Self-Excited Oscillations of Flow Past a Perforated Plate: Attenuation Via Three-Dimensional Surface Elements

Abstract

It is known that flow past a perforated plate can give rise to purely hydrodynamic, self-sustained oscillations, which have a wavelength of the order of the plate length. The present investigation demonstrates that these oscillations can be effectively attenuated by three-dimensional surface elements in the form of vortex generators. A technique of high-image-density particle image velocimetry is employed to characterize the patterns immediately adjacent to the surface of the perforated plate. These patterns are interpreted in conjunction with pressure spectra at the trailing-end of the plate, as well as velocity spectra determined from cinema sequences of images. In the absence of the three-dimensional surface elements, a well-defined front of the unstable oscillation propagates along the surface of the plate. In the presence of an appropriate surface element, the front is no longer detectable, and instantaneous and averaged defects of the streamwise velocity and surface-normal vorticity exist along the span of the plate. The magnitudes and spanwise wavelengths of these defects are directly related to the extent of attenuation of the oscillation. Since the magnitudes of these defects decay significantly with distance along the plate, the most effective attenuation typically occurs for plate lengths less than or equal to a critical length. An appropriate measure of the degree of attenuation is a reduction in the magnitude of the coherent component of the fluctuating pressure. For all cases, even in the presence of effective attenuation, a low magnitude spectral peak at the frequency of the inherent oscillation is still discernible; it may be either sharp or broadband, and is indicative of the robust nature of the self-sustained oscillation.