The occurrence and development of the dominant unsteady flow structures in a vanless centrifugal pump impeller are revealed by the proper orthogonal decomposition (POD) method. The pressure and velocity data of four radial surfaces is selected as the variables of decomposition. The results show that this method is beneficial to the analysis of flow field when there is no strong interaction of flow structures. When the flow rate starts to decrease from the design flow rate, unstable flow

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... on such as flow separation and wake begin to appear and develop in the impeller. The POD analysis reveals the influence of the main unsteady structures on the flow field when there is no mixed or have little interaction among flow structures. It outlines the development of flow separation near the suction side of impeller and the wake near the trailing edge as the flow rate changes. However, the flow field inside the impeller becomes more and more complex as the operation condition is far away from the design condition, which needs to be combined with other methods to better analyze the flow field.