Towards adaptive drag reduction of a flat-back 3D bluff body in variable pitch and crosswind by flow orientation at the base separation

Abstract

AbstractThe sensitivity of the drag to the rear design of a flat-back body is investigated under different body attitudes defined by the pitch ($$-1.5^\circ , 0^\circ , +1.5^\circ$$ - 1 . 5 ∘ , 0 ∘ , + 1 . 5 ∘ ) and yaw (up to $$15^\circ$$ 15 ∘ ). The rear design consists of taper angles at the top and bottom trailing edge varying from $$0^\circ$$ 0 ∘ (no taper) to $$12.5^\circ$$ 12 . 5 ∘ . Compared to the fixed optimal rear design that minimizes drag at the wind-aligned body attitude, the rear design adaptation to the change of attitude produces a noticeable drag reduction up to 5% depending on the pitch angle within a yaw range smaller than $$2^\circ$$ 2 ∘ . It is shown that this drag reduction is related to the vertical wake steady instability interfering with the rear design. For yaw larger than $$2^\circ$$ 2 ∘ and up to $$12^\circ$$ 12 ∘ , an almost constant drag reduction of 2% is found and shown to be a compromise between a beneficial pressure recovery on the flat base and a detrimental pressure drag on the tapers. At larger yaw angles and whatever the pitch angle is, there is no compromise anymore such that any taper angle different from $$0^\circ$$ 0 ∘ produces a drag increase leading eventually to the squareback rear design as the optimal design. Overall, the study emphasizes the potential of adaptive control of the top and bottom trailing edge tapers to arbitrary body attitude even at small yaw angles when the pitch is varied.