Proceedings of FEDSM'03 4TH ASME JSME Joint Fluids Engineering Conference FEDSM2003-45002 OBSERVATIONS OF CHAIN-REACTION BEHAVIOR AT BUBBLE COLLAPSE USING ULTRA HIGH SPEED VIDEO CAMERA Keiichi Sato, Shigemasa Shimojo and Jun Watanabe Department of Mechanical Engineering Kanazawa Institute of Technology Ishikawa, Japan ABSTRACT Collapsing behavior of cavitation bubbles is a very short phenomenon. In this study a new ultra-high-speed video camera with the maximum frame speed of 10 6 fps is used to observe the r;etailed aspects of three cavitation patterns such as separated vortex cavitation in a convergent-divergent channel, Karman- vortex-like cavitation in the wake flow of a circular cylinder and vibratory cavitation in an ultrasonic vibratory apparatus. For a convergent-divergent channel, the re-entrant motion within the separation zone was observed together with bubble collapse in the divergent part. It was found that minute bubbles collapsed in a chain-reaction manner inside the separated zone with the re-entrant motion after the shedding of cavitation cloud. For the wake flow of a circular cylinder, a collapsing motion of cavitation was observed as an axial-collapse type. The successive bubble collapses after the collapse of main bubble were caused due to pressure wave near the flow field. For cavitation in a vibratory apparatus, cavitation bubbles on the vibratory disk surface were examined in detail, especially from the viewpoint of a non-cavitation ring. The disappearance of bubbles spread radially toward the outer region through the motion of vibratory horn. Keywords: Cavitation, Bubble Collapse, Chain-Reaction, Collapse propagation, High-speed-Video Observation INTRODUCTION A cavitation bubble which occurs in a high-speed liquid flow shows a very high-speed behavior at its collapse. For a long time the growth or collapse phenomena of cavitation bubbles have been observed as a typical subject of high-speed photography [1]. On the other hand, recent developments of high speed video camera, namely new developments of solid- state image sensor can capture rapid moving images at (1,000,000 frames/s) 1Mfps [2, 3] and bring about fiarther progress in cavitation studies. In the present study various kinds of cavitation phenomena were investigated by an ultra-high- speed video camera with an in-situ storage image sensor developed by Etoh et al. [3]. Most cavitation patterns behave in a complex manner as a cloud of cavitation. For example, a kind of attached cavitation such as sheet cavitation shows an unsteady motion as cloud cavitation with some regularity [4]. Cavitation bubbles produce high impact closely related to severe erosion through the rapid deformation of cavitation bubbles or cavitation clouds when the cavitation clouds are exposed to strong pressure fluctuation with some periodicity. The typical examples of high-impact- prone cavitations are as follows; (1) Karman-vortex-like cavitation in the wake of a circular or a triangular cylinder [e.g., 5], (2) cloud cavitation in a convergent-divergent channel or on a hydrofoil [e.g., 6], (3) vibratory cavitation in a cavitation erosion apparatus [e.g., 7, 8] and (4) cavitating water jet with periodic discontinuity [e.g.; 9]. It seems to be no doubt that the high-speed interaction within the cavitation cloud makes a crucial role on the occurrence of high impact [10]. Filed, et al. [1 l, 12] made clear through a series of their investigations that the shock waves and micro-jets produce a cha;m reaction of bubble collapses by the observation of cavity behaviors in gelatin layer. Matsumoto and Shimada [13] showed by numerical simulation that a cloud of bubbles can locally produce high pressure inside of it. The recent result by Yagi et al. [14] is interesting from a practical viewpoint because they showed experimentally that the downstream collapse of bubbles can generate the upstream erosion in a long orifice for pressure reduction in a flow system. The recent progress of a CCD image sensor has made possible the high-speed observation of cavitation bubbles with little difficulty. In the present experiment the picture capturing is made in various kinds of cavitating flows using two high- speed video cameras with the framing rate of 9 Kfps to 500Kfps. The propagation of bubble collapse is mainly examined for these unsteady cavitation phenomena. EXPERIMENTAL APPARATUS AND PREVIOUS INVESTIGATIONS 1 Copyright © 2003 by ASME Proceedings of ASME FEDSM’03 4 th ASME_JSME Joint Fluids Engineering Conference Honolulu, Hawaii, USA, July 6-10, 2003 FEDSM2003-45002