Numerical and experimental studies on nonlinear parametric sound enhancement through different fluid layers

In this study, nonlinear enhancement mechanisms of parametric sound through different fluid layers have been both numerically and experimentally analyzed. In the numerical simulation, a transmittance boundary condition is imposed when solving the Khokhlov-Zaboloskya-Kuznetsov equation using a finite-difference method in the frequency domain. In the experiments, the sound pressure distribution of parametric sound at sum and difference frequencies is measured by a calibrated hydrophone to confirm the numerical results. Both numerical simulations and experiments were performed using an ethanol layer in water under the irradiation of two distinct primary frequencies from a sound source. The sound pressure distribution and the enhancement ratio of both sum and difference frequencies were evaluated. The results indicate that a different fluid layer with a length of 150 mm generates an amplitude 3.7 times larger in the difference frequency and 2.9 times larger in the sum frequency compared to that in water alone. This is due to the nonlinear fluid properties and diffraction effect in the different fluid layer. It was also found that the enhancement ratio at the sum and difference frequencies increased with increasing length of the different fluid layer, while the enhancement ratio is slightly affected by the initial water layer length variation.