TEMPORAL FLOW INSTABILITY FOR ROBIN-MAGNUS EFFECT AT HIGH ROTATION RATES

T K Sengupta, A Kasliwal, S De, M Nair.

Abstract

The lift and drag coefficients of a circular cylinder, translating and spinning at a supercritical rate is studied theoretically to explain the experimentally observed violation of maximum mean lift coefficient principle, that was proposed heuristically by Prandtl on the basis of inviscid flow model. It is also noted experimentally that flow past a rotating and translating cylinder experiences temporary instability - a fact not corroborated by any theoretical studies so far. In the present paper we report a very accurate solution of Navier- Stokes equations that displays the above mentioned and the violation of the maximum limit. The calculated lift coefficient exceeds the limit of 4-pi, instantaneously as well as in the time averaged sense. The main purpose of the present paper is to explain the observed temporal instability sequence in terms of a new theory of instability based on full Navier-Stokes equation that does not that does not require making any assumption about the flow field, unlike other stability theories.