Numerical Investigation on Flow Over Rotational and Transverse Oscillating Circular Cylinder

Abstract

Fluid flow over a circular cylinder in free stream posses a challenging fluid mechanics problem. The nature of the flow becomes more interesting under the effect of cylinder motion. It determines the flow induced forces and vibration for many engineering application. Two-dimensional free stream flow across a cylinder undergoes steady to unsteady periodic flow transition leading to vortex shedding (VS) formation at critical Reynolds number, Re=47. The VS formation causes flow unsteadiness. VS suppression stops the periodic variation of the force acting on the cylinder and thus, preventing the flow induced vibration of the cylinder. Thus, the formation and suppression of the VS are proffered according to an engineering application. The present study is to examine the wake flow pattern behind a rotational and transverse oscillating cylinder. A Numerical study is performed for unsteady flow over rotationally and transverse oscillating circular cylinder. The non dimensional forcing frequency for rotational oscillation F = 0.2, 0.5, 1, 1.5, 2, 3, 4, 7, 10 and for transverse oscillation is F = 0.05, 0.1, 0.2, 0.5, 0.7, 1, 1.2, 1.5, 2 are consider. The effect of non dimensional forcing frequency on vortex formation for rotational oscillation with amplitude of 1800 and for transverse oscillation with amplitude of 2D is examine at the different Reynolds no (100, 110, 200) using commercial software FLUENT 6.3.26. The main emphasis is placed on initially vortex formation by varying non-dimensional forcing frequency. For rotational oscillation, it is observed that, when the frequency of excitation of the cylinder in the vicinity of the natural vortex formation frequency, a lock-on regime appears. As the excitation frequency increase relative to the natural vortex formation frequency the initially formed vorticity concentration switches to the opposite side of the cylinder. The mean value of drag co-efficient is maximum at lock-on condition however it reach at minimum value by further increasing the forcing frequency and then approaches the value for the stationary cylinder. For transverse oscillation, it is observed that, As the frequency of excitation of the cylinder is increased relative to the inherent vortex formation frequency, the initially formed concentration of vorticity moves closer to the cylinder until a limiting position is reached; at this point, the vorticity concentration abruptly switches to the opposite side of the cylinder. The flow behavior in lock-on, non lock-on regime and the timing of vortex formation from the oscillating cylinder are studied. The flow patterns are understood with the help of streamlines, vorticity contours and wake-structure. Based on present study some complicated vortex pattern in periodic and non-periodic vortex formation is identified. The ensuing flow is analyzed with the help of separation point on the surface of the cylinder, strength and trajectory of the shed-vortices's and flow distribution above and below the cylinder.