Autorotation of an elliptic cylinder about an axis perpendicular to the flow

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Abstract
Autorotation of an elliptic cylinder about an axis fixed perpendicular to a parallel flow is explained in this paper by means of numerical solutions of the Navier-Stokes equations. Potential-flow theory predicts, for constant angular velocity, half a period in which a torque supports rotation and half a period in which it opposes rotation, with vanishing torque in the average. This balance is disturbed by viscous-flow effects in such a way that, for a given angular velocity, vortex shedding either damps rotation or, under certain conditions, favours rotation. The proper interplay of those conditions, which include synchronization of vortex shedding and rate of rotation, results in auto-rotation. The numerical results forRe[les ] 400 are compared with experimental data forRe= 90000 from the literature. The agreement of the force coefficients and the large-scale flow patterns is surprisingly good.

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