FLOW FLUCTUATIONS AND VORTICITY DYNAMICS IN THE NEAR WAKE OF A TRIANGULAR PRISM IN CROSS-FLOW

Abstract. The results are described of experiments carried out to investigate on the connec-tion between the flow fluctuations and the dynamics of different vorticity structures in the wake of a prism with equilateral triangular cross-section, aspect-ratio h/w = 3 (where h and w are the prism height and width, respectively), and placed vertically on a plane with its apex edge against the incoming flow. Flow visualizations, hot-wire velocity surveys and pressure measurements are analysed for the model in the original configuration and with geometrical modifications along its edges, conceived in order to interfere with the evolution of the various vorticity structures. In the wake of the original model, fluctuations at three prevailing fre-quencies are present, with different relative intensities depending on the wake region. In par-ticular, the frequency connected with alternate vortex shedding from the lateral vertical edges of the prism, with a Strouhal number St = fw/U 0.16 (HF), dominates in the zones outside the lateral boundary of the wake, for vertical positions below z/h = 0.9. A lower frequency, at St 0.05 (LF), is found to prevail in the velocity fluctuations in the whole upper wake, for downstream distances x/w 1.5; this frequency is associated with a vertical, in-phase, oscil-lation of the vorticity structures detaching from the free-end. Fluctuations are also observe