Three-dimensional Numerical Simulations On Flow Pasting A Stationary And Vibrating Cylinder Near A Wall With Different Attack Angles

Submarine pipeline is an important part of offshore oil and gas resources transportation system.When a submarine pipeline is laid on seabed,suspended spans may exist in many locations due to the current scour or seabed unevenness.And the vortex-induced vibrations of the free spans will further aggravate the fatigue damage of the pipeline.The current usually flows through the pipeline at different inclination angles owing to the changeable direction of the current,which makes the interaction between the seabed,pipeline and flow field more complex.In order to further explore the inherent mechanism of fluid-structure interaction in the suspension section of submarine pipeline under complex inflow conditions,this paper simplifies this kind of problem into the flow and vortex-induced vibration around a cylinder near a wall under different inflow inclination angles.The three-dimensional direct numerical simulation of flow pasting a stationary and vibrating cylinder near a wall under different gap ratios is carried out by using CgLES_IBM_X,an efficient three-dimensional turbulence calculation program based on immersion boundary method.The inclination angles are ranging from 0° to 60°.In the case of laminar boundary layer,three gap ratios are involved,namely 1.6,1.1 and 0.6.In the case of turbulent boundary layer,a fixed gap ratio of 0.8 is adopted.Research finds that:1.In the case of laminar boundary layer,the suppression of the vortex shedding from the lower surface of the cylinder is increasing with the decreasing gap ratio and there are three vortex shedding modes named as "2S","weak 2S" and "1S" respectively At a fixed gap ratio,the interaction between vortex shedding and roll-up of the wall boundary layer increases with the increasing inclination angle.Besides,at ?<60°,the lift and drag coefficients and the Strouhal number follow the Independent Principle(IP)well under all gap ratios.The amplitudes of the minimum mean pressure and maximum shear stress on the plane boundary decrease with the increasing gap ratio.At a same gap ratio,the distribution of mean pressure and shear stress on the downstream wall becomes more uneven with the increasing inclination angle by forming many irregular inclined strips.2.In the case of turbulent boundary layer,three distinct vortex shedding stages are observed sequentially in time,including the two-dimensional stage with only spanwise vortex tubes(Stage ?),the parallel vortex shedding stage(Stage ?)and the oblique vortex shedding stage(Stage ?)at a? 30°.However,only the former two stages are observed at a ?15°.The variation of lift and drag coefficients under the oblique vortex shedding mode violate the IP.However,IP is valid to predict the Strouhal number at a<60°.3.When the cylinder is freely vibrated in the case of turbulent boundary layer,there is only first mode both in streamwise and transverse directions.And the vibration trajectory is in oval-shape with clockwise rotation.Besides,IP is valid to predict the amplitude at a<450 and the vibration frequency at a ?60°.