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Dynamics and geometry of developing planar jets based on the invariants of the velocity gradient tensor

Journal of Hydrodynamics 2015年第6期27卷 894-906页

作者：吴楠楠 SAKAI Yasuhiko NAGATA Kouji ITO Yasumasa School of Electric Power Engineering China University of Mining and Technology Department of Mechanical Science and Engineering Nagoya University

Based on the direct numerical simulation （DNS）, the developing planar jets under different initial conditions, e.g., the con- ditions of the exit Reynolds number and the exit mean velocity profile, are investigated. We mainly focus on the characteristics of the invariants of the velocity gradient tensor, which provides insights into the evolution of the dynamics and the geometry of the planar jets along with the flow transition. The results show that the initial flow near the jet exit is strongly predominated by the dissipation over the enstrophy, the flow transition is accompanied by a severe rotation and straining of the flow elements, where the vortex structure evolves faster than the fluid element deformation, in the fully-developed state, the irrotational dissipation is dominant and the most probable geometry of the fluid elements should remain between the biaxial stretching and the axisymmetric stretching. In addition, with a small exit Re and a parabolic profile for the exit mean streamwise velocity, the decay of the mean flow field and the magnitude of the turbulent variables will be strengthened in the process of the flow transition, however, a large exit Re will promote the flow transition to the fully-developed state. The cross-impact between the exit Re and the exit mean velocity profile is also observed in the present study.

关键词： planar jet direct numerical simulation (DNS) velocity gradient tensor flow transition joint probability density function(joint pdf)

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flow Instability of Molten GaAs in the Czochralski Configuration

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Journal of Materials Science & Technology 2007年第3期23卷 395-401页

作者： Shuxian CHEN Mingwei LI Institute of Power Engineering Chongqing University Chongqing 400030 China Aviation Engineering Institute Civil Aviation Flight University of China Guanghan 618307 China

The flow and heat transfer of molten GaAs under the interaction of buoyancy, Marangoni and crystal rotation in the Czochralski configuration are numerically studied by using a time-dependent and three-dimensional turbulent flow model for the first time. The transition from axisymmetric flow to non-axisymmetric flow and then returning to axisymmetric flow again with increasing centrifugal and coriolis forces by increasing the crystal rotation rate was numerically observed. The origin of the transition to non-axisymmetric flow has been proved to be baroclinic instability. Several important characteristics of baroclinic instability in the CZ GaAs melt have been predicted. These characteristics are found to be in agreement with experimental observations.

关键词： flow transition Baroclinic instability Thermal wave Characteristics GaAs melt

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Physics of multiple level hairpin vortex structures in turbulence

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Science China(Physics,Mechanics & Astronomy) 2016年第2期59卷 91-101页

作者： Yi Qian Wang Hassan Al-Dujaly Yong Hua Yan Ning Zhao Chao Qun Liu Department of Aerodynamics Nanjing University of Aeronautics and Astronautics Nanjing 210016 China Department of Mathematics University of Texas at Arlington Arlington 76019 USA

Previous experimental and numerical studies have revealed that the hairpin vortex is a basic flow element of transitional boundary layer. The hairpin vortex is believed to have legs, necks and a ring head. Based on our DNS study, the legs and the ring head are generated separately by different mechanisms. The legs function like an engine to generate low speed zones by rotation, create shear layers with surrounding high speed neighbor fluids, and further cause vortex ring formation through shear layer instability. In addition, the ring head is Ω-shaped and separated from quasi-streamwise legs from the beginning. Contrary to the classical concept of ＂vortex breakdown＂, we believe transition from laminar flow to turbulence is a ＂buildup＂ process of multiple level vortical structures. The vortex tings of first level hairpins are mostly responsible for positive spikes, which cause new vorticity rollup, second level vortex leg formation and finally smaller second level vortex ring generation. The third and lower level vortices are generated following the same mechanism. In this paper, the physical process from A-vortex to mul- ti-level hairpin vortices is described in detail.

关键词： hairpin vortex vortex buildup multiple level vortices flow transition

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