Flapping Amplitude Research Articles

Time-resolved particle image velocimetry measurement of vortex dynamics behind tandem self-oscillating inverted flags in a channel flow

The coupled flapping behaviors of two tandem inverted flags and the resultant vortex dynamics in a turbulent channel flow were experimentally determined using time-resolved particle image velocimetry (TR-PIV). The highly unsteady flow fields near the tandem self-oscillating flags with dimensionless separation distances of G* = 1, 1.5, 2, 2.5 and 3 were measured using time-resolved particle image velocimetry. Furthermore, the instantaneous flag profiles were successfully identified by the structure boundary detection algorithm. The time histories of the displacement of the monitor points indicated that the front and rear flags flapped synchronously with a constant phase difference, which was linearly dependent on the separation distance. The flapping amplitude of the rear flag was inferior to that of the front flag. A vortex pair (P) and a single vortex (S) shedding from the front flag were observed in the phase-averaged velocity vector maps and vorticity strength contours. The leading edge vortex excited an anti-rotating wall-induced vortex, which had the potential to disturb the thermal boundary layer, enhancing the wall heat removal performance of the channel flow. The phase-averaged pressure fields were estimated to understand the interactions between the flapping flags and the surrounding fluid. The vortex destruction process decreased the pressure difference on both sides of the rear flag, resulting in the attenuation of the rear flag’s flapping amplitude. Finally, the cross-correlation between the flag’s displacement and the fluid’s velocity fluctuation demonstrated that the front flag’s trailing edge vortex traveled downstream and encountered the rear flag with a constant convecting velocity, which explained the linear dependency of the phase difference between the tandem flags on their separation distance.

Heat transfer enhancement of turbulent channel flow using a piezoelectric fan

This study experimentally investigated the flapping dynamics of a piezoelectric (PE) fan in channel flow and the resultant wall heat-removal performance enhancement. The PE fan consisted of a rigid PZT (Lead zirconate titanate) actuator and a flexible PET (Polyethylene terephthalate) blade. The PE fan in absence of electric excitation or air flow was chosen as the baseline data configuration. The mid-span of the PE fan was illuminated by a laser sheet, and the instantaneous profiles of the PE fan were captured by a high-speed camera system; the structure boundary detection algorithm and fast Fourier transform algorithm were used to identify the flapping amplitude and frequency. The time-averaged heat transfer performance across the channel wall with constant heat flux boundary condition was spatially resolved by the temperature sensitive paint measurement. The piezoelectric fan displayed the largest flapping amplitude at the fundamental resonant frequency of 43 Hz. The amplified flapping amplitude and the increased frequency were achieved with the elevation of the Reynolds number, and correspondingly the resultant heat transfer performances were improved. At a low Reynolds number of 1.37×104, the piezoelectric fan blade flapped synchronously with the PZT actuator, while the flapping amplitude was considerably intensified to 0.72 times the blade length under excitation voltage of fundamental resonant frequency, resulting in a 30% augmentation in the local Nusselt number. At a high Reynolds number of 1.91×104, the flapping frequency was locked to its flutter frequency 64.5 Hz, regardless of the excitation voltage frequency. The flapping amplitude and the resultant heat transfer performance were found to be insensitive to the excitation voltage frequency at a Reynolds number of 1.91×104, although the maximum local heat transfer was enhanced by 46% at an excitation voltage frequency 43 Hz.

Self-propelled plate in wakes behind tandem cylinders.

Fish may take advantage of environmental vortices to save the cost of locomotion. The complex hydrodynamics shed from multiple physical objects may significantly affect fish refuging (holding stationary). Taking a model of a self-propelled flapping plate, we numerically studied the locomotion of the plate in wakes of two tandem cylinders. In most simulations, the plate heaves at its initial position G_{0} before the flow comes (releasing Style I). In the typical wake patterns, the plate may hold stationary, drift upstream, or drift downstream. The phase diagrams of these modes in the G_{0}-A plane for the vortex shedding patterns were obtained, where A is the flapping amplitude. It is observed that the plate is able to hold stationary at multiple equilibrium locations after it is released. Meanwhile, the minimum amplitude and the input power required for the plate seem inversely proportional to the shedding vortex strength. The effect of releasing style was also investigated. If the plate keeps stationary and does not flap until the vortex shedding is fully developed (releasing Style II), then the plate is able to hold stationary at some equilibrium locations but the flapping plate has a very minor effect on the shedding vortices. However, in Style I, the released plate is able to achieve more equilibrium locations through adjusting the phase of vortex shedding. The effort of the preflapping in Style I is not in vain, because although it consumes more energy, it becomes easier to hold stationary later. The relevant mechanism is explored.

Bionic Flapping Pectoral Fin with Controllable Spatial Deformation

This paper presents the design of a bionic pectoral fin with fin rays driven by multi-joint mechanism. Inspired by the cownose ray, the bionic pectoral fin is modeled and simplified based on the key structure and movement parameters of the cownose ray’s pectoral fin. A novel bionic propulsion fin ray composed of a synchronous belt mechanism and a slider-rocker mechanism is designed and optimized in order to minimize the movement errors between the designed fin rays and the spanwise curves observed from the cownose ray, and thereby reproducing an actively controllable flapping deformation. A bionic flapping pectoral fin prototype is developed accordingly. Observations verify that the bionic pectoral fin flaps consistently with the design rule extracted from the cownose ray. Experiments in a towing tank are set up to test its capability of generating the lift force and the propulsion force. The movement parameters within the usual propulsion capabilities of the bionic pectoral fin are utilized: The flapping frequency of 0.2 Hz–0.6 Hz, the flapping amplitude of 3°–18°, and the phase difference of 10°–60°. The results show that the bionic pectoral fin with actively controllable spatial deformation has expected propulsion performance, which supports that the natural features of the cownose ray play an important role in designing and developing a bionic prototype.

Unsteady numerical simulation of flap vibration for 30P30N three-element airfoil

The pitching motion of NACA0012 airfoil and the flap vibration of 30P30N multi-element airfoil are simulated by using commercial fluid dynamics simulation software Fluent. The influence of flap vibration on lift coefficient and pitch moment coefficient of multi-element airfoil is analyzed. The results show that the influence of flap vibration on pitching moment coefficient of multi-element airfoil is greater. With the increase of flap amplitude, the variation range of lift and moment coefficients gradually increases, while with the increase of frequency, the variation range of lift and moment coefficients gradually decreases, and the hysteresis loop gradually becomes “thin” and its shape tends to ellipse. With the increase of flap vibration frequency, the unsteady effect of flow field increases, and the hysteresis loop may change in reverse direction.

Large amplitude flapping of an inverted elastic foil in uniform flow with spanwise periodicity

An elastic foil interacting with a uniform flow with its trailing edge clamped, also known as the inverted foil, exhibits a wide range of complex self-induced flapping regimes such as large amplitude flapping (LAF), deformed and flipped flapping. In particular, the LAF is of interest for its applications in the development of energy harvesting devices. Here, we perform three-dimensional numerical experiments assuming spanwise periodicity on the LAF response of an inverted foil at Reynolds number Re=30,000 for a relatively low mass-ratio m∗=1.0 using a variational fluid–structure formulation and large-eddy simulation (LES). We examine the role of the vortex structures particularly the counter-rotating periodic vortices generated from the leading and trailing edges of the inverted foil, and the interaction between them on the LAF. For that purpose, we investigate the dynamics of the inverted foil for a novel configuration wherein we introduce a fixed splitter plate at the trailing edge to suppress the vortex shedding from the trailing edge and thereby inhibit the interaction between the counter-rotating vortices. Unlike the vortex-induced vibration of an elastically-mounted circular cylinder, we find that the inhibition of the interaction has an insignificant effect on the transverse flapping amplitudes, due to a relatively weaker coupling between the counter-rotating vortices emanating from the leading edge and trailing edge. The inhibition of the trailing edge vortex generally reduces the streamwise flapping amplitude, the flapping frequency and the net strain energy of foil. To further generalize our understanding of the LAF, we next perform low-Reynolds number (Re∈[0.1,50]) simulations for the identical foil properties to realize the impact of vortex shedding on the large amplitude flapping. Due to the absence of vortex shedding process in the low-Re regime, the inverted foil no longer exhibits the periodic flapping. Nevertheless, the flexible foil still loses its stability through divergence instability to undergo a large static deformation. This study has implications on the development of novel control mechanisms for energy harvesting and propulsive devices.

Insights into Sensitivity of Wing Shape and Kinematic Parameters Relative to Aerodynamic Performance of Flapping Wing Nano Air Vehicles

In this work, seven wings inspired from insects’ wings, including those inspired by the bumblebee, cicada, cranefly, fruitfly, hawkmoth, honeybee, and twisted parasite, are patterned and analyzed in FlapSim software in forward and hovering flight modes for two scenarios, namely, similar wingspan (20 cm) and wing surface (0.005 m2). Considering their similar kinematics, the time histories of the aerodynamic forces of lift, thrust, and required mechanical power of the inspired wings are calculated, shown, and compared for both scenarios. The results obtained from FlapSim show that wing shape strongly impacts the performance and aerodynamic characteristics of the chosen seven wings. To study the effects of different geometrical and physical factors including flapping frequency, elevation amplitude, pronation amplitude, stroke-plane angle, flight speed, wing material, and wingspan, several analyses are carried out on the honeybee-inspired shape, which had a 20 cm wingspan. This study can be used to evaluate the efficiency of different bio-inspired wing shapes and may provide a guideline for comparing the performance of flapping wing nano air vehicles with forward flight and hovering capabilities.

Dynamic damage analysis of airfoil flutter for a generic hypersonic flight vehicle

The main aim of this paper is to establish the relationship between the airfoil flutter with the flight dynamics of a generic hypersonic flight vehicle (HFV) and analyze the airfoil damage variation during the airfoil flutter. Based on the motion equations of the two degrees of freedom airfoil model and the longitudinal dynamics of the HFV, an airfoil dynamic model is established. By using a coupling equation, the relationship between airfoil flutter and the flight dynamics is estimated. According to the stress–strain () model and the strain–fatigue damage () model, the influence of the stress on the airfoil flutter damage is analyzed. The simulation results show that the flutter of the airfoil is closely related to the flight dynamics for the HFV and the airfoil flutter damage is closely related to the flutter amplitude of the airfoil.

Fluid–Structure Interaction Numerical Simulation of Bridge Wind-Induced Vibration Based on CV Newmark-β Method

Two kinds of numerical solution of differential equations were first introduced to solve the problem of wind-induced vibration of bridge structures. Fluid-structure interaction (FSI) numerical simulation of a streamlined bridge section was realized using ANSYS Fluent as the computing platform and embedding user-defined function (UDF) in Fluent. Theoretical analysis indicated that the displacement gained by the FSI calculation model, where the grids near the bridge section were driven by conventional Newmark-beta (β) program embedded in UDF, differs from that of grid motion updated in Fluent. A corrected velocity method (i.e., CV Newmark-β) was proposed in this work to eliminate such an error. The corresponding FSI simulation model was then set up through the abovementioned method. For a concrete bridge section, the FSI simulation under low and high wind speeds was obtained by the models set up using conventional Newmark-β and CV Newmark-β programs. Result showed that the displacement time history curves obtained using different calculation models are consistent while the bridge section takes a small amplitude motion. The error from the displacement of conventional Newmark-β program and actual grid motion is small. In addition, the error of the displacement mismatch effect between this algorithm and the real motion of the grid can be ignored under low wind speed. While the bridge section takes a large amplitude flutter motion, the disparities of the displacement time history curves obtained using different calculation models are large. The error of the displacement mismatch effect between the conventional Newmark-β algorithm and the real motion of the grid must be considered under high wind speed. The displacement gained using the FSI calculation model set by the CV Newmark-β program is consistent with the real motion displacement of the grid motion updated in the Fluent program whether under low or high wind speed. Therefore, such a mismatch effect must be considered to establish a reasonable FSI calculation model while taking the numerical simulation of bridge flutter.

Experimental investigation of flutter characteristics of shallow [formula omitted] section at post-critical regime

The aeroelastic response of a bridge deck at post-critical regime of flutter is featured by limit cycle oscillation (LCO) due to nonlinearities in both fluid and structural-dynamics. In this study, wind tunnel tests are conducted on an elastically-mounted rigid section model with shallow Π cross section, which is free to vibrate in vertical and torsional directions. The aim is to explore the response characteristics of shallow Π cross section at different initial angles of attack (AOA) for wind speeds above the flutter threshold. Firstly, the structural nonlinearities of the elastically-mounted system for section model are carefully characterized in still air to evaluate their effects on aeroelastic responses of the model. Then, post-critical responses of the model subjected to flutter instability are measured at different initial AOA. Experimental phenomena in terms of the aerostatic displacement, critical velocity, steady state amplitude, time history of oscillations, frequency characteristics, phase-angle shift between heaving and torsional motions, and damping evolution are discussed in detail as well as the influence of structural damping on post-critical responses. The results show that the oscillations of Π cross section at post-critical regime of flutter grow slowly with wind speed, instead of building up to large amplitude abruptly. Both the critical flutter velocity and the steady state amplitude of flutter oscillations are sensitive to initial AOA and structural damping. One degree of freedom (DOF) motion approximation is suitable for small amplitude flutter oscillations. For large amplitude oscillation description of coupled 2-DOF motion would be necessary for the bluff Π cross section. The torsional and vertical heaving oscillations tend to be synchronous at high velocity with large amplitude. Compared to slight nonlinear structural damping in the dynamic system, the nonlinear aerodynamic damping related to higher-order terms of displacement is more significant, implying that the aerodynamic nonlinearity is the main source leading to LCO.

High intimal flap mobility assessed by intravascular ultrasound is associated with better short-term results after TEVAR in chronic aortic dissection

Thoracic endovascular aortic repair (TEVAR) in chronic aortic dissection remains controversial. We analysed whether a high intimal flap mobility (IFM) of the dissection membrane has an impact on aortic remodelling after TEVAR in chronic Type B aortic dissection. Patients undergoing TEVAR with intravascular ultrasound (IVUS) were analysed and IFM was calculated. High IFM was defined as maximum flap amplitude >3 mm. For determining aortic remodelling, the degree of true lumen (TL) expansion was analysed in the last available follow-up CT. Fifty-two patients (63.6 ± 15.4 years) with a mean follow-up of 26.6 ± 20.7 months were analysed. The mobile flap group (n = 29) showed higher absolute TL expansion at the distal stent-graft (5.9 ± 3.1 vs. 3.3 ± 5.4 mm; p = 0.036) and a higher increase in TL diameter (18 ± 10 vs. 9 ± 15%; p = 0.017) compared to the non-mobile group (n = 23). Basic TEVAR-related outcome characteristics were comparable, but the mobile intimal flap group showed a lower re-intervention rate (3 vs. 8pts.; p = 0.032) in chronic dissections. High IFM in chronic Type B aortic dissection is linked to improved aortic remodelling and is associated with a lower re-intervention rate over time. IVUS assessment of IFM in chronic Type B aortic dissection might be helpful in identifying patients with better remodelling after TEVAR.

The influence of oscillating trailing-edge flap on the dynamic stall control of a pitching wind turbine airfoil

Unsteady operating environment of a horizontal axis wind turbine can induce excessive loads on the blades, originating from rapid variations in angle of attack and consequently dynamic stall (DS) occurrence. Therefore, it is of utmost importance to control the flow around a blade by which fatigue damage is likely to happen. Using two-dimensional incompressible unsteady Reynolds-averaged Navier–Stokes equations in OpenFOAM package, a series of simulations are carried out to assess the viability of an oscillating deformable trailing-edge flap (DTEF) in load and DS control on a pitching wind turbine airfoil which experiences deep DS at Re = 420,000. Results reveal whether or not the airfoil is equipped with an oscillating DTEF, DS vortex forms at high angles of attack. The size, strength and traveling of the DS vortex, however, can be influenced by out-of-phase deflection of the DTEF. More effectively, the change in the airfoil camber line during flap oscillation can remarkably affect the pressure distribution around the airfoil, and hence, significant load alleviation and mean lift enhancement are achievable, all of which help the wind turbine performance and enhance the life span of the components. Moreover, a parametric study on flap size and amplitude of deflection together with a comparison between a discrete flap and a DTEF suggests an out-of-phase oscillation of a large gently curved DTEF, up to 30% of the total chord, with similar amplitude and frequency with respect to the airfoil is the best condition under which fatigue load control as well as enhancement in resultant load for a blade rotation can take place.

Small flags in rectangular channels: Dynamics and mean wake characteristics

This experimental study presents some aspects of the flapping dynamics and flow characteristics in the wake of small flags (M*=0.42) inserted in rectangular channels of various aspect (0.2 to 1.0) and confinement (1.0 to 5.0) ratios. The experiments were performed in the range of up to U*=48 or Redh=44,000. Results revealed that flag start-up characteristics were highly dependent on initial conditions. The hysteresis loop of the critical velocity ranged from 15% to 48% and was partially explained by the residual alternating pressure difference between the two sides of the flag. The flag flapping dynamics was largely affected by the confining walls. The flapping amplitude was constrained by the channel width and the oscillation modes shifted from simple shapes in low velocity, high confinement ratios to complex shapes in high velocity, low confinement ratios. The dimensionless oscillation frequency St initially decreased at low U* and Redh, and then converged to constant values in high U* and Redh. In addition, the presence of the flag substantially increased the flow unsteadiness and turbulence levels inside the channel, especially in the near wake. The measurements reveal a possible mechanism of vortex formation due to the flag’s oscillation. The shed vortices further interact with each other and the channel walls, then eventually decay. Confinement has a profound impact on wake behaviour. The size of the channel, which constrained the flag amplitude, largely dictated the structure of the vortex cores in the near wake and their eventual decay. The turbulence levels (I) inside the channel also depended on confinement: wider channels have higher I levels due to larger vortices produced which are convected farther before decaying. Narrow channels limit flag oscillation amplitude, leading to smaller vortex structures and their rapid decay.

Toward a Dielectric Elastomer Resonator Driven Flapping Wing Micro Air Vehicle.

In the last two decades, insect-inspired flapping wing micro air vehicles (MAVs) have attracted great attention for their potential for highly agile flight. Insects flap their wings at the resonant frequencies of their flapping mechanisms. Resonant actuation is highly advantageous as it amplifies the flapping amplitude and reduces the inertial power demand. Emerging soft actuators, such as dielectric elastomer actuators (DEAs) have large actuation strains and thanks to their inherent elasticity, DEAs have been shown a promising candidate for resonant actuation. In this work a double cone DEA configuration is presented, a mathematic model is developed to characterize its quasi-static and dynamic performance. We compare the high frequency performance of two most common dielectric elastomers: silicone elastomer and polyacrylate tape VHB. The mechanical power output of the DEA is experimentally analyzed as a DEA-mass oscillator. Then a flapping wing mechanism actuated by this elastic actuator is demonstrated, this design is able to provide a peak flapping amplitude of 63° at the frequency of 18 Hz.

Design of flexible hinges in electromagnetically driven artificial flapping-wing insects for improved lift force

This paper presents a design approach to determine the bending stiffness of the flexible hinge in electromagnetically driven artificial flapping-wing insects for improved lift force. The prototype insects are fabricated by laser cutting technique and have a wingspan of 3.6 cm and a total mass of 84 mg. To induce effective wing rotational movements for improved aerodynamic performances, the geometric parameters of the flexible hinge in the wing rotation mechanism are optimized based on lift force simulations and bending stiffness tests on the enlarged flexible hinges. With an applied AC current of 420 mA (AC voltage of 0.5 V), the prototype #4 with optimized flexible hinge can produce effective wing rotational movements with a maximum rotation angle of 50° and a flapping amplitude of ±50.6° at 65 Hz. Such wing movements can generate a measured average lift force of 384.5 µN to produce a total lift force to weight ratio of 0.47. With this magnitude of lift force, the prototype can slide on a tilted guide rail with an inclined angle of 7°. Such results validate the feasibility of the design approach of the flexible hinge in the artificial insects for improved lift force.

Effects of flapping wing kinematics on the aeroacoustics of hovering flight

Despite the recent interest in flapping wing aerodynamics, the aerodynamic sound generation mechanism of a flapping wing is inadequately understood. In this paper, the interplay between the wing motion, resulting unsteady aerodynamics, and aeroacoustics of a flapping wing flyer is investigated. The wing motion is varied in terms of the flapping amplitude, pitching amplitude, and the phase difference between flap and pitch. The unsteady flow around the flapping wing is numerically calculated using a well-validated Navier-Stokes equation solver. Acoustic pressures are computed using the Ffowcs-Williams-Hawkings equation in a three-dimensional space at varying distances from the wing. Two main sound generation mechanisms are found. The flapping motion induces the highest sound pressure level (SPL) in the stroke plane. Furthermore, the SPL peaks under the wing due to the wake induced by lift generation. Effective motions, generating the highest lift and lowest SPL, are found when the flap and pitch amplitudes are high and when the pitch rotation is delayed with respect to flap. These results suggest that the delayed rotations observed for small hovering insects may aim to minimize sound production rather than maximize lift generation.

Features of shock-induced panel flutter in three-dimensional inviscid flow

Oblique shockwaves may impinge on supersonic vehicles internally in an engine or externally on the outer mold line. They create a severe loading environment and may induce dynamic instabilities such as panel flutter. This study computationally explores the effect of shock-induced panel flutter response in 3D, inviscid, Mach 2 flow. Flutter behavior of a square panel is compared across several incident shock angles and inflow dynamic pressures. The presence of an impinging shockwave is found to produce panel flutter that is characteristically different than the shock-free condition. The response contains significantly larger local pressure gradients, larger spanwise variations, and higher-order modal activity in the panel. Results are also compared to two-dimensional inviscid flow over an infinite-span panel. The 3D centerline is found to compare closely with 2D simulations. However, away from the centerline, 3D effects have a significant influence on the solution. In general, stronger oblique shockwaves raise flutter amplitude and frequency, while weaker shockwaves stabilize the panel response. These latter findings indicate important considerations for both structural lifing and flow control applications.

Background-Oriented Schlieren of Fuel Jet Flapping Under Thermoacoustic Oscillations in a Sequential Combustor

This study deals with thermoacoustic instabilities in a generic sequential combustor. The thermoacoustic feedback involves two flames: the perfectly premixed swirled flame anchored in the first stage and the sequential flame established downstream of the mixing section, into which secondary fuel is injected in the vitiated stream from the first stage. It is shown that the large amplitude flapping of the secondary fuel jet in the mixing section plays a key role in the thermoacoustic feedback. This evidence is brought using high-speed background-oriented Schlieren (BOS). The fuel jet flapping is induced by the intense acoustic field at the fuel injection point. It has two consequences: first, it leads to the advection of equivalence ratio oscillations toward the sequential flame; second, it modulates the residence time of the ignitable mixture in the mixing section, which periodically triggers autoignition kernels developing upstream of the chamber. In addition, the BOS images are processed to quantify the flow velocity in the mixing section and these results are validated using particle image velocimetry (PIV). This study presents a new type of thermoacoustic feedback mechanism, which is peculiar to sequential combustion systems. In addition, it demonstrates how BOS can effectively complement other diagnostic techniques that are routinely used for the study of thermoacoustic instabilities.

Design of Wing Root Rotation Mechanism for Dragonfly-Inspired Micro Air Vehicle

This paper proposes a wing root control mechanism inspired by the drag-based system of a dragonfly. The previous mechanisms for generating wing rotations have high controllability of the angle of attack, but the structures are either too complex or too simple, and the control of the angle of attack is insufficient. In order to overcome these disadvantages, a wing root control mechanism was designed to improve the control of the angle of attack by controlling the mean angle of attack in a passive rotation mechanism implemented in a simple structure. Links between the proposed mechanism and a spatial four-bar link-based flapping mechanism were optimized for the design, and a prototype was produced by a 3D printer. The kinematics and aerodynamics were measured using the prototype, a high-speed camera, and an F/T sensor. In the measured kinematics, the flapping amplitude was found to be similar to the design value, and the mean angle of attack increased by approximately 30° at a wing root angle of 0°. In the aerodynamic analysis, the drag-based system implemented using the wing root control mechanism reduced the amplitude of the force in the horizontal direction to approximately 0.15 N and 0.1 N in the downstroke and upstroke, respectively, compared with the lift-based system. In addition, at an inclined stroke angle, the force in the horizontal direction increased greatly when the wing root angle was 0° at the inclined stroke angle, while the force in the vertical direction increased greatly at a wing root angle of 30°. This means that the flight mode can be controlled by controlling the wing root angle. As a result, it is shown that the wing root control mechanism can be applied to the MAV (micro air vehicle) to stabilize hovering better than the MAV using a lift-based system and can control the flight mode without changing the posture.

Vortical patterns generated by flapping foils of variable ratio chord-to-thickness

Depending upon the flapping amplitude and frequency, different types of vortex streets are shed downstream of a pitching foil. In the present experimental work, the foil aspect ratio chord-to-thickness is varied ( $$L/D=5;\,4;\,3$$ and 2) additionally to the frequency, while the amplitude is kept constant. Dye visualizations allow to find that the number of vortices shed by oscillation cycle slightly depends upon the foil aspect ratio. Moreover, whereas the von Karman street can reverse for the longest two foils, this change is not observed for the shortest two foils. Finally, when the frequency is high enough, the formation mechanism of the asymmetric reverse Karman street, which is significative of a positive thrust, is shown to be the same as the $$P+S$$ pattern that develops for an oscillating cylinder. Detailed visualizations show that a jet is produced for the longest two foils because the foil tip vorticity is maximum just when a counter-rotating vortex is ejected.