Showing papers on "Pressure gradient published in 1991"

On the Pressure Gradient Force over Steep Topography in Sigma Coordinate Ocean Models

Abstract: The error in computing the pressure gradient force near steep topography using terms following (σ) coordinates is investigated in an ocean model using the family of vertical differencing schemes proposed by Arakawa and Suarez. The truncation error is estimated by substituting known buoyancy profiles into the finite difference hydrostatic and pressure gradient terms. The error due to “hydrostatic inconsistency,” which is not simply a space truncation error, is also documented. The results show that the pressure gradient error is spread throughout the water column, and it is sensitive to the vertical resolution and to the placement of the grid points relative to the vertical structure of the buoyancy field being modeled. Removing a reference state, as suggested for the atmosphere by Gary, reduces the truncation error associated with the two lowest vertical modes by a factor of 2 to 3. As an example, the error in computing the pressure gradient using a standard 10-level primitive equation model appl...

Transient eddy formation around headlands

Abstract: Eddies with length scales of 1–10 km are commonly observed in coastal waters and play an important role in the dispersion of water-borne materials. The generation and evolution of these eddies by oscillatory tidal flow around coastal headlands is investigated with analytical and numerical models. Using shallow water depth-averaged vorticity dynamics, eddies are shown to form when flow separation occurs near the tip of the headland, causing intense vorticity generated along the headland to be injected into the interior. An analytic boundary layer model demonstrates that flow separation occurs when the pressure gradient along the boundary switches from favoring (accelerating) to adverse (decelerating), and its occurrence depends principally on three parameters: the aspect ratio [b/a], where b and a are characteristic width and length scales of the headland; [H/CDa], where H is the water depth, CD is the depth-averaged drag coefficient; and [Uo/σa], where Uo and σ are the magnitude and frequency of the far-field tidal flow. Simulations with a depth-averaged numerical model show a wide range of responses to changes in these parameters, including cases where no separation occurs, cases where only one eddy exists at a given time, and cases where bottom friction is weak enough that eddies produced during successive tidal cycles coexist, interacting strongly with each other. These simulations also demonstrate that in unsteady flow, a strong start-up vortex forms after the flow separates, leading to a much more intense patch of vorticity and stronger recirculation than found in steady flow.

Electrokinetic dissipation induced by seismic waves

Abstract: When a fluid electrolyte moves relative to a solid, an electric field is generated that migrates ions and thus dissipates energy. This 9electrokinetic9 dissipation is theoretically compared to the viscous shear dissipation for fluid flow generated by a fixed time harmonic pressure gradient in a planar quartz duct. It is shown that, regardless of frequency, it is safe to ignore the electrokinetic losses when the electrolyte molarity is on the order of 0.1 M or greater. For low molarity electrolytes (10 (super -3) M), however, the electrokinetic losses do become significant compared to the viscous losses for flow in sufficiently tight pores. The ratio of electrokinetic dissipation D E to viscous dissipation D V is always a maximum when the electrokinetic radius (the duct half-width divided by the Debye length) is nearly equal 1.5. The maximum value of D E /D V does not exceed 0.5 for the NaCl, KCl, and quartz systems considered.The generated electric field pushes on the excess ions in the duct in a direction opposite to the applied pressure gradient, thus giving rise to an apparent viscosity enhancement. This enhancement is E /D V ratio. Indeed, the central effect of a large D E /D V ratio is that the amount of relative fluid flow is reduced, and thus, the amount of wave attenuation is reduced.

Decreased hydrodynamic resistance in the two-phase flow of blood through small vertical tubes at low flow rates.

Abstract: The aggregation of red blood cells in blood flowing through small tubes at very low shear rates leads to the two-phase flow of an inner core of rouleaux surrounded by a cell-depleted peripheral layer. The formation of this layer is known to be accompanied by a decrease in hydrodynamic resistance to flow. To quantitate this effect, we measured the pressure gradient, flow rate, and the radius of the red blood cell core in suspensions flowing through tubes of 172-microns radius at mean linear flow rates (U) from 50 to 0.15 tube diameters.sec-1. Washed red blood cells were suspended in 1.5% buffered dextran 110 at hematocrits of 34-52%. Using syringe pumps, blood flowed from a stirred reservoir through a vertical 12-cm length of tube in either the upward or downward direction. The pressure drop was measured with transducers. Mean values in distributions in the core radius were obtained by analyzing cine films of flow taken through a microscope with flow in the upward direction, measuring the core radius at five equally spaced axial positions of the tube in each of 100 frames. At 34% and 46% hematocrit, the hydrodynamic resistance increased as U decreased from 50 sec-1, reaching a maximum at U-2 sec-1. It then decreased to a minimum at U less than 0.5 sec-1 as the red blood cell core formed in the tube, and the mean core radius/tube radius ratio decreased from 0.98 to 0.74 with marked axial fluctuations at the lower U. At higher hematocrits, both the increase and decrease in hydrodynamic resistance were greater. In a red blood cell albumin-saline suspension, where there is no aggregation of red blood cells and no two-phase flow, hydrodynamic resistance increases linearly with decreasing U. The experimental results were compared with the predictions of a two-phase steady-flow model, assuming axisymmetric flow of a core surrounded by cell-free suspending medium. Two models were considered, one in which the core is solid, the other in which the rheological properties of the suspension in the core are given by the Quemada equation. The effects of sedimentation of the core resulting in a zero net flow pressure gradient were taken into account. Provided that an experimentally extrapolated value for the zero pressure gradient was used, the Quemada-fluid model gave good agreement with the experimentally observed core radius as a function of U and hematocrit.

The coastal current off Western India during the Northeast monsoon

Abstract: Hydrographic data collected during December 1987 and January 1988 are used to describe the poleward coastal current along the west coast of India that flows against weak winds during the northeast monsoon. Near the southern end of the coast, at about 10°N, the current was approximately 400 km wide, 200 m deep and carried the low salinity Equatorial Surface Water. The isopycnals tilted down on approaching the coast. Near the northern end of the coast, at about 22°N, the flow was restricted mainly to the vicinity of the continental slope; the current was a narrow (100 km), 400 m deep jet with a transport of about 7 × 106 m3 s−1. Along most of the coastline, a southward moving undercurrent was inferred from the distribution of salinity, temperature and dynamic topography. To understand the driving mechanism of the current, the annual cycle of the contribution of the longshore pressure gradient and that of the winds to the near-surface momentum balance was examined using available climatologies. It is seen that the longshore pressure gradient overwhelms the winds during the northeast monsoon, whereas during the southwest monsoon the winds dominate. In the Leeuwin Current off western Australia, the only other known eastern boundary current that flows against the winds, the pressure gradient dominates the winds throughout the year. The overall structure of the northeast monsoon coastal current is consistent with that predicted in the analytic model proposed by McCreary et al. (1986, Journal of Marine Research, 44, 71–92) to explain the Leeuwin Current. In the model a poleward baroclinic pressure gradient is generated by a density gradient along the coast. The latter is known to exist along the west coast of India. However, it seems likely that the current is also influenced by a barotropic pressure gradient.

A Zonally Averaged Ocean Model for the Thermohaline Circulation. Part I: Model Development and Flow Dynamics

Abstract: A two-dimensional latitude–depth ocean model is developed on the basis of the zonally averaged balance equations of mass, momentum, heat, and salt Its purpose is to investigate the dynamics and variability of the buoyancy-forced thermohaline circulation For the time scales of interest an annually averaged model is selected, and the momentum balance is taken to be diagnostic The east-west pressure gradient, which arises upon zonally averaging the momentum equations, is parameterized in terms of the meridional pressure gradient The thermohaline circulation is driven by mixed surface boundary conditions, ie, temperatures are relaxed to prescribed values while the salt flux is held constant The dynamics of the flow is investigated in hemispheric and global geometries for both short and long time integrations, the latter extending over many thousands of years As has been noted by previous investigators, it is possible to perturb a steady state such that a diffusively dominated regime results

Gas‐solid flow in vertical tubes

Abstract: This paper reports on a computational study of fully-developed flow of gas-particle suspensions in vertical pipes which was carried out, using the model proposed recently by Sinclair and Jackson, to understand the predicted scale-up characteristics. It was shown that the model can capture the existence of steady-state multiplicity wherein different pressure gradients can be obtained for the same gas and solids fluxes. A pronounced and nonmonotonic variation of the pressure gradient required to achieve desired fluxes of solid and gas with tube diameter was predicted by the model, and this is explained on a physical basis. The computed results were compared with the experimental data. The model manifests an unsatisfactory degree of sensitivity to the inelasticity of the particle-particle collisions and the damping of particle-phase fluctuating motion by the gas.

Similarity analysis of compressor tip clearance flow structure

Abstract: A new approach is presented for analyzing compressor tip clearance flow. The basic idea is that the clearance velocity field can be (approximately) decomposed into independent throughflow and crossflow, since chordwise pressure gradients are much smaller than normal pressure gradients in the clearance region. As in the slender body approximation in external aerodynamics, this description implies that the three-dimensional steady clearance flow can be viewed as a two-dimensional, unsteady flow. Using this approach, a similarity scaling for the crossflow in the clearance region is developed and a generalized description of the clearance vortex is derived. Calculations based on the similarity scaling agree well with a wide range of experimental data in regard to flow features such as crossflow velocity field, static pressure field, and tip clearance vortex trajectory.

A Laboratory Investigation of Foam Flow in Sandstone at Elevated Pressure

Abstract: This paper described a study made of the flow of foam through sandstone at elevated backpressures. Gamma ray attenuation was used to measure local liquid saturations. Steady-state pressure gradients for foam were found to be essentially independent of gas velocity but nearly proportional to liquid velocity. This flow behavior of foam is explained in terms of changes in bubble texture.

Ballooning instabilities in tokamaks with sheared toroidal flows

Abstract: The stability of ballooning modes in the presence of sheared toroidal flows is investigated. The eigenmodes are shown to be related by a Fourier transformation to the nonexponentially growing Floquet solutions found by Cooper [Plasma Phys. Controlled Fusion 30, 1805 (1988)]. It is further shown that the problem cannot be reduced further than to a two‐dimensional partial differential equation. Next, the generalized ballooning equation is solved analytically for a circular tokamak equilibrium with sonic flows, but with a small rotation shear compared to the sound speed. With this ordering, the centrifugal forces are comparable to the pressure gradient forces driving the instability, but coupling of the mode with the sound wave is avoided. A new stability criterion is derived that explicitly demonstrates that flow shear is stabilizing at constant centrifugal force gradient.

Similarity analysis of compressor tip clearance flow structure

Abstract: A new approach is presented for analyzing compressor tip clearance flow. The basic idea is that the clearance velocity field can be (approximately) decomposed into independent throughflow and crossflow, since chordwise pressure gradients are much smaller than normal pressure gradients in the clearance region. As in the slender body approximation in external aerodynamics, this description implies that the three-dimensional steady clearance flow can be viewed as a two-dimensional, unsteady flow. Using this approach, a similarity scaling for the crossflow in the clearance region is developed and a generalized description of the clearance vortex is derived. Calculations based on the similarity scaling agree well with a wide range of experimental data in regard to flow features such as crossflow velocity field, static pressure field, and tip clearance vortex trajectory.

Reynolds shear stress measurements in a separated boundary layer flow

Abstract: Turbulence measurements were obtained for two cases of boundary layer flow with an adverse pressure gradient, one attached and the other separated. A three-component laser Doppler velocimeter system was used to measure three mean velocity components, all six Reynolds stress components, and all ten velocity triple product correlations. Independent measurements of skin-friction obtained with a laser oil-flow interferometer were used to examine the law of the wall in adverse pressure gradient flows where p(+) is less than 0.05. Strong similiarities were seen between the two adverse pressure gradient flows and free shear layer type flows. Eddy viscosities, dissipation rates, and pressure-strain rates were deduced from the data and compared to various turbulence modeling assumptions.

Forces acting in the direction of propagation in pulsed ultrasound fields

Abstract: This paper considers some non-thermal effects resulting from absorption of acoustic energy from an ultrasound beam. An experimental investigation of the location of the 'source pump', responsible for the generation of streaming in high amplitude diagnostic fields in water, is reported. Acoustically transparent membranes were inserted in the ultrasound field in order to restrict the streaming volume. It is shown that the major contribution to an acoustic stream is generated in the region near to the focus of a transducer where the intensity in the beam and the degree of non-linear distortion are both high. In the second part of the paper a simple model of non-linear propagation is used to predict the magnitude of the maximum pressure gradient induced in a medium by the absorption of acoustic energy from a beam. Propagation in water, in tissue and in amniotic fluid are considered. Within the limitations of this model it is shown that the pressure gradients induced in pulsed acoustic fields do not result in the ultimate shear stress of tissue being exceeded.

Pressure-Transient Behavior of Horizontal Wells With and Without Gas Cap or Aquifer

Abstract: In this paper analytic solutions are presented in real time and as Laplace transforms for horizontal wells in reservoirs bounded at the top and bottom by horizontal planes. Two types of boundary conditions are considered at these planes, and the Laplace-transform pressure solutions are used to include wellbore-storage and skin effects. Solutions are based on the uniform-flux, line-source solution, but differ from most existing solutions owing to the use of pressure averaging to approximate the infinite-conductivity wellbore condition and use of the correct equivalent wellbore radius for an anisotropic reservoir. New flow periods (regimes) are identified, and simple equations and existence criteria are presented for the various flow periods that can occur during a transient test.

Electron diamagnetic effects on the resistive pressure-gradient-driven turbulence and poloidal flow generation

Abstract: The nonlinear evolution of resistive pressure‐gradient‐driven turbulence with diamagnetic effects included generates a dc electric field (poloidal velocity) through the convective nonlinearity in the momentum balance equation. This radial electric field has a strong shear and contributes to the saturation of the turbulence; its effect on the saturation level of turbulence is more important than the change of the time and length scales of the modes by the direct ω* effects.

Permeation equations developed for prediction of membrane performance in pervaporation, vapor permeation and reverse osmosis based on the solution-diffusion model

Abstract: General permeation equations based on the solution-diffusion model were proposed for pervaporation (PV), vapor permeation (VP) and reverse osmosis (RO) on two different assumptions about the pressure gradient inside a membrane: a flat gradient (case 1) and a linear gradient (case 2). With these equations the permeation properties in PV, RO and VP can be estimated once the transport parameter of a membrane is known.The effect of upstream pressure on selectivity and flux in RO and PV was estimated by sample calculations for water- and ethanol-selective membranes in ethanol–water system. Flux and selectivity in RO is smaller and, reaching that in PV at infinite pressure. This ultimate value is different in cases 1 and 2, and in the latter the molar volume ratio of the permeants becomes important. The effect of downstream pressure in PV was also estimated and compared with the case of vacuum-enhanced membrane distillation (MD) with a porous membrane. With increasing pressure the separation factor approaches that of vapor–liquid equilibrium in both PV and MD. With decreasing pressure that in MD is governed by the ratio of diffusion coefficients inside the membrane. Since the Knudsen diffusion coefficient of water is larger than that of ethanol, the separation factor decreases in ethanol–water separation with decreasing downstream pressure. This was verified by experiment, using PTFE membranes.

Influence of pressure on solute retention in liquid chromatography

Abstract: In this investigation, multiple on-column detectors are utilized to measure the retention of model solutes directly on a packed, capillary column. The absolute pressure on the column is controlled using a restrictor at the column exit, while the pressure gradient and volumetric flowrate are maintained constant. Measurements obtained under reversed-phase conditions indicate that the local capacity factor changes considerably with local pressure under typical operating conditions. These results are somewhat surprising since the mobile-phase solvents used for liquid chromatography are generally considered to be incompressible.

Ion Dynamics in the Venus Ionosphere

Abstract: Dynamics play an important role in defining the characteristics of the Venus ionosphere. The absence of a significant internal magnetic field at Venus allows the ionization to respond freely to gradients in the plasma pressure. The primary response to a gradient in plasma pressure is the nightward flow of the ionization away from a photoionization source on the dayside. The flow is approximately symmetric about the Sun-Venus axis and provides the source of O+ that maintains the nightside ionosphere during solar maximum. Modelling efforts have generally been successful in describing the average nightward ion velocity. Asymmetric and temporally-variable flow is measured, but is not well described by the models. Departures from axially-symmetric flow described in this paper include ionospheric superrotation at low altitudes and an enhanced flow at high altitude at the dawn terminator. Variability that is the result of changes in the ionopause height induced by changes in solar wind dynamic pressure is especially strong on the nightside. Ion flow to the nightside is also reduced during solar minimum because of a depressed ionopause.

Theory of the expanding plasma of vacuum arcs

Abstract: Investigations of the diffuse plasma expanding from cathode arc spots into a vacuum have revealed some unexpected properties calling for physical explanation. A theoretical model of such plasmas which is based on one-dimensional time-independent hydrodynamic two-fluid equations can be solved analytically in the form of asymptotic power series approximately describing the plasma parameters as functions of the variable s=(I/r)2/5(current I, distance r). The main results give a quantitative decomposition of the force accelerating the ions into three partial forces caused by the electric field, by the ion pressure gradient and by the electron-ion friction, which are of comparable importance. Unlike the ions, the electrons are accelerated by the pressure gradient only, but are decelerated both by the field and by friction; all these forces cancel each other almost completely. The direction of the electric current is opposite to the direction of the field; therefore the resistance of the plasma is negative. The limitations of the validity of the derived solution are discussed, as are possible extensions and simplifications of the model.

Method for reducing startup time during a steam assisted gravity drainage process in parallel horizontal wells

Abstract: A method for reducing the time during which steam moves in a lateral direction between two parallel superimposed horizontal wells when utilizing a Steam Assisted Gravity Drainage (SAGD) process. Foam is added while injecting steam into an upper horizontal well once steam breakthrough occurs in an interwell region. Foam enters the interwell region thereby causing an increased pressure gradient. This increased pressure gradient adds to the gravity force thereby providing a greater interstitial oil velocity which increases oil drainage between wells during startup.

Turbulent Flow in a Channel With a Wavy Wall

Abstract: A numerical method for the solution of the Reynolds-averaged Navier-Stokes equations, together with a two-layer turbulence model, has been used to describe steady flow in a two-dimensional channel with a wavy wall. Comparisons of calculations with experiments demonstrate the effects of alternating pressure gradients induced by alternating surface curvatures, and multiple separations and reattachments. The numerical method and the turbulence model are shown to capture the overall features of such a flow, including the breakdown of the logarithmic law of the wall in strong pressure gradients and in separated flow.

Steady gradually accelerating flow in a smooth open channel

Abstract: Laboratory experiments on the structure of spatially accelerating flows in a smooth open channel are reported. The influences of acceleration (i) on the velocity profiles; (ii) on the turbulence intensity profiles; and (iii) on the longitudinal evolution of the bottom shear stress are investigated. The most important conclusions of the study can be summarized as follows: a. The velocity distributions cannot be represented entirely by the universal log-law. However, this law seems to follow the data very close to the wall, up to about y/d = 0.02 ∼ 0.05. Higher in the flow, the data increasingly deviate from this law. In the external layer, the equilibrium state of the flow was tested: some near-equilibrium flow situations exist, but mostly the flow was out of equilibrium; b. The generalization of Coles' model was tried, resulting in correlation of the wake parameter, Π, and the dimensionless pressure gradient parameter, β, for β<–0.5; c. The acceleration of the mean velocity field leads to reduction of the...

Structure and Dynamics of a Coastal Filament

Abstract: Repeated microstructure transects across filaments in the coastal transition zone (CTZ) have revealed fundamental structure and dynamics of the complicated features. The measurements allow detailed momentum and vorticity analyses and provide a possible explanations for structural asymmetry of the fronts. Observations made between July 2 and July 23, 1988, along the central meridional CTZ survey line were used to estimate terms in the meridional momentum equation. The analysis indicates geostrophic flow along the axes of the fronts with the acrosg-fr0nt pressure gradient explaining as much as 87% of the variance in the balance. Significant ageostrophic flow in the across-front coordinate was found, with the along-front pressure gradient explaining only 7!% of the variance in the momentum balance. The fronts were found to be asymmetric in relative vorticity, with stronger positive vorticity on the cooler side of the front and weaker negative vorticity on the warm side. Mean vertical velocities were estimated from the repeated transects of acoustic Doppler current profiles and the rapid sampling vertical profiler hydrographic and turbulence measurements. Regions of upwelling and downwelling are likely associated with adjustments in the relative vorticity, resulting in maximum vertical velocities of 40 m d -l . Asymmetry in the near-surface temperature and salinity extrema are explained by cross-frontal exchang e. This cross-frontal exchange modifies the relative roles of salinity and temperature in determining the density away from the coastal upwelling region, a dynamically important characteristic not revealed by advanced very high resolution radiometer imagery.

Evidence for Hygrometric Pressurization in the Internal Gas Space of Spartina alterniflora.

Abstract: Higher pressure, up to several hundred pascal relative to ambient, is generated by hygrometric pressurization within the central hollow space of the stem in Spartina alterniflora. Dilution of oxygen and nitrogen by water vapor within the plant's internal gas space results in an influx of nitrogen and oxygen from the air and a net increase in the internal gas pressure at steady state. The nature of the pressure gradient suggests that small pores exist in the plant tissues. Moreover, the compact arrangement of leaf mesophyll cells creates a high resistance for the mass flow of gases and contributes to the higher pressure within leaves. After experimentally venting the internal pressure, outside air diffused through the basal area of the adaxial side of the leaves into the internal space and elevated pressure was restored.