Enhancements to the Sediment & Scour Model
This development note written by George Wei, Ph.D., Senior Developer, highlights modifications of the sediment scour model to be included in the 2014 release of FLOW-3D.
The sediment scour model was first introduced in FLOW-3D v8.0 to predict the erosion and deposition of sediment (Brethour, 2003). The model works by emulating both the entrainment of sediment at the packed bed interface and the drifting and deposition of sediment due to gravity. Version 9.4 saw the inclusion of a bed-load transport model that accounted for the rolling motion of sediment particles over the surface of the packed bed (Brethour and Burnham, 2010). Multiple sediment species and non-linear drifting of the suspended phase were also introduced.
Although the model is comprehensive and has been successfully used by the water engineering community, a major remaining factor limiting its applicability is an approximate treatment of the interface between the packed and suspended sediment. This makes an accurate calculation of the turbulent shear stress at the packed bed surface, which is the main mechanism of sediment entrainment, quite challenging.
In the present work, the sediment scour model is modified so that the packed sediment is treated by FAVOR™ (Fractional Area Volume Obstacle Representation) which is the standard treatment for solids in FLOW-3D. To account for changes in the packed sediment, volume and area fractions are recalculated at each time step. Turbulent shear stress on the packed sediment surface is evaluated using the standard wall function for turbulent flow, which now includes the surface roughness. The coding for entrainment, deposition and bed-load transport have been rewritten to improve computational accuracy of surface properties (such as surface normal and slope directions) and transport rates for suspended- and bed-loads.
Model Validation and Application
Figure 1. Schematic of experimental setup (Chatterjee, et al., 1994)
The model was tested to predict scour caused by a submerged horizontal jet. The simulation result was compared with the experimental measurement of Chatterjee et al. (1994). Figure 1 shows the experimental flume setup. Water enters the flume through a narrow slit under pressure. The resulting two-dimensional sheet of water flows over a solid apron before contacting a packed bed. The sediment grains are sand with 0.76 mm diameter. Figure 2 shows the calculated scour profile evolution. Table 2 compares the calculated and measured maximum scour depth in the scour hole. A good match between the two results was obtained: the error range is 0 to 7.1%, and the averaged error is 3.7%.
Figure 2. Calculated scour profile and fluid velocity distribution
Table 1. Comparison of the calculated and measure maximum scour depth
Animations 1 and 2 show a 3D simulation of scouring downstream of a 10 m high weir. The packed sediment is composed of three uniformly distributed sediment species, each of sizes 5.0, 13.8 and 21.0 mm diameter and specific gravity of 2.65. The simulation time is 1.0 min. It can be seen that the location and shape of the scour hole are reasonable.
Animation 1. Velocity distribution of weir flow
Animation 2. Scour hole due to weir flow
The sediment scour model in FLOW-3D is modified so that the packed sediment geometry is treated with FAVOR™. Calculations of packed bed surface properties, turbulent shear stress on packed bed surface and sediment transport rate are modified using more accurate methods. These model enhancements will help users to simulate scour process more accurately.