Three-dimensional velocity measurements in a roughness sublayer using microscopic digital in-line holography and optical index matching

Microscopic in-line digital holography and particle tracking are used for measuring the 3D flow field in the inner part of a turbulent boundary layer over a rough surface. This paper focuses on procedures, uncertainty and data quality. Experiments are performed for a rectangular channel flow, at Re-tau = 3520, with the top and bottom surfaces containing uniformly distributed pyramidal elements. Optical accessibility through the acrylic rough walls is attained by matching the optical refractive index of the fluid with that of acrylic using a NaI solution in water. Localized particle injection ensures that the seeding is sufficient for detecting 5000-10 000 particle pairs in each hologram pair within the 3.1 x 2.1 x 1.8 mm(3) sample volume, which covers the entire roughness sublayer. The data quality is assessed by evaluating how well the data satisfy the continuity equation for varying resolutions and procedures. Mean velocity and Reynolds stress profiles are compared to 2D particle image velocimetry data, showing excellent agreement above one roughness height away from the wall, and discrepancies, some of which can be attributed to spatial resolution and bias, closer to the rough wall. Sample instantaneous flow realizations contain low-lying vortices, some with spanwise orientation, and others aligned parallel to the roughness grooves, flooding the lower part of the sublayer. Quasi-streamwise vortices with vertical inclinations of 50 degrees-60 degrees also appear, with a small fraction of them spanning the entire sublayer.