Time-resolved 3D velocity and pressure measurement around a roughness element in the Inner Part of a Rough-wall Turbulent Boundary Layer

Understanding the momentum and turbulence transport in high-Reynolds-number rough-wall boundary layers requires detailed characterization of the flow among the elements, yet 3D experimental data remain limited due to measurement challenges. These challenges are overcome by applying Microscopic Dual-View Tomographic Holography (MDTH) in a refractive index-matched water tunnel, combined with local seeding with 2 μm particles from injectors located upstream of the roughness fetch. The staggered cylindrical roughness elements have a height of 0.5 mm (k), corresponding to k/δ_ν~50-150 and δ/k~40, a diameter of 0.79 mm, and the volumetric time-resolved velocity measurements among these elements are performed at Re_τ=2000-6000. Particle tracking provides the unstructured velocity and acceleration, which are then interpolated onto a structured grid using a physics-based constrained cost minimization method (CCM). The 3D pressure distribution is computed by spatial integration of the material acceleration using a GPU-based, parallel line, omnidirectional code. Extending previously presented analysis of the time-averaged flow field, the present data characterizes the effect of unsteady flow channeling and wake meandering on the turbulence, pressure distribution, low frequency wall-shear stresses, vortical canopy engulfing the element, trailing streamwise vortices, and the 3D separated flow behind the element.