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

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Abstract
The sensitivity of the drag to the rear design of a flat-back body is investigated under different body attitudes defined by the pitch (-1.5o, 0o, +1.5o) and yaw (up the 15o). The rear design consists of taper angles at the top and bottom trailing edge varying from 0o (no taper) to 12.5o. 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 3% and 5% when the pitch is varied within a yaw range smaller than 3o. It is shown that this drag reduction is related to the vertical wake steady instability interfering with the rear design. For yaw larger than 3o and up to 12o, 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 and independently to the pitch, there is no compromise anymore such that any taper angle different from 0o 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 when the pitch is varied.

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