Vortex shedding flow patterns and their transitions in oscillatory flows past parallel-plate thermoacoustic stacks

This paper investigates vortex shedding phenomena occurring at the end of the parallel-plate thermoacoustic stacks, in oscillatory flow conditions induced by an acoustic standing wave. Phase-locked Particle Image Velocimetry (PIV) measurement technique is used to identify a range of vortex shedding flow patterns within an acoustic cycle, in particular during the "ejection stage", when the fluid flows out of the stack. As an extension of the earlier work [X. Mao, Z. Yu, A.J. Jaworski, D. Marx, PIV studies of coherent structures generated at the end of a stack of parallel plates in a standing wave acoustic field, Experiments in Fluids 45 (5) (2008) 833-846] this paper reports experimental results obtained for a much wider range of parameters in order to identify detailed relationships between vortex flow patterns and experimental conditions. In addition to the flow patterns observed previously, current results show several new flow phenomena related to the evolution of vortices shed in the ejection stage. This work also attempts to classify systematically the types of the vortex wake patterns using three dimensionless parameters: the Reynolds number (Re), Keulegan-Carpenter number (KC) and Womersley number (Wo). In the (Re, KC) plane (Re is up to 5000, KC is up to 250), four flow pattern regions are defined according to the characteristics of nine types of vortex wake "transitions". It is proposed that Wo plays an important role in determining the detailed classification of such transitions within each flow pattern region. Furthermore, the physical mechanisms behind different types of wake patterns and their relationship with the dimensionless parameters investigated have been discussed.