Experimental study of boundary layer characteristics around platoon of vehicles

Abstract

This study deals with the evaluation of the aerodynamic characteristics of ground vehicles. The aerodynamic characteristics include the velocity profile, displacement thickness, momentum thickness, shape factor, skin friction coefficient and friction drag. These parameters are investigated as the function of vehicle speed and vehicle spacing (space between two vehicles) with two and three vehicles moving in platoon maneuvers. To conduct this investigation, 1/32 scale models of a saloon car are tested in the 300x300 mm wind tunnel facility of the Department of Mechanical Engineering, BUET with three different speeds. Velocities in the vertical plane through the centre of the wind tunnel are measured by pitot static tube at different axial positions in the stream wise direction including the top surfaces of the vehicles placed centrally in the tunnel. The pressure signal being interfaced with a computer through pressure transducer. The Pitot static tube is traversed by a computer controlled 3-axes co-ordinate positioning device which is developed as a part of this research work. The position of the probe and the corresponding data are recorded simultaneously by the computer using IBM PC parallel port. For this purpose a program is developed using Turbo C. The integral analysis of the boundary layer is used to quantify the behavior of the aerodynamic characteristics and the effects of vehicle speed and vehicle spacing on the boundary layer characteristics and skin friction. The zone of flow separation approaching laminar-turbulent transition is also investigated. In the leading and trailing surface of the vehicle, the aerodynamic characteristics are found to be functions of the vehicle speed. The displacement and momentum thicknesses of the trailing vehicle in the platoon depend on the number of vehicle in the platoon and are found to be higher for large platoon. The flow strikes the front of the vehicle and is accelerated over the bonnet and is observed to be separated from the rear zon~ of the bonnet but reattaches in the leading edge of the roof. In this zone, friction coefficient becomes negative due to back flow. Hence, friction drag is maximum in the front of the vehicle and minimum in the leading edge. of the roof. The friction drag coefficient on the trailing vehic1e(s) is observed to be higher than that on leading vehicle in the platoon and increases for smaller vehicle spacing. The vehicle speed variation has no significant effect on friction drag over the cars in platoon.