Tracer particle dispersion around elementary flow patterns

被引:4
作者
Goudar, Manu, V [1 ]
Elsinga, Gerrit E. [1 ]
机构
[1] Delft Univ Technol, Lab Aero & Hydrodynam, Dept Mech Maritime & Mat Engn, NL-2628 CD Delft, Netherlands
关键词
topological fluid dynamics; turbulence theory; turbulent mixing; NUMERICAL SIMULATIONS; 2-PARTICLE DISPERSION; INTENSE VORTICITY; TURBULENCE; DIFFUSION; MOTIONS;
D O I
10.1017/jfm.2018.146
中图分类号
O3 [力学];
学科分类号
08 ; 0801 ;
摘要
The motion of tracer particles is kinematically simulated around three elementary flow patterns; a Burgers vortex, a shear-layer structure with coincident vortices and a node-saddle topology. These patterns are representative for their broader class of coherent structures in turbulence. Therefore, examining the dispersion in these elementary structures can improve the general understanding of turbulent dispersion at short time scales. The shear-layer structure and the node-saddle topology exhibit similar pair dispersion statistics compared to the actual turbulent flow for times up to 3-10 tau(eta), where, tau(eta) is the Kolmogorov time scale. However, oscillations are observed for the pair dispersion in the Burgers vortex. Furthermore, all three structures exhibit Batchelor's scaling. Richardson's scaling was observed for initial particle pair separations tau(eta) <= 4(eta) for the shear-layer topology and the node-saddle topology and was related to the formation of the particle sheets. Moreover, the material line orientation statistics for the shear-layer and node-saddle topology are similar to the actual turbulent flow statistics, up to at least 4 tau(eta) . However, only the shear-layer structure can explain the non-perpendicular preferential alignment between the material lines and the direction of the most compressive strain, as observed in actual turbulence. This behaviour is due to shear-layer vorticity, which rotates the particle sheet generated by straining motions and causes the particles to spread in the direction of compressive strain also. The material line statistics in the Burgers vortex clearly differ, due to the presence of two compressive principal straining directions as opposed to two stretching directions in the shear-layer structure and the node-saddle topology. The tetrad dispersion statistics for the shear-layer structure qualitatively capture the behaviour of the shape parameters as observed in actual turbulence. In particular, it shows the initial development towards planar shapes followed by a return to more volumetric tetrads at approximately 10 tau(eta), which is associated with the particles approaching the vortices inside the shear layer. However, a large deviation is observed in such behaviour in the node-saddle topology and the Burgers vortex. It is concluded that the results for the Burgers vortex deviated the most from actual turbulence and the node-saddle topology dispersion exhibits some similarities, but does not capture the geometrical features associated with material lines and tetrad dispersion. Finally, the dispersion around the shear-layer structure shows many quantitative (until 2-4 tau(eta)) and qualitative (until 20 tau(eta)) similarities to the actual turbulence.
引用
收藏
页码:872 / 897
页数:26
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