Wake structure visualization of a flapping-wing Micro-Air-Vehicle in forward flight

The present study provides an experimental visualization of the three-dimensional wake structure of a small flapping-wing Micro Air Vehicle (MAV) using a stereoscopic Particle Image Velocimetry (stereo-PIV) technique, in combination with simultaneous force measurements. Variations in several kinematic parameters and flow conditions (flapping frequency, flow speed and incidence angle) are examined to identify their effects on the wake topology. The three-dimensional wake structures were reconstructed using the temporal information of one transverse measurement plane by a ‘frozen flow’ approach, which was qualitatively validated by a spatial reconstruction method using information from different spatial planes. For the first time, the complete wake topology and evolution of this specific four-wing configuration MAV in forward flight condition is characterized experimentally. The general wake topology contains two predominant vortex structures: a U-shaped vortex ring shed from the trailing edges after instroke and an arc-shaped vortex shed from the wing tips during outstroke. These two vortex structures are shed periodically from the wing and appear quite isolated from each other. As expected, a higher reduced frequency will result in an increased unsteadiness of the wake behavior, reflected by a shorter wavelength and vortical connection. Application of an incident angle of the incoming free-stream introduces geometrical changes of the wake structure.