Browsing by Author "Shahroodi, Maryam"

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    Three dimensional topology and structural differences in the turbulent wake of cantilevered square and circular cylinders protruding thin boundary layers
    (2021-04-26) Shahroodi, Maryam; Martinuzzi, Robert; Mohamad, Abdulmajeed; Hassanzadeh, Hassan
    A comparative experimental study is conducted of the flow around cantilevered circular and square-section cylinders to investigate the effects of the cylinder geometry and the boundary layer state on the wake structures and flow topology. Detailed velocimetry results are presented in the wake of circular and square cylinders protruding a thin laminar boundary layer at a nominal Reynolds number of 10500. The results are compared to those for similar cylinders protruding a thin turbulent boundary layer at a nominal Reynolds number of 12000. In all cases, the wake is turbulent and characterized by quasi-periodic, K´arm´an-like vortex shedding with Strouhal numbers ≈ 0.1. For all cases, the signature of the shed vortices are dipole vortical structures imprinted on the mean wakes. The square cylinder in the laminar boundary layer, however, exhibits a more complex mean wake with additional structures: a secondary pair of descending vortices, far-wake and plate junction vortices. It is shown that the boundary layer state affects the wake dynamics. A low-frequency instability is observed at higher elevations of both cylinders in the laminar boundary layer, yet this instability is not directly related to instabilities of the laminar horseshoe vortex system. Unlike the circular cylinder, the low-frequency instabilities are shown to interact with the K´arm´an shedding process for the square cylinder wake. For the square cylinder, regions of bi-stable flow are observed. Locally, the flow switches between two-preferred states. While for laminar boundary layers, bi-modality is observed around half-height, for the turbulent boundary layer, bi-modality is observed close to the ground plane and close to the free-end of the cylinder. Surprisingly, perhaps, the mean wake structure for the circular cylinder does not appear sensitive to the boundary layer state. This work establishes the importance of the boundary layer state on the mean wake structure and topology, which ostensibly is a result to a modification of the wake dynamics. The outcome of the work also establishes that circular and square cylinder wakes differ in some important fundamental aspects. This result is potentially of applied importance as many engineering applications, for examples building standards, do not distinguish between circular and square cantilevered geometries.