Enhanced bubble-acoustic interaction via acoustic radiator adaption: Application into underwater wet welding

The role of the bubble in ultrasonic-wave-assisted underwater wet welding (U-UWW) remains largely empirical, primarily due to the lack of acoustical understanding of the bubble-acoustic interaction. Hence, numerical simulation of acoustic field, using a model considering bubble effect, was carried out to investigate the effect of acoustic radiator configuration on bubble-acoustic interaction. The pressure field characteristic and its influence on time-averaged potential and acoustic radiation force at the bubble boundary were systematically studied. The results show that the degree of bubble-acoustic interaction largely relies on the synergic effects of acoustic radiator configuration and the resonant mode of acoustic field; the latter can be accomplished by tailoring radiator height. Also, acoustic radiator with a concave surface is superior to that with cylinder and stepped-plate configuration in enhancing acoustic radiation force, in which the dimensionless maximum value of 93.7 is attained at the curvature radius of 38 mm. As the resonant height increases, the amplitude of acoustic radiation force tends to decrease, which renders the beneficial role of the first resonant mode in U-UWW, i.e., the resonant height is 50 mm. The simulated acoustic field results agree well with experimental results on bubble behavior and welding stability. When the resonant height is 50 mm and the curvature radius is 38 mm, the inverse of variation coefficient of arc voltage is increased to maximum values of 4.3 and 4.9, respectively, indicating superior welding stability. It is believed that the bubble-acoustic interaction mechanism can provide vital clues in enabling the design of ultrasonic system via acoustic radiator adaption.