Experimental investigation on the leading-edge vortex formation and detachment mechanism of a pitching and plunging plate

The flow topology and leading-edge vortex (LEV) formation and detachment mechanism of a pitching and plunging flat plate are experimentally investigated in this study. Focus is placed on three novel aspects. First, to examine the differences between start-up and cyclic motions, the flow fields of one-shot experiments are compared to cyclic cases. The results show that the start cycle has very limited effect on the cyclic LEV development and flow topology evolution. Next, the influence of the maximum effective angle of attack on the LEV development in cyclic motion is introduced. Different secondary structures that lead to the detachment of LEV are found with variation of maximum effective angle of attack. Depending on the leading-edge shear-layer angle, three different flow topologies develop on the plate: quasi-steady development, boundary-layer eruption and secondary vortex formation. Which of these three topological scenarios occurs depends entirely on the maximum effective angle of attack. A vortex Reynolds number based on the transition time of the leading-edge shear-layer angle is defined to quantitatively assess which of the flow topologies will appear. Finally, a simplified model to describe the observed LEV growth is proposed, based on the assumptions that the velocity is constant at the outer vortex boundary and that the vortex boundary is a circular arc starting from the leading edge. The LEV circulation growth is found to increase linearly with the combination of the effective inflow velocity and the effective angle of attack.