Performance and mechanism of the hydrodynamic noise reduction for biomimetic trailing-edge serrations of a submarine

As submarine speed increases, the hydrodynamic noise generated by the sail of a submarine becomes more pronounced. Inspired by the noise reduction capabilities demonstrated by the serration on the owl’s wing trailing edge, this paper proposes the serration noise reduction structure applied to the sail trailing edge of the submarine. The large eddy simulation and the Ffowcs Williams–Hawkings equation are employed to examine the impact of serration on flow and noise characteristics at the sail. The quadrupole noise source is captured by the permeable surface combined with the formulation Q1A. The physical mechanisms underlying trailing edge serration, which reduce flow noise, are revealed. The accuracy of the hydrodynamic and acoustic calculation methods is verified by experimental data. This study demonstrates that the serration exerts the double effect on the flow noise, which is a combination of dipole and quadrupole noise. Total noise is reduced by up to 4.32 dB. The impact of serration on dipole noise is the combined behavior of multiple physical mechanisms. First, the serration induces flow separation at the trailing edge and block water convergence at the serration peak, thereby diminishing turbulent fluctuations within the boundary layer; Secondly, it causes the decoherence effect on the vertical pressure fluctuations at the trailing edge, resulting in destructive interference in the dipole noise source. The serrations extend the continuous Stream vortex, delaying its evolution into the Hairpin vortex and subsequent fragmentation into small-scale vortex. This distortion of the spatial vortex structure intensifies the magnitude of the Lamb vector $\left|ℒ\right|$ and Lighthill stress, thereby enhancing the energy of quadrupole noise source. Flow noise reduction is achieved by applying sail trailing edge serration in a submarine, but the submarine hull diminishes its noise reduction performance. The reason involves turbulent interference in the boundary layer of the hull altering the incoming flow conditions at the trailing edge serration, which increases the unsteady pressure fluctuations at the serration valley, thereby amplifying the intensity of the dipole noise source.