Orbital atherectomy is a means of removing plaque from a stenosed artery which uses a
grinding head attached to a catheter to break up the accumulated plaque. The grinding
head, referred to as crown or burr, is rotated at very high rotational velocity during the
operation, from 80, 000rpm to 200, 000rpm. Because of this high rotational velocity,
there are many concerns regarding the possible harm that can be done to the patient.
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.
Since rotational fluid mechanics underlay the entire enterprise, an imperative initial
task was to deeply explore the applicability of various turbulence models to such flows.
In that regard, the situation of single-phase flow in an annular space bounded by a
rotating inner cylinder and a stationary outer cylinder was used as a standard. That
physical situation had been the subject of an in-depth experiment-based doctoral thesis
noteworthy for its care and attention to detail. The examination of the efficacy of
turbulence modeling encompassed two different categories of models.
One of these categories dealt with models based on isotropic turbulence. Models
in this category result in a turbulent viscosity that is the same for all three possible
directions of fluid flow. However, in consideration of the ultimate fluid flows to be
considered here, which include superimposed axial and rotational motions, isotropic
turbulence cannot be expected to prevail. In this light, consideration was also extended
to models in which the isotropic turbulence assumption was dropped in favor of different
turbulence intensities in each of the possible directions of fluid flow. To implement the
use of non- isotropic turbulence models, certain existing formulations were extended to
levels rarely encountered in the published literature. The outcome of the extensive study of turbulence models led to a logic-based selection
of the optimum one for the fluid mechanic investigation that is central to this
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
The possible presence of bubbles in a liquid flow creates a fluid regime termed twophase
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
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
From the experimentation in the open-topped container, both observational and
quantitative results were also extracted. Again, these findings strongly supported those
from the numerical simulations.
Overall, the four interrelated parts of this thesis provided ample opportunity to
delve deeply into highly complex fluid-mechanic phenomena. The logic-based selection
of turbulence models represents the most complete study of this category compared with
the less thoroughgoing comparable studies in the literature. The bubble nucleation and
growth models implemented here were strongly supported by experimental data. With
regard to the two-phase fluid-mechanic investigation, the overall satisfactory agreement
of the numerical predictions with the experimental data provide license for the use of the
simulation model for related problems involving the separation of particles immersed in
a liquid medium. Finally, all the fluid-mechanic issues related to rotating atherectomy
were fully resolved.