Three-dimentional Modeling Of Scour Around Underwater Pipes

According to the United Nations,half of the world’s population lives within 60 km of the sea and three-quarters of all-large cities are located on the coast.Drinking water production,collection of domestic and industrial effluent,as well as increased demand for energy will lead to increased pressure on marine ecosystems,as much of the produced effluent will be discharged in the ocean.In this regard,as an essential part of sustainable and environmentally sensitive wastewater management strategy,submarine outfalls have been widely used in coastal regions.When pipelines are laid on the seabed,not only the location and influence of the mixing zone but also the function of the waterbody and the natural ecological balance need to be considered.In addition,after the pipelines are laid on the seabed,the local flow field is changed.Sediment erosion around pipelines can lead to pipe suspension.Combined with the flow-induced vibration subject to currents and waves,it may result in pipeline failures.Therefore,the research on local scour around offshore pipelines has important economic and social significance.A numerical model has been developed for predicting the flow and local scour around offshore structures.The commercial software FLUENT is used as the flow solver,which is based on the unsteady Reynolds-averaged Navier-Stokes equations with standard turbulence closure.The bedload sediment transport equations are then solved using the finite difference method through User-Defined-Functions(UDFs).The change in the bed level is calculated from the continuity equation of sediment transport.A dynamic re-meshing algorithm is used to update the bed positions during the simulation.An efficient time-marching algorithm is adopted to speed up the computation.Using the newly-developed model,this paper simulates the bed changes adjacent to a pipeline with different undisturbed Shields parameters,which focuses on the three-dimensional flow pattern.The development characteristic of the scour hole is then summarized.At the same time,the well-validated numerical model is further applied under the situation that an outlet velocity is set at the end of the pipe.The influence of the outflow of the underwater sewage pipelines on flow field and bed morphology is studied.