关键词: 叶道涡, 动态模态分解, 本征正交分解, 相干结构, 重构

Abstract: In order to explore the spatiotemporal evolution of inter-blade vortices in a francis turbine at unsteady load conditions, the DMD method was introduced in combination with numerical simulation and experimental verification to decouple the coherent structure of inter-blade vortices. The numerical results show that inter-blade vortices have low frequency characteristics and with the main frequency which is 0.5 times the rotation frequency of the turbine. The cavity of inter-blade vortices originates from the upper crown of the impeller in the shape of vortex tube, which is connected with the sheet vortices on the back of the blades. DMD method was applied to obtain dynamic modal of inter-blade vortices velocity field, and was compared with the POD method. The results show that the inter-blade vortices flow structure is decomposed by the DMD method at a specific frequency to obtain the single-fold frequency characteristics, which is more convenient for the flow mechanism analysis of the coherent structures. Both of two methods may reconstruct the inter-blade vortices velocity field based on the average flow mode and the main modes, but there are errors in the reconstruction of local complex inter-blade vortices regions.

Key words: inter-blade vortices, dynamic mode decomposition(DMD), proper orthogonal decomposition(POD), coherent structure, reconstruct