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.