Using singular systems analysis to characterise the flow in the wake of a model passenger vehicle

As the time-dependent fluid dynamics of wakes becomes important in industri al applications such as vehicle design, so techniques need to be found that enable these dynamics to be characterised. Whilst laser Doppler anemometry and particle image velocimetry are becoming widespread in their applicatio n, they are not necessarily suitable for this application due to their low rate of data capture when air is the working fluid. In this paper a methodo logy that has already been applied successfully to low Reynolds number flow s is applied to a turbulent wake. This involves the use of hot-wire anemome try to capture a large number of time series of velocity throughout the wak e of a model road passenger vehicle. These time series are then analysed by a mathematical analysis tool known as singular systems analysis, which ena bles the low-frequency components of a noisy signal to be determined. This is done in the framework of nonlinear dynamical systems theory so that the underlying dynamics of the wake can be determined. From this it is possible to characterise those areas of the wake where coherent dynamical structure s are present and to explore the mechanism responsible for the oscillation of the wake. The paper reviews the background to singular systems analysis systems analysis and describes the application of the technique to the char acterisation of the dynamics of the wake of a model vehicle placed in an op en jet wind tunnel. Results are presented for three cross-flow planes in th e wake where the structure of the wake is revealed in a new light. In parti cular, it is clear that the traditional picture of the vortex core appear t o be present around the periphery of the vortex and in other areas where sh ear is apparent in the mean flow. The analysis technique allows the motion of these to be tracked downstream through the wake, whereas simpler analysi s techniques do not allow such tracking to be carried out. (C) 2000 Elsevie r Science Ltd. All rights reserved.