3-D Flow And Local Scour Around A Spur Dike With Bivariate Normal Surface

Spur dike is an important hydraulic structure which is widely used in channel improvement and maintenance engineering. It can narrow stream to wash riverbed and protect the river bank from being scoured by weakening flow velocity. But the flow around a spur dike has strong 3-D turbulent characteristics. It leads to the development of a local scour around the groyne inevitably, which may cause damage of the whole works.To avoid these, a new type of spur dike with bivariate normal surface is proposed to optimize the flow structure and reduce the local scour at the groyne head. Matlab and Pro/E are used to build three-dimensional model of the new spur dike, and a series of numerical simulations have been conducted in order to study the hydrodynamic characteristics and local scour of new structure.The simulations calculate the flow fields around a trapezoid spur dike and this new spur dike. The velocity, turbulent energy, vorticity and bed shear stress are analyzed. The results show that the flow parameters like velocity, turbulent energy around the new spur dike are all less than which around trapezoid groyne because of its streamline-shaped surface. The maximal velocity around the new type reduces about 12 percent from the trapezoid spur dike, the turbulence, dissipation rate and vorticity are just 1/5,1/3,3/5 of the trapezoid one.The moving mesh model in Fluent and the sediment scour model in Flow-3d are used to get the maximal scour depth of two different spur dikes. With the same flux, the maximal scour depth of the new type reduces about 20 percent from the trapezoid spur dike. The area of scour pit around new spur dike is also less than the other one. The results tell, the new design with bivariate normal surface can smooth the flow, reduce severe turbulence, and also avoid the current washing river bed directly, so it reduces the local scour obviously and must be valuable in estuary improvement.The flow regime in the scour hole is analyzed by using the data from Fluent. When the scour hole being stable, flow around spur dike becomes more complicated, with four main flow conditions existing. And some 3-D vortexes with uniform size are produced in scour pit because of the obstructing of wall and spur dike. The turbulence and vorticity around new spur dike is also less than the other one.