Simulation of Photoacoustic Wave Generation and Propagation in Fluid-Solid Coupled Media Using Finite Integration Technique

Photoacoustic (PA) waves using the transmission of a laser beam are being introduced in recent nondestructive ultrasonic testing. Not parameters related to acoustic, optic, and thermal phenomena but also spatial and time conditions of the laser irradiation affect the generation of the PA wave. Therefore, an appropriate design and preparation are required for reliable laser ultrasonic testing. Numerical simulation can be an effective tool for predicting PA signals. This study aims to develop a numerical model to simulate PA wave generation and propagation. Here, the heat conduction and the elastic wave equations were solved in a coupled manner based on discretization by the finite integration technique (FIT). We simulated two cases where the laser were irradiated on the target material in the air and underwater. The amplitude of the longitudinal PA wave propagating in the depth direction in the case of laser irradiation in water became more extensive than in the case of laser irradiation in the air. This reason was that the presence of water in the upper part caused a reaction force against the stress generated by thermal expansion. The FIT simulation was validated by experimental measurement of the PA wave.