Turbulent flow structure over rough side-wall surface under wave-against-current motion

Abstract

An experimental study of periodic surface waves acting against the turbulent currents under the influence of smooth and rough side wall is presented. Flow structure for current alone, and combined wave against current flows over smooth and rough side walls were measured with Acoustic Doppler Velocimeter to resolve the changes in the mean flow and turbulence characteristics resulting from the interaction of the flow and the wall roughness. Turbulent velocity fluctuations were analysed to characterise the turbulence and coherent structures and integral length and time scales were evaluated from the autocorrelation function estimates. Further the Kolmogorov length scales were determined using energy dissipation rate. Results show that the dissipation rate is greater for wave against current flow compared to the current alone flow for rough side walls. The streamwise integral length scale at the measured locations reduced with the increase in wave frequency acting against the current flow. Close to the wall surface, the effect of roughness attenuated the integral length scale and increased the turbulent dissipation rate. Continuous wavelet transform (CWT) was used as a tool to detect and establish the average time and length scales of largest horizontal coherent structures that existed in the wave against current combined flow.

Effect of d-type rib roughness on the turbulent structure of side wall boundary layer for wave-current combined flow

“…However, vertical bank or sidewall for canals and rivers comprises of different roughness scales that introduce additional turbulence scales at the bottom boundary layer turbulence. Hansda and Debnath 28 carried out an experimental study to look into the distribution of mean velocity, turbulence intensity, Reynolds shear stress and streamwise integral length scales over a sand-grain rough and granule rough sidewall boundary under the wave-against current flow. Investigation of turbulence structures and eddy scales close to the sand-grain and granule rough sidewall surfaces due to interaction with wave-current combined flows were presented in Hansda et al 29 By varying the submerged vegetation on a vertical wall, velocity and the turbulence structure was studied for waves, current alone and wave-current combined flow by Chen et al 30 Throughout the water column, the obtained turbulent kinetic energy was relatively larger in magnitude for wave-current combined flow than the current alone flow.…”