Measurements of the flow due to a rapidly pitching plate using time resolved high resolution PIV

The flow topology arising from unsteady airfoil motion is complex, but the benefits of understanding such flows extend to a wide variety of applications. Natural flows such as those induced by birds or insects during flapping flight, or by the rapidly moving fins of swimming fish, as well as flows over the manoeuvring wings of micro air vehicles (MAVs) or the rotors of helicopters or wind turbines exhibit behaviours typical of rapidly manoeuvring airfoils. This paper presents measurements and discussion on a set of recent experiments performed on a rapidly pitching plate in low Reynolds number flow. Time resolved particle image velocimetry (PIV) data with a high spatio-temporal resolution are presented, and the main features of the flow are identified. Measurements are taken at six Reynolds numbers ranging between 1500 and 10 000 and an analysis is given of the temporal evolution of the massively separated flow at the leading edge as well as at the trailing edge and in the wake. Individual coherent structures are located and tracked, and the variation of their strength and fluctuation are quantified as a function of time. Several key observations are made regarding the effect of Reynolds number on the topology of the wake structure of a pitching flat plate.