Superhydrophobic surface is a promising technology, but the effect of superhydrophobic surface on flow noise is still unclear. Therefore, we used alternating free-slip and no-slip boundary conditions to study the flow noise of superhydrophobic channel flows with streamwise strips. The numerical calculations of the flow and the sound field have been carried out by the methods of large eddy simulation (LES) and Lighthill analogy, respectively. Under a constant pressure gradient (CPG) condition, the average Reynolds number and the friction Reynolds number are approximately set to 4200 and 180, respectively. The influence on noise of different gas fractions (GF) and strip number in a spanwise period on channel flow have been studied. Our results show that the superhydrophobic surface has noise reduction effect in some cases. Under CPG conditions, the increase in GF increases the bulk velocity and weakens the noise reduction effect. Otherwise, the increase in strip number enhances the lateral energy exchange of the superhydrophobic surface, and results in more transverse vortices and attenuates the noise reduction effect. In our results, the best noise reduction effect is obtained as 10.7 dB under the scenario of the strip number is 4 and GF is 0.5. The best drag reduction effect is 32%, and the result is obtained under the scenario of GF is 0.8 and strip number is 1. In summary, the choice of GF and the number of strips is comprehensively considered to guarantee the performance of drag reduction and noise reduction in this work.
Hydrodynamic noise is one of the three major noise sources of underwater vehicles. The sonar dome is a device placed in front of the ship and the submarine to absorb the flow fluctuation and to reduce the hydrodynamic noise, so that the sonar inside the dome is not affected by the external fluid. However, with the increase of the velocity of ships and submarines, cavitation can usually form in the bulge of the sonar domes, which will bring high level of noise to the sonar. The internal self-noise of the sonar dome mainly comes from two areas: the leading-edge stagnation point and the transition zone of boundary layer. In the paper, we designed the leading-edge serrations and dimples in the leading-edge and transition areas of the sonar dome respectively to reduce the movement resistance and prevent the separation of the boundary layer. The research on leading-edge serrations and dimple technology is carried out by using theoretical analysis, numerical calculations. The results show that the leading-edge serrations and dimples can add energy from the outer flow into the boundary layer; the cavitation phenomenon can be delayed. The hydrodynamic noise has been suppressed by about 20dB.
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