Experimental Study of a Ground Vehicle Body Unsteady Near Wake 1999-01-0812
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. Pairing continued until the shear layers from all sides coalesced at the free stagnation point. The vortices were shed periodically into the far wake from the free stagnation point. The periodic pumping caused interaction of the upper and lower portions of the ring vortex at StH(pumping) = 0.069. This resulted in periodic base pressure fluctuations.
The effects of a base cavity on the unsteady base pressure were also investigated. The base pressure increases with increasing cavity depth and for a cavity depth ratio D/H = 0.8, the mean base pressure increase is 11%. The unsteady pressures are in-phase across the model base, indicating that the bubble pumping mode is suppressed by the base cavity.