DIRECT SIMULATION AND LARGE EDDY SIMULATION OF UNSTEADY FLOW INCLUDING SEPERATION,VORTEX MOTION AND TURBULENCE

本次合作研究的目的是建立一种精确有效的分离涡非定常流数值模拟方法，特别是湍流的直接数值模拟（DNS）和大涡模拟（LES）。为此目的，从数学角度开发了高阶精度格式和边界条件，并从湍流结构的物理分析中检验了LES湍流模型。上述研究在国内可压缩流数值方法、高阶精确格式及其在直接湍流模拟中的应用，以及日本不可压缩流数值方法、边界条件及其在大涡模拟中的应用等方面均有开展。这一合作通过双方的相互作用促进了对湍流现象更准确和有效的研究的发展。非定常流场在工程中应用较多，但目前广泛使用的雷诺平均模型对非定常流场的预测不够准确。由于它能直接模拟大范围的湍流运动，因此有望广泛而准确地应用于各种类型的流动。然而，在将其应用于工程之前，还需要研究几个问题：从数学角度来看，1)有效的时间推进分析算法，2)减少边界条件引起的数值误差，3)避免高阶精度格式中的不稳定性，4)复杂几何和物理分析的边界拟合方法，5)LES湍流模型的开发和验证，6)可视化数据分析湍流结构。最后对LES和DNS进行了精确有效的大规模计算。1995年预算年度的研究工作如下：中国的研究主要集中在两方面：一是开发具有紧凑格式的稳定高阶格式；二是可压缩湍流剪切流的直接数值模拟。日本研究人员对非定常边界条件的不稳定性和准确性进行了研究，建立了LES的动力学模型，利用边界拟合网格和非结构化网格进行了大涡模拟，并对DNS的可压缩剪切流的可视化分析进行了联合研究。G)将高阶格式应用于不可压缩流动模拟。这种合作是通过互联网不断交换信息，并通过相互访问以讨论更多细节来进行的。此次合作的部分成果已在一次国际研讨会上发表，并将在中日双方共同授权的国际期刊上发表。这一研究计划将会继续并扩展到更广泛的DNS和LES在可压缩和不可压缩湍流中的应用。少

A purpose of this co-labolation research was to establish an accurate and effective method for the numerical simultion of unsteady flows with seperation and vortexes, esppecially using a direct numerical simulation (DNS) and a large eddy simulation (LES) of turbulence. For this purpose high-order accuracy schemes and boundary conditions were developed in the mathematical view and LES turbulence models were also examined from an physical analysis of turbulence structures.The above reseach had been performed in numerical methods of the compressible flow, high order accurate schemes and their application to the direct turbulence simulation by Chinese researchers, and also in numerical methods of the incompressible flows, boundary condition and their application to the large eddy simulation by Japanese researchers. This co-laboration promoted a development of more accurate and effective research in turbulence phenomena by an interaction between the both sides.The unsteady flows with the se … More perations and votexes are popular in the engineering, but have not been predicted correctly enough by a widely used Reynolds averaging model. The LES is expected to be applied generally and accurately to verious types of flows because it simulates a large scale part of turbulence motion directily. However, several problems should be researched before it is applied to the engineering : in mathematical view, 1) effective algorithms for the time marching analysis, 2) reducing numerical errors caused from the boundary conditions, 3) avoiding instability in the high order accuracy schemes, 4) boundary fitting method to the comlicated geometry, and also in the physical analysis, 5) development and validation of the LES turbulence models, 6) analysis of turbulence structures by the visualized data. Finally a large scale computation for the LES and DNS can be performed accurately and effectively enough.In 1995 budget year these researches were proceeded as fillows :Chinese researches investigated ina) development of stable and high order schemes with compact formulation,b) direct numarical simulation of the compressible turbulent shear flow. Japanese researchers investigated inc) instability and accuracy of the unsteady boundary conditions,d) development of dynamic models for the LES,e) large eddy simulations using the boundary fitted and unstructured grids, and also joint researches are performed inf) analysis of the visualization of the compressible shear flows by the DNS.g) applying the high order schemes to the incompressible flow simulations.This co-laboration has been proceeded by exchanging information constantly through an internet and also by visiting each other for discussing more detail. A part of this co-laboration results has been presented in an international symposium and will be published in an international journal coauthorized by the researchers from both Chinese and japanese side. This research program will be continued and extended to wider application of the DNS and LES to both the compressible and the incompressible turbulent flows. Less