Effects of flow accelerations typical of thunderstorm outflows on the vortex-shedding from a square cylinder

Accelerating flows acting on a sectional model of a sharp-edged square cylinder are reproduced through the action of a multiple-fan wind tunnel. The model is equipped with 94 pressure sensors, while the wind flow around the body is characterized through three Pitot-static tubes. The accelerations generated by the 72 individually controlled fans of the facility are compatible with those typical of thunderstorm outflows. Particular attention is devoted to the acquisition of signals associated with vortex-shedding, for which tailored time-frequency analyses, based on the continuous wavelet and Hilbert transforms, are proposed. Time intervals in which the shedding frequency is constant, separated by discontinuities, are found during the transients. The number and magnitude of such discontinuities seem to be connected with the flow acceleration. The appearance of constant-frequency time cells is not strictly repetitive; moreover, the Strouhal number is seen to decrease for higher levels of acceleration.