Research On Vortex Shedding And Hydrodynamic Characteristics Of A Near-plane Circular Cylinder

When the pipeline is laid upon an erodible bed erosin occurs around the pipeline and the suspension of the pipeline take place.Votex shedding due to the scour process causes additional unsteady forces acting on the pipeline which may due to damages.In order to prevent these problems,the model of the flow around the circular cylinder laid on a horizontal plane boundary should be understood in detail.Steady flow around the circular cylinder in vicinity of a plane boundary is one of the crucial izzues in ocean engineering for its complexity and perplexity of multiple parameters with undefined effects.In the present study,the ocean outfall pipelines is chosen to be the researching object since its large scale size gives rise to the high Renolds number flow field,hence the wake sream and the hydrodynamics are more complicated.Base on the application in engineering,the vortex shedding suppression and the characteristics of the hydrodynammics have been investigated via three aspects:the momentum of the boundary layer of the circular cylinder,the three dimensional effects of the boundary layer and the energy of the boundary layer of the circular cylinder.The main objectives and the conclusions are summarized as follow:(1)The feature of the steady flow past around a circular cylinder in vicinity of a plane boundary was described and the characteristic parameters were listed.Reviewed the previous studies and summarized the development of this izzue,pointed out that the reason of perplexing the researchers is the multiple of the parameter of which the effects on the wake flow behind the cylinder cannot be confirmed.The fundamental method and turbulence model to resolving the high Renolds turbulent flow have been introduced,the studies on mesh independence and comparison of the turbulence modes have been carried out as well.For the lower Reynolds number(1.4×104),either the LES model and standard k-? model showed the good accuracy on calculating the force coefficients and St number?For the higher Reynolds number(1×105),standard k-? model performed well in advance of computational cost.(2)Three significant parameters of the flow around the near-plane cylinder which defined as the ratio of the boundary layer to the cylinder diameter,the ratio of the gap between the cylinder and the plane to the cylinder diameter and the Reynolds number were disscused in detail by analysis of the representative experiments,it has been found that these three parameters are not the control factors on affecting the vorticities shed from the surface of the circular cylinder.A new new point of view to explain the causes of vortex shedding suppression and the variation of hydrodynamic forces has been proposed.The new parameter,shape factor H was proposed on the basis of mechanism of vortex shedding and the von Karaman Momentum Integral.The shape factor presents the momentum contained by the boundary layer,the momentum decreases as the shape factor of the circular cylinder in steady flow increases,the moment that the boundary layer cannot maintain the adequate momentumto support the vorticities shed from the surface of the circular cylinder,the vortex shedding suppresses.(3)The three dimensional effects of the high Reynolds number turbulent flow is significantly strong,the wall of the computational domain may affects on the weak flow of the circular cylinder in case there is a improper scale of computational domain.The spanwise of the computational domain was studied and it has been suggested that the effects from the wall can be neglected reasonably in the computatinal domain of 25D width.The numerical simulation of the circular cylinder in vicinity of a plane boundary in oblique flow has been carried out to extend the effect of boundary layer shape factor from 2-dimensional to 3-dimensional.A well agreed explaination on vortex shedding suppression and the variation of the hydrodynamic forces was obtained via the perspective of momentum in boundary layer of cylinder.The independence principle has also been investigated at large degree(above 45°),the error of the principle can be explained by the boundary layer momentum.(4)Base on the different temperture of the current and the designing temperature specified by executive standard of the pipeline in actual engineering requirements,a numerical simulation of thermal flow around the circular cylinder near the plane has been carried out to investigate the energy exchange between the fluid and the structure,it is reported in the present study that the fluid flow includes the velocity profile of the flow field,vortex sheds and the stress on the cylinder is effected by thermal boundary layer since the energy changes in convection.The circular cylinder boundary layer with low energy caused the abnormal tendency of changing of these items.