Session G05: Boundary Layers: Roughness II

Experimental characterization of roughness-induced transition in favorable pressure gradient boundary layers

Accurate prediction of the boundary layer transition point in all but the simplest of geometries is a pervasive challenge in flow modeling. This is particularly true when the problem involves non-zero pressure gradients and 'bypass' mechanisms such as surface roughness. In this talk, we detail an experimental campaign aimed at probing the effects of favorable pressure gradients (FPG)s with constant acceleration parameter (i.e. 'sink' flows) on roughness-induced transition. Measurements are conducted in the refractive-index-matched boundary layer facility at JHU to allow unrestricted optical access to the flow around 1x1x1mm cube roughness element(s). Ongoing experiments include conventional 2D-2C PIV to establish the rough bounds of the parameter space, as well as more detailed 3D-3C microscopic dual-view tomographic holography (M-DTH) to fully characterize the wakes of individual roughness elements. It is first confirmed that the flow upstream of the roughness conforms to the appropriate laminar similarity solution both with and without injection of local seeding farther upstream. The flow downstream of the roughness is then examined in more detail to elucidate the mechanisms and conditions for disturbance amplification/suppression.   

Streamwise development of boundary layer properties over a rough wall turbulent boundary layer with pressure gradients

Previous research has shown that there are variation in the equivalent sand ground roughness with pressure gradient (PG) with varying levels of amplification reported. These variations could genuine or due to a combination of limited scale separation in previous studies where the log and wake region are not clearly delineated. In this study, we present results from an experimental study where we examine the development of a high Reynolds number flow (inlet Re_τ = 16,000) under the influence of different pressure gradients. The pressure gradient is generated by an airfoil (1.25m long) mounted in the free stream with its angle of attack imposing different PG histories. Planner PIV measurements are taken throughout the developing flow while drag balance and Hotwire measurements are carried out in the most downstream location (where the flow has recovered to ZPG state). Initial analysis shows that at the most downstream location, the sand grain roughness marginally depends on the PG history. Further analysis of the PIV data will be carried out and the results on the variation of sand grain roughness as well as flow structure will be presented.   

Rough- and Smooth-Wall Boundary Layer Similarity under Adverse Pressure Gradient Conditions

Outer layer similarity between rough- and smooth-wall boundary layers is well established for zero pressure gradient flows. Outside the roughness sublayer, which typically extends approximately 3 sand grain roughness heights above the surface, the mean velocity profile, turbulence statistics, and turbulence correlations will match between smooth- and rough-wall cases when quantities are normalized using the boundary layer thickness and the friction velocity u_τ. Recent experiments have shown, however, that if the pressure gradient in terms of the dimensionless acceleration parameter, K=(ν/U_∞²)(dU_∞/dx), is matched between rough- and smooth-wall cases in a non-equilibrium adverse pressure gradient, similarity does not hold. The present work considers whether similarity can be achieved if the Clauser pressure gradient parameter, β=(δ^*/t_o)(dP/dx), history is matched.  Experiments will be conducted in boundary layers on rough and smooth walls with different K distributions to achieve matching β histories. Profiles of mean velocity and turbulence statistics will be measured using a two-component LDV system at multiple streamwise locations and assessed for similarity.