Three Dimensional Simulation of Vortex SheddingPhenomenon and Sediment Transportaround Obstacles subjected to flow, Using Flow3D

Bridges are known as one of the most important communicational hydraulic structures. This is particularly so important in bridge piers exposed to river currents.The flow contact with bridge piers causes erosion around them and would result in damage. Therefore, controlling the erosion around the piers is one of the most important objects in the bridge safety. Many different experimental models have been prepared so far to investigate the flow pattern and local scour around this structure. Regardless of the cost and time of the experiments, problems resulting from changes in scale are also available. Nowadays, the Hydrodynamic behavior of the fluid is more extensively investigated by using the more efficient codes of computational fluid dynamic (CFD). The Flow3D software is used for this regard. This study focuses on the simulation of flow around the different bridge piers and studied the flow pattern and bed erosion around them. In this regard, the assumption of incompressible fluid is used and the sharp free surface is modeled by Volume of Fluid (VOF). Considering the sensitivity of numerical models to the number of cells in the computational domain, verifying the number of cells has been done by comparing the velocity profiles. Three different turbulence models also are investigated, and the free surface profiles, flow pattern around the pier and the Strouhal number are compared. Finally, the flow simulation is done by LES turbulence model. For investigation the two key factors causing local scouring, flow separation and downward flow in upstream of the pier, th1 10 models is selected. The countermeasures that considered in this regard include berm, cross-section and middle slot. The results show that the effect of flow separation control in reducing the bed erosion is much more than controlling the downward flow. Also the results revealed that in most cases using one countermeasure may be acts more better than situations different countermeasures was applied. In using lenticular cross-section, the scouring depth weas decreased up to 58% and with using the combined round-nosed rectangular cross-section, berm, and slot, the scouring depth showed decreasing up to 50%. Also, it was concluded that controlling the flow separation from the pier in boundary layer and the vortex shedding affect the bed erosion much more than the downward flow in pier upstream.