Showing papers by "Ephraim M Sparrow published in 2007"

Numerical Simulation of the Urine Flow in a Stented Ureter

Abstract: When a stent is implanted in a blocked ureter, the urine passing from the kidney to the bladder must traverse a very complicated flow path. That path consists of two parallel passages, one of which is the bore of the stent and the other is the annular space between the external surface of the stent and the inner wall of the ureter. The flow path is further complicated by the presence of numerous pass-through holes that are deployed along the length of the stent. These holes allow urine to pass between the annulus and the bore. Further complexity in the pattern of the urine flow occurs because the coiled "pig tails," which hold the stent in place, contain multiple ports for fluid ingress and egress. The fluid flow in a stented ureter has been quantitatively analyzed here for the first time using numerical simulation. The numerical solutions obtained here fully reveal the details of the urine flow throughout the entire stented ureter. It was found that in the absence of blockages, most of the pass-through holes are inactive. Furthermore, only the port in each coiled pig tail that is nearest the stent proper is actively involved in the urine flow. Only in the presence of blockages, which may occur due to encrustation or biofouling, are the numerous pass-through holes activated. The numerical simulations are able to track the urine flow through the pass-through holes as well as adjacent to the blockages. The simulations are also able to provide highly accurate results for the kidney-to-bladder urine flow rate. The simulation method presented here constitutes a powerful new tool for rational design of ureteral stents in the future.

Attainment of flowrate uniformity in the channels that link a distribution manifold to a collection manifold

Abstract: A logic-based systematic method of designing manifold systems to achieve flowrate uniformity among the channels that interconnect a distribution manifold and a collection manifold has been developed, implemented, and illustrated by case studies. The method is based on tailoring the flow resistance of the individual channels to achieve equal pressure drops for all the channels. The tailoring of the flow resistance is accomplished by the use of gate-valve-like obstructions. The adjustment of the valve-like obstructions is determined here by means of numerical simulations. Although the method is iterative, it may converge in one cycle of the iterations. Progress toward the goal of per channel uniformity can be accelerated by tuning a multiplicative constant. The only departure of the method from being fully automatic is the selection of the aforementioned multiplicative constant. The method is described in detail in a step-by-step manner. These steps are illustrated both generically and specifically for four case studies. As an example, in one of the case studies, an original flow imbalance of over 100% in an untailored manifold system was reduced to a flow imbalance of less than 10% in one cycle of the method.

Numerical simulation of a BPH thermal therapy--a case study involving TUMT.

Abstract: The use of numerical simulation as a means to predict the outcome of transurethral microwave thermotherapy (TUMT) is set forth in detail. The simulation was carried out as a case study of a specific TUMT procedure. The selection of the case study was based on the availability of extensive medical records which documented an extraordinary application of TUMT. Predictions were made of the time-varying temperature patterns within the prostate, the bladder, the sphincter, the pelvic floor, and the fat and connective tissue which envelop these organs. These temperature patterns provided the basis of maps which highlighted those locations where necrosis occurred. An injury integral was used to predict the extent of the necrotic tissue produced by the therapy. It was found that, for the specific case being considered, necrosis occurred not only within the prostate but also extended to the neck of the bladder and to the fatty tissue. A special feature of the simulation was the accounting of the liquid-to-vapor phase change of the interstitial water. The vapor generated by the phase change is believed to significantly enlarge the region of necrosis. By the same token, the vapor pressure is expected to cause motion of the high-temperature liquid to deep-tissue regions. The damage predicted by the numerical simulation was compared, in detail, with post-operative medical examinations and found to be corroborated.

A Quasi-Analytical Method for Fluid Flow in a Multi-Inlet Collection Manifold

Abstract: This paper sets forth a fully validated quasianalytical method for determining the fluid flow in a multi-inlet collection manifold. The method is based on first principles, which are the conservation laws for mass and momentum. Although it is necessary to use numerical means to extract results from the model, the solution task is accomplished by the use of a spreadsheet, without the need for complex software or large computer assets. The validation of the method was achieved by comparing the key results with those from a numerically exact simulation. The comparison included both local results and global results. For the local results, the accuracy of the model was found to be in the 1% range, while the global results from the model were accurate to about 4%. The investigated manifold was a case study drawn from a problem involving thermal management of electronic equipment, in which an array of coldplates discharged spent air into the manifold. It was found, from both the quasianalytical method and the numerical simulation, that there is a variation in the per-coldplate flowrate due to axial pressure variations in the manifold. These pressure variations can be attributed to the streamwise acceleration of the manifold flow due to the accumulation of the flow entering the manifold from the coldplate array. The utility of the quasianalytical method was further demonstrated by applying it to a number of other cases. In particular, the method was used to design a manifold capable of producing a uniform mass flowrate through all of its ports. DOI: 10.1115/1.2717620

Numerical Simulation of Axisymmetric, Turbulent Buoyant Plumes—Application to Displacement Ventilation

Abstract: The displacement ventilation method of providing quality indoor air relies on the use of buoyant plumes generated by discrete heat sources. A number of practical displacement ventilation techniques are based on the model of a buoyant plume generated by a point heat source. Such a model has a number of intrinsic flaws. In particular, it is inapplicable in a finite region above the source. Furthermore, the solutions for the point source, when obtained for turbulent flow, require empirical inputs for their completion. These limitations have motivated the present numerical simulation. Consideration is given here to the turbulent buoyant plume generated by a heated finite sphere. In the limit of a vanishing radius, the sphere reduces to a point source. The numerical simulations are based on a model free of simplifying assumptions, aside from axisymmetry. The resulting numerical solutions have quantified the zone in which the point-source solutions are valid. Furthermore, the virtual origins needed to give mean...

Simulation of Laminar Axisymmetric Buoyant Plumes: A Case Study of the Anatomy of Similarity Solutions

Abstract: This article is concerned with the numerical simulation of the laminar buoyant plume created by a heated sphere situated in an otherwise quiescent environment. The limit of vanishing sphere diameter corresponds to the point source of heat, for which extensive results have been obtained in the literature. The dual objectives of this work are the attainment of an exact characterization of the buoyant plume, and the diagnosis of an existing similarity solution for the limiting problem of the point source. These objectives were fulfilled by means of a numerically exact solution of the equations representing mass, momentum, and energy conservation. The solutions were carried out over a range of the radius-based Grashof number extending from 50 to 5 × 106 for a Prandtl number of 0.7, which corresponds to air. The velocities in the plume were found to increase with elevation above the sphere but approach a fully developed state characterized by congruent (similar) velocity profiles at sufficiently high elevation...