Blockage effects on flow around rectangular prisms in uniform flow

The influence of blockage ratio on the flow around a rectangular prism was experimentally investigated using a time-resolved particle image velocimetry system. Three different blockage ratios (BR = 2.5 %, 5 %, and 10 %) were tested, with a fixed streamwise aspect ratio (AR = 3) and a Reynolds number of 7500. The results indicate that, for BR = 2.5 %, the separated shear layer from the leading edge of the prism is shed directly into the wake, however, an increase of BR promotes the reattachment on the surface of the prism. The streamwise extent of the primary, secondary and wake recirculation bubbles decrease with an increase in blockage ratio. Distinct regions of positive and negative Reynolds shear stress manifest near the leading edge, with the transitioning interface shifting upstream with increasing BR. Turbulence production concentrates near the shear layer, with peak magnitude increasing and shifting upstream with BR. The growth rate of the shear layer is similar for the reattached test cases but decrease for the unattached test cases. The spatial coherence of the turbulent structures increases downstream, however, the effects of blockage significantly reduced the spatial coherence in the BR10 case. Frequency spectra and spectral decomposition of the Reynolds normal stresses revealed that increasing the blockage ratio increases the dominance of von Kármán vortex shedding. For higher blockage ratios, turbulence is primarily influenced by intermediate-scales associated with vortex shedding, with reduced contributions from low-frequency flapping and small-scale shear-layer instabilities. In contrast, the lower blockage ratio exhibits a more complex interplay of small, intermediate, and large-scales turbulence dynamics.