Dynamics of turbulent circular wall jets over smooth and rough beds: an experimental analysis

This experimental research examines the dynamics of turbulent circular wall jets developing over a smooth bed and three distinct rough beds. Acoustic Doppler velocimetry was employed to measure the instantaneous flow field. The velocity statistics for the circular wall jet over a smooth bed were consistent with those reported in previous studies, thereby confirming the reliability and accuracy of the measurements. The results indicated that, for both smooth and rough beds, the mean axial velocity profiles exhibit self-similarity in the vertical and lateral planes. However, bed roughness significantly modifies the jet development. Increasing roughness reduces the maximum mean axial velocity and accelerates its downstream decay, reflecting enhanced momentum loss due to increased bed friction and turbulence production. Roughness also promotes vertical momentum redistribution, leading to a linear increase in the vertical spreading rate while slightly suppressing lateral spreading. Specifically, doubling the bed roughness increases the vertical spreading rate by approximately 15%. The enhanced near-wall turbulence generated by roughness shifts the location of the maximum mean velocity away from the bed, thickens the inner layer, and delays the onset of self-similarity of the mean velocities. The root-mean-square velocities decrease in the presence of roughness. As the jet develops downstream, the influence of roughness progressively weakens, and the root-mean-square velocities approach self-similarity. Similarly, bed roughness lowers peak Reynolds shear stresses and broadens their spatial distribution, reflecting enhanced turbulent transport that accelerates momentum dissipation. Spectral analysis further indicates reduced energy levels over rough beds while preserving inertial-subrange scaling. Overall, the findings indicate that bed roughness primarily influences the early stages of jet development, whereas its influence decreases downstream as the flow approaches self-similarity.