Flow-field and Acoustic Interaction behind a Wall Mounted Cylinder

Abstract

The current manuscript is a subset of a larger database where fluid-acoustic and structural vibrations were obtained behind three bluff bodies (cylinder, ramp and vortex generator). The main goal of this effort was to study the effect of shape, orientation, length scales of the protrusion, Mach number and boundary layer thickness on the flow field behind the bluff body. In the current paper we discuss the correlation between fluctuating wall pressure and flow field dynamics at a low Mach number to determine the fluid-acoustic-structural interaction behind a wall mounted cylinder. The effect of the flow field on the wall pressure fluctuations were studied for various cylinder diameters and heights at different Mach numbers and free stream boundary layer thicknesses. Three different zones were identified in the flow field. These include: reversible flow zone behind the cylinder, slow/stagnation region at the reattachment point and a leading edge vortex at the separated shear layer. The fluctuating pressure level was as high as 145dB at the reattachment point, which was over 22dB higher than the baseline without the cylinder. Increasing the Mach number increased the amplitude of the fluctuations over the entire measurement region. Increasing the cylinder height from (127mm to 254mm) was found to extend the high pressure fluctuation zone in the streamwise direction. The separation length increased with increasing diameter or height of the cylinder. The separation length was unaffected by the Mach number (0.15 to 0.2) or turbulent boundary layer thickness (38mm,107mm) in the current study. I. Introduction Continued improvement of the aircraft structural performance is a focus of the airframe design engineer. The never ending push to reduce the aircraft structural weight to help improve the overall performance of the airplane is the goal of this study. Towards this goal, new and advanced measurement techniques can provide insight into flow regimes once deemed complicated. Over the past forty years, many researchers have reported measurements of cross and auto spectra of wall pressure fluctuations beneath turbulent boundary layers [1-7]. These have been aimed at solving practical engineering problems listed below [7],