Road vehicles usually operate within windy environments. The combination of typical wind distributions and vehicle speeds, imposes on such vehicles aerodynamic yaw angles, β, which are often almost uniform up to 6° and relevant up to 14°. Drag saving devices are often optimized for zero cross-wind scenarios, minimizing drag only around these design conditions. This work presents the drag saving increase that an adaptive system can provide over a classic boat-tail.
In the experimental set up employed, two flaps are located at the rear lateral edges of an Ahmed body and respectively set at angles
θ1 and
θ2 with respect to the model. To evaluate the efficacy of different flap positioning strategies under cross-wind, the model was tested in a wind tunnel,
, with and without flaps at yaw angles
β = 0°, 3°, 6° or 9°. The flap sizes tested,
δ, were 9% or 13% of the body width. For each
β and
δ, the maps of drag against the two flap angles were obtained.
The minimum drag is generally not located at the symmetric flap deflection, defined by θ1 = θ2. Such a condition is characteristic of static positioning and is equally effective for positive and negative β. The flap positioning strategies considered for comparison are the static symmetric configurations minimizing drag at each β and the adaptive configuration providing the minimum drag at every β. Except for β = 0°, the adaptive strategy consistently provides less drag than each optimal symmetric flap deflection.
Also, a static strategy cannot optimize the symmetric flap deflection at each β. When the average of each configuration drag, weighted with a realistic β distribution, is employed, the adaptive solution provides a drag reduction 40% to 70% higher than the best static positioning, depending on the flap length and exact yaw distribution.