wavy leading-edge airfoils. The latter are motivated by the wavy-shaped tubercles found in the flippers of humpback whales, which are believed to improve their maneuverability. Our attempt is, therefore, to assess the effects of leading-edge waviness now on a simpler and canonical geometry: circular cylinders. The present work relies on iLES simulations conducted with Nektar++ at a Reynolds number of 3900. Besides the straight cylinder, two wavy geometries are assessed, which are determined by a single wavelength of 37.5% for two amplitudes, 3% and 11%, based on the mean diameter of the wavy cylinder. Our results showed that, contrary to what is usually the case with traditional wavy cylinders at similar Reynolds numbers, waviness caused a reduction in the near-wake recirculation length and an increase in the mean near-wake turbulent kinetic energy compared to the straight cylinder. This was followed by a reduction in base pressure (up to about 36%) leading to a rise in lift oscillations and also to a significant increase in the mean drag coefficient of up to about 28%. An attempt to detail the flow phenomena is provided, evidencing the emergence of counter-rotating pairs of streamwise vortices between peaks. It is argued that the differences observed in recirculation length, turbulent kinetic energy, and force coefficients start even prior to the formation of these coherent structures and end up with interactions with the near wake.

Optimization and Design of a Flexible Droop Nose Leading Edge Morphing Wing Based on a Novel Black Widow Optimization (B.W.O.) Algorithm—Part II

This work presents an aerodynamic and structural optimization for a Droop Nose Leading Edge Morphing airfoil as a high lift device for the UAS-S45. The results were obtained using three optimization algorithms: coupled Particle Swarm Optimization-Pattern Search, Genetic Algorithm, and Black Widow Optimization algorithm. The lift-to-drag ratio was used as the fitness function, and the impact of the choice of optimization algorithm selection on the fitness function was evaluated. The optimization was carried out at various Mach numbers of 0.08, 0.1, and 0.15, respectively, and at the cruise and take-off flight conditions. All these optimization algorithms obtained effectively comparable lift-to-drag ratio results with differences of less than 0.03% and similar airfoil geometries and pressure distributions. In addition, an unsteady analysis of a Variable Morphing Leading Edge airfoil with a dynamic meshing scheme was carried out to study its flow behaviour at different angles of attack and the feasibility of leading-edge downward deflection as a stall control mechanism. The numerical results showed that the variable morphing leading edge reduces the flow separation areas over an airfoil and increases the stall angle of attack. Furthermore, a preliminary investigation was conducted into the design and sensitivity analysis of a morphing leading-edge structure of the UAS-S45 wing integrated with an internal actuation mechanism. The correlation and determination matrices were computed for the composite wing geometry for sensitivity analysis to obtain the parameters with the highest correlation coefficients. The parameters include the composite material qualities, thickness, ply angles, and the ply stacking sequence. These findings can be utilized to design the flexible skin optimization framework, obtain the target droop nose deflections for the morphing leading edge, and design an improved model.

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.

The Dynamics of Supersonic Flow Past a New Cusped Leading Edge Airfoil

The Dynamics of Supersonic Flow Past a New Cusped Leading Edge Airfoil

Dynamic Stall Characteristics of the Bionic Airfoil with Different Waviness Ratios

A dynamic stall will cause dramatic changes in the aerodynamic performance of the blade, resulting in a sharp increase in the blade vibration load. The bionic leading-edge airfoil with different waviness ratios, inspired by the humpback whales flipper, is adopted to solve this problem. In this study, based on the NACA0015 airfoil, the three-dimensional unsteady numerical simulation and sliding mesh technique are used to reveal the flow control mechanism on the dynamic stall of the bionic wavy leading edge. The effects of the waviness ratio on the dynamic stall characteristics of the airfoil are also investigated. The results show that the peak drag coefficient is dramatically reduced when a sinusoidal leading edge is applied to the airfoil. Although the peak lift coefficient is also reduced, the reduction is much smaller. When the waviness ratio R is 0.8, the peak drag coefficient of the airfoil is reduced by 17.14% and the peak lift coefficient of the airfoil is reduced by 9.20%. The dynamic hysteresis effect is improved gradually with an increasing waviness ratio. For the bionic airfoil with R = 1.0, the area of the hysteresis loop is the smallest.

Numerical Predictions of Turbine Cascade Secondary Flows and Heat Transfer With Inflow Turbulence

Abstract Turbine passage secondary flows are studied for a large rounded leading edge airfoil geometry considered in the experimental investigation of Varty et al. (“Vane Suction Surface Heat Transfer in Regions of Secondary Flows: The Influence of Turbulence Level, Reynolds Number, and the Endwall Boundary Condition,” ASME J. Turbomach., 140(2), p. 021010) using high resolution large eddy simulation. The complex nature of secondary flow formation and evolution are affected by the approach boundary layer characteristics, components of pressure gradients tangent and normal to the passage flow, surface curvature, and inflow turbulence. This paper presents a detailed description of the secondary flows and heat transfer in a linear vane cascade at exit chord Reynolds number of 5 × 105 at low and high inflow turbulence. Initial flow turning at the leading edge of the inlet boundary layer leads to a pair of counter-rotating flow circulation in each half of the cross plane that drives the evolution of the pressure side and suction side of the near-wall vortices such as the horseshoe and leading edge corner vortex. The passage vortex for the current large leading edge vane is formed by the amplification of the initially formed circulation closer to the pressure side (PPC) which strengthens and merges with other vortex systems while moving towards the suction side. The predicted suction surface heat transfer shows good agreement with the measurements and properly captures the augmented heat transfer due to the formation and lateral spreading of the secondary flows towards the vane midspan downstream of the vane passage. Effects of various components of the secondary flows on the endwall and vane heat transfer are discussed in detail.

Characteristics of a separated flow past a semicircular leading-edge airfoil model under different imposed pressure gradient

The effect of pressure gradient on a separated boundary layer past the leading edge of an airfoil model is studied experimentally using electronically scanned pressure (ESP) and particle image velocimetry (PIV) for a Reynolds number ( Re) of 25,000, based on leading-edge diameter ( D). The features of the boundary layer in the region of separation and its development past the reattachment location are examined for three cases of β (−30°, 0°, and +30°). The bubble parameters such as the onset of separation and transition and the reattachment location are identified from the averaged data obtained from pressure and velocity measurements. Surface pressure measurements obtained from ESP show a surge in wall static pressure for β = −30° (flap deflected up), while it goes down for β = +30° (flap deflected down) compared to the fundamental case, β = 0°. Particle image velocimetry results show that the roll up of the shear layer past the onset of separation is early for β = +30°, owing to higher amplification of background disturbances compared to β = 0° and −30°. Downstream to transition location, the instantaneous field measurements reveal a stretched, disoriented, and at instances bigger vortices for β = +30°, whereas a regular, periodically shed vortices, keeping their identity past the reattachment location, is observed for β = 0° and −30°. Above all, this study presents a new insight on the features of a separation bubble receiving a disturbance from the downstream end of the model, and these results may serve as a bench mark for future studies over an airfoil under similar environment.

Aeroacoustic design and broadband noise predictions of a fan stage with serrated outlet guide vanes

This work, realized in the framework of the European project TurboNoiseBB, presents an advanced aeroacoustic design methodology of Outlet Guide Vanes (OGVs) with leading edge serrations, including details on their broadband noise and aerodynamic performance. The serrated OGV corresponds to a modified stator from an aeroengine fan stage tested at the AneCom AeroTest’s facility (Germany). Sinusoidal leading edge patterns with varying amplitude and wavelength along the span are designed in collaboration with Safran Aircraft Engines. Serrations are adjusted to account for the turbulence characteristics provided by Reynolds-Averaged Navier–Stokes (RANS) calculations. Optimal parameters are found using simple design rules discussed in this paper. Down selection of serrated OGV designs (patent pending) is conducted through a RANS control of aerodynamic performances and in accordance with industrial specifications, ensuring acceptable penalties on the loss coefficient and isentropic efficiency. Broadband noise simulations are performed using a computational aeroacoustic (CAA) code that solves the linearized Euler equations with a synthetic turbulence model. Additionally, the acoustic response of the serrated leading edge airfoils is also estimated using the most relevant analytical formulation in the literature, based on the Wiener–Hopf technique. Numerical predictions at approach conditions are compared to available experimental measurements (on the untreated baseline case) and analytical Amiet-based and Wiener–Hopf solutions, showing satisfactory agreement in the sound power spectrum in the bypass duct. Finally, the acoustic performances estimated at the design stage are numerically assessed by both the CAA and Wiener–Hopf methods, providing around 2.5 dB and 3.5 dB reduction in the overall power level, respectively.

Robust aerodynamic morphing shape optimization for high-lift missions

Morphing wings have a great potential to enhance the overall performances of the aircraft by adapting the wing shape to various flight conditions. However, there are some issues concerning the applicability of this idea at high-lift missions. An important problem is the need for more actuators along the airfoil that may face with the weight penalty. This mainly arises from the design parameterization methods that are unable to effectively control the leading and trailing edge deformations without modifying the middle part of the airfoil. In this work, a new parameterization method is presented for optimum morphing wing section design at high-lift missions that tends to minimize the number of actuators by increasing un-morphed middle sections of the wing. Genetic algorithm is used for shape optimization and the aerodynamic objective function is calculated by the numerical solution of the compressible turbulent flow equations. Results show that the new method is capable of generating high-lift morphing geometry with minimum shape transformation. In addition, higher lift coefficient is achieved in less computational time by the new method compared with the alternative flexible leading edge airfoil.

Numerical study of airfoil with wavy leading edge at high Reynolds number regime

In this work, a numerical study of flow around an airfoil with wavy leading edge is presented at a Reynolds number of 3X106. The flow is resolved by considering the RANS (Reynolds Average Navier-Stokes)equations. The baseline geometry is based on the NACA 0021 profile. The wavy leading edge has an amplitude of 3% and wavelength of 11%, both with respect to the airfoil chord. Cases without and with wavy leadingedges are simulated and compared. Initially, studies of the numerical sensitivity with respect to the obtained results, considering aspects such as turbulence modeling and mesh refinement, are carried out as well as bycomparison with corresponding results in the literature. Numerical data such as pressure distribution, shear stress lines on the wing surface, and aerodynamics coefficients are used to describe and investigate the flowfeatures around the wavy leading airfoil. Comparisons between the straight leading edge and the wavy leading edge cases shows an increase of the maximum lift coefficient as well as stall angle for the wavy leading edge configuration. In addition, at an angle of attack near the stall, the present numerical results shows an increase of the drag coefficient with the wavy leading edge airfoil when compared with the corresponding straight leading edge case.

The effects of leading-edge tubercles on dynamic stall

The effects of leading-edge tubercles, based on the flippers of humpback whales, on the flow field around an airfoil undergoing large-amplitude dynamic changes in the angle of attack have been studied experimentally. Airfoils were pitched from an initial angle of attack of to at constant pitch rates with a chord Reynolds number of 12 000. Velocity and vorticity fields around a standard NACA 0012 airfoil and NACA 0012 modified with leading-edge tubercles were quantified using molecular tagging velocimetry. Vortex dynamics were characterized by tracking the location, core radius and circulation. The resulting velocity fields showed that the dynamics of the formation and separation of the leading-edge vortex were fundamentally different between the straight leading-edge airfoil and the tubercled airfoil. The tubercled airfoil also showed spanwise variation in dynamics of the dynamic stall vortex (DSV) formation. The characteristics of the DSV, specifically the circulation and proximity of the DSV to the airfoil suction surface, are known to have an impact on lift during dynamic pitching. The results showed that the DSV was stronger and remained closer to the airfoil longer for the modified airfoil. The baseline DSV convected away from the airfoil faster than the DSV on the tubercled airfoil once it began to separate from the airfoil. Shear-layer vortices, which form during dynamic stall near the mid-cord region, appeared to affect the convective behaviour of the DSV. The results suggest that the leading-edge tubercles observed on Humpback whale flippers act as passive flow-control mechanisms to control or delay dynamic stall.

An experimental investigation on the dynamic ice accretion and unsteady heat transfer over an airfoil surface with embedded initial ice roughness

In the present study, a comprehensive experimental study was conducted to evaluate the effects of initial ice roughness formed around the leading-edge of an airfoil model on the dynamic ice accretion and unsteady heat transfer processes over the airfoil surface. The experimental study was performed in the Icing Research Tunnel at Iowa State University, Two airfoil models with the same airfoil shape were manufactured by using a rapid protype machine for a comparative study, i.e., one test model was designed to have embedded initial ice roughness around the airfoil leading-edge and the other model having smooth airfoil leading-edge as the comparison baseline. During the experiments, while a high-speed imaging system was used to record the early-stage icing morphologies over the airfoil surfaces with and without the initial leading-edge roughness, an infrared (IR) thermal imaging system was also utilized to map the corresponding surface temperature distributions over the airfoil surfaces to quantify the unsteady heat transfer and dynamic icing, i.e., phase changing, processes under different test conditions. It was found that, the initial ice roughness formed around the airfoil leading-edge would affect the characteristics of local airflow, impingement of supercooled water droplets, collection and transport of impacted water mass, unsteady heat transfer and subsequent ice accretion processes dramatically. The initial ice roughness formed around the airfoil leading-edge would redistribute the impacted water mass, with more impacted water mass being captured and frozen over the roughness region. In addition, the initial ice roughness was also found to produce span-wise-alternating low- and high-momentum pathways (LMPs and HMPs, respectively), which can significantly affect the convective heat transfer and subsequent ice accretion processes over the airfoil surface.

Numerical Investigation on the Effects of Airfoil Leading Edge Radius on the Aerodynamic Performance of H-Rotor Darrieus Vertical Axis Wind Turbine

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.

Influence of the Airfoil Leading Edge Thickness on Turbine Cascade Losses Due to the Incidence

On the basis of experimental studies, it is established that the thickening of the airfoil leading edge at positive incidences increases the impingement resistance of the moderate convergence cascade, but it does not exert impact in the high convergence cascade, and practically does not affect losses in the cascade of any convergence at the negative incidences.