The appearance of constant-frequency time cells during vortex-shedding from a square cylinder in accelerating flows

A wind tunnel test campaign is conducted to trace the temporal variation of the shedding frequency of a square cylinder tested under the action of accelerating flows. These are characterized by flow accelerations which are consistent with those typical of thunderstorm outflows. Time intervals in which the shedding frequency is constant are found. The presence of these constant-frequency time cells reflects a local violation of the Strouhal law, and it seems to be connected with the flow acceleration. Furthermore, the ensemble mean of the Strouhal number is seen to decrease for higher levels of acceleration.