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Showing papers by "Ephraim M Sparrow published in 2008"


Journal ArticleDOI
TL;DR: In this paper, the breakdown of laminar pipe flow into transitional intermittency is predicted numerically here for the first time, and the locations in the pipe at which both laminars breakdown and fully developed attainment occur are provided.
Abstract: The breakdown of laminar pipe flow into transitional intermittency is predicted numerically here for the first time. Subsequent to transitional intermittency, a fully developed regime is achieved wherein the flow may be either intermittent or fully turbulent. Fully developed friction factors are predicted as a function of the Reynolds number throughout the intermittent regime. These predictions successfully bridge the gap between well-established laminar friction factors and turbulent friction factors. Definitive numerical information is provided about the locations in the pipe at which both laminar breakdown and fully developed attainment occur. These locations are a function of the Reynolds number. The streamwise changes in the velocity profiles reflect the complex evolution of the flow as it passes through the successive regimes. For design purposes, information is provided for the pressure drop that characterizes the evolving flow. The numerical results correspond to an inlet turbulence intensity leve...

67 citations


Journal ArticleDOI
TL;DR: The shear stress on the artery wall, a major determinant of the buildup of plaque, is found to be higher for a debulked artery than for a plaque-narrowed artery, and the outcome is favorable in that the higher the wall shear, the lower the rate of plaque formation.

41 citations


Journal ArticleDOI
TL;DR: In this paper, the authors quantify the behavior of popular turbulence models in regions where the flow is laminar according to the accepted Reynolds number criteria, and evaluate three models: 1) standard k-e, 2) k-ω, and 3) SST.
Abstract: Attention is focused on physical situations in which separate zones of laminar and turbulent flow coexist within a given device. The specific motivating application for this study is a complex fluid-flow manifold used in the thermal management of electronic equipment. The adopted approach to the solution of this fluid-flow problem was to employ a turbulent-flow model for the entire solution space but to use several turbulence models as inputs. The goal of the study was to quantify the behavior of popular turbulence models in regions where the flow is laminar according to the accepted Reynolds number criteria. Three models were considered: 1) standard k-e, 2) k-ω, and 3) SST. Evaluation of the models was based on the values of the ratio μ t /μ in nominally laminar regions, where μ t and μ are, respectively, the turbulent and molecular viscosities. Values of this ratio well below one in a nominally laminar region would indicate that a turbulence model reduces, in effect, to a laminar model. It was found tha...

41 citations


Proceedings ArticleDOI
25 Jun 2008
TL;DR: In vitro and cadaver experiments, coupled with numerical simulations, were performed to assess the possibility that orbital atherectomy might cause thermal damage of tissue as discussed by the authors, and concluded that it is extremely unlikely that the use of an orbital debulking device, the Diamondback 360°™ (Cardiovascular Systems, Inc), can lead to thermal injury of the artery wall.
Abstract: In vitro and cadaver experiments, coupled with numerical simulations, were performed to assess the possibility that orbital atherectomy might cause thermal damage of tissue. The experiments involved debulking operations on a surrogate artery and on the plaque-lined posterior tibial artery of a cadaver. Temperatures and coolant flow rates measured during these experiments enabled a numerical simulation of the debulking of a plaque-lined artery in a living human. The temperature variations from the numerical simulations were used to evaluate a thermal injury index. The resulting values of the index were found to be several orders of magnitude below the threshold value for thermal injury. It is concluded that it is extremely unlikely that the use of an orbital debulking device, the Diamondback 360°™ (Cardiovascular Systems, Inc.), can lead to thermal injury of the artery wall.Copyright © 2008 by ASME

10 citations


Journal ArticleDOI
TL;DR: In this article, numerical simulation was used to investigate natural convection in rectangular enclosures ranging in aspect ratio from tall, narrow shapes to flat, wide shapes, and two fundamental thermal boundary conditions were considered.
Abstract: Numerical simulation was used to investigate natural convection in rectangular enclosures ranging in aspect ratio from tall, narrow shapes to flat, wide shapes. For each of the selected aspect ratios, the Rayleigh number was varied over a range of approximately four decades. Two fundamental thermal boundary conditions were considered. One of these, commonly designed as heating from below, involved the imposition of a temperature at the lower bounding wall that is higher than that at the upper bounding wall. The side walls of the enclosure were maintained adiabatic for this case. The second employed boundary condition was side walls at different uniform temperatures and adiabatic upper and lower boundaries. For each aspect ratio and for both types of boundary conditions, the onset of natural convection from the regime of pure conduction occurred at a definitive value of the Rayleigh number, termed the critical Rayleigh number. Once that threshold value was exceeded, the Nusselt number increased markedly wi...

8 citations


Journal ArticleDOI
TL;DR: The method of numerical simulation has been employed to evaluate the use of a gas as a heating medium for endometrial ablation for the treatment of menorrhagia and uterine fibroids, showing the capability of the gas-based therapy to target specific zones which require treatment, while sparing healthy tissue.
Abstract: The method of numerical simulation has been employed to evaluate the use of a gas as a heating medium for endometrial ablation for the treatment of menorrhagia and uterine fibroids. The simulations encompassed fluid flow and heat transfer within the gaseous medium which serves to heat the uterine lining and the coupled heat conduction in the uterine tissue. For the case study featured here, helium at a temperature of 140 °C was employed as the heating medium. A total therapy duration of 6 min was modeled. The outcome of the simulation has provided quantitative information about the detailed necrosis depths that can be attained by the application of the therapy. In particular, necrosis depths on the order of 5 mm were achieved. It was also shown that by making use of a tailored pattern of fluid flow within the uterine cavity, particular zones on the uterine lining such as fibroids can be selectively targeted. Furthermore, the duration of the therapy needed to achieve a desired degree of necrosis can be predicted in advance. The advantages of a gas-based therapy relative to a liquid-based therapy are identified. Significant among these is the capability of the gas-based therapy to target specific zones which require treatment, while sparing healthy tissue. Another significant advantage of gas-based therapy is that it enables the use of higher temperatures which, in turn, allows the therapeutic process to be performed in a shorter time duration.

8 citations


Journal ArticleDOI
TL;DR: In this paper, a multidimensional numerical simulation has been used to investigate the creation of fine fiber by the melt-blown process, which involves highly complex fluid flow and convective heat transfer processes.
Abstract: In this paper, a multidimensional numerical simulation has been used to investigate the creation of fine fiber by the melt-blown process. The problem involves highly complex fluid flow and convective heat transfer processes. The fine fiber is created by the use of high-velocity, obliquely impinging air jets whose high shear forces stretch a polymer extrudate in the partially fluid state. High-temperature air is used to maintain the fluidity of the polymeric material as it exits the die. The model which was developed and implemented for the simulation closely reflects the physical situation employed in the actual production of fine fibers. The actual configuration of fine fiber production is a linear array of orifices from which the partially fluid polymer emerges. The array contains inherent symmetries which enable the problem to be tractable. Notwithstanding this, the numerical simulation still required multimillions of control volumes to achieve results of practical relevance. The results of the simulat...

2 citations


Proceedings ArticleDOI
01 Jan 2008
TL;DR: In this article, the accuracy of a direct contact device for measuring skin surface temperature was investigated using numerical simulation, and it was shown that a variation of thermal conductivity of the foam has a greater effect on the error rather than the variation of the blood perfusion rate.
Abstract: Numerical simulation is used to investigate the accuracy of a direct-contact device for measuring skin-surface temperature. A variation of thermal conductivity of the foam has greater effect on the error rather than a variation of the blood perfusion rate. For a thermal conductivity of zero, an error of 1.5 oC in temperature was identified. For foam pad conductivities of 0.03 and 0.06 W/m-oC, the errors are 0.5 and 0.15 oC. For the transient study, with k=0 W/m-oC, it takes 4,900 seconds for the temperature to reach steady state compared with k=0.03 W/m-oC and k=0.06 W/m-oC where it takes 3,000 seconds. The configuration without the foam and in presence of an air gap between the skin surface and the sensor gives the most uniform temperature profile.

1 citations