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UBC Theses and Dissertations

Hydrodynamic performance of an artificial aortic valve implant Aminzadeh, Mohammad

Abstract

The project aims at studying fluid dynamics of a prosthetic aortic heart valve through an organized experimental programme with a view to obtain better appreciation of factors affecting its performance and eventual failure. The problem is approached in stages representing an increasing order of difficulties in terms of experimental set-up and interpretation of data. Design, construction and calibration of the glycerol tunnel, which formed the fundamental test facility for the project, are described briefly in the beginning followed by an introduction to the test models and monitoring instrumentation. A simple theory for wedge shaped hot film anemometer is developed and velocity profiles with and without models in the test section presented. Essentially the test programme consists of three stages: (i) a single sphere by itself during stationary and oscillating conditions, R[sub n] = 74 - 5838; (ii) a stationary spherical poppet occupying different positions in the cage representing quasi-steady opening and closing of the valve, R[sub n] = 220 - 1200; (iii) poppet pulsating according to the recorded displacement-time history when implanted in a patient, R[sub n] = 290 - 650. A careful choice of reference pressure and velocity results in presentation of data in a manner that would permit reproduction and comparison of results by investigators using different test facilities. The static pressure distribution data and resulting information on the average base pressure, separating shear layers, its movement, etc. are obtained for closed and open bypass conditions of the tunnel, the latter representative of regurgitation. An extensive flow visualization programme using the dye injection procedure in conjunction with still and high speed movie photography complemented the experimental data. It resulted in better appreciation of the physical character of the flow. Studies with a single sphere vividly showed the progress of formation, elongation and instability of the vortex ring with the Reynolds number. Of particular interest was the distinct rise in the minimum and base pressures in the Reynolds number range of 240 - 475, which was found to be associated with the onset of instability of the vortex ring leading to its periodic shedding. In general, the Reynolds number effects were confined to the region near and downstream of the minimum pressure point. The experimental results clearly suggested inherent limitations in the analytical procedures for predicting pressure distribution as proposed by several investigators. On the other hand, the pressure integrated drag results compared favourably with the directly measured drag data reported in the literature thus substantiating the reliability of the pressure measurements. Coming to the pulsating sphere, effects of the oscillation frequency, Reynolds number and sphere position are investigated during both forward and reverse strokes of a cycle. Influence of the Beta number is confined to the local changes in the character of the base pressure plots without substantially affecting their average magnitudes. A decrease in y[sub s] leads to increase in the minimum and average wake pressures together with a forward movement of the separation point during the forward stroke. The general character of the pressure profiles remains the same during the reverse stroke, however, the magnitudes involved are markedly different. Next, the thesis discusses a more realistic situation, from the configuration consideration, of a stationary poppet occupying different positions in the cage. The Reynolds number influence, which is more significant for y[sub b] < 0.2, is essentially reflected in the increase of the wake pressure. A significant rise in the negative pressure coefficient at y[sub b] ≤ 0.1 would suggest large shearing stresses leading to possible deformation and destruction of the red blood cells. The presence of distributed vorticity and the associated centrifugal field in the wake are suspected to be the fundamental factors promoting dissociation of the blood into its constituents and finally their deposition on the body of the valve. Finally, the case of poppet oscillating inside the valve is considered which shows the effect of valve opening on C[sub p] to be far greater than that observed during the stationary case. The high negative pressure, at the pulsation frequency, in the wake region and the associated large periodic shear stresses are likely to cause not only coagulation of the cells leading to clotting but may be responsible for rupturing of the suture lines in the implanted prosthetic valve, as often encountered in practice.

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