Abstract

Turbulent flow across the surface of the object causes pressure fluctuation and generates sound waves. The resonance frequency and sound pressure level (SPL) of different geometries are different. In this paper, a three-dimensional cylinder with a Reynolds number of 41100 and a rod-NACA0012 profile are selected as the research objects. The large eddy simulation method is used to calculate the flow-induced noise by combining the Ffowcs Williams and Hawkings analogy and porous surface integral. The results of three kinds of methods are compared with experiment to verify the reliability. The paper compares the simulation results of cylinder and rod-NACA0012, analyzes the difference of vortex distribution using Liutex and turbulent energy distribution in the flow field. Besides, this paper analyzes the linear and nonlinear terms of SPL. Several sound pressure probes are arranged around the two models to predict the SPL. Some typical probes are selected for sound pressure frequency domain analysis. The simulation results show that the frequency domain distribution of SPL for the flow around a cylinder has a more obvious peak frequency than that of the rod-NACA array. The linear term of sound pressure is related to flow separation, and the nonlinear term is related to the distribution of turbulent flow energy.

Introduction

Turbulent flow passing through the object causes pressure fluctuation and generates sound waves. In the field of science, the study of sound source generation and propagation characteristics of flow-induced noise has instructive significance for the understanding of the flow field and sound field (Hanson, 1993). In the engineering field, noise not only affects the concealment of military ships, but also affects the comfort of ships and submarines (Kellett, 2013). Understanding the propagation mechanism of flofoil noise can not only guide the precise positioning of sonar, but also help suppress noise.

Lighthill (1952) transformed the Navier-Stokes equation into the form of wave equation, and the terms related to fluid nonlinearity are regarded as the source terms of the wave equation. Assuming that the non-linear effects related to the sound field all appear in the vicinity of the turbulence, the computational domain can be artificially divided into the near field and the far field. The near field uses computational fluid dynamics (CFD) to solve the source term of the wave equation, and the far field uses a linear propagation model to calculate noise. The most commonly used integral form of the Lighthill equation is the FW-H equation (Farassat, 1980). Bensow (2016) used OpenFOAM software and FW-H integral equation to calculate the far-field noise of the three-dimensional flow around a cylinder with Reynolds number equal to 41100, and compared it with the experimental data. Ianniello (2013) used the acoustic analogy method to predict the underwater noise of a full-scale ship model in steady flow. The results show that the acoustic analog method can predict better both in the direction of noise propagation or in the amplitude. Although researchers used a variety of different equations for computations, few people compare the computation accuracy of different methods for simple models.

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