Effects of Wall Roughness on Adverse Pressure Gradient Boundary Layers

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Mottaghian, Pouya
Separation , Recirculation region , Large-eddy Simulations , Fully-rough , Sand-grain roughness , Double-averaging , Adverse pressure gradient
Large-eddy Simulations were carried out on a flat-plate boundary layer over smooth and rough surfaces in the presence of an adverse pressure gradient, strong enough to induce separation. The inlet Reynolds number (based on freestream velocity and momentum thickness at the reference plane) is 2300. A sand-grain roughness model was implemented and spatial resolution requirements were determined. Two roughness heights were used and a fully-rough flow condition is achieved at the reference plane with roughness Reynolds numbers 60 and 120. As the friction velocity decreases due to the adverse pressure gradient the roughness Reynolds number varies from fully-rough to transitionally rough and smooth regime before the separation. The double-averaging approach illustrates how the roughness contribution decreases before the separation as the dispersive stresses decrease markedly compared to the upstream region. Before the flow detachment, roughness intensifies the Reynolds stresses. After the separation, the normal stresses, production and dissipation substantially increase through the adverse pressure gradient region. In the recovery region, the flow is highly three dimensional, as turbulent structures impinge on the wall at the reattachment region. Roughness initially increases the skin friction, then causes it to decrease faster than on a smooth wall, generating a considerably larger recirculation bubble for rough cases with earlier separation and later reattachment; increasing the wall roughness also leads to larger separation bubble. In addition, roughness causes early flow reversal upstream of the real separation (which occurs when the zero-velocity line moves away from the wall) because the small-scale separation regions downstream of the roughness elements become larger and merge together as a result of the APG. However, this flow reversal remains below the roughness crest. The reasons for the earlier separation are larger momentum deficit in rough-wall flows and the shutting down of the production of shear both before and after the separation, mainly due to the decrease in the velocity gradient in the outer layer. After the separation, roughness effects can be felt throughout the boundary layer because of the advection of near-wall fluid around the recirculation region.
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