No abstract

Direct numerical simulations are used to investigate the generation and growth of disturbances in the near-and far-wake of a distributed roughness. Particular emphasis is placed on the interaction of disturbances induced due to the tallest portions of the roughness. It is observed that the disturbances may constructively or destructively interfere with each other depending on the relative orientation of the tallest regions of the roughness in close vicinity and that such interference affects the location of laminar-turbulent transition. This behavior is investigated in detail by considering the interaction of disturbances induced due to two discrete elements with varying orientation.

Laminar flow over a periodic array of cylindrical surface roughness elements is simulated with an immersed boundary spectral method both to validate the method for subsequent studies and to examine how persistent streamwise vortices are introduced by a low Reynolds number roughness element. Direct comparisons are made with prior studies at a roughness-based Reynolds number Rek (=U(k) k/ν) of 205 and a diameter to spanwise spacing ratio d/λ of 1/3. Downstream velocity contours match present and past experiments very well. The shear layer developed over the top of the roughness element produces the downstream velocity deficit. Upstream of the roughness element, the vortex topology is found to be consistent with juncture flow experiments, creating three cores along the recirculation line. Streamtraces stemming from these upstream cores, however, have unexpectedly little effect on the downstream flowfield as lateral divergence of the boundary layer quickly dissipates their vorticity. Long physical relaxation time of the recirculating wake behind the roughness remains a prominent issue for simulating this type of flowfield.