## 1 Introduction
Based on this two-way coupling method, cavitation bubble elongation along the vortex core trajectory and interaction between cavitation bubbles and re-entrant jet on the suction side of NACA0015 were simulated [26]. However, most of these methods were applied to problems involving relatively simple geometries. In this study, the cavitation inception and noise of underwater propellers are numerically and experimentally investigated with a consideration of the full body of a submarine of a DARPF Suboff. The effects of inflow due to the submarine body on the cavitation of the underwater propeller are fully included in all of the presented numerical and experimental results so that the results in the current study are of great practical importance to the actual developers. Reliable and efficient numerical methodology is developed to predict cavitation inception and cavitation noise of submarine propellers. The proposed method is based on the one-way coupled Eulerian-Lagrangian method, of which the validity was confirmed through its application to the prediction of the tip vortex cavitation and cavitation noise of a NACA16-020 wing in the previous study [27]. Since the cavitation pattern that first occurs on the propeller is generally the TVC, the current numerical methods are focused on the prediction and analysis of the TVC and cavitation noise of underwater propellers. Two propellers of the same specifications except for the sweep angle are designed and manufactured to investigate the sweep angle effects on cavitation inception and noise. The sweep angle of one propeller is 17\({}^{\circ}\), and that of the other is 34\({}^{\circ}\), which are named Model-A and Model-B, respectively. The main contributions of the present study are four-fold. First, the systematic numerical methodology is presented, which can provide reliable predictions of the tip vortex cavitation inception and noise of underwater propellers with full consideration of the entire hydrodynamic flow field of a submarine body. Second, the effects of sweep angle of the underwater propellers on cavitation inception and noise are analyzed quantitatively. It is found in both numerical and experimental results that the propeller with a higher sweep angle induces the tip vortex of less strength, generates weaker cavitation bubbles, and produces less cavitation noise. Third, the effects of incident wake from the boundary layer flow of the submarine body on the cavitating flow of underwater propellers are quantitively analyzed, with a focus on the effects of the wake flow of the rudders and the sail. The wake flow is found to reduce the incident axial flow velocity, increase the local angle of attack, and induce the cavitation on the propeller surface. Fourth, the effects of initial nuclei distributions on the cavitation flow and noise are quantitatively examined by varying the nuclei's number densities according to their size. The result reveals that the water quality described by the nuclei distribution is critical to the reliable assessment of cavitation flow and noise of underwater propellers. The present paper is organized in the following way: In Section 2, the overall numerical procedures are presented. The Eulerian approach consisting of the RANS solver and the vortex model are described. Then, the Lagrangian approach based on the bubble dynamics model and the acoustic analogy is explained. In Section 3, the experimental method is described, which is used to capture the propeller's cavitating tip vortex and to measure hydro-acoustic pressure caused by cavitation. The numerical results are presented with the experimental results in Section 4. The effects of the inflow distortion due to submarine body, especially, the rudders and the sail on the tip vortex flow of underwater propellers are investigated in detail. Then, the tip vortex flow reconstructed by using the vortex model is presented.