Laboratory for Water and Turbine Machines, University of Ljubljana, Askerceva 6, 1000 Ljubljana, Slovenia.
Ultrason Sonochem. 2013 Jul;20(4):1113-20. doi: 10.1016/j.ultsonch.2013.01.011. Epub 2013 Jan 31.
Previous investigations showed that a single cavitation bubble collapse can cause more than one erosion pit (Philipp & Lauterborn [1]). But our preliminary study showed just the opposite - that in some cases a single cavitation pit can result from more than one cavitation event. The present study shows deeper investigation of this phenomenon. An investigation of the erosion effects of ultrasonic cavitation on a thin aluminum foil was made. In the study we observed the formation of individual pits by means of high speed cameras (>1000 fps) and quantitatively evaluated the series of images by stereoscopy and the shape from shading method. This enabled the reconstruction of the time evolution of the pit shape. Results show how the foil is deformed several times before a hole is finally punctured. It was determined that larger single pits result from several impacts of shock waves on the same area, which means that they are merely special cases of pit clusters (pit clusters where pits overlap perfectly). Finally it was shown that a thin foil, which is subjected to cavitation, behaves as a membrane. It was concluded that the physics behind erosion depends significantly on the means of generating cavitation (acoustic, hydrodynamic, laser light) and the specimen characteristics (thin foil, massive specimen), which makes comparison of results of materials resistance to cavitation from different experimental set-ups questionable. Further development of the shape from shading method in the scope of cavitation erosion testing will enable better evaluation of cavitation erosion models.
先前的研究表明，单个空化泡的崩溃可能会导致不止一个侵蚀坑（Philipp 和 Lauterborn [1]）。但我们的初步研究结果恰恰相反——在某些情况下，单个空蚀坑可能是由多个空化事件引起的。本研究对此现象进行了更深入的调查。对超声空化对薄铝箔的侵蚀效应进行了研究。在研究中，我们通过高速摄像机（>1000 fps）观察单个蚀坑的形成，并通过立体视觉和阴影形状法对一系列图像进行定量评估。这使得能够重建蚀坑形状的时间演化。结果表明，在最终穿孔之前，箔片会发生多次变形。结果表明，较大的单个蚀坑是由同一区域的多次冲击波冲击造成的，这意味着它们只是蚀坑群的特殊情况（蚀坑完全重叠的蚀坑群）。最后表明，受到空化作用的薄箔片表现为膜。可以得出结论，侵蚀背后的物理学原理在很大程度上取决于空化的产生方式（声空化、动空化、激光空化）和试样特性（薄箔片、块状试样），这使得来自不同实验装置的材料抗空化性能的结果比较值得怀疑。在空蚀测试范围内，阴影形状法的进一步发展将能够更好地评估空蚀模型。
