Large-eddy simulation of the near wake of a 5:1 rectangular cylinder in oscillating flows at Re=670

Abstract In this study, three-dimensional Large-Eddy Simulations (3D LES) are conducted to investigate the near-wake characteristics of a 5:1 rectangular cylinder in oscillating flow. The oscillating flow is generated by introducing a sinusoidal velocity perturbation to the smooth flow at a low Reynolds number. It is observed that the dynamics of the vortices on the upper and lower surfaces are synchronously affected by the oscillating flow, and thus interact with the wake flow. Consequently, the vortex shedding patterns that appear are less regular than those in uniform flow, resulting in a non-synchronous phenomenon between the shedding of the wake vortices and the leading-edge vortices. Interestingly, the complex near-wake dynamics in the oscillating flow are characterized by multiple peaks of the wake velocity in the frequency domain. The energy contributions of each frequency component are studied through POD analysis. The results indicate that the flow structures of Modes 1 and 2 are of Karman vortices which dominate the velocity fluctuations in the wake and exhibit the most prominent peak in the velocity spectra. In contrast, Modes 3 and 4 are controlled by the inflow oscillations. The higher modes result from the interactions between the above two patterns. It is worth noting that the shedding frequency, longitudinal spacing and convective speed of the Karman vortices are synchronously influenced by the inflow frequency and amplitude.