【作者】 罗坤；

【导师】 樊建人； 岑可法；

【作者基本信息】 浙江大学 ， 工程热物理， 2005， 博士

【摘要】 众所周知,湍流问题是个世纪难题。经过众多科学家百余年来的艰辛探索,迄今为止这一问题尚未得到彻底解决。对于更加复杂和广泛存在的气固两相流动问题,人类势必需要付出更多的辛勤耕耘。传统上,对湍流和气固两相流的数值模拟研究主要采用的是基于雷诺平均的湍流模型和气固两相流模型。这样得到的数值解是一种近似的平均解,无法了解流动的瞬时特性,更无从深入探索气固两相流动的内在物理机制。另一方面,现代高性能计算机的出现和应用,为湍流和气固两相流的研究提供了一个新的思路—直接数值模拟(DNS)。它不引入任何湍流模型,而是通过数值求解完整的Naviver—Stokes方程组,能得到包括Kolmogorov微尺度脉动运动在内的所有湍流瞬时流动量在三维空间中的演变。从而为湍流和气固两相流的研究注入新的活力,带来新的发展机遇。 在自然界和实际的工程应用中,气固两相自由剪切流动比较典型。对它的研究既有助于对湍流的根本机理和气固两相间相互作用机理的理解,又可以为相关的工程应用提供指导,还可以为更加复杂的气固两相流的理论发展和实验研究提供参考。因此,开展对气固两相自由剪切流动的研究不仅具有重要的学术理论意义,也有广泛的工程应用价值。在这样的背景下,本学位论文集中开展对气固两相自由剪切流动进行直接数值模拟的研究。具体涉及到对三维气固两相平面混合层、二维气固两相平面射流和三维气固两相平面射流的研究。着重考察自由剪切流动中气相流体拟序结构的演化规律、不同Stokes数颗粒的扩散特性和扩散机理、以及颗粒对气相流动特性的影响所造成的湍流变动。 在对三维气固两相平面混合层的研究中,假定流动为时间发展模式的、不可压缩的流动,颗粒初始时刻仅布置在流场上半侧的高速区。流场控制方程的求解采用拟谱算法,颗粒的跟踪分别采用单向、双向耦合的拉格朗日方法。结果发现,初始时刻位于流场上半侧的颗粒向下半侧的扩散程度跟颗粒的Stokes数成反比。颗粒的Stokes数越小,向流场下半侧的扩散越多,意味着颗粒跟流体的混合越充分。流场大涡结构对Stokes数为1的量级的中等颗粒的影响最强,使得这种颗粒大量聚集在涡结构的外围,浓度分布最不均匀;而其它颗粒在流场中的分布都比较均匀。双向耦合的模拟中,不仅观测到跟以往研究结果相似的现象,而且有新的发现。以往的研究大都认为,小尺寸颗粒由于耗散会衰减湍流,大尺寸颗粒由于尾迹作用会增强湍流。而本部分的DNS结果则表明,在混合层大涡结构的卷起和配对过程中,Stokes数为5的同一种颗粒对气相湍流特性的影响是完全不同的。在大涡结构的卷起过程中,颗粒削弱了流体基波、亚偕波能量、总的湍动能和湍流强度等;而在大涡结构配对过程中,趋势则相反,颗粒都增强了这些量。颗粒的这种削弱和增强作用跟颗粒的质量携带率成正比关系。更多还原

【Abstract】 It is well-known that turbulence is a difficult problem over centuries. So far, this problem has not been thoroughly solved yet even if after more than one hundred years of hard exploring by lots of scientists. As to the gas-solid two-phase flow problems which are even more complicated and widely existent, Mankind has to pay more efforts. Traditionally, the numerical simulation studies of the turbulence and gas-solid two-phase flows are all based on the Reynolds-Averaged turbulence and gas-solid two-phase flow models. The numerical solution obtained by this method is an approximate mean result. It can not tell the instantaneous characteristics of the flows, and not to mention being used to deeply explore the inner physical mechanism in the gas-solid two-phase flows. On the other hand, the appearance and application of the modern high-performance computers offer a new way to study the turbulence and gas-solid two-phase flows—Direct Numerical Simulation(DNS). It doesn’t introduce any turbulence model, but numerically solve the full Navier-Stokes equations, and can obtain the evolutions of all the instantaneous flow variables including the Kolmogorov micro scale fluctuation in the three-dimensional space. Therefore, DNS adds the new energy and brings new developing opportunity to the studies of turbulence and gas-solid two-phase flows.In the nature and practical engineering applications, the gas-solid two-phase free shear flows are typical. Studies on them are helpful to understand the turbulence mechanism and the interaction mechanism between the gas and the solid particles, are directive to the associated engineering applications, and can provide references to the theory-developing and experiment-investigating of the more complicated gas-solid two-phase flows. So, it is not only of academic theoretical significance but also has the extensive engineering application value to carry out the study on the gas-solid two-phase free shear flows. Under these backgrounds, this dissertation focuses on the study of the DNS of the gas-solid two-phase free shear flows, including DNS of the three-dimensional gas-solid two-phase plane mixing layer, the two-dimensional gas-solid two-phase plane jet and the three-dimensional gas-solid two-phase plane jet. The main objective is to investigate the evolution of the gas-phase coherent structures, the dispersion characteristics and dispersion mechanism of the particles with different Stokes numbers, as well as the turbulence modulation by the effects of the particles on the gas-phase flow characteristics.In the DNS of the three-dimensionally gas-solid two-phase plane mixing layer, the flow is assumed to be temporally evolving and incompressible. The particles are initially arranged in the upper region of the higher speed of the flow field. The governing equations of the gas-phase are directly solved by using the pseudo-spectral method. The particle trajectories are traced by using the one-way and two-waycoupled Lagrangian method, respectively. The results show that the dispersion extent to the nether region of the particles placed initially in the upper region of the flow field is in inverse proportion to the particle Stokes number. The smaller the Stokes number of the particles is, the more the dispersion to the nether region is, meaning that the mixing between particles and fluid is more sufficient. Due to the strongest effects by the large-scale vortex structures, the particles at the Stokes numbers of the order of unity concentrate largely in the outer boundaries, thus the particle concentration distribution is the most un-uniform. But the particles at other Stokes numbers distribute evenly in the How field. In the two-way coupling simulation, besides the similar phenomena to those of the previous studies, there are some new findings. In previous studies, it is usually thought that the particles with smaller scale can attenuate turbulence due to the dissipation, but the particles with the larger scale will enhance turbulence due to its wake effect. However, the present DNS results show that the same particles at the Stokes number of 5 have thoroughly different effects on the gas-phase flow characteristics during the rolling up and the pairing courses of the large-scale vortex structures in the mixing layer. In the rolling up of the large-scale vortex structures, the particles reduce the encrpy at fundamental wavenumber and subharmonic wavenumber, the total turbulent kinetic energy and the turbulence intensity of the fluid; but in the pairing course, the trends are opposite and the particles increase all the above variables. This reducing and increasing effects by the particles are direct proportion to the particle mass loading.In the DNS of the two-dimensionally gas-solid two-phase plane jet, the flow is spatially developing and weakly compressible. The fourth-order compact finite difference schemes are used to discretize the space derivatives in the governing equations of the gas-phase. The efficient fractional-step integration scheme is adopted to integrate time. The boundary conditions are also elaborately treated. In the two-way coupling, the momentum effect on the fluid by a particle is described by a point force. The results of the one-way coupling DNS display the transition of the flow field from a symmetric mode to an asymmetric mode in the near field of the jet, capture the consecutive pairing between two or three vortex structures, find the local-focusing phenomena of the dispersion of the particles at the intermediate Stokes numbers, and reveal the dispersion mechanism of particles at different Stokes numbers. These outcomes reinforce the conclusions of the previous researchers. As to the turbulence modulation by the effects of the particles on the gas-phase jet, the study of the two-way coupling DNS shows that the particles at the Stokes number of 0.01 and 50 promote the evolving of the coherent structures in the flow field and make the profiles of the turbulence intensity wider and lower, but the particles at the Stokes number of 1 delay the developing of the coherent structures and reduce the turbulence intensity, making the profiles of the turbulence intensity narrower and lower. Under the same mass loading, the influences on the coherent structures and the jet velocity half-width更多还原