The Fluid Dynamics of Cavity Flow Induced Oscillations and the Boundary Layer Effect

Abstract

This thesis presents experiments, and numerical modelling of incompressible open cavity flow. A review identifies research gaps for effects of cavity width, external geometry, upstream (U/S) boundary layer (BL), and pressure gradient (dp/dx) on cavity response. These are quantified through experiment in a low-speed wind tunnel, measuring unsteady pressure on cavity surfaces, and velocity with hot-wire anemometry, to map the U/S BL. To modify the BL, a series of nose cones were fitted U/S of the cavity, and datum-d to a flat-plate, whilst cavity geometry was varied for L/D = 1-3, and L/W = 1-6.

From analysis, findings showed, shorter cavities sustained high pressure on all surfaces, and longer had distinct low and high regions, peaking at a primary recirculation point on the floor. For narrow (L/W > 1) cavities, narrower presented less coherent 3D flow than wider, which had more distinct 3D flow, presenting as spanwise nodes, flanking primary recirculation.

Varying U/S BL thickness, and dp/dx, altered internal cavity response significantly. The thinnest laminar BL, of largest dp/dx < 0, presented a slight shift in cavity frequency response, and notable increase in magnitude response. In contrast, the thickest turbulent BL, with dp/dx = 0, had a much lower response. Thus, reducing BL thickness, and increasing dp/dx < 0, amplified flow periodicity, and internal cavity mechanisms for a more volatile, high magnitude pressure response, to expel flow downstream through the shear layer mass-exchange cycle.

Based on results, derivation of a viscid incompressible open cavity flow model is presented, to empirically estimate frequency, with respect to the U/S BL, superimposing cavity vorticity and feedback modes. The model also estimates pressure amplitude, at a specified location and wave number, with respect to a source and frequency, to update a past model, including the BL, and superposition of vorticity and feedback. Experimental St and SPL are estimated within 5% error.