Experimental study on the confined 3D laminar flow past a square prism with a high blockage ratio

An experimental PIV study is presented that addresses the confined 3D laminar flow behaviour past a square prism. The Reynolds number (Re), based on prism cross-section height varies between 100 and 256. The channel aspect ratio is 1/1 and the blockage ratio is 1/2.5. This geometry is representative of a passive method to enhance mixing in otherwise highly ordered laminar channel flow. It is found that the lateral walls exert a strong effect on the flow behaviour with two main consequences: (a) the onset of vortex shedding is delayed to a Re in the vicinity of 170, as opposed to the unconfined case where the critical Re is reported to be between 50 and 60 and (b) transition from the steady closed recirculation bubble regime to the vortex shedding regime is not abrupt. In particular, there is a range of Re for which the closed recirculation bubble pulsates with increasing amplitude prior to the onset of the Karman street regime. The experimental results are supported by numerical computations, and the relation of the results with the practical design of engineering systems is also discussed.