Similitude method for self-noise under boundary layer pressure fluctuation excitation.

In this paper, the authors propose a similitude method based on the Π-theorem to predict the noise generated by pressure fluctuations along the boundary layer of a full-scale model by using a small-scale model. We begin by investigating the theory of the similitude method and introducing an approach to control the flow regime. We also provide an account of active and passive methods of controlling the flow regime. We analyze a plate model and a rotary model by using the computational fluid dynamics method to validate the effectiveness of the proposed similitude method and subject it to error analysis. The results demonstrate that it can accurately predict flow-induced noise in the full-scale model based on the noise observed in the small-scale model. We also provide methods to identify transitions in flow in the Appendix.

Measurement of Underwater Acoustic Energy Radiated by Single Raindrops.

Underwater noise produced by rainfall is an important component of underwater ambient noise. For example, the existence of rainfall noise causes strong disturbances to sonar performance. The underwater noise produced by a single raindrop is the basis of rainfall noise. Therefore, it is necessary to study the associated underwater noise when drops strike the water surface. Previous research focused primarily on the sound pressure and frequency spectrum of underwater noise from single raindrops, but the study on its sound energy is insufficient. The purpose of this paper is to propose a method for predicting the acoustic energy generated by raindrops of any diameter. Here, a formula was derived to calculate the underwater sound energy radiated by single raindrops based on a dipole radiation pattern. A series of experiments were conducted to measure the underwater sound energy in a 15 m × 9 m × 6 m reverberation tank filled with tap water. The analysis of the acoustic energy characteristics and conversion efficiency from kinetic to acoustic energy helped develop the model to predict the average underwater sound energy radiated by single raindrops. Using this model, the total underwater sound energy of all raindrops during a rainfall event can be predicted based on the drop size distribution.

A Flow Velocity Measurement Method Based on a PVDF Piezoelectric Sensor.

To measure the flow velocity of a fluid without affecting its motion state, a method was proposed based on a polyvinylidene fluoride (PVDF) piezoelectric film sensor. A self-made PVDF piezoelectric sensor placed parallel with the flow direction was used to measure the flow velocity. First, the piezoelectric characteristics of PVDF were obtained theoretically. Next, the relationship between flow velocity and sound pressure was verified numerically. Finally, the relationship between flow velocity and the electrical output of the PVDF piezoelectric film was obtained experimentally. In conclusion, the proposed method was shown to be reliable and effective.