A DANS model for flows in gabion channels

Chi-Wai Li, JiaJia Li

Tuesday 30 june 2015

9:30 - 9:45h at Europe 1 & 2 (level 0)

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

Parallel session: 4E. Engineering - Computational

Gabion mattresses and gabion walls are sometimes used to stabilize conveyance channels to prevent erosion. The surface roughness of a gabion channel is large and the vertical profile of the longitudinal velocity will deviate from the logarithmic profile. Also in the layer consists of gravels and water the velocity variation is large due to the blockage effect of stones. In the modeling of flow over this layer using the Reynolds Stress Navier Stokes equation (RANS) approach the governing equations need to be spatially averaged. This process generates the dispersive stress which is generally neglected in the models. In the present work a Double Averaged Navier Stokes equation (DANS) model is proposed to simulate flows in gabion channels. Turbulence is modelled by the Spalart-Allmaras closure with a modified length scale which is dependent on a zero-plane-displacement parameter relating to the stone characteristics. The dispersive stress is modelled by a sub-length scale closure, similar to the Smagorinsky model used in Large Eddy Simulation. The model has been applied to several cases of flow over gravels. The velocity and Reynolds stress profiles are predicted correctly. The dispersive stress peaks at the correct level and with proper magnitude through calibration of the Smagorinsky coefficient. The effect of dispersive stress on the velocity profile and Reynolds stress profile generally is small. This is mainly because the peak dispersive stress is approximately one order of magnitude smaller than the peak Reynolds stress. The DANS model however is useful if the turbulent mixing phenomenon in the gravel layer is of interest. The dispersive stress can be comparable to the Reynolds stress within this region. The vertical distribution of porosity in the stone layer is also investigated and the effect is minor. The model is a concluded to be a suitable tool for the study of the flow and solute dispersion in gabion channels, especially in the region close to the gravel layer.