不同雷诺数下隔水管绕流模型仿真

Simulated research for the flow around the model of a riser under different Reynolds numbers

为了研究深海钻井过程中不同海况下海流对隔水管振动情况的影响,利用COMSOL软件,建立了圆柱绕流二维仿真模型,对刚性圆柱展开绕流仿真,研究了不同雷诺数(2.5×104<Re<4.0×105)下隔水管绕流的漩涡脱落形态、升力系数和阻力系数、涡泄频率,斯特哈尔数(Strouhal)的变化规律。结果表明,随雷诺数增大,升、阻力系数减小,涡泄频率增大且近似呈线性关系,斯特哈尔数(Strouhal)在亚临界流区域及亚临界流向临界流过渡区域内基本不变。仿真结果与相关文献及经验值吻合良好,验证了仿真方法对隔水管振动研究的适用性。

The present paper is inclined to make an exploration of the changing regularity of the various related parameters of the riser under the different Reynolds numbers（2.5 × 10^4 〈Re〈 4.0 ×10^5） in the case of deep-level drilling under-water conditions.For the research purpose,we have established a 2-D simulation model of the liquid flowing directions around the cylinder in the flow field based on a software named COMSOL,with the riser diameter of 50 D × 20 D.What is more,we have also done the experiment for the flow liquid around the rigid cylinder,with the whole model meshed by a free subdivision triangular grid.At the same time,we have also built up the boundary layer properties and the mesh refinement for the interface between the fluid and the riser.Actually,it is of great necessity to identify and determine the vortex shedding pattern change tendency,the lift and the drag coefficients,the vortex shedding frequency and the Strouhal number of the riser flow under the different Reynolds numbers,for they should have to account for lots of accidents,such as the riser fracture and its collapse,in which the natural frequency of the riser may overlap the vortex shedding frequency,as is shown in the changes of the above parameters in the paper.The results of the experiments shown above demonstrate that：（1） Whenever the fluid is close to the leading edge of the riser,there would appear a high pressure area in the first half of the riser whereas the negative pressure area appearing in the backside and the pressure difference may lead to the separation at the maximum width of the riser section in the boundary layer,and the vortex is generated and released alternately on the both sides of the rear which may lead to ＂Carmen vortex＂phenomenon;（2） With the increase of the Reynolds number,the lift and the drag coefficients are likely to get reduced,and the drag coefficient cycle may turn to be reduced by about 1/2 of the lift coefficient.At the same time,the drag coefficient of the pulsate force amplitude will be much smaller than the lift coefficient,so as to make the drag effect far less than the lift;（3） With the increase of the Reynolds number,the vortex shedding frequency would grow to a position approximately linear to the Reynolds number,whereas the Strouhal number may remain essentially constant in the subcritical flow region with the flow region changing from the subcritical flow to the critical one.Thus,the simulation results can be found well in accord with the relevant literatures and empirical values,which can prove the applicability of the simulation method to the reduction of the riser vibration.From what is said above,it can be concluded that the instructive significance for the safety deep-water drilling riser analysis and the off-shore engineering structures design.In addition,the results of our study can also help to change the natural frequency of the riser rationally in getting rid of the ＂riser resonance＂.