Experimental investigation on the wake generated from a low aspect-ratio triangular prism in cross-flow

Abstract

The velocity fluctuations experimentally detected in the wake of a prism ha- ving triangular cross-section and an aspect ratio H/w = 3.0 are characterized. It is shown that, besides the fluctuations induced by an alternate vortex shedding with Strouhal number St ≈ 0.16, further components are present, with different relative intensities in different wake regions. A spectral contribution at St ≈ 0.05 is found to dominate all the velocity signals in the upper-wake and it is attributed to a vertical, in-phase oscillation of a couple of counter-rotating axial vortices de- taching from the front edges of the model free-end. An intermediate component is also ascertained occurring at St ≈ 0.09. The analysis of a previously available LES simulation was fundamental for the interpretation of the physical mechanism giving rise to this flow fluctuation, which is associated with the oscillations of a transversal shear layer detaching from the rear edge of the model free-end. Pro- ceeding downstream it bends downwards into the wake in such a way to be reversed upstream impinging the rear surface of the model. Consequently, a recirculation region is delimited by this transversal shear layer. This feature is also assessed from the pressure measurements carried out on the model surfaces; indeed, a pres- sure maximum was ascertained on the rear surface at z/H = 1/3 and fluctuations at St ≈ 0.09 are singled out just at the locations below the recirculation region. Furthermore, the statistics of this frequency are comparable to the ones related to the same spectral component singled out in proximity to rear edge of the free-end, and thus most probably are generated from the same vorticity structure, viz. the transversal shear layer. From the numerical visualizations of the vorticity field it is observed that the fluctuations of the recirculation region are strictly connected with the vortex shedding. Lateral vorticity sheets are dragged in the upper wake generating in correspondence to the wake symmetry plane a vertical \action" on the transversal shear layer directly. Most probably this intricate wake morphology is the physical mechanism giving rise to the oscillations of the recirculation region. Furthermore, it is experimentally assessed that modifications on the vertical edges of the model generate a variation of the vortex shedding frequency comparable to the one produced on the fluctuation frequency of the transversal shear layer. Howe- ver, no variations were found in the fluctuations at the lower frequency in the upper part of the wake, which suggests that they are likely to be essentially connected with an instability of the axial vortices originating from the free-end