Flow past a square cylinder in the vicinity of a plane wall has received considerable research attention. However, most of the previous studies focused on a cylinder immersed in an upstream turbulent boundary layer with boundary layer thickness much smaller than the cylinder height. The present study investigates wall proximity effects on the wake dynamics behind a square cylinder subjected to a thick turbulent boundary layer. The experiments were performed in a recirculating open water channel using particle image velocimetry. All experiments were conducted at a water depth of 430 mm and a free-stream velocity of 0.5 m/s. The Reynolds number based on the free-stream velocity and the cylinder height (h&nbsp;= 25 mm) was 12500 while the thickness of the oncoming turbulent boundary layer was 3.6h. The Froude number based on the free-stream velocity and the water depth was 0.24 so that the effects of surface waves on the wake flow downstream of the square cylinder were negligible. The gap distance (G) between the bottom face of the cylinder and the wall was varied, resulting in gap ratios (G/h) of 0, 0.3, 0.5, 1 and 8. Two side-by-side cameras were simultaneously used to capture 9000 images at a frame rate of 16 Hz.Preliminary results show a symmetric flow pattern about the cylinder centerline for G/h&nbsp;= 8. As the cylinder approaches the wall, the mean flow topology becomes asymmetric and the size of the mean separation bubbles increases. In the case of the wall-mounted cylinder, the mean flow reattached onto the bottom wall at 11.4h&nbsp;downstream of the leading edge. For test cases with G/h&nbsp;&gt; 0, the magnitude of spanwise vorticity in the upper shear layer is higher than that in the lower shear layer. Furthermore, the magnitudes of the Reynolds stresses decreased with decreasing gap ratio. Contrary to the offset cases, the magnitude of the streamwise Reynolds normal stress in the wall-mounted test case was consistently higher than the vertical Reynolds normal stress. The distributions of the third order moments are analyzed to provide insight into the transport of turbulence in the streamwise and vertical directions. The results showed that turbulence transport by vertical fluctuating velocity near the wall decreased with decreasing gap ratio.The final paper will include detailed literature review, description of the test facility and measurement procedure and measurement uncertainty. The flow topological differences among the various gap ratios will be analyzed using the mean flow and higher order turbulence statistics, &nbsp;two-point correlations and probability density functions. Furthermore, quadrant and proper orthogonal decompositions will be employed to elucidate the impact of thick turbulent boundary layer on the wake dynamics.

Influence of Wall Proximity on the Wake Dynamics Behind a Square Cylinder

Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2021/volume/85284

ASME 2021 Fluids Engineering Division Summer Meeting (FEDSM2021)

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