Study of the numerical simulation method for unsteady cavitating flow

叶栅中的思维流包含了从微观气泡动力学到宏观非定常流等多种物理性质。因此，这种两相流的实验或理论分析存在困难。近年来，数值研究有望成为一种合理的工具，特别是在工业上的应用。本研究的目的是建立一种实用的方法来分析叶栅流动中的非定常和不稳定现象。FY2001，我们在Chen方法的基础上提出了一种修正的空化模型来处理具有非定常空化的宏观流场，并将其应用于三维计算。结果合理再现了再入射流与云空化相互作用等特征结构。在2002财年，我们的方法的可靠性得到了提高。特别考虑了低马赫数假设方法的精度和鲁棒性以及非反射边界条件的守恒性。此外，还对二维叶栅流道间的空化相互作用现象进行了分析。2003财年，继续对计算模型进行一些改进。以广义曲线坐标系下的大涡模拟为例，建立了新的亚网格尺度湍流模型。虽然在本项目中没有将其与空化分析结合起来，但我们相信它有希望进行下一步的开发。在二维模拟中，成功再现了旋转空化和流动分离两种传播模式，验证了方法的充分性。

Cogitating flows in cascades contain wide variety of physics, ranging from microscopic bubble dynamics to macroscopic unsteady flow. Thus experimental or theoretical analysis of such two-phase flows contains difficulties. Recently, numerical investigation is expected to become a reasonable tool, especially for application in industry. An objective of this study is to construct a practical method to analyze unsteady and unstable phenomena in cascade flow. In FY2001, we proposed a modified cavitations model based on Chen's method to deal with macroscopic flow field with unsteady cavitations and applied it to three-dimensional computation. As a result, characteristic structure such as interaction between re-entrant jet and cloud cavitations was reasonably reproduced. In FY2002, the reliability of our method was improved. Namely, the accuracy and robustness of low Mach-number assumption method and the conservation properties of non-reflection boundary condition were particularly considered. In addition, the analysis of interactive phenomena of cavitation among flow passages in two-dimensional cascade was started. In FY2003, some improvements for computational model were continued. For instance, new sub-grid scale turbulence model for the large eddy simulation (LES) on generalized curvilinear coordinate system was developed. Although it was not combined to the cavitation analysis in the term of this project, we believe it is hopeful for next development. As for the two-dimensional simulation, the two propagation modes due to the rotating cavitation and the flow separation were successfully reproduced and the result confirmed the adequacy of our method.