Max F. Platzer

Flapping Wing Aerodynamics: Progress and Challenges

It is the objective of this paper to review recent developments in the understanding and prediction of flapping-wing aerodynamics. To this end, several flapping-wing configurations are considered. First, the problem of single flapping wings is treated with special emphasis on the dependence of thrust, lift, and propulsive efficiency on flapping mode, amplitude, frequency, and wing shape. Second, the problem of hovering flight is studied for single flapping wings. Third, the aerodynamic phenomena and benefits produced by the flapping-wing interactions on tandem wings or biplane configurations are discussed. Such interactions occur on dragonflies or on a recently developed micro air vehicle. The currently available two- and three-dimensional inviscid and viscous flapping-wing flow solutions are presented. It is shown that the results are strongly dependent on flapping frequency, amplitude, and Reynolds number. These findings are substantiated by comparison with the available experimental data.

Experimental and Computational Investigation of the Knoller -Betz Effect

The ability of a sinusoidally plunging airfoil to produce thrust, known as the Knoller-Betz or Katzmayr effect, is investigated experimentally and numerically. Water-tunnel experiments are performed providing flow visualization and laser Doppler velocimetry data of the unsteady wakes formed by the plunging foils. Vortical structures and time-averaged velocity profiles in the wake are compared with numerical computations from a previously developed inviscid, unsteady panel code that utilizes a nonlinear wake model

Jet characteristics of a plunging airfoil

Water-tunnel tests of a NACA 0012 airfoil that was oscillated sinusoidally in plunge are described. The flowered downstream of the airfoil was explored by dye flow visualization and single-component laser Doppler velocimetry (LDV) measurements for a range of freestream speeds, frequencies, and amplitudes of oscillation. The dye visualizations show that the vortex patterns generated by the plunging airfoil change from drag-producing wake flows to thrust-producing jet flows as soon as the ratio of maximum plunge velocity to freestream speed, i.e., the nondimensional plunge velocity, exceeds approximately 0.4. The LDV measurements show that the nondimensional plunge velocity is the appropriate parameter to collapse the maximum streamwise velocity data covering a nondimensional plunge velocity range from 0.18 to 9.3

Computational prediction of airfoil dynamic stall

Abstract The term dynamic stall refers to unsteady flow separation occurring on aerodynamic bodies, such as airfoils and wings, which execute an unsteady motion. The prediction of dynamic stall is important for flight vehicle, turbomachinery, and wind turbine applications. Due to the complicated flow physics of the dynamic stall phenomenon the industry has been forced to use empirical methods for its prediction. However, recent progress in computational methods and the tremendous increase in computing power has made possible the use of the full fluid dynamic governing equations for dynamic stall investigation and prediction in the design process. It is the objective of this review to present the major approaches and results obtained in recent years and to point out existing deficiencies and possibilities for improvements. To this end, potential flow, boundary layer, viscous–inviscid interaction, and Navier–Stokes methods are described. The most commonly used numerical schemes for their solution are briefly described. Turbulence models used for the computation of high Reynolds number turbulent flows, which are of primary interest to industry, are presented. The impact of transition from laminar to turbulent flow on the dynamic stall phenomenon is discussed and currently available methods for its prediction are summarized. The main computational results obtained for airfoil and wing dynamic stall and comparisons with available experimental measurements are presented. The review concludes with a discussion of existing deficiencies and possibilities for future improvements.

Thrust generation due to airfoil flapping

Thrust generation on a single flapping airfoil and a flapping/stationary airfoil combination in tandem is studied parametrically. A multiblock Navier-Stokes solver is employed to compute unsteady flowfields. The unsteady flowfield around a single flapping airfoil is also computed by an unsteady potential flow code. The numerical solutions predict thrust generation in flapping airfoils and a significant augmentation of thrust in flapping/stationary airfoil combinations in tandem. The propulsive efficiency is found to be a strong function of reduced frequency and the amplitude of the flapping motion. At a flapping amplitude of 0.40 chord lengths and a reduced frequency of 0.10, the propulsive efficiency of a single NACA 0012 airfoil was computed to be more than 70 %. For the airfoil combination in tandem, the propulsive efficiency was augmented more than 40% at a reduced frequency of 0.75 and a flapping amplitude of 0.20 chord lengths when the airfoils are separated by about two chord lengths.

Computational Study of Flapping Airfoil Aerodynamics

Unsteady, viscous, low-speed e ows over a NACA 0012 airfoil oscillated in plungeand/orpitch at various reduced frequency,amplitude, andphaseshift arecomputed. Vortical wakeformations, boundary-layere owsat theleading edge, the formation of leading-edge vortices and their downstream convection are presented in terms of unsteady particletraces.Flowseparationcharacteristicsandthrust-producingwakeproe lesareidentie ed.Computedresults compare well with water tunnel e ow visualization and force data and other computational data. The maximum propulsive efe ciency is obtained for cases where the e ow remains mostly attached over the airfoil oscillated in a combined pitch and plunge.

Wake Structures Behind Plunging Airfoils: A Comparison of Numerical and Experimental Results

Comparisons are made between numerically and experimentally produced wake structures behind airfoils undergoing rapid, oscillatory plunging motions. Numerical simulations are performed using an unsteady panel code. Inviscid, incompressible ows about arbitrary moving airfoils are computed with the unsteady wake approximated by discrete point vortices, tracked using a Lagrangian mesh scheme. Numerically computed results are visualized using an interactive, graphical-animation interface. Experimental data are obtained from a low-speed water tunnel. Two-color dye injection is used to visualize unsteady wake structures, and velocity data are acquired using laser doppler velocimetry. Comparisons of vortex location agree well with linear theory for low amplitude motions. For large amplitude, high frequency motions, results diverge from linear theory, but wakes from the two approaches compare well with each other, including highly non-linear, non-symmetric wakes obtained for high amplitude, high frequency motions. Computed velocity pro les and integrated thrust coe cients for both approaches agree well.

Bio-inspired design of flapping-wing micro air vehicles

In this paper the development and flight testing of flapping-wing propelled, radio-controlled micro air vehicles are described. The unconventional vehicles consist of a low aspect ratio fixed-wing with a trailing pair of higher aspect ratio flapping wings which flap in counterphase. The symmetric flapping-wing pair provides a mechanically and aerodynamically balanced platform, increases efficiency by emulating flight in ground effect, and suppresses stall over the main wing by entraining flow. The models weigh as little as 11g, with a 23cm span and 18cm length and will fly for about 20 minutes on a rechargeable battery. Stable flight at speeds between 2 and 5ms –1 has been demonstrated, and the models are essentially stall-proof while under power. The static-thrust figure of merit for the device is 60% higher than propellers with a similar scale and disk loading.

An investigation of the fluid-structure interaction in an oscillating-wing micro-hydropower generator

in Fluid Structure Interaction II, Eds. Chakrabarti, S.K., Brebbia, C.A., Almorza, D. and Gonzalez-Palma, R., WIT Press, Southampton, UK, 2003, pp. 73-82.

Numerical Simulation of Fully Passive Flapping Foil Power Generation

A fully passive flapping foil turbine was simulated using a two-dimensional Navier–Stokes solver with two-way fluid-structure interaction at a Reynolds number based on freestream flow Re=1100 and 1.1×106 with a NACA 0012 foil. Both pitch angle and angle-of-attack control methodologies were investigated. Efficiencies of up to 30% based on the Betz criterion were found using pitch control, which is commensurate with values reported in the literature for prescribed motion studies. Nonsinusoidal foil pitching motions were found to be superior to sinusoidal motions. Efficiencies exceeding 41% were found using angle-of-attack control, and nonsinusoidal angle-of-attack profiles were found to be superior. The key to improving the efficiency of energy extraction from the flow is to control the timing of the formation and location of the leading-edge vortex at crucial times during the flapping cycle and the interaction of the vortex with the trailing edge. Simulations using Reynolds-averaged Navier–Stokes turbulenc...