Browsing by Subject "Cavitation"
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Item Fluid mechanics of cavitation in orbital atherectomy.(2012-06) Ramazani-Rend, RezaOrbital 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 work. 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 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 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. 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.Item LES of turbulent cavitating flows using the homogeneous mixture model(2020-09) Bhatt, MrugankThe objective of this dissertation is to develop LES (large-eddy simulation) capabilities to study cavitation in complex hydrodynamic geometries. A fully-compressible homogeneous mixture model with a finite rate mass transfer is used in the simulations. The ability of the homogeneous mixture approach to capture resolved small-scale vapor bubbles is evaluated by a vapor bubble collapse problem. The effects of physical length scale, surface tension, driving pressure, and dimensionality of the problem are assessed using the parametric study. The finite rate effects of the cavitation model are discussed using the non-dimensional parameters and compared to the flow advection time scales. The expression for the finite rate mixture speed of sound is derived. Partial cavitation over incipient, transitory, and periodic regimes in the experimental sharp wedge configuration of Ganesh et. al. (2016) is investigated. The vapor void fractions obtained from LES shows very good agreement with X-ray measurements in each of the regimes. Physical mechanisms of cavity transition, both re-entrant jet and bubbly shock waves are captured in the LES. Conditions favoring the formation of either the re-entrant jet or the bubbly shock waves are studied through a detailed analysis of streamline curvature, vapor production, and vorticity transport. Flow over a five-bladed marine propeller is studied at design conditions. The assessment of propeller shaft orientation, numerical dissipation, the pressure drop in vortex cores, free-stream nuclei, and grid resolution revealed that the propeller performance is sensitive to the free-stream nuclei content, lower values showing a better comparison to the experiments. A numerical approach based on the preconditioning and the DTS is proposed to address the acoustic stiffness; thereby, enabling the low free-stream nuclei calculations. The novelty of the method lies in the application of preconditioning to a fully-compressible cavitation solver; where the characteristic-based filtering is modified based on the all-speed Roe-type scheme in addition to the traditional time-derivative matrix. The results are demonstrated for the unsteady flow over a cylinder under wetted and cavitation inception conditions, and the LES of low over a propeller under wetted conditions.Item Numerical modeling and simulation of cavitating flows in different regimes(2023-03) Brandao, FilipeThe objective of this dissertation is to develop numerical methodologies for large eddysimulation (LES) of multiphase cavitating flows over different cavitation regimes. Unstructured grids are considered to enable complex geometries to be considered. The dissertation has the following major components: (i) a compressible homogeneous approach for a mixture of water–vapor–gas is developed to study the effect of non–condensable gas on bluff–body cavitation. (ii) Incipient cavitation in the shear layer of a backstep is studied using incompressible simulations of the flow field coupled with a continuum equation for vapor volume fraction. (iii) The inception model is extended to account for multiple groups of bubbles of different sizes and used to investigate the effects of water quality on tip vortex inception. A numerical method based on the homogeneous mixture model, fully compressible formulation and finite rate mass transfer developed by Gnanaskandan and Mahesh [1] is extended to include the effects of non–condensable gas (NCG). We then investigate cavitation over a circular cylinder at two different Reynolds numbers (Re = 200 and 3900 based on cylinder diameter and free–stream velocity) and different cavitation numbers. Two different cavitation regimes are observed depending on free–stream pressure: cyclic and transitional. In the cyclic regime, the cavitated shear layer rolls up into vortices, which are then shed from the cylinder, forming the K´arm´an vortex street, similar to a single phase flow. In the transitional regime, a cavity is formed behind the cylinder, and is only detached after the passage of a condensation shock. As a consequence, there is a drastic drop in shedding frequency. Dynamic mode decomposition (DMD) is performed to explain this change in behavior. DMD reveals that cavitation delays the first transition of the Karman vortex street. The effects of the non–condensable gases on this flow is discussed for both regimes, and it is found that the gas decreases the strength of the condensation shock. It is observed that vapor and gas uniformly introduced in the free–stream, distributed themselves differently in the wake of cylinder depending upon local flow conditions, particularly at lower cavitation numbers as the pressure in the wake dropped below vapor pressure. Vapor and NCG distribution in the boundary layer suggest that cavitation as a mass transfer process only occurs inside a fine layer in the near–wall region, while the remaining boundary layer only undergoes expansion of both vapor and gas. The levels of free–stream void fraction are found to have an impact on the boundary–layer separation point. Vortex stretching and baroclinic torque are greatly reduced in the transitional regime compared to the cyclic regime. Next, the development of a method to simulate cavitation in the incipient regime is presented. The main idea is that since inception is a stochastic process that generates small amounts of vapor for short periods of time, the effects of these small regions of vapor on the liquid density and dynamics can be neglected. Therefore, vapor is treated as a passive scalar in an incompressible liquid. Thus, the equations solved are the incompressible Navier–Stokes equation along with a advection–diffusion equation with source terms for the transport of vapor. The scalar field, however, is advanced in time with a different time step than the one used to advance the velocity field. The model is used to investigate inception in the shear layer of a backward–facing step at Reτ = 1500 (based on skin friction velocity and boundary layer thickness). Statistics are computed for both pressure and vapor volume fraction, and the likelihood of inception is determined. The locations of the preferred sites for cavitation are compared to experimental results and good agreement is achieved. The effects of finite rate evaporation and condensation are revealed by the probability density functions of pressure and volume fraction. The flow topology is investigated and inception is found to occur in the core of the stretched tubular vortical structures with a rotation rate four times higher than the stretching rate. These cavitating tubular structures are elongated two to three times more in their most extensive principal direction than in their intermediate principal direction, and are most likely aligned with the streamwise direction. The model developed for cavitation inception is extended to account for multiple groups of bubbles of different sizes, effectively making it a polydisperse model. This allows us to investigate the effects of water quality on inception. The model is used to simulate inception in a tip vortex of an elliptic hydrofoil at 12 degrees angle of attack and Reynolds numbers of 9 × 105 and 1.4 × 106 based on root chord length and free–stream velocity. It was found that inception is strongly dependent on the amounts of nuclei in the freestream. When the flow is depleted of nuclei, inception is an intermittent event confined to a position very close to the hydrofoils tip. However, when the flow is rich in nuclei, a larger portion of the tip vortex cavitates, as well as part of the suction side very close to the leading edge of the hydrofoil. Probability density functions reveal that cavitation occurs in any region experiencing a pressure field lower than vapor pressure when the flow is rich in nuclei, while extremely low values of pressure (usually kPa of tension) are required to make a flow deplete of nuclei cavitate. The topology of a flow poor in nuclei was investigated and inception was found to occur in regions dominated by irrotational straining with high stretching rates. Particles were released from the hydrofoil tip and tracked. It is seen that at the higher Reynolds number, the particles are more likely to experience low pressures. However, the amount of time they are subject to very low pressures is shorter at the higher Reynolds number.Item Topics in Viscous Potential Flow of Two-Phase Systems(2010-02) Padrino Inciarte, Juan CarlosTwo-phase flows are ubiquitous, from natural and domestic environments to industrial settings. However, due to their complexity, modeling these fluid systems remains a challenge from both the perspective of fundamental questions on the dynamics of an individual, smooth interface, and the perspective of integral analyses, which involve averaging of the conservation laws over large domains, thereby missing local details of the flow. In this work, we consider a set of five problems concerning the linear and non-linear dynamics of an interface or free surface and the study of cavitation inception. Analyses are carried out by assuming the fluid motion to be irrotational, that is, with zero vorticity, and the fluids to be viscous, although results from rotational analyses are presented for the purpose of comparison. The problems considered here are the following: First, we analyze the non-linear deformation and break-up of a bubble or drop immersed in a uniaxial extensional flow of an incompressible viscous fluid. The method of viscous potential flow, in which the flow field is irrotational and viscosity enters through the balance of normal stresses at the interface, is used in the analysis. The governing equations are solved numerically to track the motion of the interface by coupling a boundary element method with a time-integration routine. When break-up occurs, the break-up time computed here is compared with results obtained elsewhere from numerical simulations of the Navier–Stokes equations, which thus keeps vorticity in the analysis, for several combinations of the relevant dimensionless parameters of the problem. For the bubble, for Weber numbers 3 [less than or equal] We [less than or equal] 6, predictions from viscous potential flow shows good agreement with the results from the Navier–Stokes equations for the bubble break-up time, whereas for larger We, the former underpredicts the results given by the latter. Including viscosity increases the break-up time with respect to the inviscid case. For the drop, as expected, increasing the viscous effects of the irrotational motion produces large, elongated drops that take longer to break up in comparison with results for inviscid fluids. In the second problem, we compute the force acting on a spherical bubble of variable radius moving within a liquid with an outer spherical boundary. Viscous potential flow and the dissipation method, which is another purely irrotational approach stemming from the mechanical energy equation, are both systematically implemented. This exposes the role of the choice of the outer boundary condition for the stress on the drag, an issue not explained in the literature known to us. By means of the well-known “cell-model” analysis, the results for the drag are then applied to the case of a swarm of rising bubbles having a certain void fraction. Computations from the dissipation method for the drag coefficient and rise velocity for a bubble swarm agree with numerical solutions; evaluation against experimental data for high Reynolds and low Weber numbers shows that all the models considered, including those given in the literature, overpredict the bubble swarm rise velocity. In the next two problems, we apply the analysis of viscous potential flow and the dissipation method to study the linear dynamics of waves of “small” amplitude acting either on a plane or on a spherical interface separating a liquid from a dynamically inactive fluid. It is shown that the viscous irrational theories exhibit the features of the wave dynamics by comparing with the exact solution. The range of parameters for which good agreement with the exact solution exists is presented. The general trend shows that for long waves the dissipation method results in the best approximation, whereas for short waves, even for very viscous liquids, viscous potential flow demonstrates better agreement. Finally, the problem of cavitation inception for the flow of a viscous liquid past a stationary sphere is studied by means of the theory of stress-induced cavitation. The flow field for a single phase needed in the analysis is found from three different methods, namely, the numerical solution of the Navier–Stokes equations, the irrotational motion of a viscous fluid, and, in the limit of no inertia, the Stokes flow formulation. The new predictions are then compared with those obtained from the classical pressure criterion. The main finding is that at a fixed cavitation number more viscous liquids are at greater risk to cavitation.Item Toughness in block copolymer modified epoxies(2014-09) Declet-Perez, CarmeloOne of the major shortcomings preventing the widespread use of epoxy resins in engineering applications is the inherent brittleness of these materials. The incorporation of small amounts of amphiphilic block copolymers into the formulation is one of the most promising strategies to toughen epoxies. These molecules are known to form nanostructures in the epoxy resin that can be preserved upon curing. This strategy is very attractive since significant enhancements in toughness can be obtained without detrimental effects on other properties of the matrix. Despite many examples of successful implementation, an in-depth understanding of the factors that lead to toughness in block copolymer modified epoxies is still elusive. The goal of this dissertation is to understand, first, the deformation mechanisms leading to toughness and, second, how different formulation parameters affect these processes.In this work we used two types of block copolymer modifiers, which produced nanostructures with different physical properties. These block copolymers self-assembled into well-dispersed spherical micelles with either rubbery or glassy cores in various epoxy formulations. Both of these modifiers toughened different epoxy formulations, although to different extents. The rubbery core micelles consistently outperformed the glassy core micelles by roughly a factor of two. While the toughening afforded by the rubbery core micelles was consistent with the current understanding of toughening, the results from the glassy core micelles could not be explained with the same reasoning.In order to understand the deformation mechanisms leading to different levels of toughness, we performed small-angle x-ray scattering experiments while simultaneously deforming our material. This combination of techniques, referred to as in-situ SAXS, allowed us to monitor changes in the structure of the block copolymer micelles as a result of the applied load. With this technique, we showed that the rubbery core micelles undergo a dilatational process while the glassy core micelles deform with constant volume. These results provide definitive evidence of cavitation in rubbery nanodomains, a result anticipated by theoretical calculations. The notion of cavitation is useful in understanding the toughness enhancement of the rubbery core micelles; however, it does not explain the toughening from the glassy core micelles. To explain the toughening afforded by the glassy core micelles we proposed the idea of network disruption in the region spanned by the corona block. We suggested that this mechanism is also capable of initiating plastic deformation of the matrix, although to a lesser extent than cavitation. Accordingly, the main toughening mechanism in block copolymer modified epoxies is plastic deformation of the matrix initiated by either cavitation of rubbery domains or by the zone of disrupted network depending on the properties of the micelle core. Having established that the matrix is responsible for dissipating the most amount of energy during fracture, we also investigated the effect of varying the crosslink density and flexibility of the network by means of in-situ SAXS. In networks formulated with different crosslink densities, but the same type of molecules, we found a correlation between different levels of toughness provided by either, rubbery or glassy core micelles, and differences in deformability of the epoxy network. In networks formulated with a different crosslinker, which incorporates flexible groups into the matrix, we found that the properties of the network strongly influence the type of deformation the block copolymer micelles undergo. In conclusion, this work has established a connection between different extents of toughening enhancement, the physical properties of the block copolymer micelles, and the properties of the epoxy network. Judicious selection of all of these formulation parameters is needed to obtain an optimal toughening effect.