激励小尺度模式在湍流圆管射流中的应用

APPLICATION OF STIMULATED SMALL SCALE MODEL IN TURBULENT ROUND JET FLOWS

采用非涡黏性的激励小尺度（Ｓｔｉｍｕｌａｔｅｄ Ｓｍａｌｌ Ｓｃａｌｅ）模式对空间发展的轴对称湍流圆 管射流进行了大涡模拟，以雷诺数为 １０ ０００的流动为例，考证了激励小尺度模式在自由剪切流 模拟中的可行性，描述了湍流强度、雷诺应力和湍流耗散量的变化，同时与标准的Ｓｍａｇｏｒｉｎｓｋｙ 涡黏性模式的计算结果进行了比较．数值结果显示，激励小尺度模式能够更为合理地描述湍流 的耗散特性和能量传输特性，从而较为准确地展示出空间发展射流中由于流动不稳定而出现的 旋涡产生、发展、破碎及合并等过程。

The large eddy simulation of the axisymmetric turbulent round jet flow is presented. The non--eddy viscosity stimulated small scale (SSS) model is adopted to simulate the spatially developing jet flow at a Reynolds number of 10 000. The round jet flow is governed by incompressible, unsteady Navier-Stokes equations in a cylindrical coordinate system and the projection method proposed by Chorin is applied to solve them. Third--order and fourth-order compact finite difference schemes are used to calculate the first and second derivatives in the convective and viscous terms respectively. The Poisson equation for t,he pressure is solved using the Gauss--Chebyshev transform in the streamwise direction and then solving the tridiagonal matrixes in the radial direction. The time integration is conducted by the third--order Runger-Kunter scheme. The convective boundary condition for velocity at the outlet is imposed to ensure less effect of noise on the upstream flow. A staggered grid arrangement is adopted, where the pressure and other scale variables are defined in the center of the cell while velocity components are defined on the surfaces. Uniform meshes are used in the streamwise and circumferential direction. The grid spacing in the radial direction is nonuniform with the grid points clustered near the jet orifice. Due to computational limit, the computational domain is taken equal to 25 orifice diameters and 15 orifice diameters in the streamwise and radial direction respectively. The grid system consists of 514 x 150 points in the streamwise, radial direction, respectively. The comparison between Smagorinsky's model and SSS model implies that the former model underestimates the turbulence intensity with Smagorinsky's constant of 0.l, while the results obtained by SSS model show a better agreement with the experiment. SSS model does not require the homogeneity and permits backscatter of energy from small to large scales. Furthermore, it not only captures the dissipative nature of turbulence hut also provides a good representation of instantaneous energy transfer between the large and small scales. Hence, SSS model is able to describe more exactly the generation, development and breaking of the vortex in round jet.