Turbulent flows interacting with groups of obstacles

Abstract

This thesis presents extensive experimental data that systematically examines the wakes and the drag forces of canopy patches with different densities, immersed in turbulent boundary layers.The patches are circular (with outer diameter D) and are made of several identical circular cylinders (height, H and diameter, d). The bulk aspect ratio of all the patches (AR = H~D)was fixed at 1 and the patch density ([phi] = Ncd2~D2) is altered by varying the number of cylinders(Nc) in the patch. Drag measurements show that the patch drag coefficient increases with increasing density. The drag coefficient of the highest investigated density ([phi]= 0.25) is greater than the drag coefficient of a solid patch (i.e. [phi] = 1, which is a solid cylinder with AR = 1). PIV measurements were carried out along streamwise-wall-normal (x - y) plane along the centreline of patch and in the streamwise-spanwise (x - z) plane at its mid height (i.e. y = H~2). Mean velocity fields show that the porosity of the patch promotes bleeding along all directions. It was observed that bleeding along the vertical/horizontal direction increases/decreases with increasing [phi]. Furthermore, it was also observed that bleeding along the lateral direction dictates the point of flow separation along the sides of the patch and makes it independent of [phi]. All these aspects make wakes for porous patches markedly different from their solid counterpart and provide a basis to explain the observed trends in the drag coefficient. The wakes generating behind three-dimensional porous patches can be divided in two distinct regions: the very near wake, where the wake properties are constant along the height of the model,as a first approximation, and the near wake, where the velocity profiles for different patches collapse, regardless of patch density. Scaling laws and parameters for the velocity profiles are introduced. Subsequently, an analysis of the trend of these parameters with [phi] and y is carried out and, where possible, a predictive model is evaluated. The wakes are also found to be self similar at the mid height horizontal plane, if scaled with appropriate scaling parameters, and the trend of these parameter with patch density is analysed. The analysis of fluctuating quantities confirms the presence of two distinct regions in the patches’ wakes, and suggests the presence of an alternate vortex street whose intensity and coherence increases with increasing patch density.

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ORCID for Bharath Ganapathisubramani: orcid.org/0000-0001-9817-0486

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