DOPPLER VELOCIMETRY IN CAVITATING MEDIA

A numerical model has been developed to simulate propagation of ultras onic beams in inhomogeneous moving media. The model is based on the ra y theory of propagating waves, valid in the limit of high frequencies. The resulting equations depend only on local values of the velocity f ield and the speed of sound. In its implementation, the model assumes that the interactions of sound with the surrounding flow field are dec oupled. This allows for applying the model in a post processing mode t o flows computed by other means. The model was used to investigate bea m behavior in unsteady cavitating flows. The study was motivated by re ports of cavitation occurring in mitral bi-leaflet mechanical heart va lves. The flow field and cavitation physics were simulated using a gen eral purpose computer code, CFD-ACE. The ultrasonic beam model was the n used to calculate the beam path, orientation, and frequency changes in the transient cavitating region. Results show that the presence of cavitation can fundamentally alter the beam propagation characteristic s. Simple models that assume rectilinear propagation cannot, by defini tion, handle such flows. Cavitation incurs very large variations in th e local sound speed, which in turn can induce very large distortions i n the beam. This fact has strong ramifications regarding the accuracy of ultrasonic velocimetry when simple models are used to interpret Dop pler data gathered under such flow conditions.