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. / text

The linear instability of laterally diverging/converging flows is an important aspect towards understanding the laminar-transition process in many viscous flows. In this work the linear instability of constant pressure laterally diverging/converging flow has been investigated. The laminar velocity field for laterally diverging/converging flows, under the source/sink approximation, has been reduced to two-dimensional flows. This reduction is alternative to the Mangier transformation used earlier. For a constant pressure laterally strained flow, the laminar velocity is found to be governed by the Blasius equation for flow over a flat plate. The non-parallel linear instability of constant pressure laterally strained flows has been examined. The instability equation is found to be same as that for the Blasius flow. This implies that the stability is same as that for the Blasius flow. A lateral divergence/convergence is shown to alter the Reynolds number from that in a two-dimensional flow. The instability of a laterally converging/diverging flow thus can be obtained from the available results for the Blasius flow by scaling the Reynolds numbers. This leads to the result that while a diverging flow is more unstable than the Blasius flow, a converging flow is more stable. Some additional relevant results are also presented.