Recent progress in flapping wing aerodynamics and aeroelasticity

1. Introduction 2. Fixed, rigid wing aerodynamics 3. Flexible wing aerodynamics 4. Flapping wing aerodynamics.

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Aerodynamics of Low Reynolds Number Flyers

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.

Flapping Wing Aerodynamics: Progress and Challenges

Significant progress has been made in understanding some of the basic mechanisms of force production and flow manipulation in oscillating foils for underwater use. Biomimetic observations, however, show that there is a lot more to be learned, since many of the functions and details of fish fins remain unexplored. This review focuses primarily on experimental studies on some of the, at least partially understood, mechanisms, which include 1) the formation of streets of vortices around and behind two- and three-dimensional propulsive oscillating foils; 2) the formation of vortical structures around and behind two- and three-dimensional foils used for maneuvering, hovering, or fast-starting; 3) the formation of leading-edge vortices in flapping foils, under steady flapping or transient conditions; 4) the interaction of foils with oncoming, externally generated vorticity; multiple foils, or foils operating near a body or wall.

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Review of experimental work in biomimetic foils

Numerical simulations are used to investigate the effect of aspect ratio on the wake topology and hydrodynamic performance of thin ellipsoidal flapping foils. The study is motivated by the quest to understand the hydrodynamics of fish pectoral fins. The simulations employ an immersed boundary method that allows us to simulate flows with complex moving boundaries on fixed Cartesian grids. A detailed analysis of the vortex topology shows that the wake of low-aspect-ratio flapping foils is dominated by two sets of interconnected vortex loops that evolve into distinct vortex rings as they convect downstream. The flow downstream of these flapping foils is characterized by two oblique jets and the implications of this characteristic on the hydrodynamic performance are examined. Simulations are also used to examine the thrust and propulsive efficiency of these foils over a range of Strouhal and Reynolds numbers as well as pitch-bias angles.

Wake topology and hydrodynamic performance of low-aspect-ratio flapping foils

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

Experimental and Computational Investigation of the Knoller-Betz Effect

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.

C. M. Dohring

Papers

Experimental and Computational Investigation of the Knoller-Betz Effect

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