Numerical simulation of flow over a rough bed

This paper presents results of a direct numerical simulation (DNS) of turbulent flow over the rough bed of an open channel. We consider a hexagonal arrangement of spheres on the channel bed. The depth of flow has been taken as four times the diameter of the spheres and the Reynolds number has been chosen so that the roughness Reynolds number is greater than 70, thus ensuring a fully rough flow. A parallel code based on finite difference, domain decomposition, and multigrid methods has been used for the DNS. Computed results are compared with available experimental data. We report the first- and second-order statistics, variation of lift/drag and exchange coefficients. Good agreement with experimental results is seen for the mean velocity, turbulence intensities, and Reynolds stress. Further, the DNS results provide accurate quantitative statistics for rough bed flow. Detailed analysis of the DNS data confirms the streaky nature of the flow near the effective bed and the existence of a hierarchy of vortices aligned with the streamwise direction, and supports the wall similarity hypothesis. The computed exchange coefficients indicate a large degree of mixing between the fluid trapped below the midplane of the roughness elements and that above it

Force and torque acting on particles in a transitionally rough open channel flow

Direct numerical simulation of open channel flow over a geometrically rough wall has been performed at a bulk Reynolds number of approximately 2900. The wall consisted of a layer of spheres in a square arrangement. Two cases have been considered. In the first case the spheres are small (with diameter equivalent to 10.7 wall units) and the limit of the hydraulically smooth flow regime is approached. In the second case the spheres are more than three times larger (49.3 wall units) and the flow is in the transitionally rough flow regime. Special emphasis is given on the characterisation of the force and torque acting on a particle due to the turbulent flow. It is found that in both cases the mean drag, lift and spanwise torque are to a large extent produced at the top region of the particle surface. The intensity of the particle force fluctuations is significantly larger in the large-sphere case, while the trend differs for the fluctuations of the individual components of the torque. A simplified model is used to show that the torque fluctuations might be explained by the spheres acting as a filter with respect to the size of the flow scales which can effectively generate torque fluctuations. Fluctuations of both force and torque are found to exhibit strongly non-Gaussian probability density functions with particularly long tails, an effect which is more pronounced in the small-sphere case. Some implications of the present results for sediment erosion are briefly discussed.Comment: accepted for publication in J. Fluid Mech. (2011

Spatially-averaged momentum fluxes and stresses in flows over mobile granular beds : a DNS-based study

Acknowledgements The authors gratefully acknowledge the Centre for Information Services and High Performance Computing (ZIH), Dresden, and the Jülich Supercomputing Centre (JSC) for providing computing time. The authors thank Markus Uhlmann and Clemens Chan-Braun for stimulating discussions on bed-load transport. Funding The present work was funded by the German Research Foundation (DFG) via the project [FR 1593/5-2] and was partly supported by the Engineering and Physical Sciences Research Council (EPSRC) UK, Grant [EP/G056404/1].Peer reviewedPublisher PD

Momentum balance in flows over mobile granular beds : application of double-averaging methodology to DNS data

The authors gratefully acknowledge the Centre for Information Services and High Performance Computing (ZIH), Dresden, and the Jülich Supercomputing Centre (JSC) for providing computing time. The authors thank Markus Uhlmann and Clemens Chan-Braun for stimulating discussions on bed-load transport.Peer reviewedPublisher PD

Development and application of a three dimensional numerical model for predicting pollutant and sediment transport using an Eulerian-Lagrangian marker particle technique

A computer coded Lagrangian marker particle in Eulerian finite difference cell solution to the three dimensional incompressible mass transport equation, Water Advective Particle in Cell Technique, WAPIC, was developed, verified against analytic solutions, and subsequently applied in the prediction of long term transport of a suspended sediment cloud resulting from an instantaneous dredge spoil release. Numerical results from WAPIC were verified against analytic solutions to the three dimensional incompressible mass transport equation for turbulent diffusion and advection of Gaussian dye releases in unbounded uniform and uniformly sheared uni-directional flow, and for steady-uniform plug channel flow. WAPIC was utilized to simulate an analytic solution for non-equilibrium sediment dropout from an initially vertically uniform particle distribution in one dimensional turbulent channel flow