ULTRASOUND CHARACTERIZATION OF DISTURBED FLOW PATTERNS
Abstract
There is a great deal of interest in detecting disturbed flow patterns from physiological as well as fluid mechanics standpoints. It is well known that vortex shedding flow patterns occur distal to constrictions in occlusive cardiovascular disease. The non-invasive detection of such flow patterns is of diagnostic value. Fluid mechanicians too are interested in these flow patterns because they occur in a variety of non-biological fluid flow situations. Vortex shedding occurs in jets, aircraft wakes and around bluff bodies. The flow patterns influence the mechanical properties of the system. In this study techniques to observe flow patterns using pulsed RF Doppler ultrasound are investigated and experimental measurements on disturbed vortex shedding flows are carried out. The study starts out with an analysis of vortex structures and the possible characteristics that are manifest in the Doppler signals. It is found that the best geometry is to observe the flow pattern with the ultrasound beam normal to the flow direction. A new method for implementing a directional pulsed RF Doppler system is investigated and a scheme involving hybrid operation is devised. A stable and accurate phase shifter is obtained by implementing the discrete Hilbert transform. The Autoregressive method of spectral estimation is implemented together with the Final Prediction Error criterion for the selection of the correct autoregressive model order, to obtain accurate estimates of the Doppler frequencies over short time intervals. The temporal spectral analysis is successfully used to obtain the development of the flow patterns with time. In order to obtain the characteristics of vortex structures that are manifest in Doppler signals a realistic acoustic scattering model is developed from which the Doppler signals pertaining to a known Karman vortex velocity field are simulated on the computer. The characteristics of the vortices that are detected from the simulation studies are used in studying experimental flow patterns. Experimental measurements are made on steady channel flows. The flow patterns detected from experimental Karman Vortex Streets match the simulations very closely. The ability of the ultrasonic techniques to detect isolated vortices is also experimentally determined. Experimental investigations of in vitro bio-fluid flows are also carried out under steady and pulsatile conditions. Stenosis sections of fifty percent and seventy-five percent occlusion are introduced into a tube flow and the flow patterns distal to the stenoses are measured. Under steady flow conditions vortices are shed distal to both the fifty percent and seventy-five percent stenoses. The milder the stenosis the more distinct and regular is the vortex shedding. Under pulsatile flow conditions the flow patterns are studied on a beat by beat basis. The development of the flow pattern during the beat is investigated. It is found that vortex shedding is a sensitive indication of a mild stenosis whereas high frequency velocity fluctuations or eddies are present in moderate stenoses. A decrease in the frequency parameter has been found to cause a destablization of the flow pattern.
Degree
Ph.D.
Subject Area
Electrical engineering
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