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Acoustic Doppler velocimeter measurements of a submerged three-dimensional offset jet flow over rough surfaces

ABSTRACT The flow characteristics of a submerged three-dimensional offset jet over transverse square ribs and gravel are experimentally investigated using an acoustic Doppler velocimeter. The pitch-to-height ratio of the ribs was varied to achieve d-type, intermediate, and k-type roughness. The Reynolds number based on the nozzle height and jet exit bulk velocity was 53,000. The mean flow properties and turbulent statistics are compared to those obtained over a smooth surface. Both decay and wall-normal spread rates were independent of surface roughness with estimated values of 0.66 ± 0.03 and 0.138 ± 0.01, respectively, with these values influenced by the presence of lateral confinement of the flow. The distribution of the mean flow, within the inner shear layer, revealed a dependence on surface roughness after reattachment. A representation of the Reynolds normal stresses within the symmetry and lateral planes revealed the highly anisotropic nature of the turbulence field of offset jets.