3D Numerical Modeling of Near Bed Flow over an Artificial Gravel Bed

Vincenzo Sessa, Nils Rüther, Carlo Gualtieri

Tuesday 30 june 2015

8:30 - 8:45h at Europe 1 & 2 (level 0)

Themes: (T) Water engineering, (ST) Computational methods

Parallel session: 4E. Engineering - Computational

The understanding of flow structures over static armor layers is essential for the development of modern sediment transport models. Over a rough boundary, such as in a gravel bed channel, friction created by individual gravel bed particles or cluster of particles retards the flow velocity creating near-bed turbulence, which plays an important role in the dynamics of a river. Very recently, the use of an artificial gravel bed in river engineering studies has proved to be highly advantageous for the investigation of steady flow and for a more easy comparison with the results obtained elsewhere. The present paper investigated the possibility to simulate the flow over an artificial gravel bed. Numerical results were compared with the experimental data collected in an experimental study using Particle Image Velocimetry (PIV) to quantify the turbulent flow over a gravel bed. The commercial code STAR-CCM+ was chosen to simulate the flow over the gravel bed. The flow was calculated by solving the Reynolds-Averaged Navier-Stokes equations in combination with the standard k-_ model. The free surface was simulated by the Volume-of-fluid method. Five different discharges of the measurements were chosen to be simulated to see whether the numerical model can reproduce flow characteristics. The numerical results for the streamwise component of the velocity were in good agreement with the experimental data. In particular, the match was very good for the velocities above the roughness height. However, within the roughness height it was observed that with an increasing simulated discharge, the deviation of the simulated velocities increased. Here, the calculated velocities were overestimated by 4.0 % to 7.5 %. This error analysis indicates a dependency on the inaccuracy of grid duplication in the numerical model. Despite this deviation, the results of the numerical gave insight in the flow over gravel bed and a detailed comparison is presented.