# Research on Characteristics of Flow Noise and Flow-Induced Noise
## 1 Introduction  
The flank array sonar is a passive sonar with a large-scale hydrophone base array mounted on the side of a submarine, which can provide remote passive detection, classification, and localization [1]. However, when the submarine is in motion, the noise generated by the interaction between the flow field and the hull can interfere with sonar detection. Consequently, it is of vital significance to investigate the noise characteristics of submarines, mitigate the impact of submarine-generated noise on flank array sonar, and optimize the arrangement of flank array sonar to enhance its anti-noise performance [2]. The noise of the flank array sonar platform is mainly divided into three parts: mechanical noise, propeller noise, and hydrodynamic noise [3]. As a result of the high speed of underwater navigation, as the navigation speed increases, the proportion of hydrodynamic noise gradually increases. Therefore, we focused our research and analysis on hydrodynamic noise. Hydrodynamic noise, as the dominant noise at relatively high navigation speeds, can be divided into flow noise and flow-induced noise [4]. Among them, flow noise is mainly caused by the combination of turbulent boundary layer disturbance outside the structural shell and wall pulsating pressure. On this basis, flow-induced noise is the superposition of structural vibration and noise generated by the interaction between the shell and fluid [5]. In order to achieve a credible study of its underwater performance, a clear understanding of its surrounding flow field must be obtained. The study of flow field in the past was to directly obtain relevant data through underwater experiments, and then to calculate and analyze its structural parameters; however, obtaining data directly through experimentswill consume relatively high resources. In recent years, with the development of computational fluid dynamics (CFD) and computational acoustics, it has become possible to study the noise of underwater targets using numerical simulation methods. [PERSON] et al. [6] computed the flow field of the SUBOFF submarine using Fluent software and then utilized Fluent's acoustic module to calculate the flow noise. However, due to the inclusion of compact source assumptions, although it accurately calculated far-field acoustic information, it introduced significant errors into the calculations near the submarine's surface. [PERSON] [7] employed a combination of computational fluid dynamics and the boundary element method to calculate the flow noise of a fully appended submarine, obtaining the total sound pressure level at characteristic points. [PERSON] et al. [8] used the k-\(\epsilon\) model in conjunction with the FW-H equation to numerically predict the underwater radiated noise of LNG ships. [PERSON] et al. [9] combined Large Eddy Simulation (LES) with the Infinite Element Method (I-FEM) to perform frequency-domain numerical predictions of the flow noise from propellers in uniform flow and the flow noise from submarines. [PERSON] [10] used the SUBOFF scaled model to simulate and model the hull command deck line shape and hull shell thickness. He studied the acoustic radiation characteristics of the hull flow-excited structural vibration under the action of pulsating pressure, but did not conduct a comparison with the flow noise. While many scholars have conducted research on submarine noise, previous studies on flow-induced noise assumed that structural deformations of the hull during underwater movement could be neglected [11]. However, under the effect of pulsating pressure, the shell still undergoes certain structural displacement changes resulting in vibration noise [12; 13]. Therefore, analyzing the flow-induced noise generated during the underwater shell motion based on one-way fluid\(-\)structure interaction has significant research value [14; 15]. In this study, we not only calculated flow noise, but also computed flow-induced noise, comparing it with flow noise. For submarines, the proportion of sonar array size is small.