Author(s): John E. Cater; John J. R. Williams
Linked Author(s): John J.R. Williams
Keywords: Computational hydraulics; Large Eddy Simulation; Open channel flow
Abstract: An important aspect in momentum transfer and bed-stress in turbulent channel flow is the role of persistent structures that may have characteristic lengths in excess of the channel depth. In this paper such structures are investigated using spectral and two-point correlation analysis of the threedimensional vorticity data available from detailed time and space resolved Large Eddy Simulation. Data from a period spanning ten large eddy turnover times has been used to establish velocity and vorticity correlations throughout the flow domain of a long compound open channel with one floodplain. The simulated length is thirty times the depth and a periodic boundary condition was used in the streamwise direction. In this simulation the Reynolds number of the bulk flow is approximately42,000 and the free surface has been treated as fully deformable. The floodplain has a depth ratio of0.5 and the Froude number of the flow is 0.39. Details are reported of the time-varying floodplain flow and flow at the junction of the channel and the floodplain. The extent of a previously identified secondary flow at the internal corner that is responsible for increased bed stress on the floodplain has been quantified. Spectral analysis has also been applied to the vorticity in the flow domain to aid the identification of the flow features particular to flood plain geometries. In the centre of each part of the compound channel the total circulation is dominated by rolling structures generated at the channel bed with vorticity aligned across the span; at the sides of the channel coherent vorticity is predominantly vertically aligned. The mechanism for fluid transport to the floodplain is identified as primarily low wave-number vertically aligned structures above the change in depth generated by the internal surface with length scales less than the channel depth.
Year: 2010