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

Abstract As the time-dependent fluid dynamics of wakes becomes important in industrial 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 application, 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 methodology that has already been applied successfully to low Reynolds number flows is applied to a turbulent wake. This involves the use of hot-wire anemometry to capture a large number of time series of velocity throughout the wake of a model road passenger vehicle. These time series are then analysed by a mathematical analysis tool known as singular systems analysis, which enables the low-frequency components of a noisy signal to be determined. This is done in the framework of non-linear 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 structures 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 characterisation of the dynamics of the wake of a model vehicle placed in an open jet wind tunnel. Results are presented for three cross-flow planes in the wake where the structure of the wake is revealed in a new light. In particular, it is clear that the traditional picture of the vortex core appear to be present around the periphery of the vortex and in other areas where shear is apparent in the mean flow. The analysis technique allows the motion of these to be tracked downstream through the wake, whereas simpler analysis techniques do not allow such tracking to be carried out.