Flow fluctuations and vorticity dynamics in the near-wake of a triangular prism in cross-flow

The results are described of experiments carried out to investigate on the connection 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 frequencies are present, with different relative intensities depending on the wake region. In particular, 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, oscillation of the vorticity structures detaching from the free-end. Fluctuations are also observed at an intermediate frequency St ≈ 0.09 (IF), and prevail in positions corresponding to the downstream boundary of the recirculation region in the central part of the near wake. Measurements of the mean and fluctuating pressures over the upper and rear surfaces of the model confirm the suggestion that the origin of the IF may be an oscillation of the transversal vorticity sheet bounding the recirculation region behind the body. Geometrical modifications to the lateral edges of the model, introduced to alter the vortex shedding, produce a lowering of the HF, directly related to the increase of the mean wake width, and the IF is found to follow a similar decreasing trend. Conversely, small plates inserted along the front edges of the model free-end do not alter the frequency, energy and regularity of the LF fluctuations in the upper part of the wake; this result probably derives from the fact that, in spite of the highly irregular free-end edge, the mechanism of roll-up of the vorticity shed from the sides of the plates is still strong enough to generate the axial vortices, even if with a different formation process. The forces acting on the various models are also measured.