# Analysis of the Guide Vane Jet-Vortex Flow and the Induced Noise in a Prototype Pump-Turbine
## 1 Introduction  
A reversible pump-turbine is the key component in pumped storage power station [1]. It has complex varying operation conditions due to frequent starting and stopping [2]. The rotating runner, adjustable guide vane, and stay vane may cause strong rotor-stator interaction and stalled flow [3, 4]. Undesirable flow regimes, including secondary flow, vortex, back flow, and submerged jet flow, could occur inside a pump-turbine [5, 6, 7]. The noise induced by undesirable flow structures is one of the bad factors occurring during operation [8]. It also represents a part of the energy dissipation [9]. During the operation of the prototype pump-turbine, high intensity noise is often found, especially in the pump mode's start-up process with a small guide vane opening angle. When focusing on this noise phenomenon, complex sources can be found, including flow-induced noise and mechanical noise [10]. The mechanical noise can be reduced by conducting better design and manufacturing. However, the flow-induced noise is more complex to handle. There are different types of flow-induced noise--mainly, the turbulent-flow-induced noise [11] and the cavitation bubble collapse noise [12]. The turbulent-flow-induced noise exists in almost all the flow conditions and needs specific studies. In bladed turbomachinery, the acoustic topic has received long-time extensive investigations [13; 14; 15]. For numerical simulations, there are two main ways to predict the flow-induced noise. One is the direct solving of the Navier-Stokes equations. This is accurate but requires a high computer cost. Another one is the acoustic analogy method [16]. This predicts the near-field by the computational fluid dynamics (CFD) method and extends to the far-field by computational acoustic (CA) method. Based on the methods above, researchers conducted numerical studies for the acoustic characters in bladed turbomachinery. It was found that the rotor-stator interaction could induce flow noise [17]. The distance between rotor and stator would strongly affect the noise intensity [18]. A rotating stall cell was also proved as one of the source of flow noise [19; 20]. The jet flow was found as another local high noise region [21; 22]. The blade exit wake also generated noise due to periodic shedding [23; 24]. Generally, the local flow regime is complex in turbomachinery and may cause the noise problem. In this study, a prototype reversible pump-turbine was studied on its turbulent-flow field and induced noise during pump mode's start-up. [PERSON] [25] introduced the simulation method of turbulent-flow-induced noise, considering the turbulence isotropic assumption. Based on a prediction of the turbulence kinetic energy and dissipation rate, the local sound power distribution can be evaluated [26]. The reason for turbulent-flow-induced noise in a pump-turbine can be clarified in detail. As a widely-used, robust, and accurate method, CFD simulation is useful for predicting the turbomachinery flow cases. Currently, a lot of CFD based studies have been conducted in the pump-turbines for the flow regime [27; 28], performance [29; 30], cavitation [31], and stability problems [32; 33]. These numerical results were compared with the tests and proved reliable and convenient. In this case, CFD simulation was used for the prototype pump-turbine. The partial-load condition with a small guide vane opening angle during pump mode's start-up was studied. The noise caused by the special jet-vortex flow regime and its transient variation is discussed in detail. As a difficult issue in hydraulic turbomachinery flow cases, the flow mechanism at the off-design condition is also discussed in this study. The results provide a basis for understanding the flow noise and energy dissipation at partial-load in pump mode. It also reveals the flow complexity when a pump-turbine operates off-design. The relationship between the operation condition and the turbulent flow regime is also clarified.