Reynolds and dispersive shear stress in free-surface turbulent channel flow over square bars

Reynolds and dispersive shear stresses in turbulent flow over spanwise-aligned square bars in an open channel flow are examined. Results of large-eddy simulation of flow over two different bar spacings corresponding to transitional and k-type (reattaching flow) roughness are analyzed. The Reynolds shear stress contribution to the momentum loss (or the friction factor, respectively) is greater than the dispersive shear stress contribution. By increasing the bars spacing, however, the contribution of the dispersive shear stress increases while the Reynolds shear stress contribution decreases, which is due to a standing wave at the water surface in the flow over k-type roughness which results in significant spatial variations in the time-averaged velocities. Strong sweep events take place and contribute to the friction coefficient. Investigating the dynamics of the flow reveals that there is momentum source below the crest of the bars and momentum sink above them, leading to acceleration or deceleration of flow, respectively. The contribution of dispersive shear stress is significant only in the deceleration of the flow near the crest of the bars and in the acceleration of the flow under the water surface. Quantification of the three components of total kinetic energy, i.e. mean, turbulent, and wake kinetic energy, reveals that the largest contribution is that of the mean flow in both geometries. By increasing the bar spacing, the contributions of turbulent and wake kinetic energy, which are localized at the bar height, increase, while the kinetic energy of the mean flow decreases.