Optimisation of Boat-Tails for Heavy Vehicles

Boat-tails offer significant promise in reducing long haul heavy vehicle aerodynamic drag, and hence fuel consumption and greenhouse gas emissions. This paper presents results from a basic numerical and experimental investigation of drag coefficient reductions for various boat-tail configurations. The vehicle chosen is an arbitrary streamlined front end with a width to height ratio of 64%. No tractor to trailer gap is modeled. Approximate model scale is 12% and test Reynolds Number is ∼850,000. Experimental data and numerical simulations are presented for the zero yaw angle condition. For boat-tail angles where the flow remains largely attached a relationship is observed between drag reduction and the minimum boat-tail area. An optimum boat-tail angle (in the range of 15 degrees) is identified for 0 degrees yaw from both experimental and numerical data. For boat-tail angles greater than optimum a distinct increase in drag is observed in the experimental data, which is associated with flow separation. Comparison of experimental and numerical results show reasonable agreement for attached flow cases, and indicate a similar optimal boat-tail angle.