Simulations and experiments on the accelerating/decelerating flow on a square cylinder

We investigate the high-Reynolds accelerating/decelerating flow around a square cross-sectional cylinder. The square cross-section is a classic shape for wind-engineering applications, e.g. high-rise buildings and towers. The flow is characterized by shear-layer separation at the upstream edges. The separated shear layers undergo Kelvin-Helmholtz instability, but they do not lose coherence until they form the von Karman vortex street in the wake. The flow around a square cylinder has been well characterized when the inflow is steady. Much less is known when the inflow is accelerating or decelerating. These conditions are relevant, e.g., for civil structures in thunderstorms. This work may be considered a first step toward the characterization of the wind loads on buildings in variable wind conditions and to a better appraisal of the limits of the wind-load predictions obtained under the assumption of steady-wind conditions.

We perform high-fidelity Large-Eddy Simulations (LES) with the spectral-element code Nek5000 to investigate the effect of Gaussian-type inflow accelerations and decelerations in the Reynolds range Re=1.720x10^4–6.536x10^4 and an excellent agreement with the experiments in Brusco et al. (2022) is found. The flow exhibits discrete changes in the vortex-shedding frequency, with the presence of constant-frequency time cells. For the accelerating flow, the vortex-shedding Strouhal number witnesses a decrease within the time cells, followed by a sudden increase between the cells, pointing out the presence of discontinuities. The opposite for the decelerating flow. In addition, we perform a parametric study by considering different acceleration/deceleration intensities. Constant-frequency time cells are present for all the investigated cases. The increasing intensity of the flow variation reduces the time length of the time cells, and it decreases the vortex shedding Strouhal number inside each cell, but a similar behavior of St with Reynolds is found.