二维环空管道后台阶突扩流动特性数值模拟分析

Numerical investigation of two-dimensional flow over a backward-facing step of an annular tube

环空管道后台阶突扩流动是空气正循环钻井过程中十分重要的关键部分,直接决定了钻探岩屑是否能够顺利上返地面.该模型中对再附着过程的演变进行了大涡模拟(LES).指出在层流状态下主回流区长度随雷诺数Re的增加而增加;过渡流状态时出现内壁二次回流区,角部二次回流区和外壁三次回流区;湍流状态时,随着角部二次回流和外壁三次回流的消失,外管内壁和内管外壁处出现大尺度涡;得出了台阶上游和下游较远处流场层流时为抛物线分布,湍流时近似为对数分布.在此基础上进一步研究了湍流情况下流场中大尺度涡结构的瞬时发展和演变过程,以期实现对湍流的有效控制,并为进一步研究气体钻井环空管道内颗粒和大涡的相互作用规律奠定基础.

The evolution of the reattachment process in annular backward-facing step flow was simulated by large-eddy simulation（LES）.It is shown that there appear successively the states of laminar flow,transitional flow and turbulent flow with the increase of Reynolds number.It is pointed out that the length of the main recirculation region increases with Reynolds number in the laminar flow state.The secondary reverse flow region is found at the inner wall of the outer pipe at Re704.15 in the transitional flow state.Moreover,the small secondary reverse flow region at the corner of the channel and the third reverse flow region at the outer wall of the inner pipe are found at Re1 005.92.Some large-scale eddies appear at the inner wall of the outer pipe and the outer wall of the inner pipe with the disappearance of the two reverse flow regions mentioned above.It is reported that the velocity distribution at the upstream and downstream of backward-facing step is parabolic in laminar flow state and logarithmic in turbulent flow state respectively.Based on these the large eddy coherent structures evolving spatially and temporally under the turbulent flow with high Reynolds number were visualized.The instantaneous reattachment process of the large eddy coherent structure was discussed in detail at turbulent flow state.The present research paves the way for further study of the interaction among particles and the eddy in annular pipe of air drilling.