Direct numerical simulations of flow past quasi-random distributed roughness
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low about a periodic array of quasi-random distributed roughness is examined using an immersed boundary spectral method. Verification of the code used in the simulations is obtained by comparing solutions to LDA wake survey and flow visualization experiments for a periodic array of cylinders at a roughness height-based Reynolds number of 202 and a diameter to spanwise spacing d/[lambda] of 1/3. Direct comparisons for the quasi-random distributed roughness are made with experiments at roughness height-based Reynolds numbers of 164, 227, and 301. Near-field details are investigated to explore their effects upon transition. Vortices formed as the flow moves over the roughness patch create three distinct velocity deficit regions which persist far downstream. Simulated streamwise velocity contours show good agreement with experiments. Additional geometries are simulated to determine the effects of individual components of the full roughness geometry on near-field flow structures. It was found that the tallest regions of roughness determine the overall wake profile.