Aerodynamic flow over cuboids and the noise generated

Abstract

This thesis is motivated by the need to reduce train aerodynamic noise, especially the noise from pantographs and bogies which are comprised of bluff bodies including cuboids. The underlying physics of the flow over cuboids with different cross-sections and the emitted noise are studied. The effects of wall proximity and corner radii on the flow field and noise of a cube as well as the effect of the aspect ratio of cuboids are the main focus. The Delayed Detached Eddy Simulation method is adopted to investigate the flow features and obtain the noise source information. The far-field noise is then predicted using the Ffowcs Williams-Hawkings acoustic analogy. Benchmark comparisons on the flow over a wall-mounted cube and a sphere are conducted to validate the feasibility of the adopted methodology for describing the flow field; a separate simulation on a square cylinder is also implemented to gain confidence in predicting surface pressure fluctuations and the far-field noise. Good agreement has been achieved between results from the benchmark cases and those from the literature.

It is found that the wall proximity mainly affects the behaviour of the separated flow from the lower leading edge of the cube and also the flow separation along the ground. Large pressure fluctuations are observed close to the trailing edge and locations of the flow reattachment on the bottom of the cube. With the increase of the elevation height, the sound from the cube increases rapidly in the vertical direction and the maximum sound is observed when the cube is elevated by one quarter of the edge length above the ground. For rounded cubes, the flow separation is delayed with the increase of the corner radius; the shear layer moves towards the lateral surfaces and the recirculation length in the wake reduces. Flow features of the rounded cube vary with the Reynolds number, although no systematic dependence is observed. Large pressure fluctuations are found in the recirculating region on the lateral surface and near the trailing edge due to reduced recirculation length in the wake. Rounding the cube with a suitable radius can be an effective way to reduce the emitted noise. The minimum noise is found for a radius one third of the edge length. Cuboids with different aspect ratios are studied, where the aspect ratio is defined as the ratio of the width in the spanwise direction to the length in the streamwise direction. The flow separated from the upper leading edge only reattaches to the top surface for aspect ratios smaller than 1 whereas the vortex shedding in the wake tends to be more evident as the aspect ratio is increased. The Reynolds number has negligible effect on the mean flow behaviours of the cuboid.

Large pressure fluctuations happen with the flow reattachment and the formation of the secondary vortex near the trailing edge. The emitted noise from the cuboid is always higher in the cross-flow direction than that in the streamwise and spanwise directions. The maximum sound is found for the aspect ratio of 6.

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Identifiers

ORCID for David Thompson: orcid.org/0000-0002-7964-5906

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