The dynamics of variable-density turbulence
The dynamics of variable-density turbulence
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变密度湍流动力学
DOI：
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发表时间：
1995
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通讯作者：
D. L. Sandoval
中科院分区：
文献类型：
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作者：
D. L. Sandoval
采用直接数值模拟方法研究了变密度湍流流体的动力学特性。流动是不可压缩的，使得声波与问题解耦，并意味着密度不是热力学变量。由于分子混合，密度发生变化。速度场一般是发散的。采用伪谱数值技术求解运动方程。使用128{sup 3}网格点的网格大小进行三维模拟。研究了两类问题：（1）各向同性变密度湍流的衰减，（2）浮力产生的湍流在流体中具有大的密度波动（这样的Boussinesq近似是无效的）。在各向同性，变密度湍流的情况下，整体的统计衰减行为，研究的情况下，是相对不受密度变化的存在时，初始密度和速度场是统计独立的。这种情况下的结果与以前的数值和实验室结果定量一致。在这种情况下，初始密度场具有双峰概率密度函数（pdf），其随时间向高斯分布演化。密度场的概率密度函数在整个演化过程中关于其平均值是对称的。然而，如果初始速度场和密度场是统计相关的，则衰减过程受密度涨落的影响很大。在这种情况下，密度的pdf在其演化的早期阶段关于其平均值变得不对称。有人认为，这些不对称的密度场的pdf是由于不同的夹带率，进入混合区，有利于高速流体。
The dynamics of variable-density turbulent fluids are studied by direct numerical simulation. The flow is incompressible so that acoustic waves are decoupled from the problem, and implying that density is not a thermodynamic variable. Changes in density occur due to molecular mixing. The velocity field is, in general, divergent. A pseudo-spectral numerical technique is used to solve the equations of motion. Three-dimensional simulations are performed using a grid size of 128{sup 3} grid points. Two types of problems are studied: (1) the decay of isotropic, variable-density turbulence, and (2) buoyancy-generated turbulence in a fluid with large density fluctuations (such that the Boussinesq approximation is not valid). In the case of isotropic, variable-density turbulence, the overall statistical decay behavior, for the cases studied, is relatively unaffected by the presence of density variations when the initial density and velocity fields are statistically independent. The results for this case are in quantitative agreement with previous numerical and laboratory results. In this case, the initial density field has a bimodal probability density function (pdf) which evolves in time towards a Gaussian distribution. The pdf of the density field is symmetric about its mean value throughout its evolution. If the initial velocity and density fields are statistically dependent, however, the decay process is significantly affected by the density fluctuations. For this case, the pdf of the density becomes asymmetric about its mean value during the early stages of its evolution. It is argued that these asymmetries in the pdf of the density field are due to different entrainment rates, into the mixing region, that favor the high speed fluid.