Transient aerodynamics of a square cylinder under downburst-like accelerating flows reproduced in a multiple-fan wind tunnel

Aerodynamic and pressure coefficients of a sectional model of a sharp-edged square cylinder, at zero incidence angle, are experimentally evaluated in accelerating flow conditions through the use of a multiple-fan wind tunnel. Different unsteady flows are reproduced, characterized by flow parameters which dictate the initial and target velocities, as well as the flow acceleration and deceleration, and the temporal spacing between the ramp-up and the ramp-down.The accelerating turbulent flows reproduced during the campaign are consistent with those typical of fullscale downbursts. The ensemble mean of the mean drag coefficient in unsteady conditions reveals to be either comparable or definitely lower than the one relevant to the steady case. The drop seems to increase for higher levels of the acceleration and to be enhanced for the ramp-up conditions, while the ramp-down case suggests a lower level of reduction. This non-symmetric pattern appears to be mitigated when the starting wind velocity increases, reflecting a higher initial Reynolds number. Same comments, but with greater discrepancies compared to the steady conditions, might be addressed for the standard deviation of the lift coefficient connected with vortex shedding. Both these findings point out the presence of fluid-memory effects associated with the acceleration, which might affect the development of vortex shedding and the consequent bluff-body aerodynamics in transient conditions. This is also confirmed by the analyses of the mean pressure distributions. In particular, the detailed scrutiny of specific pressure coefficients reflects their different level of interference with the wake and the model edge.