Author(s): Razieh Jalal Abadi; Thorsten Stoesser

Linked Author(s): Razieh Jalal Abadi, Thorsten Stoesser

Keywords: Free surface flow; Large-eddy simulation; Rough-bed flow; Turbulent flow; Transitional flow; Laminar flow

Abstract: Free surface flow over spanwise aligned square bars are studied using Large-eddy simulations for laminar, transitional and turbulent flows at constant Froude number (Fr). Two different bar spacings corresponding to transitional and k-type (reattaching flow) roughness are selected. Validating the turbulent flow simulations with available experimental data shows convincing agreement for water surface elevations and streamwise velocity profiles. The deformation of the water surface is in the form of mild undulation for the transitional roughness and distinct standing waves for the k-type roughness. Increase in Reynolds number (Re) leads to larger deformations of water surface especially for the k-type roughness. The presence and extension of recirculation zones in the trough between two consecutive bars are seen in the contours of the mean streamwise and wall-normal velocities, the total shear stress and the streamfunction. This recirculation bubble occupies the entire trough between the bars for the transitional roughness while for the k-type roughness it extends to a reattachment point and after that the flow recovers to a boundary layer. The variations of local Fr at the free surface is quite small for the flows over transitional roughness similar to the variations of the water depth. In flow over k-type roughness the local Fr experiences the increase and decrease due to the local increase and decrease of the streamwise velocity under the free surface. The pressure coefficient over bed, Cp, in both cases has a similar distribution with a peak at the leading edge of the bars but Cp is generally smaller for the laminar flows. The friction coefficient Cf is significantly smaller in the turbulent case than in the transitional and laminar cases for flow over both roughness types. The friction and pressure forces do not change drastically by the bar spacing and Re but in general the pressure drag dominates in this flow. In all flow types the flow resistance (8 / f)1 / 2 is greater for the transitional roughness. Darcy–Weisbach friction factor f is derived for turbulent flows using double averaging (in time and space) of the Navier–Stokes equation (Nikora et al., Journal of Fluid Mechanics, 2019). The obtained relationship includes three components: viscous stress, turbulent stress and dispersive stress where the dispersive stress represents the roughness effects. The contribution of these components to f are calculated and compared for both roughness types. The viscous stress contribution is the smallest and turbulent stress contribution is the largest for both cases. However, the contribution of dispersive stress is larger for flow over k-type roughness leading to a decrease in the contribution of turbulent stress to f for this case.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022138

Year: 2022