The simulation results of the fluid model. (a) Thermo–acoustic source. (b) Current density modulus distribution. (c) Acoustic pressure distribution. (d) Conductivity, thermo–acoustic source, current density modulus, and the y component of the electric field intensity for the white straight line. (e) The impulse response and amplitude–frequency characteristic of the ultrasonic transducers. (f) Simulation convolved acoustic pressure waveform.