Time Resolved Holographic PIV in the Inner Part of a Rough-Wall Turbulent Boundary Layer

Flow structures as well as the mean skin friction in the inner part of turbulent boundary layers are significantly altered by the presence of roughness elements. Due to technical difficulties to access the flow inside roughness sublayer, there is no 3D time resolved experimental data on the turbulence inside this important region. To address this challenge, we have performed fully resolved volumetric measurements of unsteady flow and pressure field around cylindrical roughness elements using Microscopic Dual-View Tomographic Holography (MDTH) in a refractive index matched water tunnel. The friction Reynolds number ranges from 700 to 8000, and the uniform cylindrical roughness elements have a diameter of 0.79 mm, and height of 0.5 mm (k), resulting in k/δ_ν varying between 50 to 100. The sample volume is 7k×3k×3k, and data are acquired at 10-25 kHz at a magnification of 8×. Aiming to track more than 20000 particles instantaneously for mapping the velocity field around the roughness element, local injection of particles “far” upstream of the sample volume establishes the required particle concentration. The pressure distribution is calculated by interpolating and then spatially integrating the material acceleration. Sample of recently obtained data will be demonstrated.