Abstract
This paper investigates vortex shedding processes occurring at the end of a stack of parallel plates, due to an oscillating flow induced by an acoustic standing wave. Here the hot-wire anemometry measurement technique is applied to detect the velocity fluctuations due to vortex shedding near the end of the stack. The hot-wire fast time response enables detailed frequency spectra of the velocity signal to be obtained, which can be used for identifying the dominant frequencies associated with vortex shedding, and thus allow calculation of the corresponding Strouhal numbers. By varying the stack configuration (the plate thickness and spacing) and the acoustic excitation level (the so-called drive ratio), the impact of the stack blockage ratio and the Reynolds number on the Strouhal number has been studied in detail. Furthermore, in the range of Reynolds numbers between 200 and 5000 a correlation between the Strouhal number and Reynolds number has been obtained and compared with analogous relationships in the steady flow. Particle Image Velocimetry (PIV) is also used to visualize the vortex shedding processes within an acoustic cycle, phase-by-phase, in particular during the part of the cycle when the fluid flows out of the stack—selected cases are shown for comparisons with hot-wire measurements.
Highlights
Vortex shedding from bluff bodies, and the associated vortex shedding frequency, have been a subject of many investigations since the pioneering work carried out by Vincenc Strouhal and published in 1878 [1]
The current study addresses the problem of vortex shedding from the stack of parallel plates in the oscillatory flow condition imposed by an acoustic standing wave
It should be noted that the vortex shedding processes in the oscillatory flow are much more complicated than those in steady flows and as a result there is a much wider selection of characteristic vortex patterns and their evolution paths
Summary
Vortex shedding from bluff bodies, and the associated vortex shedding frequency, have been a subject of many investigations since the pioneering work carried out by Vincenc Strouhal and published in 1878 [1]. Kovasznay [3] carried out some of the earliest quantitative investigations of the regular vortex street pattern behind a circular cylinder using hot-wire technique His studies of Strouhal number covered Reynolds numbers up to 104. The latter work tried to derive vortex shedding Strouhal number: the frequency was estimated by “counting” the vortex structures on PIV images taken at two different phases of the acoustic cycle, while the reference velocity was taken as the convection velocity of the vortices. It should be noted that vortex shedding processes could be investigated using much simpler hot-wire anemometry techniques, which could provide high temporal resolution in order to investigate the spectra of velocity fluctuations behind the stack and subsequently obtain the corresponding Strouhal number values.
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