Institute of Shaanxi Key Laboratory of Ultrasonics, Shaanxi Normal University, Xi'an 710119, China.
Institute of Shaanxi Key Laboratory of Ultrasonics, Shaanxi Normal University, Xi'an 710119, China.
Ultrasonics. 2023 Jul;132:106992. doi: 10.1016/j.ultras.2023.106992. Epub 2023 Mar 30.
The generation and evolution of bubble clusters in ultrasound fields were studied using high-speed photography. The transition of a spherical bubble cluster to a layer-like bubble cluster was demonstrated in detail. At a distance of half a wavelength to the water surface, the rising spherical cluster oscillated strongly and its equilibrium size grew. The speed was about 0.4 m/s and had a tendency to decrease. A jet caused by the last collapse of the spherical cluster rushed to the water surface, creating a bulge on the surface. Subsequently, due to the primary acoustic field, bubbles accumulated again below the bulge, and a layer-like bubble cluster gradually formed. The effects of acoustic frequency and intensity on the layer-like cluster were considered. It was found that the clusters located at a distance-to-wavelength ratio of about 0.08 to 0.13, very close to the water surface. The flickering bubble clusters were easy to be observed at 28 kHz and 40 kHz, while the accumulation of bubbles and their flicker were relatively weak at 80 kHz. The higher the frequency, the shorter the wavelength, the closer the structure to the water surface. However, at 80 kHz, the cavitation threshold is supposed to be higher and the resonance size of the bubbles is smaller, so the bubble oscillations and their interactions were weaker, and the phenomenon was different from the cases of 28 kHz and 40 kHz. Multiple structures mainly exist at 40 kHz. The formation and evolution of the layer-like cluster are closely dependent on the adequate supply of bubble nuclei from the water surface and the surrounding liquid. A Y-shaped bifurcation was used to model the branch streamers, which provided a path of bubbles accumulate into the clusters. The secondary Bjerknes forces between bubbles were adapted to analyze the interactions, and the results proved that it plays an important role in the appearance and evolution of the substructures.
采用高速摄影技术研究了超声场中气泡簇的产生和演化。详细演示了球形气泡簇向层状气泡簇的转变。在距水面半个波长的距离处，上升的球形簇强烈振荡，其平衡尺寸增大。速度约为 0.4 m/s，并有减小的趋势。球形簇最后一次坍塌产生的射流冲向水面，在表面形成凸起。随后，由于初级声场的作用，气泡再次在凸起下方积聚，逐渐形成层状气泡簇。考虑了声频和强度对层状簇的影响。发现簇位于距波长比约为 0.08 至 0.13 的位置，非常接近水面。在 28 kHz 和 40 kHz 下，闪烁的气泡簇很容易被观察到，而在 80 kHz 下，气泡的积聚和闪烁相对较弱。频率越高，波长越短，结构越接近水面。然而，在 80 kHz 下，空化阈值应该更高，气泡的共振尺寸更小，因此气泡的振荡及其相互作用较弱，现象与 28 kHz 和 40 kHz 的情况不同。在 40 kHz 下主要存在多个结构。层状簇的形成和演化与从水面和周围液体中获得足够的气泡核密切相关。Y 形分叉用于模拟分支射流，为气泡积聚到簇中提供了途径。气泡之间的二次 Bjerknes 力用于分析相互作用，结果证明它在亚结构的出现和演化中起着重要作用。
