ABSTRACT

Underwater noise (URN) is the focus of academic research, and cavitation is an important source of underwater noise. This paper takes NACA66 (mod) two-dimensional hydrofoil as the research object, and uses the open source software OpenFOAM to simulate the sheet cavitation and sound field. The turbulence model is DDES, and the cavitation model is the Schnerr-Sauer model. The sound field is predicted by the FW-H formulation. Unlike the traditional method, this paper solves the quadrupole term (non-linear term) by direct volume integration, so the nonlinear term can be predicted more accurately. At the same time, a new method of changing sound wave velocity is proposed considering the two-phase medium problem caused by cavitation. Four methods are compared including two-phase volume integration, direct volume fraction, object surface integration and penetrable formulation. It was found that the influence of two-phase flow is greater near the closure area of the cavity, which needs to be considered separately. The closer to the cavity closure zone, the greater the nonlinear effect.

INTRODUCTION

The underwater noise not only causes harm to marine life, but also affects the stealth of military equipment. The International Maritime Organization (IMO) issued non-mandatory noise standards for commercial ships (IMO, 2014). More and more attention is paid to the acoustic environment. At this stage, the prediction of such noise becomes a hot topic (Deane, 2010; Ianniello, 2013; Bensow, 2016).

The underwater noise can be divided into three parts: structure vibration noise, propeller noise and flow-induced noise. Flow-induced noise is caused by pressure fluctuations. As the speed of structures increases, the impact of flow-induced noise also increases. Theoretically, prediction of the flow-induced noise needs to directly solve the compressible N-S equation, but the solution strategy in aerodynamics cannot be applied to the water medium, and this method is computationally expensive (Cianferra, 2019). In recent years, acoustic analogy has been adopted in most researches (Epikhin, 2015; Schmalz, 2015; Choi, 2016). Within this framework, the flow field is regarded as the sound source, and the free space Green's function is used to describe the sound pressure propagation in the far field. The acoustic analogy was first proposed by Lighthill (1952) and developed by Curle (1955), Ffowcs Williams (1969) and others. By now, it has achieved great development. The commonly-used formulation is called FW-H equation, which regards the various motion of solid objects as sound sources.

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