California Partners for Advanced Transportation Technology

This report summarizes wind tunnel experimental measurementson the aerodynamic interaction between members of misaligned platoons. Experiments are conducted at the University of Southern California's Dryden Wind Tunnel Facility. All experiments are made using 1/8 scale models of 1991 Chevy Lumina minivan. Models are placed above a ground plane with a porous surface, through which slight suction is applied to remove the boundary layer. Refurbishing of the ground plane surface, and its repositioning to a 1 degree angle of attack produce significantly improved air flow through the test section. Automatization of the testing procedures allow measurements of drag, side force and yawing moment with extremely fine position resolution. The measured quantities are presented in the form of coefficient ratios by ratioing the forces and moment with the value of drag experienced by a vehicle in isolation. The results of two separate experiments are presented in this report. First, aerodynamic forces on misaligned three-vehicle platoons are presented for all possible platoon configurations resulting from a longitudinal separation range of 0 to 0.72 vehicle lengths and a lateral displacement range of 0 to 1.1 vehicle widths for the middle vehicle. Results are presented in the form of color maps of the drag, side force and yawing moment coefficient ratios for each individual vehicle in the platoon. Experimental results from a complete set of symmetric configurations and five sets of non-symmetric configurations, associated with five fixed separations between the leading vehicle and the trailing vehicle, are presented in detail. A second experiment consists of a detailed investigation of aerodynamic forces on a two-vehicle platoon in back-to-back geometry. Following previous observations, the present experiment investigates a two-fold increase in drag force occurring at specific separations between the two vehicles. Color maps for the drag, side force and yawing moment coefficient ratios document the presence of a hysteresis loop-the drag on the leading vehicle as separation increases is different from drag as separation narrows. It is argued that the resonance with hysteresis represents a matching between the wavelength of turbulent flow structure and the spacing itself. A dimensional analysis relates the drag increase phenomenon to longitudinal separations between different types of vehicles.