Interaction between a vortex wake and an immersed rectangular plate

Air jet experiments and water flow visualization experiments to investigate the relationships between the characteristics of flow over a tandem array of two thick plates, force fluctuations on the downstream plate, and sound generated by the array are described. The leading edge of the upstream, vortex-generating, plate was elliptical and the trailing edge square with sharp corners; the downstream plate was of rectangular cross section. Flow visualization shows that as the gap between the plates is increased, the flow within the gap undergoes transition from one with stationary vortices at small gaps (the “trapped vortex” regime) to one with vortex shedding from the trailing edge of the upstream plate at large gaps (the “vortex street” regime). With progressive increase in gap, the flow changes progressively, over a range of gap widths, through a transitional regime in which weak convected vortices are formed at the downstream end of the gap; then, at a critical value of gap-to-thickness ratio G, vortex formation shifts abruptly upstream to a position close to the trailing edge of the upstream plate, and the vortex street regime is suddenly established. In the vortex street regime, the formation length and spacing of the vortices vary quasi-periodically with G, in accord with the number of vortices occurring within the gap. Radiated acoustic power at vortex frequencies is found to be proportional to the 5.7th power of flow velocity, indicating that the sound source is essentially dipole in character and supporting a previous conclusion that force fluctuations generated on the downstream plate by its interaction with vortices formed in the gap between the two plates constitute the main sound source. The radiated power reaches a maximum at a value of G slightly less than the critical transition value and varies quasi-periodically with G in the vortex street regime according to the number of vortices in the gap.