几何对称性处理对自由平面湍射流大涡模拟的影响

Influence of Symmetrical Assumption of Computational Domain on LES of a Free Planar Jet

研究了几何对称性处理自由平面湍射流大涡模拟的影响。以 Re数为 1130 0的平面不可压缩湍射流流动为例 ,采用 Chorin的分步投影法求解大尺度涡运动的 Navier- Stokes方程 ,小尺度涡采用标准 Smagorinsky亚格子模式模拟。初始条件采用平面射流无粘流动解 ,出流速度边界使用 Sommerfeld辐射开边界条件处理 ,计算域的横向外边界使用自由裹入边界条件 ,对计算域的横向对称中心平面分别采用对称性条件和直接求解两种方法。模拟结果显示 ,采用对称性条件处理 ,会抑制自由平面湍射流中拟序结构的生长 ,阻碍大尺度涡从中心平面的穿透 ,长时间的统计平均不能给出合理的湍流低阶矩的时均结果。相反 ,对中心平面进行直接求解的做法能真实再现自由平面射流中涡的合并与破碎过程 。

The influence of symmetrical assumption on the LES of an unsteady spatially developing free planar jet at a moderate Reynolds number, Re=11300, is investigated. The spatially filtered continuity and Navier-Stokes equations (N-S) in non-dimensional forms for the incompressible flow are solved by using a fractional-step method. Simulations start from the inviscid solution of the jet. Non-reflective Sommerfeld open boundary conditions are used at the outflow, while traction-free boundary conditions are implemented at the lateral boundaries. Two methods are adopted to treat the central symmetrical plane of the jet. In method I the symmetrical assumption is used, of which both the normal velocity component and the normal derivatives of all other variables are set equal to zero on the central plane. In method II, all the variables (u,v, p etc.) on the central plane is solved directly. However in method I, the natural growth of the coherent structures in the jet is inhibited. While all the coherent structures undergo the generation, grow-up , mutual coalescence and break-up naturally in method II. The time-average results of the jet flow resulting from the above two methods are also different obviously. The present work shows that although the symmetric assumption works well in some numerical simulations with time-averaged turbulence models, it should be more careful to adopt such a kind of prescription in the LES work of shear flows with symmetrical characteristics.