Numerical simulation of scour and flow field around side by side piers using SSIIM

Scour around pier in the flow is an Inevitable issue. Estimation of scour depth and understanding the flow field around pier would help us to design with safer factor. The most important factor of scour around pier is changing of streamelines that leads to a system so called local scour. It consist of two vortices: horseshoe vortices and wake vortex. The obstacle creates downflow jet in front of pier that collide with bed sediments and carry them to the downstream, making the horseshoe vortice. The wake vortex is caused by splitting the streamelines and formation of low pressure flow field region and absorption of flow in rear of pier and pick up the bsd sdiments in this district.
In this study we used the numerical model SSIIM as a CFD model to simulate flow and scour pattern Simultaneously around a group piers. This model can be used in hydraulic and environment engineering and has the ability of sediment transport calculation in bed transient movement with temporal dependent as the most important advantage in compare with the other CFD models. The verification of this model was implemented by data and results reported for side by side piers examinations as one ofe the group categorize. In this model we considered the k-ε and k-ω seperately as a turbulence model to solve the eddy viscousity of 3D Navier-Stokes flow equations and use their outputs as inputs of sediment transition equations, we used Power-Law scheme as one of the descritization method of First-Order upstreame scheme to solve the flow and sediment equations on the grids. The pressure term of Navier-Stokes equations in cells was calculated by SIMPLE algorithm which is the First-Order upstreame scheme too. Also by changing the G⁄D distant ratio on the other simulation runs, we generated the diagrams with comparative situation with experimental diagrams.
Results in the last time of simulation showed there is much more value of horizontal and vertical velocity between the piers than the other sides. It was 57% of final maximum scour depth in first hour of calculation. Similarly to velocity, The final scour patterns showed there is more scour depth counters between the piers. In details it was deriven that the scour depth pattern was symmetric in early time of calculation, but with time passing it appeared more in the region of between the piers. Although Numerical results show the SSIIM model have calculated the erosion depth in front of piers with high accuracy resulted from good calculation of downflow, comparisons between model results and data show the scour depth pattern that the model calculated the wake vortices behind the piers and Interference the horseshoe vortex between the piers with overestimate value and there are deeper countors of scour depth than experiment diagram. Also the RMS index of scour depth has been calculated in the grid and it represented the values of 0.0353 for k-ε model and 0.0899 for k-ω model. Therefore, the k-ε turbulence model resulted better scour depth pattern calculated in compare with k-ω turbulece model.