Session: 7.2.2 - Cavitation II

Paper Number: 167063

167063 - Ultra-High Speed Photography of Shock Waves During Collapse of Cloud Cavitation Induced by Submerged Water-Jet 

Abstract: 

A cloud of cavitation bubbles and cavities sometimes shows collective growth and collapse behavior, and shock waves are released associated with its collapse, which causes noises, vibrations, and erosion in hydromachinery. Such a destructive form of cavitation is called cloud cavitation, and it is important to understand how the shock wave is generated when the cloud collapses.

In early study, much effort has been made to measure the impact pressures associated with the cloud collapse in practical flows. Due to the recent development of technology, the shock waves from the cloud cavitation have been observed by using a high-speed camera. For example, Brujan, Ikeda, and Matsumoto. (2012) used a streak camera at up to 20,000,000 fps to capture the shock waves released from the hemispherical cloud on the wall and estimated the pressure decay close to the cloud based on the experimental result. Johnston et al. (2014) recorded periodical shock wave emissions from the cloud driven by the ultrasound at 1,000,000 fps. Khavari et al. (2021) performed the high-speed photography of the cloud-collapse-induced shock waves in the ultrasound and measured their impact pressures, then considered that the prominent frequency peak is associated with accumulative effect of shock wave emissions. In our previous study (Ushioku and Yoshimura, 2025a), we made simultaneous observations of the unsteady behavior of the cloud and the shock waves by using two high-speed cameras operating at 450,000 fps. We finally found that the shock wave emission occurs when the size of high-void fraction area, which is referred as “the nucleus of the cloud”, becomes minima. This implies that the shock wave is generated by the collapse of the cloud nucleus.

In this way, the high-speed photography of the shock waves and the cloud collapse have been performed. However, how the shock wave is generated associated with the cloud collapse has not been fully understood. For example, as reported by Johnston et al. (2014), the multiple shock emissions were observed when the larger cloud collapses. In addition, our numerical study (Ushioku and Yoshimura, 2025b) shows that weak pressure waves are released before the principal shock wave generation. Therefore, the cloud-collapse-induced shock wave is considered to be more complex phenomenon, and the further experimental study is required to clarify the mechanism.

The purpose of this study is to explore the how the shock wave is generated associated with the collapse of the cloud by the high-speed photography. In our experiment, we produced the isolated cloud cavitation by pulsed submerged water-jet injection, which was forced out from a fine nozzle by a Ho:YAG laser ablation. The shock waves during the cloud collapse were visualized by the Schlieren method and recorded by a high-speed camera operating at 1,700,000 fps. Simultaneously, four hydrophones were set near the cloud to measure the impact pressures. 

In particular, we will report the following points: first, the multiple shock waves are emitted when the nucleus of the cloud collapses, which is the high-void fraction region and plays a measure role in the unsteady behavior of the cloud. Second, weak pressure waves are released from the cloud before the shock waves are generated, which is consistent with our previous numerical work (Ushioku and Yoshimura, 2025b). These experimental results suggest that the main shock wave is formed by the accumulation of these weak pressure and multiple shock waves. Finally, the multiple shock waves propagate asymmetrically, which may lead significant dispersion in impact pressure measurements as reported by Khavari et al. (2021) and Ushioku and Yoshimura (2025a). 

References
・Brujan, E. A., Ikeda, T., Yoshinaka, K., and Y. Matsumoto, Y. [2011], Ultrason. Sonochem., 18(1), pp. 59–64. 
・Johnston, K. et al. [2014], Ultrasonics, 54(8), pp. 2151–2158. 
・Khavari, M. et al. [2021], J. Fluid Mech., 915, p. R3. 
・Ushioku, T. and H. Yoshimura [2025a] (in press), Exp. Comput. Multiph. Flow.
・Ushioku, T. and H. Yoshimura [2025b] (in press), Exp. Comput. Multiph. Flow.

Presenting Author: Takahiro Ushioku Waseda University

Presenting Author Biography: