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The unsteady near wake of a ground vehicle body was investigated using hot wire anemometry and an unsteady pressure measurement system. A three dimensional bluff body model was used to simulate the time dependent, three dimensional near wake flow field generated by trucks, buses, and automobiles. Coherence and coherence phase were effective methods to analyze the unsteady pressure field and to relate different pressure signals. Spectral analysis of the velocity and pressure signals was used to identify periodic wake flow structures. The time averaged near wake contains a ring vortex enclosed by shear layers which start where the model boundary layer separates from the body. At the start of the shear layer, vortex shedding was measured at a dimensionless frequency, StH(shed) = 1.157. As these vortices convected along the shear layer, vortex pairing was observed which approximately halves the characteristic frequency.

The effect of rear splitter plates and base cavities on the near wake behind a ground vehicle body above a moving ground plane were studied. Static pressure taps on the base of the body were used to measure both the mean and fluctuating pressures acting on the model base. Hot wire anemometry and flow visualization were used to study the effect of splitter plates and cavities on the near wake velocity field. The effects of varying ground clearance and of the moving ground on the base pressure were also studied. It was determined that some splitter plate configurations affect the base pressure distribution by forming a low pressure vortex on one side of the plate with high pressure fluctuations and altering the flow on the opposite side of the plate to raise the mean pressure. The suction region could be eliminated by moving the splitter plate to the model edge to form a side of a cavity. A full four sided cavity was able to increase the overall base pressure by 11% on the model.