Aerodynamic characteristics of a square cylinder in transverse oscillating flows

The present study investigates the aerodynamic characteristics of a square cylinder subjected to transverse oscillating flows. The inflow oscillating frequencies range from 0.25 to 2.00 times the natural Karman vortex shedding frequency. The inflow oscillating intensities are within 5%–20%. Flow resonance between the inflow oscillation and Karman vortex shedding is identified by lift force spectra and verified by dynamic mode decomposition of velocity fields. The range of inflow frequencies conducive to flow resonance is centered around the natural Karman vortex shedding frequency and widens as the inflow fluctuating intensity increases, reaching a span of 0.5 to 1.5 times the natural Karman vortex shedding frequency at the intensity of 20%. The fluctuating lift force coefficients exhibit significant enhancement within the central region of the resonance frequency range, contrasting with lower values observed at the boundaries of this range (similar to non-resonance cases). The enhancement is attributed to intensified shear-layer flapping motion, which is quantitively estimated by the velocities at leading edges. In non-resonance cases and resonance cases with lower inflow frequencies, the root mean square pressure coefficients increase at the center of the leeward wall, resulting in a more uniform distribution of base pressures. Conversely, in resonance cases with higher inflow frequencies, the root mean square pressure coefficients exhibit a sharp decrease from the trailing edge to the center of the leeward wall. This phenomenon coincides with an expanded vertical separation between the Karman vortices of opposite directions in the wake region.