KZK Texas Time Domain page
Simulations of an electromagnetic acoustic source
(EMAS). The EMAS is modelled after a source built by Bruce Mortimer at The
University of Witwatersand, South Africa. A schematic of the source is
shown in the figure on the right. A helical coil is wrapped onto a base
which has a spherically concave surface. The coil is covered by an
insulating membrane and then a metallic membrane. When a current is
quickly discharged through the coil the metallic plate is repulsed and a
focused acoustic shock wave is generated.
The device has a radius of curvature of 60 mm and an aperture diameter of 75 mm; the distance from the front face of the source to the focal point is approximately 47 mm. Two source waveforms are shown in the figure. The upper one models the measured waveform near the device. The lower waveform is an idealised model for comparison purposes. The duration of the leading positive pulse is about 10 µs and so a characteristic frequency is 50 kHz.
The shock wave was propagated in two fluids: water and a fluorocarbon liquid (FC-43). The latter is of interest because it has has a low sound-speed and the highest coefficient of nonlinearity reported in the literature. The relevant acoustic properties for the two fluids are:
| Property | FC-43 | Water |
| Density (kg/m3) | 1880 | 1000 |
| Sound-speed (m/s) | 655 | 1500 |
| Beta (1+B/2A) | 13.2 | 5 |
| Viscosity (centistokes) | 2.8 | 0.9 |
The simulations were carried out using the KZK Texas time domain code at the Boston University Scientific Computing and Visualisation centre. The source condition was modelled by phasing the source condition at the plane z=0 at the mouth face of the source. The source waveform was either of the two shown. The amplitude of the source waveform across the face of the source was shaded in three way:
Some of the results from the simulations are given here:
KZK Texas Time Domain page
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| robinc@bu.edu | http://people.bu.edu/robinc/kzk/emas | September 1999 |