The multi-mode stretched spiral vortex in homogeneous isotropic turbulence
The multi-mode stretched spiral vortex in homogeneous isotropic turbulence
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DOI：
10.1017/s0022112007009251
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发表时间：
2008-01-25
影响因子：
3.7
通讯作者：
Fujisawa, Takeharu
中科院分区：
文献类型：
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作者：
Horiuti, Kiyosi;Fujisawa, Takeharu
利用直接数值模拟（DNS）数据识别了均匀各向同性湍流中的拉伸螺旋涡，并对其性质进行了研究。阐明了它的发生、生长和湮灭，并指出了它在湍流产生中的作用。除了前人研究的两种对称构型模式外，还发现了第三种不对称构型模式，即沿两个螺旋片和核心区域涡管的涡度排列。对称模式和非对称模式中的一种不是由单个漩涡片的传统卷起产生的，而是通过几个漩涡片之间的相互作用产生的。由两层板引起的滞止流通过与沿第三层板的涡的相互作用而收敛形成再循环流。这种循环流动使薄板绷紧并拉伸。涡旋管是通过轴向应变、压力的降低和循环区内旋流运动的加剧而形成的。由于管诱导的差动旋转和板自诱导的差动旋转，涡流板被管夹带，形成螺旋旋转。研究了三种模式之间的跃迁。初始构型是两种对称模态中的一种，但由于沿拉伸薄片在涡度方向发生重定向，它转变为另一种两种模态。在两种转换模式中，对称模式往往比非对称模式更持久。螺旋片的螺旋转动的收紧产生了小尺度的速度波动级联，并产生了强烈的间歇耗散场。为了精确地捕捉螺旋旋转，需要至少k(max)(eta) / bar近似于4.0的网格分辨率（k（max）是最大波数，(eta) / bar是平均Kolmogorov标度）。在较高雷诺数下，不稳定叶栅沿拉伸的涡片连续产生自相似螺旋涡。一个由长度范围很广的螺旋涡组成的团簇形成了一个能量级联。
The stretched spiral vortex is identified using direct numerical simulation (DNS) data for homogeneous isotropic turbulence and its properties are studied. Its genesis, growth and annihilation are elucidated, and its role in the generation of turbulence is shown. Aside from the two symmetric modes of configurations with regard to the vorticity alignment along two spiral sheets and the vortex tube in the core region studied in previous works, a third asymmetric mode is found. One of the two symmetric modes and the asymmetric mode are created not by a conventional rolling-up of a single vortex sheet but through the interaction among several sheets. The stagnation flow caused by the two sheets converges to form recirculating flow through its interaction with the vortex along the third sheet. This recirculating flow strains and stretches the sheets. The vortex tube is formed by axial straining, lowering of pressure and the intensification of the swirling motion in the recirculating region. As a result of the differential rotation induced by the tube and that self-induced by the sheet, the vortex sheets are entrained by the tube and form spiral turns. The transition between the three modes is examined. The initial configuration is in one of two symmetric modes, but it is transformed into another set of two modes due to the occurrence of reorientation in the vorticity direction along the stretched sheets. The symmetric mode tends to be more persistent than the asymmetric mode, among the two transformed modes. The tightening of the spiral turns of the spiral sheets produces a cascade of velocity fluctuations to smaller scales and generates a strongly intermittent dissipation field. To precisely capture the spiral turns, a grid resolution with at least k(max)(eta) over bar approximate to 4.0 (k(max) is the largest wavenumber, (eta) over bar is the averaged Kolmogorov scale) is required. At a higher Reynolds number, self-similar spiral vortices are successively produced by the instability cascade along the stretched vortex sheets. A cluster consisting of spiral vortices with an extensive range of length scales is formed and this cluster induces an energy cascade.