Abstract:
A liquid-ring pump is one of the commonly-used rotating positive-displacement machines. This study aims to analyze the transient flow characteristics of complex high-speed jet flow field in the ejector of liquid ring pump, in order to reveal the frequency characteristics of jet wake vortex shedding and shock wave self-excited oscillation. Dynamic Mode Decomposition (DMD) and Spectral Proper Orthogonal Decomposition (SPOD) were used to decouple the jet coherent structure using Large Eddy Simulation (LES). Two modes of decomposition were compared to extract the jet flow features. The results show that the vortex band was formed in the mixing chamber for the strong shear effect of the jet shear layer. The shape of shear vortex was changed to fall off at the trailing edge of jet, due to the influence of vortex zone. The periodic oscillation was found in the shear layer of the jet under the interaction with the shock waves. As such, the periodic shedding wake vortex was gradually formed at the trailing edge of the jet. The decoupling analysis of the high speed jet flow field was also achieved to obtain the spatiotemporal single frequency coherence structure using DMD and SPODs. The flow field at the target time was accurately predicted using the first five-order DMD density modes, where the Root Mean Square Error (RMSE) and absolute percentage error were 3.5% and 1.5%, respectively. The DMD mode with the frequency of 0 presented the highest energy proportion, indicating the steady-state characteristics of unsteady flow. The first order DMD mode with the frequency of 1 250 Hz was utilized to capture the shock wave and the shock string structure that formed by the interaction between the shock wave and the jet shear layer. The spatial structure shared the periodic evolution characteristics of jet shear vortexes in the second and third order DMD modes with the frequencies of 17 500 and 18 000 Hz, respectively. In addition, there was the similarity of spatial structure in the DMD modes in the high frequency band, indicating a multi-frequency coupling shedding of jet wake vortexes. The dominant mode of SPOD was better reflected by the frequency amplitude. The characteristic values of SPOD were arranged in the descending order at each frequency. In the frequency of 1 250 Hz, the first order SPOD mode also reflected the shock wave structure similar to the first order DMD mode. Meanwhile, there was the similar characteristic structure in the SPOD and DMD mode in the high frequency band. The characteristic scale of the second order SPOD mode was smaller than that of the first order mode. It infers that the coherent structure of turbulent jet was constantly evolved into the multi-scale flows during the process of development. The SPOD can be expected to accurately obtain the spatiotemporal single frequency dynamic modes for the evolution characteristics of turbulent jet, compared with the DMD. In addition, the SPOD can also effectively avoid the DMD in the selection of dominant mode. Therefore, the SPOD has more advantages than the DMD in the coherent structure decoupling analysis of turbulent jets.