| The unsteady near wake behind ground vehicle bodies was investigated to further understand the aerodynamics affecting vehicle fuel consumption and operational safety. A three dimensional bluff body model was mounted above a moving ground belt in a wind tunnel facility to simulate the time dependent, three dimensional near wake flow field generated by trucks, buses, and automobiles.; The velocity field was measured with hot wire anemometry. The time averaged and dynamic pressures on the model base were measured with pressure taps and transducers. Spectral analysis of the velocity and pressure signals identified periodic wake flow structures.; The time averaged near wake contains a ring vortex and is enclosed by shear layers which starts where the model boundary layer separates on the body. At the start of the shear layer, vortex shedding was measured at a dimensionless Strouhal frequency, {dollar}Stsb{lcub}H{rcub} = 1.157.{dollar} As these vortices are convected along the shear layer, their separation distance decreases and vortex pairing occurs, halving the frequency. Pairing continues until the shear layers from the three other sides coalesce at the free stagnation point, which separates the near wake from the developing far wake. At the free stagnation point, the vortices are shed periodically into the far wake. This periodic pulsing causes interaction of the upper and lower portions of the ring vortex and was measured at {dollar}Stsb{lcub}H{rcub} = 0.069.{dollar}; The effects of ground clearance, model aspect ratio, model yaw angle, and ground simulation on the near wake and base pressure distribution were also studied.; The applicability of splitter plates and cavities for vehicle drag reduction was investigated. It was determined from a parametric study of splitter plate lengths and positions that the overall mean base pressure could be raised up to 7 percent. This was accomplished by producing a low velocity region on one side of the plate. A four sided base cavity increased the overall mean base pressure up to 11 percent by reducing the near wake velocity fluctuations up to 50 percent and creating a quiescent body of fluid between the base and the unsteady near wake. |
