Experimental Investigations of Vehicle Base Drag Reduction Using Passive Jet Boat-Tail Flow Control 2014-01-2448

This study is focused on the detailed experimental investigation of jet boat-tail (JBT) passive flow control bluff body models to reduce the base pressure drag. The JBT technique is employed through an open inlet at the leading edge of the bluff body along with a circumferential jet at the trailing edge in order to energize the base flow using the high kinetic energy flow from freestream. As a consequence, entrainment of the main flow into base flow region is initiated earlier downstream. A reduction in the turbulent fluctuation of the wake can be observed in addition to a decrease of the recirculation region velocity. Using 2D/3C Particle Image Velocimetry (PIV), two models with different inlet sizes are tested. The large flow rate model is designed with an inlet area 4.7 times greater than the other JBT prototype. The wind tunnel experimental results show a substantial reduction in the wake width and depth for the two models, which indicates a significant drag reduction. Moreover, mean velocity vector plots from PIV measurements at the mid-plane location suggest their flow fields differ significantly due to the nature of the passive jets employed. The Jet1 model initiates the large coherent structures and flow entrainment earlier than the baseline model even though the jet momentum is small. The Jet2 generates paired vortices in the shear layer due to the high jet momentum and entrain main flow to the base region. The experimentation is performed at a Reynolds number of Re = 2.55×10⁵. In order to investigate the effects at higher Reynolds numbers, Computational Fluid Dynamics (CFD) is used to model the flows using Large Eddy Simulation, which shows that the drag reduction is more effective at high Reynolds number.