Acoustic field directivity of longitudinal wave generated by oblique laser based on combustion wave

Acoustic field directivity patterns of longitudinal wave are theoretically and experimentally analyzed, when the oblique laser supported combustion wave exists. Based on laser ultrasonic generation mechanism, the normal displacement of particle in the far field is deduced under the condition that splash materially vertically impacts on the surface of workpiece. The directivity function is obtained, and the relevant factors are analyzed. A measurement system for directivity patterns is established to verify the function. Longitudinal wave is received by piezoelectric transducer whose peak frequency is 5 MHz in a state of laser supported combustion. The experimental data and theoretical data agree well with each other. For the longitudinal wave under the same size of spot, the directivity patterns become narrower with the increase of the inclined angle, which means that the energy of the longitudinal wave field is also more concentrated, but the angle of the acoustic beam axis remains unchanged.