Search Results

This paper conducts experimental study and numerical large eddy simulation for the drag reduction effect of jet boat-tail passive flow control on bluff body models. The jet boat-tail for bluff bodies operates by surrounding a converging duct around the end of a bluff body where the base surface is located. The duct captures free stream and forms a high speed jet angled toward the center of the bluff body base surface circumferentially to have the effect of a boat tail. A rectangular prism bluff body representative of various motor vehicle shapes such as trucks, vans, SUVs is used in this study. The numerical Large Eddy Simulation shows that the jet boat-tail sucks in the forebody boundary layer due to the low base pressure and significantly thins the boundary layer. The jet interacts with the shear layer and creates large vortex structures that entrain the freestream to base flow and energize it.

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.

1 This paper introduces and proves a novel automotive mirror base drag reduction method using passive jet flow control. The new concept is to open an inlet at the front part of the mirror, introduces the airflow via a converging duct, and ejects the jet surrounding the mirror surface at an angle toward the center of the mirror. The jet harnesses the energy from the free stream by jet mixing with the main flow via large coherent structures, entrains the main flow to energize the base flow, reduces the wake size and turbulence fluctuation, and ultimately significantly decreases the drag. Above phenomena are proved by wind tunnel testing with PIV and drag force measurement and CFD large eddy simulation (LES) calculation. Two jet mirrors with different inlet areas are studied. The jet mirror tunnel 1 has a smaller inlet area, and the jet mirror tunnel 2 has a 4.7 times larger inlet area. The wind tunnel testing is only done for the baseline and jet mirror tunnel 1.

This paper conducts numerical simulation and wind tunnel testing to demonstrate the passive flow control jet boat tail (JBT) drag reduction technique for a heavy duty truck rear view mirror. The JBT passive flow control technique is to introduce a flow jet by opening an inlet in the front of a bluff body, accelerate the jet via a converging duct and eject the jet at an angle toward the center of the base surface. The high speed jet flow entrains the free stream flow to energize the base flow, increase the base pressure, reduces the wake size, and thus reduce the drag. A baseline heavy duty truck rear view mirror is used as reference. The mirror is then redesigned to include the JBT feature without violating any of the variable mirror position geometric constraints and internal control system volume requirement. The wind tunnel testing was conducted at various flow speed and yaw angles.