L. Tsuei, J. K. Hedrick, Ö. Savas
California PATH Research Report UCB-ITS-PRR-99-29, 1999.

In order to provide reliable control algorithms on PATH automated highway system, knowledge of the transient aerodynamic forces is important from the point of the controllability of the platoon and individual vehicles. In a passing maneuver, the platoon members may experience severe aerodynamic forces and yaw moment when another vehicle overtakes. Similar phenomenon can be seen in our daily life as one car passes the other but much more complicated flow field is expected when the interaction involves in a multi-vehicle platoon. It is the purpose of this study to quantify the unsteady aerodynamic forces and moment so as to assure the safety of the system.

Wind tunnel experiments are conducted to study the transient aerodynamic effects experienced by every member of a platoon during passing maneuvers. Four identical 1/20-scale passenger car models are aligned as a platoon with an inter-vehicle separation of 2/5 car length. To simulate a passing maneuver, a fifth identical model is driven longitudinally by a linear motion system which is located parallel to the platoon at various lateral spacings. When the mobile model moves forward, the simulation represents a vehicle passing the platoon. When it moves backward, a vehicle overtaken by a platoon is simulated. The drag force, side force, and yaw moment coefficients on each of the four vehicles in the platoon are determined simultaneously using strain gauge balances during the maneuver to characterize the transient effects.

The results show that each member in the platoon experiences significantly increased drag when the mobile model moves to the proximity of it. When the mobile vehicle is in the neighborhood of the rear half of a platoon member, the side force on it is found to be directed away from the mobile model. The side force reverses its direction and points toward the mobile model when the mobile model is in the proximity of the front half of the platoon member. The lateral spacing between the platoon and the mobile model is found to be an important factor in the passing maneuver. The closer the mobile model to the platoon is, the greater the forces experienced by the platoon members are. The influence of the relative velocity between the platoon and the mobile model is investigated, and it is found that lower relative velocities generate higher force coefficients on each member of the platoon. When a rectangular box is used as the mobile model, the platoon members experience much greater forces. Similar trends in force coefficients are observed in both simulations of a vehicle passing a platoon and a platoon overtaking a vehicle. The measured yaw moments are negligible compared with the magnitudes of drag and side forces. These data are summarized as dimensionless coefficients and the results are presented in graphical forms.

Keywords : platoon aerodynamics, bluff-body aerodynamics, bluff-body flow, transient aerodynamics, vehicle aerodynamics, passing maneuver.