Research on Acoustic Characteristics of Flow inside 90° Bends for Ship Fire-fighting.

To explore the underwater flow acoustic characteristics of ship pipelines and improve the acoustic stealth of ships, taking the 90° bends used in ships as the research object, the method combining the LES and Lighthill acoustic analogy method is used for numerical calculation and experimental verification of flow field and acoustic field of 90° bends with different radii of curvature. The results show that, with the increase of the radius of curvature (R/D), the vortex-like low-pressure area inside the 90° bend decreases, and the pressure distribution inside the bend becomes uniform. The 90° bend with R/D=1 has the largest change in average pressure and velocity, with a decrease of 4.2% in average pressure and an increase of 6.7% in average velocity. The flow noise source in the pipeline is mainly the medium-frequency and low-frequency noise, the main frequency of flow noise in the inlet and outlet sections of the 90° bend is close to the natural frequency of the second-order structure, while the main frequency of flow noise in the turning section is close to the first-order coupled natural frequency. The flow noise in the inlet section of the 90° bend has a circular distribution, the flow noise at the turning point has a dipole distribution, and the flow noise at the outlet has a circular distribution at a radius of curvature R/D=1 and R/D=2.5, while the rest has a dipole distribution. [ABSTRACT FROM AUTHOR]