Cavitation microjet and shock wave in a signal from impact load measurement

Nugzari Kardava

Master's thesis

Abstract:

This master thesis work is done to try and describe processes that occurred during cavitation bubble collapse near a solid boundary, to try and separate cavitation microjet and shock wave impact from the signal received by PVDF sensors. The thesis starts with the introduction of the general terms connected to the cavitation phenomenon itself. Literature related to the bubble interaction phenomena near solid wall and usage of PVDF films is also reviewed. The experimental setup, with the method of bubble generation and used equipment are also described. Two PVDF sensors were fixed to two metal plates in order to receive signals from the bubble collapse, generated by the underwater spark discharge. Calibration of the sensors was done with the ball drop method. The experiments were observed by the Nanosense MKIII CCD camera at 10000 fps. Bubbles were generated at different dimensionless stand-off distances and impacts were further analyzed and compared to different articles. A small chapter is also dedicated to describe the possible problems that can occur during the experiments using PVDF films. Inverse squares law was used to check if the theoretical and experimental shock wave propagation were the same. This law works only for spherical shock sources, results of the experimental values during specific ranges are in the acceptable level with the theoretical model. Maximum impact forces and loading stresses (pressures) are shown and discussed. The exact values of the cavitation microjet are not recorded as a significant event, but several important assumptions were made, especially when the dimensionless stand-off (proximity) parameter is in the range between 0.8 and 1. Double peak-shaped signals are received in this range and these peaks are created by the splash effect (accompanied by the shock waves) due to the microjet and by the moving flow towards the ring (as well can be interpreted as a bubble ring collapse).

Abstract:

This master thesis work is done to try and describe processes that occurred during cavitation bubble collapse near a solid boundary, to try and separate cavitation microjet and shock wave impact from the signal received by PVDF sensors. The thesis starts with the introduction of the general terms connected to the cavitation phenomenon itself. Literature related to the bubble interaction phenomena near solid wall and usage of PVDF films is also reviewed. The experimental setup, with the method of bubble generation and used equipment are also described. Two PVDF sensors were fixed to two metal plates in order to receive signals from the bubble collapse, generated by the underwater spark discharge. Calibration of the sensors was done with the ball drop method. The experiments were observed by the Nanosense MKIII CCD camera at 10000 fps. Bubbles were generated at different dimensionless stand-off distances and impacts were further analyzed and compared to different articles. A small chapter is also dedicated to describe the possible problems that can occur during the experiments using PVDF films. Inverse squares law was used to check if the theoretical and experimental shock wave propagation were the same. This law works only for spherical shock sources, results of the experimental values during specific ranges are in the acceptable level with the theoretical model. Maximum impact forces and loading stresses (pressures) are shown and discussed. The exact values of the cavitation microjet are not recorded as a significant event, but several important assumptions were made, especially when the dimensionless stand-off (proximity) parameter is in the range between 0.8 and 1. Double peak-shaped signals are received in this range and these peaks are created by the splash effect (accompanied by the shock waves) due to the microjet and by the moving flow towards the ring (as well can be interpreted as a bubble ring collapse).

Keywords

cavitation, cavitation bubble, bubble collapse, shock waves, microjet, PVDF sensor.

Language used: English

Date on which the thesis was submitted / produced: 4. 6. 2021

Thesis defence

• Supervisor: Ing. Jan Hujer, Ph.D.

Technical University of Liberec

Faculty of Mechanical Engineering

Master programme / field:
Mechanical Engineering / Machines and Equipment Design