Fluid Mechanics of Cavitation in Orbital Atherectomy.

摘要

The original motivation for this thesis was to explore and quantify the fluid mechanics of rotational atherectomy. However, as the topic developed, it broaden appreciably to encompass the exploration of the efficacy of turbulence modeling, bubble nucleation and growth, two-phase flow in rotational systems, and means for determining difficult to measure fluid flow characteristics. The work that finally emerged was a synergistic blending of experimentation and numerical simulation. In some instances, the simulation guided the experimental work, while in others the experiments served to guide and validate the simulation models.;Another issue dealt with in preparation for the study of the rotational atherectomy device is the nucleation and growth of bubbles. The need for this focus was the concern, often suggested by certain medical practitioners, that the high-rotational velocities of the device would give rise to locally low pressures in the flowing medium (blood and additives). The existence of pressures below the vapor pressure of the medium would give rise to cavitation bubbles. The bursting of such bubbles is known to create a high-velocity jet which, if impinged on an artery wall, would cause necrosis.;Bubbles may be created by a number of different physical processes other than cavitation. In particular, the presence or absence of nucleation sites is a major factor in the creation of bubbles. To gain a thoroughgoing understanding of the entire process of bubble creation and collapse, a theoretical development was pursued. That development was guided by experimental results present in the literature. The model that was created for the numerical simulation yielded results that were consonant with the experimental data.;The possible presence of bubbles in a liquid flow creates a fluid regime termed two-phase flow. To adhere to the rotational fluid theme, experiments and corresponding modeling was performed for an impeller-driven flow in a contained fluid environment. This physical situation is closely aligned with rotational atherectomy. The investigated situation was designed to enable an initial configuration in which the liquid interfaced with a gas at a horizontal free surface to metamorphize into a curved free-surface interface. In particular, a method of dealing with two-phase flows was evaluated and then successfully implemented.;The main focus of the work was a synergistic fluid-mechanic analysis of the rotating atherectomy device positioned in two independent environments: (a) a transparent horizontal tube whose diameter was chosen to model that of the superficial femoral artery and (b) a large open-topped transparent container. The atherectomy device consisted, in essence, of a shaft on which is mounted an enlarged section called the crown. The crown is coated with an abrasive material whose function is to grind hardened plaque and thereby rejuvenate the arterial function.;The tube-based experimentation provided both observational and quantitative data. With respect to former, flow visualizations implemented by means of a tracer medium did not reveal the presence of bubbles. With regard to this finding, it is relevant to convey the caveat that inherent optical constraints provided a bound on the smallest observable bubbles. The extracted quantitative information included velocity magnitudes which were compared with those of the numerical simulations and virtual congruence was found to occur. The injected tracer medium also enabled the observation of patterns of fluid flow. These patterns were found to be in close accord with those predicted by the simulations. An additional product of the experimentation was the opportunity provided to investigate situations which were beyond those that could be modeled numerically. These situations included the case in which the crown was positioned eccentrically and in which the shaft was flexible rather than rigid. These two realities brought in laboratory experimentation into close accord with the operational experience. (Abstract shortened by UMI.).