The 3D flow in the inner part of a turbulent boundary layer over a rough surface, with Reτ = 3400, is measured using digital in-line microscopic holography and particle tracking. Experiments are performed in a special facility, in which the optical refractive index of the transparent rough wall is matched with that of the working fluid. Holograms are recorded in a sample volume covering the roughness sublayer. Using localized particle injection, each hologram pair contains 5000–10,000 matched particle traces, providing the 3D velocity field. Profiles of mean velocity are compared to 2D PIV data, recorded under the same flow conditions. Sample instantaneous flow realizations elucidate some of the typical vortical structures encountered in the sublayer, such as low-lying vortices, some with spanwise and others with roughness groove parallel orientations, and quasi-streamwise structures with vertical inclinations of 50°–60°, some of which extend from the surface to the top of the sublayer. Conditional sampling indicates that characteristic structures have a preferred alignment in the spanwise direction close to the wall. However, with increasing elevation, these structures turn towards the streamwise direction due to roughness-induced flow channeling, and then rise in sharp angles of about 55° to the mean flow.

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