Study on the Subgrid Scale Modeling for Particle-Laden Turbulent Flows

为了在含颗粒湍流大涡模拟（LES）中考虑亚网格尺度（SGS）模型，对流固两相湍流流动进行了直接数值模拟（DNS）.首先，发展了一种有限差分格式，以解决流体湍流和颗粒运动之间的相互作用.在我们的方法中，绕长度比计算网格间距大几倍的固体颗粒的流动，直接模拟，颗粒的运动由压力、粘性应力和重力等体积力的表面积分控制，该格式不包括流体湍流模型，本文首先介绍了一种新的计算方法，该方法能有效地计算流体中颗粒的运动和相互作用，然后将其应用于三维绕流问题，计算结果与实验结果相吻合。对于阻力系数和非定常旋涡脱落，数值计算结果与实验结果吻合较好。当旋涡脱落时，湍流能量的增加会导致旋涡脱落，从而引起湍流能量的增加。在此基础上，提出了一种新的计算方法，该方法能有效地计算流体中颗粒的运动和相互作用 关于我们 通过尾流的非平衡性，在颗粒附近区域，颗粒的阻力变化后，这种非平衡性增强。接下来，我们模拟了含球形颗粒的平面通道中的受迫各向同性湍流和湍流流动。颗粒形成更大的结构，如团簇，导致流体湍流的能量源的额外尺度。在另一个装置中，固定颗粒的相对位置，观察无颗粒团的湍流调制.在这种情况下，总能量有时会由于能谱向高波数区移动而降低，即使颗粒诱导了额外的能量.在壁湍流中，颗粒在通道中是不均匀分布的.在这种情况下，这些含颗粒湍流流场的DNS结果揭示了SGS模拟中需要考虑的一些重要特征，即非定常旋涡脱落的非平衡性，流体湍流能量注入的多尺度性，例如，颗粒分布对湍流长度尺度的调制，以及由于脱落涡的拉伸而产生的雷诺应力。少

We carried out a direct numerical simulation (DNS) of fluid-solid two-phase turbulent flows to consider the subgrid scale (SGS) modeling in the large-eddy simulation (LES) of particle-laden turbulence.First, we developed a finite-difference scheme to resolve the interaction between fluid turbulence and particle motion.In our method, the flow around a solid particle having a length scale several times larger than the spacing of computational grid, is directly simulated.The particle motion is governed by the surface integral of pressure and viscous stress together with the body force such as gravity.This scheme includes no models in fluid turbulence, particle motion or interaction between them.Then, we examined our new method by applying it to the three-dimensional flow past a sphere fixed in a uniform stream.As for the drag coefficient as well as the unsteady vortex shedding, numerical results agreed with experimental results.When the vortex shedding took place, turbulence energy induce … More d through wake was not fully dissipated in the region near a particle.This non-equilibrium nature was enhanced just after the change in drag force of a particle.Next, we simulated a forced isotropic turbulence and turbulent flow in a plane channel including sphere particles.Particles formed larger structure such as clusters, resulting in an additional scale of energy source for fluid turbulence.In an another setup, the relative position of particles was fixed to observe the turbulence modulation without particle clusters.In this case, total energy was sometimes reduced due to the shift of energy spectra into high wavenumber range, even though particles induced extra.In the wall turbulence, particles were distributed non-uniformly in the channel.In this case, the shed vortices was stretched into the streamwise direction due to the velocity gradient, resulted in the production of Reynolds shear stress.These DNS results of particle-laden turbulent flows suggested some important characteristics to be accounted for in SGS modeling.They are non-equilibrium nature due to unsteady vortex shedding, multiple scale of energy injection to fluid turbulence, the modulation in turbulence length scale by particle distribution, and the Reynolds stress production due to the stretch of shed vortices, for example. Less