Experimental Characterization of the 3D Flow Field around a Roughness Element Embedded in the Inner Part of a Rough-Wall Turbulent Boundary Layer*

Knowledge of detailed flow structure around roughness elements in a high Reynolds number rough wall boundary layer (BL) is essential for understanding the transport of momentum and turbulence away from the wall. However, there is very little 3D experimental data in the flow in the roughness sublayer due to technical challenges in accessing this region. To address this issue, Microscopic Dual-View Tomographic Holography (M-DTH) is applied in a refractive index-matched water tunnel to fully resolve the 3D flow in the inner part of the rough wall turbulent boundary layer (Re_τ~ 3300). The array of 0.5 mm high (k), 0.79 mm diameter cylindrical roughness elements correspond to k/δ_ν~35 and δ/k~90. 3D velocity field is obtained by interpolating particle tracks onto a structured grid. The time-averaged distributions of velocity, vorticity, wall shear stress, and Reynolds stresses reveal many phenomena, e.g. flow channeling between elements, which causes high wall shear stresses, flip-flopping wakes behind the cylinders, roll-up of a horseshoe vortex, which entrains most of the BL vorticity, 3D open separation on the sides and behind the cylinder, formation of a very turbulent vortical canopy on top and in the near wake behind the cylinder, formation of multiple axial vortices originating from the horseshoe vortices and tilting of the canopy legs.