Distributions Of Fluid Research Articles

Significance of the spinal cord position in spinal cord stimulation.

The effects of the antero-posterior and medio-lateral positions of the spinal cord in the dural sac on the perception threshold and paresthesia coverage in spinal cord stimulation were analyzed. The distributions of the dorsal cerebrospinal fluid (CSF) layer thickness, measured from transverse MR scans of normal subjects at various spinal levels, were used to calculate the distributions of threshold voltages for the stimulation of spinal nerve fibers by a computer model. These theoretical threshold distributions were shown to fit well to the corresponding distributions of perception threshold measured in patients. It is concluded that the thickness of the dorsal csf layer is the main factor determining the perception threshold and paresthesia coverage in spinal cord stimulation: an increasing thickness raises the threshold and reduces the coverage, and vice versa. The effects of an asymmetrical electrode position with respect to the spinal cord midline were also analyzed by computer modeling. It is concluded that a lateral asymmetry of less than 1 mm gives a significant reduction of perception threshold and may result in unilateral paresthesiae.

Effects of curvature variations on the nonlinear evolution of Goertler vortices

The nonlinear development of Goertler vortices over walls of variable curvature is studied. The parabolized disturbance equations governing the problem for small curvature, high Reynolds number, and order unity Goertler number are integrated numerically. Cases with concave, convex, and zero curvatures are analyzed. The ‘‘mushroom-shaped’’ distributions of low-momentum fluid riding above high-momentum fluid that are subject to secondary instability are predicted. The results show significant stabilization of disturbances introduced from a concave into a convex region, where new sets of vortices are successively created with opposite rotation to the preceding set. The convex curvature tends to eliminate the inflection points from the spanwise and normal profiles of the streamwise velocity and, hence, suppresses the oscillatory secondary instability that leads to turbulence. Decay of Goertler vortices in a flat region is found to be less rapid than in a convex region.

Cartilage Stresses in the Human Hip Joint

The total surface stress measured in vitro on acetabular cartilage when step-loaded by an instrumented hemiprosthesis are partitioned into fluid and cartilage network stresses using a finite element model of the cartilage layer and measurements of the layer consolidation. The finite element model is based on in situ measurements of cartilage geometry and constitutive properties. Unique instrumentation was employed to collect the geometry and constitutive properties and pressure and consolidation data. When loaded, cartilage consolidates and exudes its interstitial fluid through and from its solid network into the inter-articular gap. The finite element solutions include the spatial distributions of fluid and network stresses, the normal flow velocities into the gap, and the contact network stresses at the cartilage surface, all versus time. Even after long-duration application of physiological-level force, fluid pressure supports 90 percent of the load with the cartilage network stresses remaining well below the drained modulus of cartilage. The results support the "weeping" mechanism of joint lubrication proposed by McCutchen.

Digital chest radiography with storage phosphor systems: Potential masking of bilateral pleural effusions

A photostimulable storage phosphor (PSP) computed radiography imaging system was analyzed for its potential to mask pleural effusion during normal image processing. This phenomenon has been observed in several clinical cases in our hospital. To better understand the relationship between pleural effusion and the PSP radiograph appearance, portable radiographs of an athropomorphic chest phantom were acquired with the PSP system in conditions simulating various quantities and distributions of pleural fluid. It was observed that the optical density of the film in one hemithorax was significantly influenced by whether or not fluid was present in the opposite hemithorax. This optical density dependence was determined to be a system-induced effect that results from the image processing (histogram analysis) technique used by the PSP system during image plate readout. It is important to recognize that the PSP system's normal optical density (sensitivity) adjustment can obscure the presence of bilateral pleural fluid accumulation, particularly if the opposite hemithorax contains fluid in an equal or greater amount.

Differential diagnosis of intraperitoneal fluid by ultrasound.

AbstractA review of 40 clinical cases of proven intraperitoneal fluid was made to assess the accuracy of ultrasonography for diagnosing the nature or distribution of intraperitoneal fluid and for differentiation between fluid collection and other conditions.Since the distributions of intraperitoneal fluid depend on the action of gravity, capillary attraction, adhesion and so forth, localized fluid collection or unusual distribution of fluid poses a problem in differential diagnosis. An unusual or irregular collection of fluid may suggest a malignant ascites.Deviations from a homogenous, echo‐free space suggest an inflammatory, mucinous or malignant fluid.The smallest amount of fluid identified by ultrasound was thought to be approximately 10 ml.

An experimental study of confined jet mixing

Measurements are reported of the mixing between a primary gas flow in a rectangular-sectioned duct and a secondary flow injected at an oblique angle through an orifice in one wall, for injection angles of 45°, 90° and 135° and ratios of secondary to primary mean velocities of 0.3, 1, 5 and 10. The quantities measured include the local concentration distributions of the secondary fluid at a number of cross-sections, the inlet velocity profiles of both streams, and the static pressure variation along the main duct. The results show that, within the range of parameters investigated, the degree of mixing increases with increasing injection angle or velocity ratio, being particularly effective at higher values of the latter when the jet impinges on the opposite duct wall and provokes a more favourable flow structure. The pressure drop data and a simple theory suggest that, from a practical point of view, downstream-oriented injection may be preferable because the jet pump effect will then reduce, if not eliminate, the pressure drop.

Solar thermochemical hydrogen: The heat source-process interface

A discussion of issues and considerations related to the interface between a solar heat source and a thermochemical hydrogen process and some details of a tubular heat exchanger operating as such an interface in a cavity-type receiver are presented. The issues include the temperature and heat input requirements for the endothermic reaction, type of receiver, heat storage, transient operations, and control. A thermal performance analysis of a tubular reactor/heat exchanger operating in a cavity-type solar receiver is applied to SO 3 decomposition. The analysis produces axial distributions of temperature tube wall and process fluid, reaction rate, conversion, velocity, density, pressure and residence time. Process fluid conditions at the inlet, tube characteristics, reaction kinetics and cavity operating temperature are inputs. The cavity temperature affects average heat flux and, therefore, heat-exchanger cost and receiver efficiency and, therefore, mirror field cost. A design which minimizes the combined cost may be found and examples are shown.

Spherical charged fluid distributions in general relativity

Formal features of Einstein–Maxwell equations for spherically symmetric distributions of a charged perfect fluid in equilibrium are discussed. An exact solution of the system of equations for a specified choice of matter density and fluid pressure, representing a charged perfect gas is presented.

A HEAT EXCHANGER COMPUTATIONAL PROCEDURE FOR TEMPERATURE-DEPENDENT FOULING

Abstract A novel heat exchanger computational procedure is described which provides a means of rapidly calculating the distributions of fluid and wall temperatures, deposit formation, and pressure loss at various points in a heat exchanger. The procedure is unique in that it is capable of treating wide variations in heat exchanger geometry without recourse to restrictive assumptions concerning heat exchanger type (e.g., co-flow, counterflow, cross flow devices, etc.). The analysis has been used extensively to predict the performance of cross-counterflow heat exchangers in which one fluid behaves as a perfect gas (e.g., air) while the other fluid is assumed to be a distillate fuel. The model has been extended to include the effects on heat exchanger performance of time varying inflow conditions. Heat exchanger performance degradation due to deposit formation with time can be simulated, making this procedure useful in predicting the effects of temperature-dependent fouling.

Heat transfer and temperature distributions in an agitated tank equipped with helical ribbon impeller.

Temperature distributions of fluid in an agitated tank equipped with a helical ribbon impeller are visualized by means of liquid crystal particles and the relation between temperature distribution and circulation flow is observed. It is concluded that non-uniformities in temperature in an agitated tank should not be neglected in the laminar flow region and that, in this case, a flow model realistic enough to represent the actual flow behavior is necessary. A flow model is developed and applied in order to analyze the steady-state heat transfer system. The introduction of this flow model has been found very effective in expressing the thermal mixing process in an agitated tank with a helical ribbon impeller.

Derivation of transit time distribution of a stationary flow system from velocity and length characteristics

With a particular probability space established for a stationary flow system it is shown that transit-time distribution of nondiffusible tracers is mathematically related to the distributions of path lengths and fluid flowing velocities. Functional forms of the transit-time distribution can be derived by first determining the distributions of path lengths and fluid velocities from basic structural characteristics of a system and then using the techniques of probability theory to convert them to transit-time distributions. The procedure is illustrated by an example.

Configurations with a vector field in the Einstein-Cartan theory

The properties of self-gravitating distributions of an ideal fluid charged by a vector field which is either massive or massless, i.e., electromagnetic, are considered taking account of the spin properties of the vector field in the framework of the Einstein-Cartan theory of gravitation. Conditions for equilibrium are found and the corresponding exact solutions are obtained. A complete system of first integrals of the corresponding equations of motion is found for dynamical distributions in the absence of pressure. A theorem on the correspondence between the dynamics of an electrically charged ideal fluid with a limiting equation of state and the dynamics of a free massive vector field is also proved.

Problems of classical and quantum-gravitational collapse

Various aspects of the statics and dynamics of spherical gravitational distributions of an ideal fluid with distributed sources of zero-mass and massive vector fields are investigated. Both unrestricted configurations and isolated configurations with the appropriate matching to the external geometry are considered.

Experimental Study of Forced Convective Heat Transfer in Annular Passage with Axially Varying Cross Sectional Area

An experimental investigation of forced convective heat transfer in a passage with variable cross sectional area in the axial direction is presented. Experiments were performed with air flowing acceleratedly in the annular space constructed with a uniformly heated core tube and an outer tube with variable inner diameter. The results show that acceleration parameter K becomes a controlling characteristic quantity of heat transfer in this case. For this parameter below about 2×10-6, the local Nusselt number varies proportionally to the 0.77 power of the local Reynolds number, that is the same trend as in the turbulent case of uniform cross sectional area. Depending on the temperature distributions of fluid at the inlet, the heat transfer coefficient decreases and then increases, or monotonously increases with axial distance. Local heat transfer coefficients with K≤2×10-6 and in the axial region, where they increase with axial distance, are smaller than the values predicted by substitution of the same local flow conditions and diameter ratio into the fully developed heat transfer correlation for uniform cross section. These experimental results are correlated in the empirical formula.

Transport Phenomena in Zonal Centrifuge Rotors: I. Velocity and Shear-Stress Distributions of Fluid During Acceleration

An analysis is presented for the evaluation of velocity components and shear-stress distributions of fluid in zonal centrifuge rotors during acceleration. Analytical expressions for the distribution of tangential and radial velocity components and the tangential shear-stress and the radial shear-stress distributions of fluid are obtained for the transient case. Characteristics of each distribution for a typical density gradient liquid in a zonal centrifuge rotor are computed from the relations derived, and are presented as figures. An unusual phenomenon-the tangential velocity of the gradient exceeding the velocity of the rotor during a particular period of acceleration-is demonstrated.

SIMULATION OF HORIZONTAL TURBULENT DIFFUSION OF PARTICLES UNDER WAVES

By oscillation of an array of turbulence-generating grids in still water, the turbulent fluid velocity field m shoaling waves near the bottom is simulated in a laboratory channel. Solid particles with fall velocities varying between 1 and 40 mm/sec are introduced into the test volume from above. Multiple- image photography using ultraviolet lighting techniques and a suitably placed mirror allow recording of the gram trajectories as functions of time and three space dimensions simultaneously. The Lagrangian intensities of turbulence and diffusion coefficients are then directly measured from the photographic data. The scale times, scale lengths, and the frequencies of the power spectra modes can then be calculated. Properties of the fluid turbulence are inferred from the quasi-neutral particles. The analysis, which is restricted to the component of diffusion in the hori2ontal direction normal to the grid motion, shows that the turbulent velocity distributions of both fluid and heavy particles are Gaussian, and that their standard deviations (intensities of turbulence) increase regularly with increasing grid Reynolds numbers (grid speeds). Diffusion coefficients likewise generally increase with increasing grid Reynolds numbers. Diffusivities of the heavy particles relative to the fluid are a function of both particle fall velocity and the structure of the fluid turbulence itself.

A Dosage System for Interstitial Radium Therapy. Part II—Physical Aspects

Part II is concerned with the physical basis of the dosage rules given in Part I. The guiding principles for the manipulation of single- and multiple-plane implants are derived from previous results relating to surface applicators. The foundation of the more general volume dosage lies in a study of the radiation fields created by distributions of a hypothetical radioactive fluid throughout the volume considered. The treatment has been restricted to cases in which two regions of different fluid density, the so-called rind and core, are permitted. The radiation fields have been evaluated for the sphere, circular and elliptic cylinders, ellipsoids and cuboids (rectangular parallelepipeds). The mean dosage-rate within solids of the same volume and radium content is independent of the shape, except for a small correction factor determined by the elongation. Dosage is given by the formula milligramme-hours per 1,000 r = 21·57 × (Volume)⅔ × e0·07 (Elongation — 1) The modifications introduced into dosage and distribution by the substitution of actual radium needles for the fluid are described.