Flapping Airfoil Research Articles

Simulation of Aerodynamic Performances of Flexible Flapping Wing Airfoils

Compared with fixed wings, the flapping wing has a significant aerodynamic performance advantage at low speeds and low Reynolds numbers, which draws more and more attentions. But most previous studies focus on rigid flapping airfoils, the aerodynamic performances of flexible flapping airfoils are still unclear. A fluid-solid coupling model for the flexible elliptical airfoils was developed to analyze the flow field around the airfoil, the airfoil deformation and the aerodynamic characteristics of airfoils, at different wind speeds and attack angles. Compared with the rigid airfoil, the flexible airfoil can delay the shedding time of the wake vortex and reduce the oscillation frequency of the disturbance on the lift force. The flexible airfoil significantly suppresses the disturbance of the wake flow and reduces the oscillation amplitude of disturbance. Even, the airfoil disturbance oscillation can be completely eliminated at an appropriate Young’s modulus of the airfoil. These results provide a theoretical guidance for the design of soft aircraft.

Limit of the two-dimensional linear potential theories on the propulsion of a flapping airfoil in forward flight in terms of the Reynolds and Strouhal number

Numerical simulations have been used to estimate the validity range of the linear potential theories of Fernandez-Feria and the classical Garrick model on the propulsion of a pitching airfoil and a heaving airfoil in forward flight, in terms of the Reynolds, Re, and Strouhal number, St.

Design of Hybrid Airfoils for Icing Tunnel Tests Based on Reduced-Order Modeling Methods

A hybrid airfoil is a scaled model for generating a full-scale ice shape for icing wind tunnel tests. This is possible by matching full-scale properties such as the distributions of collection efficiency and heat transfer coefficient. Previous studies have used indirect methods using full-scale stagnation point location or tangent droplet trajectories. Therefore, these methods can cause a discrepancy between the full-scale and hybrid airfoil ice shapes under glaze ice conditions. To cope with the issue, this paper proposes a new approach to match the distributions of the full-scale collection efficiency and heat transfer coefficient on the leading edge, using a viscous turbulent computational fluid dynamics icing simulation. For computational efficiency, reduced-order modeling based optimization was used to match the distributions. The optimization process was applied to the glaze ice condition with a high liquid water content and temperature. The results indicate that matching the distribution of the heat transfer coefficient is recommended to minimize the error between full-scale and hybrid airfoil ice shapes for the glaze ice condition. Finally, a hybrid airfoil flap geometry, which can be applied to various angles of attack, was designed using the optimization design process.

Extended Implicit PIS-ALE Method to Efficient Simulation of Turbulent Flow Domains with Moving Boundaries

In this work, an implicit finite-volume-element (FVE) method is extended to efficiently simulate the vortical structure of unsteady turbulent flows in domains with moving meshes. The arbitrary Lagrangian-Eulerian (ALE) approach is used to consider the motion of a hybrid mesh distributed in the solution domain. Conventional turbulence models are applied to simply confirm the sample achieved efficiency and accuracy in solving complex turbulent flow domains with moving boundaries. In this regard, the advective terms in the Navier-Stokes equations, including those in the transport equations for the applied turbulence models, are treated in a rather innovative manner. In other words, an advanced physical influence scheme (PIS) is suitably introduced in the context of extended ALE formulations. The accuracy and efficiency of the extended method are carefully evaluated by simulating various turbulent flows, including the fluid flow in stationary domains, separated turbulent flow over a bluff body problem, and the dynamic stall of fluid flow over a flapping airfoil. Comparing the current solutions with experimental data, it is shown that the current PIS-ALE method provides better accuracy and efficiency than those of past numerical methods, which used similar turbulence models in their algorithms.

Numerical Investigation of Aerodynamic Characteristics of NACA 4312 Airfoil with Gurney Flap

This paper presents a two-dimensional Computational Fluid Dynamics (CFD) analysis on the effect of gurney flap on a NACA 4312 airfoil in a subsonic flow. These numerical simulations were conducted for flap heights 1.5%, 1.75%, 2% and 3% of chord length at fixed Reynold Number, Re (5×105) for different angle of attack (0o ~16o). ANSYS Fluent commercial software was used to conduct these simulations. The flow was considered as incompressible and K-omega Shear Stress Transport (SST) model was selected. The numerical results demonstrate that lift coefficient increase up to around 12o AoA (angle of attack) for NACA 4312 with and without gurney flap. For every AoA lift coefficient and drag coefficient presented proportionate behavior with flap height. However, lift co-efficient was decreased after around angle of attack due to flow separation. Maximum lift to drag ratio was found at around 4o AoA for every flap length and airfoil with flap of 1.5%C (chord length) had shown the most optimized aerodynamic performance through the analysis. This study concluded that airfoil with gurney flap displayed enhanced aerodynamic performance than the airfoil without gurney flap due to the delay in flow separation.

Acoustic and flow characteristics of an airfoil fitted with morphed trailing edges

An experimental study of a simple NACA 0012 airfoil fitted with two different flap profiles was carried out to characterize their aerodynamic and aeroacoustic performance. The airfoil with a flap deflection angle of β = 10° was tested for a wide range of angles of attack at a chord-based Reynolds number of Rec=2.6×105. The aerodynamic lift and drag measurements show improved lift-to-drag performance for the morphed flap airfoil compared to the hinged flap airfoil at low angles of attack. Surface flow visualization and boundary layer measurements on the suction surface of the flap show delayed separation for the morphed flap airfoil. Higher-order moments of the wall pressure fluctuations were also used to observe the flow separation over the flap. Additionally, Particle Image Velocimetry was also used to study the flow over the flap and at the airfoil wake. Flow measurements showed that the downstream wake development could be significantly influenced by the flap camber. The mean velocity contours at the wake showed increased wake velocity deficit and turbulent kinetic energy for the morphed flap airfoil. The turbulent kinetic energy results displayed a characteristic double peak behavior which was also the dominant characteristics of the streamwise Reynolds shear stress component. Near-field unsteady surface pressure fluctuations and far-field noise measurements show reduced point spectra and noise levels for morphed flap configuration at low angles of attack but considerably increased noise levels at high angles of attack compared to hinged flap configuration.

The Contribution of Added Mass Force to Formation of Propulsive Force of Flapping Airfoil in Viscous Fluid

This paper investigates the formation mechanism of the propulsive force of a flapping airfoil, which performs harmonic angular oscillations in a continuous medium flow, in the light of the recently proved general theorem on the added mass of bodies in a viscous incompressible fluid. The flow is described by the Navier–Stokes equations. The calculations were performed by the mesh-free numerical method of viscous vortex domains. This work explains the formation mechanism of a reverse vortex street in the wake behind the flapping airfoil in the forward thrust mode. The dominant contribution of the added mass force to the value of the propulsive force is detected. The obtained results reveal to some extent the hydrodynamic action mechanism of the tail fin of underwater animals.

Effect of Gurney flap on the aerodynamic behavior of an airfoil in mutational ground effect

In this research, the effect of Gurney flap on the aerodynamic behavior of an airfoil in mutational ground effect is investigated. To perform this, lift and drag coefficients of NACA 4412 airfoil section in the presence and absence of Gurney flap in mutational ground effect are evaluated and compared. To provide a better illustration of the effect of Gurney flap on the aerodynamic behavior of the airfoil section in the mutational ground effect, results are obtained during the takeoff and landing processes. Validation of the used numerical model is also performed by comparison of the obtained results with those of other works and reasonable agreements were seen. Results show that inclusion of Gurney flap to the airfoil leads to higher variations of lift and drag coefficients during takeoff and landing process. During the takeoff process, the flapped airfoil results in a higher lift decrement and drag increment although an increase in distance of airfoil from the ground or angle of attack causes the lift decrement of the flapped airfoil to get close to that of clean airfoil. It is shown that during takeoff process, downwash is generated around the airfoil as the airfoil leaves the step and as the airfoil gets away from the step, the generated downwash decreases. During the landing process, at each distance of airfoil form the ground and angle of attack, the lift decrement and drag increment of the flapped airfoil are significant compared to that of the clean airfoil. Results exhibit that during landing process, upwash is generated around the airfoil as the airfoil reaches the step and further airfoil move on the step leads to the decrease in the upwash. The decrement in lift coefficient and also the increment in drag coefficient during landing process are more remarkable than those in takeoff process.

Impact of the ground effect on the energy extraction properties of a flapping wing

The energy harvesting device with flapping foil has the characteristic of environmental adaptability, particularly in shallow water. This study applies a transient numerical calculation method based on a dynamic grid technique in an absolute coordinate system to analyze the flow fields and pressure distributions of a flapping airfoil near a ground wall. The effects of Reynolds number (Re), average ground clearance, and effective angle of attack on the power extraction efficiency of the flapping airfoil are investigated with respect to variations in the oscillation frequency. The simulation results indicate that the ground effect can greatly increase the power extraction efficiency of the flapping airfoil, and that the highest efficiency is obtained at an optimum average ground clearance. This ground effect is asymmetric over the course of flapping airfoil motion toward and away from the ground wall, and the efficiency is most increased as the airfoil moves far from or closely to the ground wall. The formation of the corresponding divergent or convergent tunnel effects alters the flow fields and pressure distributions around the airfoil near the leading edge, and, in particular, decreases the pressure on the suction side, which results in an overall increase in the heaving force and a correspondingly increased power harvesting capability. With or without the ground effect, the efficiency of the flapping airfoil firstly increases and then decreases with increasing oscillation frequency or effective angle of attack at constant Re and average ground clearance values. Moreover, the positive impact of the ground wall effect on the power extraction efficiency of the airfoil becomes increasingly obvious when the oscillation frequency and the angle of attack are increased simultaneously. Finally, the oscillation frequency obtaining the highest power extraction efficiency in conjunction with the ground effect decreases as the flow fields transition from laminar to turbulent flow with increasing Re.

A Parametric Study of Trailing Edge Flap Implementation on Three Different Airfoils Through an Artificial Neuronal Network

Trailing edge flaps (TEFs) are high-lift devices that generate changes in the lift and drag coefficients of an airfoil. A large number of 2D simulations are performed in this study, in order to measure these changes in aerodynamic coefficients and to analyze them for a given Reynolds number. Three different airfoils, namely NACA 0012, NACA 64(3)-618, and S810, are studied in relation to three combinations of the following parameters: angle of attack, flap angle (deflection), and flaplength. Results are in concordance with the aerodynamic results expected when studying a TEF on an airfoil, showing the effect exerted by the three parameters on both aerodynamic coefficients lift and drag. Depending on whether the airfoil flap is deployed on either the pressure zone or the suction zone, the lift-to-drag ratio, CL/CD, will increase or decrease, respectively. Besides, the use of a larger flap length will increase the higher values and decrease the lower values of the CL/CD ratio. In addition, an artificial neural network (ANN) based prediction model for aerodynamic forces was built through the results obtained from the research.

Numerical simulation of flapping airfoil with alula

Bionic micro aerial vehicles have become popular because of their high thrust efficiency and deceptive appearances. Leading edge or trailing edge devices (such as slots or flaps) are often used to improve the flight performance. Birds in nature also have leading-edge devices, known as the alula that can improve their flight performance at large angles of attack. In the present study, the aerodynamic performance of a flapping airfoil with alula is numerically simulated to illustrate the effects of different alula geometric parameters. Different alula relative angles of attack β (the angle between the chord line of the alula and that of the main airfoil) and vertical distances h between the alula and the main airfoil are simulated at pre-stall and post-stall conditions. Results show that at pre-stall condition, the lift increases with the relative angle of attack and the vertical distance, but the aerodynamic performance is degraded in the presence of alula compared with no alula, whereas at post-stall condition, the alula greatly enhances the lift. However, there seems to be an optimal relative angle of attack for the maximum lift enhancement at a fixed vertical distance considering the unsteady effect, which may indicate birds can adjust the alula twisting at different spanwise positions to achieve the best flight performance. Different alula geometric parameters may affect the aerodynamic force by modifying the pressure distribution along the airfoil. The results are instructive for design of flapping-wing bionic unmanned air vehicles.

Active Flow Control of Flapping Airfoil Using Openfoam

ABSTRACTThe concept of the fixed wing Micro Air Vehicles (MAVs) has received increasing interest over the past few decades, with the principal aim of carrying out the surveillance missions. The design of the flapping wing MAVs still is in infancy stage. On the other hand, there has been increasing interest over the flow control using plasma actuators in worldwide. The aim of this research is to study the flow control of a flapping airfoil with and without plasma actuation in OpenFOAM. The OpenFOAM CFD platform has been used to develop our own plasma solver. For the plasma induced turbulence in the flow regime, k-ε turbulence model was adopted to address the interaction between plasma and fluid flows. For the plasma-fluid interaction, we use reduced-order modelling to solve the plasma induced electric force. A two dimensional NACA0012 flapping airfoil without plasma actuation study has been benchmarked with previous published literature. We have not only focused on the active flow control but also analyzed the important parameter reduced frequency at different values, those are 0.1, 0.05 and 0.025. Reduced frequency (κ) is very important parameter of an airfoil in the unsteady motion. Our major contribution is testing the several reduced frequencies with the plasma actuation. The positive and beneficial effects of the plasma actuator for all cases have been observed. From the observed results, the flapping with plasma actuation at reduced frequency of 0.1 showed the 14.285 percent lift improvement and the 16.19 percent drag reduction than the flapping without plasma actuation at the respective dynamic stall angles. The maximum lift coefficient is increased with the increase in reduced frequency. In overall, plasma actuators are effective in the flow control of a flapping airfoil. In future, the combination of the flapping with plasma actuators will be a promising application to boast the maneuverability of MAVs.

고해상도 수치기법을 이용한 플래핑 날개 주위 비정상 아음속 유동해석

Many numerical and experimental researches have been done for the unsteady flow around a flapping airfoil. The main research topics are vortex shedding, dynamic stall phenomenon, and MAV''s lift and thrust generation for the flow over a flapping airfoil. Although many researches have been studied to analyze the flow for the variation of frequency and amplitude, much research is still needed. In this study, thrust generation, wake structures, and acoustic wave propagation characteristics were studied for a flapping airfoil. The governing equation was the Navier-Stokes equation with LES turbulence model, and the Optimized High-Order Compact (OHOC) scheme and 4th order Runge-Kutta method were used. The Mach number and Reynolds number were 0.3 and 1000, respectively. The analysis of mean lift and drag coefficients showed the thrust generation characteristics according to the phase angle and the reduced frequency. The wake vortex shedding and propagation characteristics of acoustic waves could be simulated using the high order method.

Pressure–velocity coupling algorithm-based pressure reconstruction from PIV for laminar flows

In this study, we propose a method to reconstruct pressure fields from planar particle image velocimetry measurements for laminar flows by employing semi-implicit method for pressure-linked equations algorithm to solve governing equations where measured velocities are inherently used as boundary conditions. The method starts with interpolating the measured velocity field on a staggered computational grid. The continuity equation, in the form of pressure equation for incompressible flows, is solved with this velocity data to reach an initial pressure field. Momentum equations are solved with calculated pressure field and then pressure correction equation is solved for calculating the mass imbalance in continuity equation. Using this mass imbalance, the current velocity field is corrected. Repeating this procedure until convergence provides not only the pressure field that satisfies governing equations, but also error reduction on the measured velocity. The method is validated with theoretical flows and its sensitivity to the velocity field error is assessed. This method reconstructs the pressure fields exactly for error-free velocity fields and can correct the flow-fields to accurate values even with extremely high experimental errors. Application on a flapping airfoil experiment showed that the method can successfully reconstruct pressure field by slightly altering the measured velocity field. The proposed method offers a non-intrusive, global pressure measurement for laminar flows with minimum end user input.

Comparative study of flow separation control using suction and blowing over an airfoil with/without flap

This study mainly focused on the comparison of the effect of single and simultaneous suction and blowing jets on the aerodynamic performance of an airfoil with/without flap to evaluate the most effective flow control configuration using computational fluid dynamics (CFD) method. Moreover, the effect of applying single and simultaneous jets have been conducted on delaying and controlling the flow separation. The results were obtained using two-dimensional incompressible Unsteady Reynolds-Averaged Navier–Stokes (URANS), and the turbulence was simulated with SST k-ω turbulence model. Also, different parameters including two jet locations (Ljet), three jet velocity ratios (Rjet), three jet angles (θjet) and three flap deflections (δf) were analyzed to find the most effective case of applying flow control jets to delay the boundary layer separation. It was concluded that applying a single suction jet and simultaneous suction and blowing jets on the flapped airfoil was more effective to improve the lift-to-drag ratio (CL/CD) than applying these jets to the without flap case. The maximum value of CL/CD was achieved by single suction jet for the without flap case which was equal to 73.7. The maximum increment of stall angle over the without flap airfoil and flapped airfoil was obtained by applying single suction jet, which increased the stall angle from 14° to 20° and 14° to 16° for the suction angle of −90° and suction velocity ratio of 0.15, respectively.

New type of motion trajectory for increasing the power extraction efficiency of flapping wing devices

Flapping wing devices represent a new type of renewable energy extraction technology that has the advantageous characteristics of simple structure, strong adaptability to surroundings, and little impact on the environment. This study numerically investigates the effects of vertical and elliptical airfoil trajectories on the power extraction efficiency of a flapping airfoil device using a transient numerical method based on the overset grid technique. The results are employed to propose a novel reversed-D airfoil trajectory that represents a composite of an elliptical trajectory in the first half of the motion cycle and a standard vertical trajectory in the second half of the motion cycle. The results show that for the elliptical trajectory, when the length of the half-axis in the vertical direction is fixed, the total power harvesting efficiency decreases with the increase of the horizontal half-axis length. In a certain frequency range, the decreased orders of the power extraction ability of the flapping wing are the upstream half-cycle elliptical trajectory, the vertical linear and the downstream half-cycle elliptical trajectory. Based on this understanding, we propose a new type of reversed-D motion trajectory. The power extraction efficiency of flapping wing devices operating in the reversed-D trajectory are investigated using both single and double airfoil designs. The results show that the power extraction efficiency of the single airfoil design moving along the reversed-D trajectory is greater than that obtained for the single airfoil moving along the standard vertical reciprocating trajectory within a specific range of frequency, and the increase is due mainly to an increase in the heave force. The power extraction efficiency of each airfoil in the double airfoil model moving along the reversed-D trajectory is less than that of a single airfoil moving along the vertical trajectory. However, the overall average power extraction efficiency is greater than that of a single airfoil moving along the vertical trajectory, and this increased efficiency is obtained over a larger frequency range than that of a single airfoil moving along the reversed-D trajectory. In addition, the increased efficiency of the double airfoil model is greater in the low frequency region. The proposed reversed-D trajectory facilitates the flexible arrangement of multiple airfoils in a flapping wing design. As such, the proposed reversed-D trajectory provides a promising new methodology for designing flapping wing devices with high power extraction efficiencies.

Numerical Investigation of the Effects of Nonsinusoidal Motion Trajectory on the Propulsion Mechanisms of a Flapping Airfoil

The effect of nonsinusoidal trajectory on the propulsive performances and the vortex shedding process behind a flapping airfoil is investigated in this study. A movement of a rigid NACA0012 airfoil undergoing a combined heaving and pitching motions at low Reynolds number (Re = 11,000) is considered. An elliptic function with an adjustable parameter S (flattening parameter) is used to realize various nonsinusoidal trajectories of both motions. The two-dimensional (2D) unsteady and incompressible Navier–Stokes equation governing the flow over the flapping airfoil are resolved using the commercial software starccm+. It is shown that the nonsinusoidal flapping motion has a major effect on the propulsive performances of the flapping airfoil. Although the maximum propulsive efficiency is always achievable with sinusoidal trajectories, nonsinusoidal trajectories are found to considerably improve performance: a 110% increase of the thrust force was obtained in the best studied case. This improvement is mainly related to the modification of the heaving motion, more specifically the increase of the heaving speed at maximum pitching angle of the foil. The analysis of the flow vorticity and wake structure also enables to explain the drop of the propulsive efficiency for nonsinusoidal trajectories.

Optimal performance of adaptive flap on flow separation control

The stall caused by flow separation of airfoil seriously compromises the efficiency and safety of wind turbines. Inspired by bird feathers, a passive flow control strategy was investigated by configuring adaptive flaps on the suction surface of the airfoil to delay flow separation. The Computational Fluid Dynamics (CFD) software ANSYS Fluent 17.0 using SST k–ω turbulence model was performed to study the performance of the adaptive flap with different length and location. The performance was evaluated by the comparison of total lift coefficient between the flap airfoil and clean airfoil. In addition, the performance of the double flaps configuration was investigated by Fluid-Solid Interaction (FSI) method. The results show that when the separation point is at the upstream of flap hinge point, the total lift coefficient increases with the flap rising, and when the flap reaches the optimal flap angle, the lift coefficient begins to decrease. When the flap operates in optimal angle, the closer the flap is to the trailing edge of the airfoil, the more the lift enhancement for moderate flow separation, and the closer the flap is to the leading edge of the airfoil, the more the lift enhancement for serious flow separation. For the flaps with the same location, the longer flap can block more backflow resulting in a better performance, but when the flap is long enough to block the fluid, the shorter flap has better performance. Considering the variations of the flap aerodynamic moment with the flap angle, an external hinge moment was recommended to improve the swinging flap performance.

Control of Flow Separation over a Flapped Airfoil Using Low-Aspect-Ratio Circular Pins

The implementation of an array of finite-span, low-aspect-ratio circular pins as a flow control method to mitigate separation over a flapped airfoil was investigated experimentally in a wind tunnel. The effect of an array of static cylinders, placed just upstream of a deflected control surface, on the separated flow over the control surface was explored via surface pressure and stereoscopic particle image velocimetry measurements. The array of pins was embedded in a turbulent boundary layer, where the height of the pins was approximately 2/3 of the local boundary-layer thickness. The static array of pins demonstrated a large reduction of the separated flow over the control surface, as well as a change in the global circulation, quantified by an enhanced sectional lift coefficient of and reduction in drag coefficient of . A secondary dataset was collected, which studied the effect imposed on the nearfield from a single pin that was either static or underwent dynamic motion (in the wall-normal direction), placed upstream of the deflected control surface hinge line. Results indicate that even a single pin (with diameter-to-airfoil span ratio of ) had a noticeable impact on the delay of separation, enhanced global circulation, and drag reduction. Overall, the features discussed have provided strong evidence of the potential that finite-span, low-aspect-ratio cylindrical pins provide as new actuators for flow control.

Virtual and Physical Surface Modifications as Means for Flow Control

This paper reviews recent work on several flow control actuators, as well as a new hybrid actuator, and their interaction with a laminar boundary layer over a flat plate. The focus is on 1) a finite-span synthetic jet, where the effects of the jet orifice aspect ratio and its skew and pitch angles relative to the flow are presented; 2) a finite-span low-aspect-ratio static or dynamic pin (with similar height to the local boundary layer), where the effect of the pin’s aspect ratio is discussed; and 3) a hybrid actuator, which incorporates the previous two actuators. First, the formation and evolution of flow structures associated with a synthetic jet issued into a laminar boundary layer, where the effect of the orifice’s aspect ratio, as well as its pitch and skew angles, are explored. Next, the effects of the pin’s aspect ratio and its dynamic motion are shown. Finally, the interaction of the hybrid actuator with the flow has shown that the combined effects of the synthetic jet and the pin resulted in alteration of the vortical structures with minimal penalty. This suggests that the hybrid actuator might be efficient for controlling different types of flowfields. An example of controlling separation over a flapped airfoil was also included, where an array of passive cylindrical pins yielded delayed or complete mitigation of separation. This was mostly due to the introduction of streamwise vortices, which redirected the flow toward the surface and energized the boundary layer. Moreover, the effect of a single dynamic pin on flow separation was larger than that of a single static pin.