Numerical Investigation On The Three-dimensional Local Scour Of Seabed Near A Submarine Pipeline

The marine environment in which submarine pipelines are located is generally very hard.Under the coupling of waves and currents,the sediment at the bottom of pipelines is susceptible to erosion and transport,causing the pipelines to suspend and damage.In this paper,a high-performance three-dimensional turbulent flow calculation program Cg LES?IBM based on the iterative immersion boundary method is used to directly numerically study the three-dimensional flow field around the near-wall pipe.Based on this,large-eddy numerical simulation is used to study three-dimensional local scour of seabed near a submarine pipeline and focus on bed load.To study the development process of the three-dimensional local scour of the sandy seabed around the pipeline,reappear the three-dimensional spiral flow field structure at the shoulder,and analyze the changes in the development speed,scouring depth,and shape of the scour hole with the initial gap ratio and different inflow angles.The gap ratio is 0 and0.2,and the inflow angle is 0°,15°,30°,and 45°.The main research results are as follows:1.Under the condition of cross flow,when the pipeline is placed on the bed,the development of the suspension can be divided into three stages according to the development speed: the rapid erosion phase,the slow erosion phase,and the stable erosion phase.Corresponding to the pipeline suspension development,the distribution curve of the Shields number in the scour hole gradually changed from the shape of‘middle high,low at both ends' to ‘middle low,high at both ends',and after the scour entered stability,the distribution curve of the distribution tends to be uniform throughout the entire span.The three stages under the condition of oblique incoming flow becomes less obvious as the angle increases.At 15 °,the rapid and slow development stages are still visible.2.When the incoming flow is orthogonal,the spiral incoming flow at the shoulder is the main reason for the development of the suspension.As the angle increases,the effect of the spiral incoming flow weakening and the effect of the tangential speed gradually increased.Among them,at 30 ° and 45 °,the three-dimensional spiral inflow across the shoulder disappears.3.In the conditions with a gap ratio of 0 and 0.2,the vortex shedding below the pipeline is suppressed,and the clockwise vortex falling off the pipeline will trigger a counterclockwise vortex at the downstream bed surface.The occurrence of this reverse vortex limits the development of scour hole.