On Diffraction of a Sawtooth Nonlinear Wave by a Narrow Circular Aperture in a Screen

The paper presents the experimental and theoretical studies of diffraction effects in intense acoustic beams diffracted by a narrow circular aperture in a screen. The characteristic acoustic pressure achieved in the experiment at the emitter aperture is 1 MPa. The operating frequency of the emitter is 2 MHz. Acoustic signals in the experiments were detected in the frequency range up to 100 MHz. Special attention in the experiments was paid to the case of passage of a nonlinear wave beam through a aperture with a diameter significantly smaller than the characteristic beam width. Theoretical studies in the paper are based on numerical simulation using the Khokhlov–Zabolotskaya–Kuznetsov equation. It is shown that the propagation of intense acoustic beams diffracted by a narrow circular aperture in the screen is accompanied by degeneration of a sawtooth wave into a sequence of short pulses. The high-frequency components of the spectrum predominate in the wave that passed through the aperture owing to spatial filtering, which leads to a change in the law of decay of the harmonics in the spectrum. The dependence of the law of decay of the harmonics in the spectrum of a sawtooth wave passing through a aperture in the screen on the ratio of the aperture diameter and characteristic width of an incident intense acoustic beam is investigated.