High Speed Observation of Damage Created by a Collapse of a Single Cavitation Bubble
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One of the remaining open questions in the cavitation erosion research is the one on the importance of the microjet and shock wave on the formation of the pit. Up until now no successful attempt was made to study this in detail, mainly because the damage could only be detected and evaluated after several successive bubble collapses. Based on our previous work on laser induced shock waves and on simultaneous observation of the dynamics of cavitation clouds and cavitation damage, we developed a technique which enables simultaneous observation of one single cavitation bubble collapse and the damage it creates.
A bubble with a maximum diameter of up to 5 mm was formed by a Nd:YAG laser. The damage was observed on a 9 μm thick aluminum foil which was attached to the glass substrate. Two high speed cameras were simultaneously used. One captured the dynamics of the bubble, while the other recorded the damage on the foil.
We also observed the collapse of a bubble in the presence of shear flow. Most of damage was created by the microjet mechanism. Sometimes, the collapse of the bubble rim, at rebound of the initial bubble caused pits in a well-known circular pattern.
From the recordings at the very fastest acquisition rate, we determined that the material deforms and then partially relaxes, while a significant deformation remains. The whole process is only 2-3 μs long.