Prof. Dr.-Ing. Reinhard Lerch
Final exam :
February 03, 2003
High power ultrasound sources have found their way into a wide variety of applications, ranging from medical ultrasound, like Lithotripsy or HIFU-therapy (High Intensity Focused Ultrasound), to ultrasonic cleaning or welding and sonochemistry. In contrast to ultrasonic applications with sources which radiate low amplitude pressure waves, the appearance of nonlinear effects like sawtooth and shock formation is observed at high power ultrasonic sound generators. The appearance of these nonlinear effects take account for speeding up chemical and metalurgical reactions, making medical imaging more efficient or simply influence the application flow in satisfactory manner.
Because of the fact that there are no appropriate design tools, especially for high power applications, available, simple design loops are used for the development in this area. In order to speed up the rapid development of this kind of ultrasound sources we are using numerical simulation schemes based on the finite element method. Based on a nonlinear wave equation first derived by Kuznetsov is this simulation sceme generally applicable to 3D nonlinear sound field calculations in lossy, viscous fluid media. As a practical application a HIFU-source, like used in medical thermo-therapy, is built up.
High and low power measurements are performed in the focus region of the source, were pressure amplitudes of serveral MPa are determined. Nonlinear behavior of the propagation medium is observed.
For the verification of our finite element method analytical Fay and Fubini solutions are compared with simulation results. The force density calculations, based on Reynolds stress calculations,
are compared to analytical solutions for plane wave models. Furthermore an ultrasonic pulse source as well as sound fields of plane pistons are calculated. The measured data is compared with corresponding simulation results.
The good agreement shows the applicability of our method.