Computer simulation of Doppler spectra associated with moving scatterers for different source configurations and arbitrary trajectories

A finite element method is developed for calculation of fields excited by focused ultrasonic radiators and scattered by moving particles, and the associated Doppler spectra. The algorithm was tested for known cases, e.g. the field along the axis of rectangular and circular aperture transducers. In all cases the algorithm generated data which adequately approximated solutions of simple canonical problems, and results published in the literature. The known phenomenon of Doppler spectra broadening in the presence of disturbed flow is simulated here. It is shown that broadening and ripple are related to nonlinear motion of particles within the sampling volume. Moreover, the results show that Doppler spectra broadening occurs around the Doppler central frequency and that the spectrum shape depends on the particle´s motional characteristics and the sampling density. The results show that broadened spectra attributed to flow reversal are similar to spectra resulting from a disturbed flow within the sampling volume