Experimental observation of waveforms with two shocks per cycle in the nearfield of intense CW circular piston source

It is well-known that a plane sinusoidal acoustic wave propagating in nonlinear medium is transformed to a sawtooth wave with one shock per cycle. The distorted waveform may look quite different in the nearfield of intense source due to strong effect of diffraction. Two theoretical models and corresponding numerical algorithms for the description of nonlinear acoustic beams radiated from intense CW sources were previously reported. In the first model, diffraction effects are included using Rayleigh integral, nonlinearity and thermoviscous absorption are accounted for in a quasi plane approximation. The second model is based on the KZK equation, which, in contrary to the first model, accounts for diffraction in the parabolic approximation. Numerical simulations show that both models predict the possibility of development of two shocks in each wave cycle in the nearfield of a circular piston source. The motivation of this work was to experimentally verify the presence of these two shocks and their further collision. The measurements were conducted in water with a 47 mm diameter transducer working at the frequency of 1 MHz at various source pressure amplitudes up to 1 MPa. For the pressure amplitudes higher than 0.5 MPa, two shocks were observed in the waveform starting from some distances from the source. With the increase of the observation distance, the two shocks collided and formed one shock (per cycle), i.e. the waveform developed to usual sawtooth wave. The agreement between results and simulation is excellent, which is a guaranty of our two numerical models which will be used for others applications