Flow Separation and Control

The study of time-varying inflow velocity on square and rectangular cylinders is crucial in wind engineering to understand the effects of thunderstorm outflow on structures. Numerical simulations in Lunghi et al. (2024), carried out with Nek5000, demonstrated that, under accelerating inflow conditions, vortex shedding from a square cylinder occurs within constant-frequency time cells, with vortex packets shed at consistent frequencies. Transitions between these cells introduce discontinuities in the shedding process. Morello et al. (2025) extended the investigation to rectangular cylinders with chord-to-depth ratios of 3:1 and 5:1, observing similar behavior. We complemented these simulations with experiments involving Gaussian-type and constant-magnitude acceleration and deceleration profiles. By progressively intensifying the severity of these profiles, we identified thresholds beyond which the flow diverges from quasi-steady behavior.

For both the simulations and experiments involving the square cylinder, the cell-averaged Strouhal number closely matches the values observed under steady inflow at the same Reynolds numbers. A similar trend was found for the elongated rectangular cylinders: despite their more complex wake dynamics, constant-frequency time cells are present under strong acceleration/deceleration. In this case, the cell-averaged Strouhal number increased significantly with Reynolds number and diverged from the steady-inflow reference values.