Locked-on vortex shedding modes from a rotationally oscillating circular cylinder

Numerical simulations of unsteady two-dimensional flow around a rotationally oscillating circular cylinder, placed in a uniform cross flow of a constant property Newtonian fluid, are performed at a fixed Reynolds number of 200. The investigation is based on the solutions of stream function-vorticity formulation of Navier-Stokes equations on non-uniform polar grids using a higher order compact (HOC) formulation. The flow field is mainly influenced by Reynolds number, Re, maximum angular velocity of the cylinder, alpha(m), and the frequency ratio, f/f(0), which represents the ratio between the frequency of oscillation, f, and the natural vortex shedding frequency, f(0). The ranges considered for these parameters are 0.5 <=alpha(m)<= 6.0 and 0.50 <= f(0)<= 3.0. The resulting vortex formation modes and lock-on phenomena behind the cylinder as well as the fluid forces acting on the cylinder are analyzed. Occurrence of new multiple lock-on regions is demonstrated in detail by the variation off and alpha(m). The instances of high drag reduction at high values of alpha(m) are confirmed. Comparisons with previous numerical and experimental results verify the accuracy and validity of the present study.