Experimental and Computational Study of Unsteady Wake Flow Behind a Bluff Body with a Drag Reduction Device

Simple devices have been shown to be capable of tailoring the flow field around a vehicle and reducing aerodynamic drag. An experimental and computational investigation of a drag reduction device for bluff bodies in ground proximity has been conducted. The main goal of the research is to gain a better understanding of the drag reduction mechanisms in bluff-body square-back geometries. In principle, the device modifies the flow field behind the test model by disturbing the shear layer. As a consequence, the closure of the wake is altered and reductions in aerodynamic drag of more than 20 percent are observed. We report unsteady base pressure, hot-wire velocity fluctuations and Particle Image Velocimetry (PIV) measurements of the near wake of the two models (baseline and the modified models). In addition, the flows around the two configurations are simulated using the Reynolds Averaged Navier-Stokes (RANS) equations in conjunction with the V2F turbulence model. In order to capture the oscillating behavior of the wake the equations are solved in their unsteady form.

The mean pressure results show a significant increase in the base pressure with the drag reduction device. For the present geometries, the drag reduction device suppresses large-scale turbulent motions in the wake. The results show a reduction of the turbulence intensity as well as a rapid upward deflection of the underbody flow with the device in place. The effect of the drag reduction device on the length of the recirculation region in the near wake is small. Furthermore, the results confirm that the separated flow from the trailing edge of the model reattaches on the extended plate (add-on device) which is similar to that of a boat-tail effect. This boat-tail effect is documented by mean flow streamlines that show the dividing streamlines originating at the tip of the plates making the recirculation region narrower even though the main recirculation length does not change. Finally, both the measurements and simulations reveal that the instantaneous flow fields differ significantly from the averaged ones.