On the early stagnation point during transient acoustic cavitation

We explore an acoustofluidic phenomenon, in an aqueous environment, of an emerging early stagnation point that consistently positions itself at a distance of two times the ultrasonic horn tip diameter 2D, regardless of the tip size. This was initially captured numerically in a two-dimensional domain of horn-type reactors of diameters D = 3, 6, 13, and 16 mm in a 107 × 50 mm cuvette. We deduced that the axial extension of the bubble cluster influences the rate of decay of axial flow; however, it does not affect the stagnation point. Cavitation attenuation was scrutinized by mathematically modeling the time-averaged axial flow during the cavitation transient state and solving the flow using Newman's subroutine. During fast streaming, acoustic force attenuation α decreases exponentially at a maximum rate of ≈1.70 with the doubling of Reynolds number Re. However, an inverse trend was demonstrated by the dimensionless attenuation Γ=−2αD, as it increased by a factor of ≈1.28. Similarly, Γ exponentially increased with the doubling of Re during slow streaming suggesting direct proportionality between Γ and Re. This emphasized the underlying role of the term 2D in amplifying attenuation induced by morphing structures of inertial bubble clusters. Moreover, tracking the bubble population along the horn axis revealed that mushroom-like structures formed under small horn tips have a linear bubble distribution, while cone-like structures under larger tips maintained an exponential distribution. This may suggest that a linear distribution may enhance attenuation and justify the aforementioned trends.