Transient Platoon Aerodynamics and Bluff Body Flows

L. Tsuei
PhD dissertation, 2000.

There are two components of this experimental work: transient vehicle platoon aerodynamics and bluff-body flows. The transient aerodynamic effects in a four-vehicle platoon during passing maneuvers and in-line oscillations are investigated. A vehicle model is moved longitudinally parallel to a four-car platoon to simulate passing maneuvers. The drag and side forces experienced by each platoon member are measured using strain gauge balances. The resulting data are presented as dimensionless coefficients. It is shown that each car in the platoon experiences a repulsive side force when the passing vehicle is in the neighborhood of its rear half. The side force reverses its direction and becomes an attractive force when the passing vehicle moves to the neighborhood of its front half. The drag force experienced by each platoon member is increased when the passing vehicle is in its proximity. The effects of the lateral spacing and relative velocity between the platoon and the passing vehicle, as well as the shape of the passing vehicle, are also investigated. Similar trends are observed in simulations of both a vehicle passing a platoon and a platoon overtaking a vehicle. During the in-line oscillation experiments, one of the four platoon members is forced to undergo longitudinal periodic motions. The drag force experienced by each platoon member is determined simultaneously during the oscillations. The effects of the location of the oscillating vehicle, the shape of the vehicles and the displacement and velocity amplitudes of the oscillation are examined. The results from the transient conditions are compared to those from the steady tests in the same setup. In the case of a four-car platoon, the drag variations experienced by the vehicles adjacent to the oscillating vehicle are discussed using a cavity model. It is found that when the oscillating car moves forward and approaches its upstream neighbor, itself and its downstream neighbor experiences an increased drag, but its upstream neighbor experiences a decreased drag. Opposite results are obtained when the oscillating car moves backward and away from its front neighbor. In the case of a four-box platoon, a critical spacing of about 0.3 vehicle length is observed when the rectangular boxes are used as platoon members, where the drag force experienced by the upstream box shows a dramatic drop.
Bluff-body flow fields are studied systematically in order to understand the physics of the inter-cavity and wake flows encountered in the platoon operations. A novel particle image velocimetry technique is developed for this purpose. This technique is capable of analyzing flow fields near walls, boundaries, and interfaces, where conventional particle image velocimetry method has difficulty in measuring the velocity. Experiments are conducted in a water towing tank. The wakes and the inter-cavity flows of various cylindrical models in canonical configurations are studied. Three types of cylinder with circular, square, and diamond cross-sections are used. The wake behind a single cylinder, two tandem arranged cylinders, and two side-by-side cylinders are investigated, and the widths of wake are measured. It is found that the flow patterns are similar for all three types of cylinders with slightly different sizes of wake. Shear layer reattachments are discovered in two cylinders in a tandem arrangement. In-phase and anti-phase vortex sheddings are observed behind two side-by-side cylinders.