The investigation on calculating and measuring the flow noise from underwater jets

Abstract

Based on the combination of large eddy simulation theory and Lighthill's acoustic analogy equation, numerical simulation of underwater flow noise is investigated. The calculation model of nozzle and fluid is built up by finite element software, ANSYS. The property of fluid field is calculated by the software, Fluent, which is in the ANSYS. The fluid property changing with time is gotten and dealt with by FFT, then, introduced into ACTRAN. Therefore, sound field of flow noise can be calculated. The flow noise of a circular nozzle is carefully investigated. The diameter of the nozzle is 20 mm, and the velocity of the fluid is 10 m/s. The frequency is in the range from 20 to 5000 Hz. The results demonstrate that sound radiation from flow noise is mainly in transition area. The distance between the nozzle and transition area is in the range from 8 to 10 D. Here, D is maximum dimension of the nozzle. Meanwhile, the directivity of radiated noise in transition area is fourth polar. Based on the principle of reverberation method, a measurement system of underwater flow noise is built up. One kind of nozzle is a steel pipe, and inner diameter is 200 mm. The pipe is placed in a reverberation water pool. In the pool, there are two parts: one part is flow area; the other part is test area. In test area, there are 32 hydrophones, which are disposed at different depth. The other kind of nozzle is a circular nozzle with coverage formation. The diameter of the nozzles is 10 mm, 20 mm, 30 mm, respectively. The nozzle is placed in a reverberation water tank, which is made by steel plates and supported by separated points at the bottom. In the top, there are three ducts. The ducts can hinder sound propagation from the tank to the outside, and vice versa. The tank is also separated into two parts: one part is flow area; the other part is test area. In test area, there are four hydrophones, which are treated as a line array to receive the signal caused by the flow. All the designs can protect hydrophones from the knock by the flow. The flow noise source is a tank in high place. And the flow is generated by the gravity. The total level of sound radiation power from flow noise is proportional to the eighth power of the velocity. The results demonstrate that total level of flow noise from the nozzle of uniform type is only decided by the pressure of flow noise source, and not related to the section area of nozzle. The results are coincided with the simulation.