Flow around a Suddenly Accelerated Rotating Plate at Low Reynolds Number

The study explores the evolution of flow field and forces of a low-aspect-ratio flat plate undergoing an accelerated rotating surge motion from rest. The measurements were performed in a water tank at Reynolds numbers of 20,000, based on the chord length and terminal velocity at 75% span. A tomographic Particle Image Velocimetry (Tomographic-PIV) technique was used in order to capture three-dimensional velocity fields at different phases of the rotational motion, in combination with direct force measurements with a six-component water submergible force sensor. Experiments were performed for angles of attack of 30°, 45° and 60°. The results show the temporal development of the generation of lift and drag in conjunction with the development of vortical structures around the wing. The force measurements reveal the temporal variation of the forces during the motion: initial added mass peak at the end of the acceleration phase; subsequent decrease and increase to the maximum with circulatory effects; and decrease to steady state values. Although the general trend is similar for the different angles of attack, the magnitudes and phasing of the circulatory peak differs. Three dimensional flow fields show the evolution of vortical structures, starting from the formation of coherent and well-defined vortices (i.e. leading edge vortex, trailing edge vortex and tip vortex) to a stalled wing flow field with several small-scale structures. The leading edge vortex moves downstream on the top of the wing surface while it bursts into small scale structures. Surprisingly, this bursting and loss of vortex coherence is not reflected in a loss of lift. The spanwise flow structure also changes in accordance with the behavior of vortex formations such that initially it is mostly confined in the cores of leading and trailing edge vortices, however, as the motion progresses, it occurs around the trailing edge.