Numerical investigations of lift suppression by feedback rotary oscillation of circular cylinder at low Reynolds number

This article describes a strategy of active flow control for lift force reduction of circular cylinder subjected to uniform flow at low Reynolds numbers. The flow control is realized by rotationally oscillating the circular cylinder about its axis with omega(t) =-lambda C-L(t), where omega(t) is the dimensionless angular speed of rotation cylinder, lambda is the control parameter and C-L(t) is the feedback signal of lift coefficient. The study focuses on seeking optimum lambda for the low Reynolds numbers of 60, 80, 100, 150, and 200. The effectiveness of the proposed flow control in suppressing lift force is examined comprehensively by a numerical model based on the finite element solution of two-dimensional Navier-Stokes equations. The dependence of lift reduction on the control parameter lambda is investigated. The threshold of lambda, denoted by lambda(c), is identified for the Reynolds numbers considered in this work. The numerical results show that the present active rotary oscillation of circular cylinder is able to reduce the amplitude of lift force significantly as long as lambda <= lambda(c), at least 50% for the laminar flow regime. Meanwhile, the present active flow control does not result in the undesirable increase in the drag force. The Strouhal number is observed to decrease slightly with the increase of lambda. As for a specific Reynolds number, the larger lambda gives rise to the larger amount of lift reduction. The lift reduction reaches the maximum at lambda = lambda(c). The mechanism behind the present lift reduction method is revealed by comparing the flow patterns and pressure distributions near the active rotationally oscillating circular cylinder and the stationary circular cylinder. It is found that the critical value lambda(c) generally increases with Reynolds number. Two types of lift shift are observed in the numerical results for the cases with lambda >lambda(c). The first is characterized by the regular fluctuation of lift coefficient but with nonzero mean value, while the second is associated with the sustaining increase of lift coefficient. The phenomenon of lift shift is found to be related closely to the evolution of vortex pattern in the near wake of circular cylinder. (C) 2011 American Institute of Physics. [doi:10.1063/1.3560379]