Investigation of cavitating sound fields
Ultrasound has many fields of application in medicine and technology. In fluids, the effects achieved are often caused by cavitation which may produce extreme conditions locally. Due to the high negative pressure during the expansion part of the ultrasound wave the medium ruptures especially at distorting particles (cavitation nuclei) and the bubble obtained oscillates and implodes, generating high temperature and pressure or strong micro streaming. This process depends, however, on many parameters and is, in addition, of a very stochastic nature. These circumstances make the control and optimisation of application processes extremely difficult. The operation parameters are, for example, deduced to a large extent empirically or set "by experience". An objective description is still missing as well as the possibility of comparing processes with each other.
One possibility of an objective description is the measurement of the sound field as the driving force of cavitation. The measurement of sound fields is a well-established technique and provides reliable results, but the high pressure amplitudes and the destruction potential of the sound field make an application in cavitating fields difficult. Furthermore, clear relations between the obtained sound field parameters and the generated effects of cavitation do not exist and the question as to the possible profit of a sound field measurement has not been sufficiently answered yet.
Within the scope of a project of Working Group 1.63, which is still running, a completely new measurement system has been established at PTB which acquires sound field parameters in cavitating fields in three spatial dimensions with high local resolution. Different sound detecting techniques, such as fiber-optical sensors or piezo-electric hydrophones, can be applied. In addition, detection techniques for determining cavitation effects were developed which also allow spatially resolved measurements. Both techniques allow the direct combination of sound field measurements with effect detection. With these data, it is now possible to answer the question in what way sound field measurements can be used for an analysis of the application processes.