Study On Mechanics And Numerical Simulation Of Flow And Local Scour Around Hydraulic Structures

Local scour is a common occurrence around hydraulic structures like spur-dikes and bridge foundations. This affair is a main reason of failure of bank protections and bridges. The flow structure around structures is in strength three-dimensional character and the formed vortex is the chief attribution of local scour. In this dissertation, the flow field and the equilibrium scour hole are simulated with numerical model. And the stability of the riprap at foundations is studied. The main achievements are as follows:A non-equilibrium k-εturbulent model is introduced to flow model to improve the simulating precision of the circumfluence. The flows around an abutment in a flat bed and scoured bed are simulated respectively. The computed results are in good agreement with the experimental data; and the whirl flow velocity field distribution and development of vortex around the abutment are well reflected.The flow around an experimental cylindrical pier in the measured developing scour beds is simulated and the mechanism of local scour is explored simply. The simulated results show that the flow becomes almost uniform in the vertical direction near the nose of the cylinder, and the turbulence is fierce in this region. When the scour hole occurs, the horseshoe vortex is formed at the scoured bed. The horseshoe vortex is a forced vortex type of flow, as the swirl velocity increases in the outward direction from the center of the vortex. Along with the increase of the scour hole and the horseshoe vortex falling into the scour hole, a new turbulent core appears in the scour hole and the turbulence disperses into the scour hole under the transport of the horseshoe vortex. The turbulent energy exhausts in the scour hole and the scouring power of the flow becomes weaker and weaker, until the scour ceases.A new method is promoted to correct the sediment transport rate on slopes, and a sediment transport model is developed. The new modification method can consider both the quantitative and directional influence of gravity on the sediment transport at the same time. The result shows good performance than former correction methods when compared with experimental data. The new method has an advantage that it can avoid the occurrence of infinite value in numerical computation. In the sediment transport model, the bed load transport rate is interpolated separated in the x and y directions rather than interpolated as an integer scalar. This change can improve the interpolation precision. The model is applied in simulating of local scour around an abutment and spur-dike. The computed scour holes are in good agreement with the experimental results. Analysis of the scour hole shows the tail vortex has important effect on the scour-hole shape downstream of the structures.The Rankine vortex model is used to simplify the flow with vertical vortex. The adsorption affinity of the vortex towards the rocks around a spur dike, The resulting stress on the rocks was used to derive the critical incipient velocity formula for rocks. The critical incipient velocity of the rock is lower than that in the classical formula, which will lead to the rock being rolled away in slow-speed flow. According to the vortex dimension and strength around the experimental spur-dike, a new critical incipient velocity formula is obtained. In this formula, the sediment diameter, length of spur-dike, flow depth and restriction ratio of the flow width are considered. This research has consulting value for the realistic projects.