非定常瞬态流动过程中的Lagrangian拟序结构与物质输运作用

摘要

Unsteady transient phenomena in flow over impulsively started circular cylinder, such as the generation of separation, burst of separation bubble, vortex shedding, etc., are studied from Lagrangian viewpoint. The transient flow is solved numerically by using characteristic-based split scheme with dual time stepping. Then Lagrangian coherent structures (LCSs) are extracted to study the transport and mixing of these transient phenomena. Results show that the variation of drag is closely related to the evolutions of separation bubbles and vortex shedding. The evolutions of the symmetric bubbles in streamwise induce high pressure distribution at rear of cylinder and result in drag reduction of the circular cylinder. As separation bubbles become asymmetric, the transport between separation bubbles and main flow is enhanced and thus can reduce the separation region and suppress flow separation as well. The results also show that the shedding vortices are induced by the transport between separation bubble and main flow. Compared with streamline patterns, LCSs have huge advantages in describing the dynamic features of the unsteady phenomena.%从Lagrangian角度数值分析了圆柱瞬时起动过程中的非定常瞬态流动现象，如分离泡产生、破裂和涡脱落等及其产生的非定常效应，揭示了所列现象诱导的物质输运和迁移效应。首先采用双时间步长的特征线算子分裂算法数值模拟了圆柱起动过程中的瞬时流场，然后采用数值方法从流场中提取出Lagrangian拟序结构(LCSs)，并根据非线性动力学理论研究了流动分离和旋涡演化过程中的物质输运作用。结果表明，圆柱瞬时起动后所产生的非定常阻力与相应瞬态现象中的物质输运有密切的关系：对称分离泡产生及其在流向方向的生长，能够使分离泡内压力升高且分布均匀，从而减小阻力；对称分离泡的失稳增强了分离泡与主流之间的物质输运作用，最终导致涡的脱落，并有利于推迟流动分离和减小分离区域。非定常流动中LCSs所描述的物质输运和迁移作用对流动控制的机理研究具有一定指导意义。