Flow noise radiated from a cylinder above a plate

Abstract

The flow noise of an infinite span circular cylinder has been well documented over the past decades. In reality, cylinder flows are rarely isolated from any nearby boundaries. The addition of a large flat plate to one side of the cylinder created an asymmetric configuration, termed as the cylinder above a plate (CAP). Examples include roof racks and rail pantographs. Flow diagnostics of a CAP have been lacking when compared to the baseline unbounded cylinders, while the corresponding noise measurements have been practically absent. Recently, the rigid CAP configuration was systematically tested in subsonic, air flow. Cylinders with various cross-sectionalshapes were tested. Microphone measurements were used to characterize the noise generated by a CAP, while particle image velocimetry (PIV) was used to characterize the cylinder wake. When the gap between the cylinder and plate was reduced, the amplitude of the tonal noise radiated from the cylinder flow generally reduced, while acoustic directivity pattern was heavily modified. The correlated flow structures were weakened and pushed away from the plate, which strongly suggested a modulation of the fluctuating forces exerted on the cylinder. By Curle’s acoustic analogy, this was related to the modified acoustic noise and directivity.