Measurements of the three-dimensional topological evolution of a dynamic stall event using wavelet methods

This paper concerns the development of highly complex three-dimensional flow structures within the vortex system produced when a flat plate, pitching rapidly to high amplitude, undergoes dynamic stall at its leading edge. A single stall event is studied, produced by a single non-periodic, simplified, pitch-hold-return motion of the plate. This differs from many previous studies on pitching airfoils which measure exclusively periodic, oscillatory motions. In this way, a single event can be measured and characterised, without regard for any preceding cycles or hysteresis in the dynamical system. Two-component, two-dimensional particle image velocimetry measurements of the flow over the upper surface of the pitching plate are taken in a water tunnel. The plate is pitched at a rate relevant to rapidly perching or manoeuvring biological flyers, and at Reynolds numbers ranging from Re = 5000 to Re = 10000. Due to the non-periodic, transient, nature of the flow it becomes necessary to carefully select an appropriate analysis tool and, in this instance, a discrete wavelet approach is chosen. A wavelet based multi-resolution analysis of the coherent structures in the dynamic stall vortex system is undertaken in two dimensions, separating flow features into multiple scales. Discussion of the topology and energy content of each scale is presented, with consideration of temporal evolution and dependence on chord-based Reynolds number.