圆柱绕流尾迹中涡量与温度标量的相平均分析

Three-dimensional vorticity and heat transport in a circular cylinder wake using phase-averaging method

实验研究了圆柱绕流尾迹中的三维涡拓扑结构特性以及湍流动量、热量输运特性。利用8支热线和4支冷线相结合的三维涡量探针分别测量从x/d=10~40的速度和温度脉动,采用相平均方法将圆柱绕流尾迹中的大尺度相干结构提取出来,剩余部分包括中等尺度相干结构和随机结构。结果发现：在x/d=10处ωz与ωx的等值线具有很强的相似性;随着流向的发展,ωx和ωy两分量受到更多来自两涡之间的流向＂肋＂结构的影响而呈现相似特征,证明了涡结构的三维特性。由于受到来自展向涡和流向肋结构的共同作用,导致展向热输运珦w＊珓θ＊在不同位置表现不一样,正涡中的珦w＊珓θ＊受到流向肋的拉伸作用而沿着分形线朝着鞍点偏移;而负涡内的珦w＊珓θ＊由于受到展向涡的卷起使得珦w＊珓θ＊和珘ω＊z形状相似。随着流向的发展,展向热输运移向涡边界处,对卡门涡内净热量传递到周围流体中有贡献。同时还发现相干结构对横向和展向热输运的影响很相似。

This work aims to provide insight into the three-dimensional aspects of momentum and heat transport in a turbulent cylinder wake.All three components of the velocity and vorticity vectors,along with the fluctuating temperatures,were simultaneously measured with an 8hot-wire vorticity probe and four cold-wire probes.Measurement was made at x/d=10~40at a Reynolds number of2540.A phase-averaging technique has been developed to detect reliably the coherent events,thus allowing the coherent contributions from various quantities associated with the vectors as well as passive scalar fields to be quantified accurately.The results show that there is a close similarity between the contour of spanwise vorticity and that of the streamwise vorticity at x/d=10,due to the strong rolling up effect of the spanwise vortex.As x/dincreases,the similarity betweenωxandωycontours can still be observed,resulting from the contributions of the rib-like structures.It exhibits significant threedimensional qualities of the vortex structure in the near wake.While the coherent heat flux is largely associated with the spanwise vortex rolls,the incoherent（or‘remainder＇）heat flux occurs between two successive opposite-signed vortices,rather than in the saddle region.Due to the combined effect of the spanwise vortex and the rib-like structure,the spanwise heat flux behaves quite differently at different locations.The珟w＊珓θ＊contour inside the negatively signed vortex is close to the vortex center,while the珟w＊珓θ＊inside the positive one is stretched to the saddle point aligning with the diverging separatrix.As x/dincreases,the negative珟w＊珓θ＊contour becomes dominant and moves towards the vortex border,which contributes to the net heat transport out of the spanwise vortex.In addition,coherent motion contributes similarly to the longitudinal and spanwise heat flux components.