Biofluidmechanics of Avian Flight: Recent Numerical and Experimental Investigations

Abstract

A three dimensional mechanical and numerical avian model with identical geometry was developed to investigate the aerodynamic performance of flapping flight for varying flow velocities and wing beat frequencies. The corresponding reduced frequencies range from k=0.22 to k=1.0 providing turbulent and unsteady flow. The model consists of a rigid body and elastic wings. Its shape was inspired by birds, but restricted by manufacturing and numerical specifications. Using a sinusoidal flapping about an off-centre axis parallel to the body axis and a phase-shifted pitching about the moving lateral wing axis the wing beat motion was realized. Wind tunnel tests with Particle Image Velocimetry (PIV) were performed to capture the velocity field around and behind the mechanical model for different reduced frequencies. Furthermore, simulations for the corresponding numerical model have been conducted by means of fluid-structure-interaction (FSI) simulation techniques providing a fully resolved flow field. The results were used to analyze the flow configurations and to validate the numerical and experimental setup for further investigations. The results of the numerical simulations and wind tunnel experiments are in good agreement and facilitate a reconstruction of the three dimensional vortex structures in the wake. The results show, that for all reduced frequencies, the wakes consist of a chain of interlocked vortex rings behind each wing. For high reduced frequencies, a shedding of small-scale vortices composing vortex sheets generates oppositely rotating upstroke (UVS) and downstroke (DVS) vortex structures which contain starting, stopping, tip and root vortices. For decreasing reduced frequencies, the upstroke becomes more aerodynamically active leading to a diffusion of the upstroke vortex structures.