Vorticity dynamics and transient force generation during the leading-edge vortex formation on a revolving wing at low Reynolds number*

Given previous efforts on explaining the stability of leading-edge vortex (LEV) when it reaches the steady state, its formation process and its contribution to the transient force generation remain largely underexplored. Here, we examine the vorticity dynamics and transient force generation during the LEV formation on a revolving wing with AR=3 and Re=1500, operating in a mineral-oil tank. The wing starts with a constant acceleration and then rotates at a constant velocity. The accelerating distance is based on the chord (c) length of travel and is varied from 0.25c to 2c, and the AoA is varied from 15  to 60 degs. The 'Shake-The-Box' Lagrangian Particle Tracking Velocimetry system together with a volumetric patching process are employed to reconstruct the entire flow generated by the wing. Results show that the LEV reaches the steady state after approximately 4c of travel regardless of the accelerating distance or the AoA. However, the circulatory lift peaks around the end of acceleration, resulted from a combined effect of the LEV growth and the expansion of the region outlined by the LEV, starting vortex and tip vortex. Our findings indicate an indispensable role of transient LEV dynamics in understanding insect flight.