Showing papers on "Pressure gradient published in 1985"

Continuous-wave Doppler echocardiographic assessment of severity of calcific aortic stenosis: a simultaneous Doppler-catheter correlative study in 100 adult patients.

Abstract: Studies of the correlation of aortic valve gradient determined by continuous-wave Doppler echocardiography and that determined at catheterization have, to date, involved young patients and nonsimultaneous measurements. We therefore obtained simultaneous Doppler echocardiographic and catheter measurements of pressure gradient in 100 consecutive adults (mean age 69, range 50 to 89 years). In 63 patients pressure measurements were obtained with dual-catheter techniques and in 37 they were obtained by withdrawal of the catheter from the left ventricle to the ascending aorta. Forty-six of these patients also underwent an outpatient Doppler study 7 days or less before catheterization. The simultaneous pressure waveforms and Doppler spectral velocity profiles were digitized at 10 msec intervals and maximum, mean, and instantaneous gradients (mm Hg) were derived for each. The correlation between the Doppler-determined gradient and the simultaneously measured maximum catheter gradient was r = .92 (SEE = 15 mm Hg), that between the Doppler-determined and mean catheter gradient was r = .93 (SEE = 10 mm Hg), and that between the Doppler and peak-to-peak catheter gradient was r = .91 (SEE = 14). The correlation between the nonsimultaneously Doppler-determined gradient and the maximum gradient measured by catheter was not as strong (r = .79, SEE = 24). The continuous-wave Doppler echocardiographic velocity profile represents the instantaneous transaortic pressure gradient throughout the cardiac cycle. The best correlation with continuous-wave Doppler-determined gradient was obtained with maximum and mean gradients measured by catheter. Continuous-wave Doppler echocardiography can be used to reliably predict the pressure gradient in adults with calcific aortic stenosis.

Pressure Drop Correlations for Inclined Two-Phase Flow

Abstract: Presentation de correlations pour la perte de charge de differents types d'ecoulements diphasiques dans une conduite inclinee

Three-dimensional flow beneath a slow spreading ridge axis: A dynamic contribution to the deepening of the Median Valley toward fracture zones

Abstract: The axial valley along a slow spreading mid-ocean ridge may be explained by a vertical pressure gradient due to viscous flow of asthenosphere upwelling in a relatively narrow conduit beneath the ridge axis. Along a ridge segment, the axial valley floor deepens by as much as 2–3 km over distances of several tens of kilometers approaching a ridge-transform intersection. This deepening may be explained, in part, by a pressure gradient associated with horizontal flow in the ridge axis conduit. Vanishing vertical velocity on a conduit end wall at a ridge-transform intersection results in reduced vertical flow within about one conduit width of the intersection. Horizontal flow along the conduit, toward the intersection, must occur to form lithosphere at a uniform rate along the ridge axis. A simple model with a conduit of uniform width that terminates with vertical planar endwalls at ridge-transform intersections is considered. The model is based on an analytical solution for flow in a narrow conduit and boundary layer approximations for the flow structure near the vertical end walls. This model indicates that the induced horizontal flow and pressure gradient extend for a distance along the ridge axis of several lithosphere thicknesses from an intersection. For reasonable values of the conduit width and asthenosphere viscosity the model predicts that induced horizontal flow can contribute significantly to the deepening of an axial valley approaching an intersection.

Characteristics of the flow around conventional and supercritical airfoils

Abstract: Measurements of the mean and fluctuating velocities have been obtained with pressure and hot-wire probes in the attached boundary layers and wakes of two airfoil models at a low Mach number. The first model is a conventional airfoil at zero incidence and the second an advanced supercritical airfoil at an angle of attack of 4°. The mean-flow and Reynolds-stress data and related quantities are presented with emphasis on the trailing-edge region. The results indicate that the flow around the conventional airfoil is a minor perturbation of a symmetric flat-plate flow with small wake curvature and weak viscous–inviscid interaction. The flow around the supercritical airfoil is in considerable contrast with strong streamwise pressure gradients, non-negligible normal pressure gradients, and large surface and streamline curvatures of the trailing-edge flow. The near wake is strongly curved and intense mixing occurs between the retarded upper-surface boundary layer and strongly accelerated lower-surface boundary layer.

Relaxation effects observed for turbulent flow over a wavy surface

Abstract: Measurements are presented for different flow rates of the time-averaged wall shear stress and of the root-mean-square value of the turbulent fluctuations along a small-amplitude sinusoidally shaped solid surface. The stresses are found to have a variation along the wave surface which is also sinusoidal. The influence of flow rate and of wavelength on the amplitude and phase angle can be correlated by using a wave-number α+ made dimensionless with wall parameters.It is found that for α+ > 10−2 a frozen-turbulence assumption can be made whereby the influence of the wave-induced variation of the mixing length can be ignored. For α+ < 10−4 the flow can be described by assuming the Reynolds stresses are given by an equilibrium assumption. The relaxation from this equilibrium condition is characterized by a sharp change in the phase angle for 6 × 10−4 < α+ < 10−3.This relaxation is associated with physical processes in the viscous wall region which are not yet understood. It is argued that these are principally related to the wave-induced variation of the pressure gradient.The wave-induced variation of the turbulent fluctuations in the wall shear stress also indicate a relaxation in that the maximum turbulence intensity is located in a region of favourable pressure gradient.

Effects of pressure gradients between branches of the left coronary artery on the pressure axis intercept and the shape of steady state circumflex pressure-flow relations in dogs.

Abstract: When steady state pressure-flow relations are studied in the circumflex coronary artery, pressure gradients develop between it and other branches of the left coronary artery. To assess the effects of these pressure gradients, we compared the pressure axis intercept and shape of steady state circumflex pressure-flow relations in the presence and absence of gradients after autoregulation was abolished, both in the beating heart and during long diastoles in dogs. We used peripheral coronary pressures and radionuclide-labeled microspheres to assess arterial collateral flow. In the beating heart, interarterial pressure gradients reduced the curvature at low circumflex pressures, and overestimated the mean pressure axis intercept by 7.8 mm Hg (P less than 0.05). The results were similar for the pressure-flow relations derived during long diastoles. This overestimation exaggerates the difference between the pressure axis intercept and coronary sinus pressure. The peripheral coronary pressure and microsphere results indicate that these effects are mediated largely by arterial collateral flow.

The Goertler vortex instability mechanism in three-dimensional boundary layers

Abstract: The two dimensional boundary layer on a concave wall is centrifugally unstable with respect to vortices aligned with the basic flow for sufficiently high values of the Goertler number. However, in most situations of practical interest the basic flow is three dimensional and previous theoretical investigations do not apply. The linear stability of the flow over an infinitely long swept wall of variable curvature is considered. If there is no pressure gradient in the boundary layer the instability problem can always be related to an equivalent two dimensional calculation. However, in general, this is not the case and even for small values of the crossflow velocity field dramatic differences between the two and three dimensional problems emerge. When the size of the crossflow is further increased, the vortices in the neutral location have their axes locally perpendicular to the vortex lines of the basic flow.

Pressure behavior of layered reservoirs with crossflow

Abstract: Experience with a large number of well tests has shown that the analysis of some field tests using currently available interpretation models is not satisfactory. Marked departure from homogeneous behavior is evident and none of the available heterogeneous models, such as the double porosity models, yields a convincing interpretation. This is particularly apparent when the pressure derivative behavior is examined. In this paper, a new analytical solution is presented that describes the pressure response of a well intercepting a layered reservoir with crossflow. The model is designed to describe a system of alternating beds of relatively high permeability contrast. The interpreation models currently in use (homogeneous reservoir, two layers without crossflow, double porosity reservoir) are shown to be limiting forms of the new solution which is therefore a more general description of transient pressure responses. The layered reservoir behavior is discussed in terms of the pressure and the derivative of pressure. The limits of applicability of the homogeneous and the double porosity solutions for analysis of tests in layered formations are clearly defined. The new model is used to analyze an actual field case and a good match is obtained when no other solution is found to be applicable. Inmore » addition to the usual well and reservoir parameters (total permeability thickness product, skin and wellbore storage constant), analysis yields quantitative information on the storativity and permeability of the contrasting layers.« less

Validation of instantaneous pressure gradients measured by continuous-wave Doppler in experimentally induced aortic stenosis

Abstract: The relation between catheter-measured and Doppler-derived aortic pressure gradients was examined in 8 open-chest dogs. A snare was placed around the proximal ascending aorta and adjusted to provide a wide range of gradient to left ventricular (LV) outflow. A continuous-wave Doppler transducer was placed above the level of the obstruction and angled to optimize the audio and spectral signals. Pressure tip transducer catheters recorded LV and ascending aortic pressures simultaneously with the Doppler signal. In 120 randomly selected sinus beats, Doppler-derived maximal gradient correlated well with maximal instantaneous catheter gradient from 4 to 179 mm Hg (r = 0.99). Mean gradients also were closely related (r = 0.98). For gradients above 100 mm Hg, the correlation remained good (r = 0.98), but for gradients below 50 mm Hg, the correlation was not as precise (r = 0.81). All 120 cycles were digitized at 10-ms intervals to examine the correspondence between the Doppler and catheter data throughout systole. For the 2,742 pairs of points so obtained, the correlation was excellent (r = 0.95). The close relation between Doppler-derived pressure gradient and that measured simultaneously by catheterization provides further validation of the use of continuous-wave Doppler in the assessment of aortic stenosis, not only at maximal gradient, but throughout the period of LV ejection.

A pressure‐gradient‐driven tokamak ‘‘resistive magnetohydrodynamic’’ instability in the banana‐plateau collisionality regime

Abstract: The moment equation approach to neoclassical processes is used to derive the linearized electrostatic perturbed flows, currents, and resistive MHD‐like equations for a tokamak plasma. The new features of the resultant ‘‘neoclassical magnetohydrodynamics,’’ which requires a multiple length scale analysis for the parallel eigenfunction, but is valid in the experimentally relevant banana‐plateau regime of collisionality, are: (1) a global Ohm’s law that includes a fluctuating bootstrap current resulting from the ‘‘parallel’’ electron viscous damping (at rate μe) of the poloidal flow due to the perturbed radial pressure gradient; (2) reduction of the curvature effects to their flux surface average because Pfirsch–Schluter currents cancel out the lowest‐order geodesic curvature effects: (3) an increased polarization drift contribution with B−2, replaced by B−2Θ where BΘ is the poloidal magnetic field component. An electrostatic eigenmode equation is determined from ∇⋅J=0. For the unstable fluid‐like eigenmode...

Nature and distribution of vascular resistance in hypoxic pig lungs.

Abstract: We used the vascular occlusion technique in pig lungs isolated in situ to describe the effects of hypoxia on the distribution of vascular resistance and to determine whether the resistive elements defined by this technique behaved as ohmic or Starling resistors during changes in flow at constant outflow pressure, changes in outflow pressure at constant flow, and reversal of flow During normoxia, the largest pressure gradient occurred across the middle compliant region of the vasculature (delta Pm) The major effect of hypoxia was to increase delta Pm and the gradient across the relatively noncompliant arterial region (delta Pa) The gradient across the noncompliant venous region (delta Pv) changed only slightly, if at all Both delta Pa and delta Pv increased with flow but delta Pm decreased The pressure at the arterial end of the middle region was independent of flow and, when outflow pressure was increased, did not increase until the outflow pressure of the middle region exceeded 89 Torr during normoxia and 188 Torr during hypoxia Backward perfusion increased the total pressure gradient across the lung, mainly because of an increase in delta Pm These results can be explained by a model in which the arterial and venous regions are represented by ohmic resistors and the middle region is represented by a Starling resistor in series and proximal to an ohmic resistor In terms of this model, hypoxia exerted its major effects by increasing the critical pressure provided by the Starling resistor of the middle region and the ohmic resistance of the arterial region

Developing turbulent boundary layers with system rotation

Abstract: Measurements are presented for low-Reynolds-number turbulent boundary layers developing in a zero pressure gradient on the sidewall of a duct. The effect of rotation on these layers is examined. The mean-velocity profiles affected by rotation are described in terms of a common universal sublayer and modified logarithmic and wake regions.The turbulence quantities follow an inner and outer scaling independent of rotation. The effect appears to be similar to that, of increased or decreased layer development. Streamwise-energy spectra indicate that, for a given non-dimensional wall distance, it is the low-wavenumber spectral components alone that are affected by rotation.Large spatially periodic spanwise variations of skin friction are observed in the destabilized layers. Mean-velocity vectors in the cross-stream plane clearly show an array of vortex-like structures which correlate strongly with the skin-friction pattern. Interesting properties of these mean-flow structures are shown and their effect on Reynolds stresses is revealed. Near the duct centreline, where we have measured detailed profiles, the variations are small and there is a reasonable momentum balance.Large-scale secondary circulations are also observed but the strength of the pattern is weak and it appears to be confined to the top and bottom regions of the duct. The evidence suggests that it has minimally affected the flow near the duct centreline where detailed profiles were measured.

Values and limitations of transstenotic pressure gradients measured during percutaneous coronary angioplasty

Abstract: The pressure gradient across coronary stenoses is measured routinely during angioplasty. Due to the finite size of the angioplasty catheter within the stenotic cross section, the remaining luminal area is further reduced and the transstenotic gradient may be overestimating the "true" pressure drop. This "true" pressure gradient can be approximated from the mean coronary blood flow and the stenosis geometry from theoretical models. Goal of this study was to assess the values and limitations of the in vivo measurements of the pressure gradient versus the calculated values. Therefore, flow in the great cardiac vein was measured in 13 patients before and/or after angioplasty of a proximal left anterior descending stenosis, not filled by collaterals. The Poiseuille and turbulent contributions to flow resistance were determined from stenosis geometry assessed by quantitative coronary angiography. A fourfold increase in the luminal area (from 0.7 mm2 pre- to 2.8 mm2 post angioplasty) was associated with a fourfold decrease in the in vivo measured transstenotic gradient (from 59 mm Hg pre- to 13 mm Hg post angioplasty). The occlusion area and the measured gradient were linearly correlated: gradient = 69-17 X occlusion area (r = 0.76). However, as expected, the transstenotic gradient systematically overestimated the theoretical gradient calculated from the laws of fluid dynamics. A nonlinear relation was found between the calculated gradient P and the occlusion area As: P = 15 X As-2 (r = 0.87).

Method and apparatus for detecting leaks in a gas pipe line

Abstract: A method and apparatus for measuring a gas pressure gradient in each of the upstream and downstream parts of a gas pipe line and detecting a leak in the gas pipe line in accordance with the pressure gradients. When a gas leak occurs, the gas pressure at the position of the leak is decreased and at least the pressure gradient upstream of this position is increased. When the difference between the upstream pressure gradient and the downstream pressure gradient is greater than a predetermined value, the occurrence of a gas leak is determined. Also, the occurrence of a gas leak is determined when each of the displacement of the upstream pressure gradient and the displacement of the downstream pressure gradient attains a predetermined relation.

Heat release effects on shear-layer growth and entrainment

Abstract: The effects of heat release were studied in a planar, gaseous reacting mixing layer formed between two subsonic freestreams; one containing hydrogen in an inert diluent, the other containing fluorine in an inert diluent. Sufficiently high concentrations of hydrogen and fluorine reactants were employed to produce adiabatic flame temperature rises of up to 940 K (adiabatic flame temperature of 1240 K absolute). Although the displacement thickness of the layer for a zero streamwise pressure gradient showed an increase with increasing heat release. the actual thickness of the mixing layer at a given downstream location was not observed to increase and, in fact, was characterized by a slight thinning. The overall entrainment into the layer was seen to be substantially reduced by heat release. The large-wale vortical nature of the flow appeared to persist over all levels of heat release in this investigation. Imposition of a favorable pressure gradient, though resulting in additional thinning of the layer was observed to have no resolvable effect on the amount of chemical product formation and hence on the mixing.

On the cellular instability of flames near porous-plug burners

Abstract: Two-dimensional burner-flame stability is discussed with arbitrary gas expansion. Density variations are allowed for by fully coupling the continuity and momentum equations. The flame is assumed to be close to a porous-plug-type flameholder so that the conventional hydrodynamic zone upstream of the flame cannot be included. Instead, the flow is assumed to obey a Darcy-type law within the holder, relating pressure gradient and velocity. It is shown that the influence of the holder and the acceleration due to gravity are important factors governing the onset of cellularity in porous-plug burner flames. Further, the balance of the transverse and longitudinal Darcy constants used to describe the upstream hydrodynamic zone within the holder have a vital effect on stability predictions. Experimental observations are confirmed by the theory presented.

An LDA study of the backward-facing step flow, including the effects of velocity bias

Abstract: The subsonic, backward-facing step flow was studied experimentally to provide structural information and test data for modellers. A single-component laser-Doppler anemometer was the primary measurement instrument. The effects of velocity bias on the LDA results were examined in detail. It was concluded that velocity bias was small (less than 4%) in the uncorrected measurements and nonexistent when a periodic sampling strategy was employed. The measurements show that the pressure gradient at reattachment is lower in the present experiment than in most previous work, due to the thick boundary layer at separation. Turbulence levels in the separated shear layer are also reduced by the thick upstream boundary layer. Scaling of the streamwise coordinate on the reattachment length produced the best agreement with previous data. Agreement of turbulence quantities was particularly good downstream of reattachment.

Particle and field stress balance within a planetary magnetosphere

Abstract: A technique is developed for experimentally estimating the local tensor stresses within a planetary magnetic field configuration characterized by local spacecraft measurements. Key to the technique is the determination of the shapes of field lines using the symmetry properties of the system coupled with local and instantaneous measurements of the field line inclination angles. The technique is applied here to the inner and middle Saturnian magnetosphere using data returned by the Magnetic Field Experiment on the Voyager 1 spacecraft. It is concluded that the ring current has substantial radial structure, heretofore not shown. Outside about 13 R(s) the newly derived field stresses match remarkably well the funtional variation of the centrifugal corotation stresses of the cool particle population measured previously by the Plasma Science Experiment. Inside about 13 R(s) the key structure in the derived field stresses, a prominent local maximum, matches the approximate position of an apparent strong pressure gradient in the energetic particles characterized by the Low-Energy Charged Particle detectors.

Mixed‐Layer Deepening in Lakes after Wind Setup

Abstract: Laboratory experiments on wind‐driven mixed‐layer deepening in a wind flume are described for the situation where the wind shear stress has become balanced by a streamwise pressure gradient so that the mean mixed‐layer velocity has vanished. The results were obtained at two lengths of the flume, and include visual observations, entrainment rates, density profiles, velocity profiles and profiles of turbulence intensities. An intermediate upwind wedge formed by accumulation of mixed water was observed in all experiments. The entrainment law obtained agrees with Kraus and Turner's relationship, and is almost independent of the length of the flume.

Wind-Tunnel Experiments on Blowing Snow

Abstract: Blowing snow was produced artificially in a cold wind-tunnel, and various measurements were conducted including particle diameters, concentrations, saltation lengths heat transport and electric charge The mean diameter of blowing snow particles decreased only slightly with increasing height; in the saltation layer, standard deviation was large and velocities were scattered in a wide range, suggesting the complex dynamic process on taking-off The mean saltation length ranged from a few cm to 40 cm increasing with wind velocity When wind blew without snow drifting, the static air pressure on the snow surface was smaller at higher levels, the vertical pressure gradient being negative The pressure gradient became positive when blowing snow was initiated eg +96 Pa/m at 112 m/s and -83 °C The magnitude of a downward force acting on a saltating snow partice caused by the pressure gradient was not large enough to explain the downward acceleration found from photographic analyses of particle trajectories Blowing snow particles were charged negatively the magnitude of charge increased with lowering temperature Increase in vertical heat transfer was found in blowing snow by measuring the temperature of the air at various levels; the increase is reflected on that in the apparent turbulent diffusion coefficient

Reinforced piezoelectric transducer and pressure sensor using such a transducer

Abstract: A piezoelectric transducer including a polarized piezoelectric material film, which also has a reinforcing material incorporated into the mass and which prevents any deformation of the film in certain given directions. The invention also relates to a pressure sensor using such a transducer. This sensor can be used both as a pressure sensor and as a pressure gradient measuring device.

Modeling the pressure gradient-velocity correlation of turbulence

Abstract: The modeling of the pressure gradient–velocity correlation of turbulence is considered. Two distinctly different approaches have been proposed in the turbulence literature: one in which the pressure gradient–velocity correlation is decomposed into a pressure‐strain correlation and a pressure‐diffusion correlation, and another in which the pressure gradient–velocity correlation is split into its deviatoric and isotropic parts. By examining the limit of two‐dimensional turbulence, it is demonstrated that the models obtained from the former approach are inconsistent with the Navier–Stokes equations in a fundamental way, whereas the models obtained from the latter approach are not. Consequently, it appears that the direct modeling of the pressure gradient–velocity correlation in its deviatoric and isotropic parts should be favored. The implications that this result has on turbulence modeling are discussed briefly.

Effects of the condition of the approach boundary layer and of mainstream pressure gradients on the heat transfer coefficient on film-cooled surfaces

Abstract: Etude experimentale realisee a l'aide d'une analogie chaleur-masse, pour differents parametres d'insufflation, des angles d'injection de 35 et 90°. Etude de l'influence des gradients de pression legerement defavorable, legerement favorable et tres favorable

Active transition fixing and control of the boundary layer in air

Abstract: Active transition of flow over an airfoil surface and feedback control by sound is investigated in a wind tunnel at Caltech. Laminar instability forced by localized time dependent nonintrusive narrow heating strip causes abrupt changes in velocity to trigger instant transition at favorable pressure gradient. At zero and adverse pressure gradients these changes are only marginal. Feedback control by sound interaction at nearly normal incidence produces significant reduction in velocity perturbation in regions of transition. This reduction is in part at the expense of an increase in background disturbance since it is not possible to restore the flow to its undisturbed state.

Numerical Simulations of a Transverse Indirect Circulation and Low-Level Jet in the Exit Region of an Upper-Level Jet

Abstract: A numerical study was performed of a severe weather event (tornado) which occurred on May 10, 1973 in the Ohio region. The situation was modeled with a primitive equation mesoscale dynamic formulation. Account was taken of precipitation, the planetary boundary layer parameters as bulk quantities, the vertical pressure gradient, and lateral boundary conditions based on radiosonde data. Two 12-hr simulations, adiabatic and nondivergent, respectively, were analyzed for relationships between upper and lower level jets. In the adiabatic formulation, a transverse circulation with a low level jet formed at the exit region of the upper level jet. The nondivergent situation led to similar, but weaker, phenomena. Both forms suggest that indirect circulation in the exit zone of an upper level jet is strongly influenced by the initial structure of the jet.

Marine acoustic gradient sensor

Abstract: A pressure gradient sensor is disclosed for optically sensing the direction and magnitude of an acoustic wave propagating through a fluid. Each sensor consists generally of at least three fiber-optic displacement sensors disposed within a circular array oriented perpendicular to the longitudinal axis of a streamer. Each sensor phase-modulates a coherent light signal indicative of the water pressure sensed outside the streamer. The output of the signals include a component due to transient pressure waves and a component due to hydrostatic pressure differences between the sensors.

Pressure build‐up during the packing stage of injection molding

Abstract: A thorough study of an isothermal fluid motion within the mold cavity during the packing stage is presented. The fluid is considered Newtonian, and its compressible behavior is assumed to obey the Spencer-Gilmore equation of state. Mathematical results indicate that the pressure built up during the packing stage is strongly dependent on the melt viscosity and the boundary of the cavity. The effects of pressure gradient and distribution, during this stage, on the shrinkage of the final products are also discussed.