Drag Reduction using Base Flaps Combined with Vortex Generators and Fluidic Oscillators on a Bluff Body

The potential of drag reduction on a generic model of a heavy vehicle using base flaps operated in combination with flow control devices is investigated experimentally. Base flaps are well known as drag reduction devices for bluff bodies and heavy road vehicles. However, for optimal performance their deflection angle should typically not exceed 12°. In this paper the primary goal is to increase the usable range of the deflection angles by applying flow control. The secondary goal is to find the most suitable method for flow control. A comparison is made between triangular vortex generators and fluidic oscillators as passive and active flow control methods, respectively. Vortex generators have the advantage of being very simple devices but produce drag. Fluidic oscillators are also quite simple devices but require additional air supply. Their advantages are that they can be activated when needed and that they do not generate additional drag. The wind tunnel model used corresponds to the geometrical dimensions of a 10% scaled model yielding a Reynolds number of 7·10⁵. Various flap angles with a length of 100mm were attached to the base. Other geometrical parameters such as the height of the vortex generators were also varied as well as their axial position. The results show that base flaps deflected by 20° combined with vortex generators reduce drag by 26% compared to the baseline. At deflection angles of 22.5° the passive and active concepts show similar drag reduction. Furthermore, possibilities for performance improvement of the active concept are identified.