A supplement to this thesis containing tabulations of all acoustic data is available from the Department of Mechanical Engineering, University of Adelaide / Bibliography: leaves 109-113 / ix, 118 leaves, 9 plates : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Mechanical Engineering, 1984

A combined experimental, computational and theoretical study is presented on the dynamics and stability characteristics of turbulent flow past a circular cylinder placed near and parallel to a moving ground. The study consists of four main parts: (i) wind tunnel experiment, (ii) numerical simulation, (iii) linear stability analysis, and (iv) proper orthogonal decomposition (POD) analysis. The main focus of the study is on the cessation of large-scale, von Karman-type vortex shedding in 'ground effect', i.e., the cessation observed when the cylinder comes close to the ground. The experiments, performed at upper-subcritical Reynolds numbers of 0.4 and 1.0 x 105, show that the cessation of von Karman-type vortex shedding and an attendant critical drag reduction of the cylinder (equipped with end-plates) occurs at the gap-to-diameter ratio h/d of around 0.35, at which point the flow through the gap between the cylinder and the ground is till not blocked at all due to the ground moving at the same speed as the free stream. It is subsequently shown that detached-eddy simulations (DES) can correctly reproduce these critical phenomena, whereas unsteady RANS simulations predict them at much smaller h/d of between 0.1 and 0.2, despite the fact that the unsteady RANS simulations are 'overly dissipative' compared with the DES. The linear stability analysis of analytical wake profiles then provides a possible explanation for the above experimental and computational results; that is, the cessation of the von Karman-type vortex shedding in ground effect may also be largely explained by the change of inviscid instability characteristics in the near wake region from 'absolutely unstable' to 'convectively unstable', in analogy with the case for a cylinder equipped with a backward splitter plate in a free stream. Finally, the near wake structure of the cylinder in ground effect is further investigated with the POD analysis. The results show that about 60% of the total kinetic energy in the near wake region (in the time-averaged sense) is contained only in the first three POD modes even when the energetically dominant, von Karman-type vortex shedding becomes intermittent at h/d = 0.4. It is also shown that both shedding and non-shedding states at this gap ratio can roughly be reproduced from the combination of these three POD modes.

An investigation of the flow physics of a diffuser equipped bluff body in ground effect has been undertaken. Situated at the rear of a racing car undertray, the diffuser is an important component and the least understood part of the vehicle. Diffuser performance can change dramatically with vehicle ride height. This includes a significant loss in performance at low ride heights which can also be a serious vehicle safety issue. An increased understanding of the diffuser behaviour in ground effect is required to assist design improvements. An accurate experimental database of the flow field is necessary both to aid this understanding and also to provide information against which the continuing development of computational simulations may be assessed. The present research is two-fold; experimental and computational. Model tests were conducted on a generic 3D bluff body equipped with a fixed angle diffuser representative of current racing car diffusers. Extensive experimental tests in wind tunnels equipped with moving belts included mean forces, surface pressures, oilflow visualisation, laser doppler anemometry and particle image velocimetry. The 3D diffuser flow field has been measured for the first time and the results are used to analyse the behaviour of the diffuser in ground effect. Complementary RANS simulations provide valuable insight into the modelling requirements. It is known that the diffuser generates down-force by accelerating air underneath the model through the channel formed by the model underside and the ground. The diffuser flow is characterised by a counter rotating vortex pair. The present research presents a new understanding of the diffuser flow field and the mechanisms causing its behaviour in ground effect. It has been found that the behaviour of the vortices alters according to the model ride height and the pressure gradient inside the diffuser. Additional down-force is generated due to the low pressure zones associated with these vortices. At relatively large ground clearances, the vortices are coherent and strong with a high axial speed core. At these heights the down-force experienced by the model increases with reducing model ride height. This behaviour is terminated at lower ground clearances by the advent of a plateau in the down-force curve and the occurrence of breakdown in the vortices inside the diffuser. The vortex breakdown results in large, diffusive and weak vortices. Maximum down-force on the model occurs at the lowest ride height of this type of flow at the end of the plateau. A sharp reduction in the down-force occurs thereafter, due to the complete breakdown of one of the vortices. The resulting asymmetric flow consists of a single coherent vortex to one side of the flow and significant flow reversal at the other side. At very low ride heights the vortices are asymmetric and weak Down-force reduction is believed to occur as a result of the steep pressure gradient inside the diffuser which advances the vortex breakdown inside the diffuser upstream as the model ride height is reduced. At the point of down-force reduction one of the vortices breaks down completely. At very low ride heights the boundary layers at the model underside and at the moving ground are believed to merge to restrict flow through the diffuser inlet. The experimental database is comprehensive and provides the necessary tool for validation of computational modelling. A computational simulation of the flow at a high ride height successfully predicts force and surface pressure coefficients and the main flow features.

The noise produced by low Mach number (M ≤ 0.4) laminar and turbulent flows is studied using computational and experimental techniques. The emphasis is on the development and application of numerical methods to further the understanding of noise generation and far field radiation. Numerical simulations are performed to investigate the tonal noise radiated by two- and three-dimensional cavities submerged in low-speed turbulent and laminar flows. A numerical approach is developed that combines near field flow computations with an integral radiation model to enable the far field signal to be evaluated without the need to directly resolve the propagation of the acoustic waves to the far field. Two basic geometries are employed in these investigations: a plane rectangular cavity and a rectangular cavity with a lip. Results for the two geometries show good agreement with available experimental data, and highlight the sensitivity of the amplitude and directivity of the radiated sound to geometry, flow speed and the properties of the incoming boundary layer. The cavity with a lip is shown to behave as a Helmholtz resonator. The plane cavities are characterized by the more familiar Rossiter modes. Both geometries are characterized by intense near field oscillations and strong noise radiation. To quantify the effects of three-dimensional phenomena on the generation and radiation of sound, a fully three-dimensional simulation is performed. The Navier-Stokes equations are solved directly using an optimized prefactored compact scheme for spatial discretization. Results are compared with those from a two-dimensional simulation and the effects of the three-dimensional phenomena are discussed. Finally, wind tunnel tests are performed to quantify the effects of geometry and flow speed on the velocity and pressure fields within a plane rectangular cavity. Velocity measurements are made using the Laser Doppler Anemometry and Particle Image Velocimetry techniques. Instantaneous and statistical data are employed to probe the flows. Although coherent vortical structures are found to characterize the shear layer, their intermittent nature prevents self-sustaining oscillations developing and consequently the pressure field is broadband in nature.