Simulation of Flow over a Rotationally Oscillating Square Cylinder at Low Reynolds Numbers

Simulation of uniform flow past a square cylinder undergoing rotational oscillations at various frequencies is performed using the ANSYS Fluent CFD package. The frequency ratios FR chosen for the study are 0.4, 0.8 and 1.0. At each frequency ratio, a number of angular amplitudes are selected, and the simulations are performed using a dynamically deforming mesh. The results are obtained from the simulations in terms of the non-dimensional drag and lift forces, vorticity contours, power spectral density plots of lift coefficients and Strouhal number. A general increase in lift and drag coefficients with increasing frequency ratios as well as with increasing amplitudes at a particular frequency ratio, except a few anomalies, is seen. The flow features in the wake and the vicinity of the cylinder are compared for the various parameters. The use of convective boundary condition for the problem is demonstrated.

Effect of rounded corners on two degree of freedom naturally oscillating square cylinder

Purpose The paper aims to study the influence of rounded corners on the flow-induced oscillation of a square cylinder that is free to oscillate in two degrees of freedom. Design/methodology/approach The finite volume code in conjunction with the moving mesh scheme was implemented via a user-defined function to carry out the computations in two dimensions. The Reynolds number (Re) chosen for the present study is fixed at 100, and the frequency ratio, Fr = fs/fn (where fs is the vortex shedding frequency and fn is the natural frequency of cylinder) is used as a varying parameter. The computational model was validated for flow past a stationary cylinder with R/D = 0 and 0.5, and the results showed good agreement with the literature. Findings The aerodynamic characteristics, amplitude response, trajectories of cylinder motion and vortex shedding modes are obtained by conducting a series of simulations under different frequency ratios of the cylinder. It was found that the minimum transverse amplitude, drag force and lift force obtained for a naturally oscillating square cylinder are quite different when compared with a stationary and forced oscillating cylinder, where the maximum drag and lift forces were observed for a square cylinder and a minimum around R/D = 0.2 was observed. Originality/value The present work identified the significant effect of the varying frequency ratio and R/D on the VIV modes of the cylinder. It was observed that the cylinder wake exhibits the (2S) vortex shedding mode for R/D = 0 to 0.2 at all Fr, whereas the C (2S) mode appeared for R/D > 0.2 at Fr = 1.1.

Experimental Investigation of Flow Over a Transversely Oscillating Square Cylinder at Intermediate Reynolds Number

This paper presents an experimental study of flow over a square cylinder oscillating in transverse direction. The Reynolds number selected for present study is 485. Limited study has also been made for two other Reynolds numbers, namely, 295 and 775. The objective of the present study is to modify the near-wake flow structure using actuation of the cylinder for possible reduction in drag force. Transverse oscillations to the cylinder are provided using electromagnetic actuators. The flow field is investigated using two-dimensional (2D)-particle image velocimetry (PIV) system, hotwire anemometer (HWA), as well as flow visualization techniques. The effect of oscillation frequency and the amplitude on parameters like Strouhal number, drag coefficient, recirculation length, power spectrum, and Reynolds stress are studied. It is observed that the recirculation length is reduced significantly with increase in forcing frequency, and consequently drag coefficient is also reduced. For a constant forcing frequency, the vortex strength is reduced with the increase in the amplitude. Further, variation of instantaneous spanwise vorticity shows that separated shear length decreases with increase in forcing frequency. As a result, vortices are moved closer to the cylinder. These phenomena affect the forces acting on the cylinder. Lock-on is also observed at a frequency close to the vortex shedding frequency of the stationary cylinder.