Abstract
The present study experimentally investigated near-bed coherent structures of turbulence over rough surface in the presence of surface waves over the steady current by replicating it in a laboratory flume. The rough surface was modelled by using wooden ribs of square cross section extended across the whole channel width of flume. The instantaneous velocities at different positions in the flow field were measured by using an acoustic Doppler velocimeter. The recorded velocities data were examined to investigate the coherent structures of turbulence near rough surface using the spatio-temporal averaging approach. Pre-multiplied velocity spectra, co-spectral, and coherency were evaluated in frequency domain at different vertical positions under different flow conditions. Length scales were determined to quantify the eddy size within the flow domain, and the anisotropy invariant maps were also obtained to characterize the anisotropic flow under different roughness conditions in wave-current flow cases. Results show increased correlation between velocity fluctuations in the large-scale low-frequency region due to the addition of surface waves, which accounts for the major part of Reynolds stress. Furthermore, turbulence dissipation in the near-bed region was found to be strongly dependent on roughness characteristics.
Published Version
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