Showing papers on "Pressure gradient published in 1997"

Lattice boltzmann model for the incompressible navier-stokes equation

Abstract: In this paper a lattice Boltzmann (LB) model to simulate incompressible flow is developed. The main idea is to explicitly eliminate the terms of o(M 2), where M is the Mach number, due to the density fluctuation in the existing LB models. In the proposed incompressible LB model, the pressure p instead of the mass density ρ is the independent dynamic variable. The incompressible Navier–Stokes equations are derived from the incompressible LB model via Chapman–Enskog procedure. Numerical results of simulations of the plane Poiseuille flow driven either by pressure gradient or a fixed velocity profile at entrance as well as of the 2D Womersley flow are presented. The numerical results are found to be in excellent agreement with theory.

Moderate Reynolds number flows through periodic and random arrays of aligned cylinders

Abstract: The effects of fluid inertia on the pressure drop required to drive fluid flow through periodic and random arrays of aligned cylinders is investigated. Numerical simulations using a lattice-Boltzmann formulation are performed for Reynolds numbers up to about 180.The magnitude of the drag per unit length on cylinders in a square array at moderate Reynolds number is strongly dependent on the orientation of the drag (or pressure gradient) with respect to the axes of the array; this contrasts with Stokes flow through a square array, which is characterized by an isotropic permeability. Transitions to time-oscillatory and chaotically varying flows are observed at critical Reynolds numbers that depend on the orientation of the pressure gradient and the volume fraction.In the limit Re[Lt ]1, the mean drag per unit length, F, in both periodic and random arrays, is given by F/(μU) =k1+k2Re2, where μ is the fluid viscosity, U is the mean velocity in the bed, and k1 and k2 are functions of the solid volume fraction ϕ. Theoretical analyses based on point-particle and lubrication approximations are used to determine these coefficients in the limits of small and large concentration, respectively.In random arrays, the drag makes a transition from a quadratic to a linear Re-dependence at Reynolds numbers of between 2 and 5. Thus, the empirical Ergun formula, F/(μU) =c1+c2Re, is applicable for Re>5. We determine the constants c1 and c2 over a wide range of ϕ. The relative importance of inertia becomes smaller as the volume fraction approaches close packing, because the largest contribution to the dissipation in this limit comes from the viscous lubrication flow in the small gaps between the cylinders.

A finite‐volume integration method for computing pressure gradient force in general vertical coordinates

Abstract: A finite-volume integration method is proposed for computing the pressure gradient force in general vertical coordinates. It is based on fundamental physical principles in the discrete physical space, rather than on the common approach of transforming analytically the pressure gradient terms in differential form from the vertical physical (i.e., height or pressure) coordinate to one following the bottom topography. The finite-volume discretization is compact, involving only the four vertices of the finite volume. The accuracy of the method is evaluated statically in a two-dimensional environment and dynamically in three-dimensional dynamical cores for general circulation models. The errors generated by the proposed method are demonstrated to be very low in these tests.

The role of inertial instability in determining the location and strength of near‐equatorial convection

Abstract: There are two major organized cloud configurations in the vicinity of the equator. Where there is a small cross-equatorial surface pressure gradient, convection is close to the equator and is generally tied to the location of the lowest sea-level pressure (SLP) and warmest sea-surface temperature (SST), in agreement with arguments based upon simple thermodynamical considerations. However, when there is a substantial cross-equatorial pressure gradient, such as occurs in the monsoon regions, organized convection appears off the equator in the summer hemisphere, equatorward of the SLP minimum and not necessarily collocated with the warmest SSTs. Thus, in this instance, simple thermodynamical considerations alone cannot explain 2he location of the convection. In this situation, the zero absolute vorticity contour (η = 0) also lies in the summer hemisphere. Therefore, between the equator and the η = 0 contour is a region of locally-anticyclonic absolute vorticity and an inertially unstable regime. It is argued that the convection results from the low-level divergence-convergence doublet centred about the η = 0 contour which is the mitigating response to the inertial instability. The associated latitude-height secondary circulation should provide subsidence (suppressed convection) over the equator and rising motion (enhanced convection) to the north of the zero absolute vorticity contour. Signatures of the inertial instability predicted by theory are found in observations supporting the hypothesis. Wherever a strong cross-equatorial pressure gradient exists, the η = 0 contour bisects a maximum in the divergent wind field. Divergence is found equatorward of the zero contour and convergence on the poleward side. Latitude-height cross sections show strong local meridional circulations with maximum rising motion on the poleward side of η = 0. As the regions where the rising motions occur are conditionally unstable, there is deep convection and the vertical circulations extend throughout the troposphere. It is noted that the intensity of the off-equator convection is deeper (and probably stronger) than convection located at the equator. This is probably because the convection associated with the inertial instability is more efficient. Necessary conditions for the location of near-equatorial convection are listed. Arguments are presented whereby inertial instability is established as the cause, rather than an effect, of off-equatorial convection. These include an outline of the sequence of processes leading up to the convection. The factors that limit the encroachment of the η = 0 contour into the summer hemisphere are discussed and an explanation for the existence of the low-level westerly monsoon wind maximum is suggested. The possible role played by the instability mechanism (or the lack of it) in coupled model simulations that produce seasonally migrating and/or double ITCZs in the eastern Pacific Ocean is discussed. Finally, it is proposed that the instability mechanism is important in the initiation of westward-moving disturbances found in the eastern Pacific and in determining active and break periods in the summer Indian monsoon.

Two Types of Nonlinear Pressure-Drop Versus Flow-Rate Relation Observed for Saturated Porous Media

Abstract: Previous reports of experiments performed with water (Fand et al., and Kececioglu and Jiang) indicated that beyond the Forchheimer regime the rate of change of the hydrostatic pressure gradient along a porous medium suddenly decreases. This abnormal behavior has been termed transition to turbulence in a porous medium. We investigate the relationship between the hydrostatic pressure gradient of a fluid (air) through a porous medium and the average seepage fluid velocity. Our experimental results, reported here, indicate an increase in the hydrostatic pressure rate beyond a certain transition speed, not a decrease. Physical arguments based on a consideration of internal versus extemal incompressible viscous flow are used to justify this distinct behavior, a consequence of the competition between a form dominated transition and a viscous dominated transition. We establish a criterion for the viscous dominated transition from consideration of the results of three porous media with distinct hydraulic characteristics. A theoretical analysis based on the semivariance model validation principle indicates that the pressure gradient versus fluid speed relation indeed departs from the quadratic Forchheimer-extended Darcy flow model, and can be correlated by a cubic function of fluid speed for the velocity range of our experiments.

The force balance of sea ice in a numerical model of the Arctic Ocean

Abstract: The balance of forces in the sea ice model of Hibler [1979] is examined. The model predicts that internal stress gradients are an important force in much of the Arctic Ocean except in summer, when they are significant only off the northern coasts of Greenland and the Canadian Archipelago. A partition of the internal stress gradient between the pressure gradient and the viscous terms reveals that both are significant, although they operate on very different timescales. The acceleration term is generally negligible, while the sum of Coriolis plus sea surface tilt is small. Thus the seasonal average force balance in fall, winter, and spring is mostly between three terms of roughly equal magnitudes: air drag, water drag, and internal stress gradients. This is also true for the monthly average force balance. However, we find that there is a transition around the weekly timescale and that on a daily basis the force balance at a particular location and time is often between only two terms: either between air drag and water drag or between air drag and internal stress gradients. The model is in agreement with the observations of Thorndike and Colony [1982] in that the correlation between geostrophic wind forcing and the model's ice velocity field is high. This result is discussed in the context of the force balance; we show that the presence of significant internal stress gradients does not preclude high wind-ice correlation. A breakdown of the internal stress gradient into component parts reveals that the shear viscous force is far from negligible, which casts strong doubt on the theoretical validity of the cavitating fluid approximation (in which this component is neglected). Finally, the role of ice pressure is examined by varying the parameter P*. We find a strong sensitivity in terms of the force balance, as well as ice thickness and velocity.

Effects of pressure gradients on turbulent premixed flames

Abstract: In most practical situations, turbulent premixed flames are ducted and, accordingly, subjected to externally imposed pressure gradients. These pressure gradients may induce strong modifications of the turbulent flame structure because of buoyancy effects between heavy cold fresh and light hot burnt gases. In the present work, the influence of a constant acceleration, inducing large pressure gradients, on a premixed turbulent flame is studied using direct numerical simulations. A favourable pressure gradient, i.e. a pressure decrease from unburnt to burnt gases, is found to decrease the flame wrinkling, the flame brush thickness, and the turbulent flame speed. It also promotes counter-gradient turbulent transport. On the other hand, adverse pressure gradients tend to increase the flame brush thickness and turbulent flame speed, and promote classical gradient turbulent transport. As proposed by Libby (1989), the turbulent flame speed is modified by a buoyancy term linearly dependent on both the imposed pressure gradient and the integral length scale l t . A simple model for the turbulent flux uc is also proposed, validated from simulation data and compared to existing models. It is shown that turbulent premixed flames can exhibit both gradient and counter-gradient transport and a criterion integrating the effects of pressure gradients is derived to differentiate between these regimes. In fact, counter-gradient diffusion may occur in most practical ducted flames.

Coupling of shear flow and pressure gradient instabilities

Abstract: The nonlinear dynamics of a shear flow and its subsequent evolution in the equatorial plane of the inner plasma sheet is studied. A linear analysis of the ideal MHD equations reveals a hybrid vortex instability which appears because of the coupling of Kelvin-Helmholtz (KH) and Rayleigh-Taylor instabilities. The hybrid vortex mode grows faster than a KH mode, extracts ambient potential energy, and leads to vortex cells that have a larger spatial extent than a simple KH vortex. In the nonlinear stage, vortices become surge-like and may destroy the shear flow region. The relevance of this model to vortex generation and auroral arc intensifications at the inner edge of the plasma sheet is discussed.

A two-dimensional simulation of the radial and latitudinal evolution of a solar wind disturbance driven by a fast, high-pressure coronal mass ejection

Abstract: Using a hydrodynamic simulation, we have studied the two-dimensional (symmetry in the azimuthal direction) evolution of a fast, high-pressure coronal mass ejection (CME) ejected into a solar wind with latitudinal variations similar to those observed by Ulysses. Specifically, the latitudinal structure of the ambient solar wind in the meridional plane is approximated by two zones: At low latitudes (< 20°) the solar wind is slow and dense, while at higher latitudes the solar wind is fast and tenuous. The CME is introduced into this ambient wind as a bell-shaped pressure pulse in time, spanning from the equator to 45° with a speed and temperature equal to that of the high-latitude solar wind. We find that such an ejection profile produces radically different disturbance profiles at low and high latitudes. In particular, the low-latitude portion of the ejecta material drives a highly asymmetric disturbance because of the relative difference in speed between the fast CME and slower ambient solar wind ahead. In contrast, the high-latitude portion of the same ejecta material drives a much more radially symmetric disturbance because the relative difference in pressure between the CME and ambient background plasma dominates the dynamics. The simulations reveal a number of other interesting features. First, there is significant distortion of the CME in the interplanetary medium. By ∼ 1 AU the CME has effectively separated (in radius as well as latitude) into two pieces. The radial separation is due to the strong velocity shear between the slow and fast ambient solar wind. The latitudinal separation arises from pressure gradients associated with rarefaction regions that develop as the CME propagates outward. Second, there is significant poleward motion of the highest-latitude portion of the CME and its associated disturbance. The main body of the CME expands poleward by ∼ 18°, while the forward and reverse waves (produced by the overexpanding portion of the CME) propagate all the way to the pole. Third, the simulations show that the high-pressure region, which develops at low latitudes as the fast CME ploughs through the slow ambient solar wind, penetrates significantly (∼ 10°) into the high-latitude fast solar wind. We compare the simulation results with a CME-driven interplanetary disturbance observed at both low and high latitudes and find that the simulation reproduces many of the essential features of the observations.

On the newtonian cosmology equations with pressure

Abstract: The basic equations describing a Newtonian universe with uniform pressure are reexamined. We argue that in the semi-classical formulation adopted in the literature the continuity equation has a misleading pressure gradient term. When this term is removed, the resulting equations give the same homogeneous background solutions with pressure and the same evolution equation for the density contrast as are obtained using the full relativistic approach.

Laser Anemometer Measurements of the Flow Field in a 4:1 Pressure Ratio Centrifugal Impeller

Abstract: A laser-doppler anemometer was used to obtain flow-field velocity measurements in a 4:1 pressure ratio, 4.54 kg/s (10 lbm/s), centrifugal impeller, with splitter blades and backsweep, which was configured with a vaneless diffuser. Measured through-flow velocities are reported for ten quasi-orthogonal survey planes at locations ranging from 1% to 99% of main blade chord. Measured through-flow velocities are compared to those predicted by a 3-D viscous steady flow analysis (Dawes) code. The measurements show the development and progression through the impeller and vaneless diffuser of a through-flow velocity deficit which results from the tip clearance flow and accumulation of low momentum fluid centrifuged from the blade and hub surfaces. Flow traces from the CFD analysis show the origin of this deficit which begins to grow in the inlet region of the impeller where it is first detected near the suction surface side of the passage. It then moves toward the pressure side of the channel, due to the movement of tip clearance flow across the impeller passage, where it is cut by the splitter blade leading edge. As blade loading increases toward the rear of the channel the deficit region is driven back toward the suction surface by the cross-passage pressure gradient. There is no evidence of a large wake region that might result from flow separation and the impeller efficiency is relatively high. The flow field in this impeller is quite similar to that documented previously by NASA Lewis in a large low-speed backswept impeller.

H-mode pedestal characteristics, ELMs, and energy confinement in ITER shape discharges on DIII-D

Abstract: The H-mode confinement enhancement factor, H, is found to be strongly correlated with the height of the edge pressure pedestal in ITER shape discharges. In discharges with Type I ELMs the pedestal pressure is set by the maximum pressure gradient before the ELM and the width of the H-mode transport barrier. The pressure gradient before Type I ELMs is found to scale as would be expected for a stability limit set by ideal ballooning modes, but with values significantly in excess of that predicted by stability code calculations. The width of the H-mode transport barrier is found to scale equally well with pedestal P(POL)(2/3) or B(POL)(1/2). The improved H value in high B(POL) discharges may be due to a larger edge pressure gradient and wider H-mode transport barrier consistent with their higher edge ballooning mode limit. Deuterium puffing is found to reduce H consistent with the smaller pedestal pressure which results from the reduced barrier width and critical pressure gradient. Type I ELM energy loss is found to be proportional to the change in the pedestal energy.

Signal processing of diurnal and semidiurnal variations in radon and atmospheric pressure : A new tool for accurate in situ measurement of soil gas velocity, pressure gradient, and tortuosity

Abstract: Signal processing of diurnal and semidiurnal variations of both atmospheric pressure and radon concentration in soil gases is shown to be useful for estimating soil gas transport parameters. The two daily-cycle peaks at 12- and 24-hour periods in the Power Spectral Density (PSD) of atmospheric pressure seem to be present everywhere on Earth's surface, and it is the effect of these regular pressure variations on the radon concentration in soil gases that makes it possible to determine three soil gas transport parameters which can be used to estimate real gas velocity; i.e. tortuosity τ, the ratio k/n between intrinsic permeability and effective porosity (that part of porosity involved in gas transport), and the pressure gradient α. The parameters k and n can be determined independently if the gas flux at the surface is measured at the same time. The method is robust, representative, and accurate: since it allows reliable estimation of transport parameters, it can provide relevant information about the depth of the radon source and the time it takes for information to reach the surface when radon bursts occur at depth. Radon is an appropriate soil gas tracer because it exists in all soils. Moreover, the measurement of radon concentration requires only passive sensors that do not hamper the rising gas column. Gas flux data obtained in Andalusia, Spain, in connection with mineral exploration are processed as examples. Determining the complete set of transport parameters helps in the interpretation of recorded radon outbursts, which are found to be correlated with regional seismic activity.

Solar wind and chromospheric network

Abstract: A physical model of the transition region, including upflow of the plasma in magnetic field funnels that are open to the overlying corona, is presented. A numerical study of the effects of Alfven waves on the heating and acceleration of the nascent solar wind originating in the chromospheric network is carried out within the framework of a two-fluid model for the plasma. It is shown that waves with reasonable amplitudes can, through their pressure gradient together with the thermal pressure gradient, cause a substantial initial acceleration of the wind (on scales of a few Mm) to locally supersonic flows in the rapidly expanding magnetic field ‘trunks’ of the transition region network. The concurrent proton heating is due to the energy supplied by cyclotron damping of the high-frequency Alfven waves, which are assumed to be created through small-scale magnetic activity. The wave energy flux of the model is given as a condition at the upper chromosphere boundary, located above the thin layer where the first ionization of hydrogen takes place. Among the new numerical results are the following: Alfven waves with an assumed f -1 power spectrum in the frequency range from 1 to 4 Hz, and with an integrated mean amplitude ranging between 25 and 75 km s4, can produce very fast acceleration and also heating through wave dissipation. This can heat the lower corona to a temperature of 5× 105 K at a height of h=12,000 km, starting from 5× 104 K at h=3000 km. The resulting thermal and wave pressure gradients can accelerate the wind to speeds of up to 150 km s-1 at h=12,000 km, starting from 20 km s-1 at h=3000 km in a rapidly diverging flux tube. Thus the nascent solar wind becomes supersonic at heights well below the classical Parker-Type sonic point. This is a consequence of the fact that any large wave-energy flux, if it is to be conducted through the expanding funnel to the corona, implies the building-up of an associated wave-pressure gradient. Because of the diverging field geometry, this might lead to a strong initial acceleration of the flow. There is a multiplicity of solutions, depending mainly on the coronal pressure. Here we discuss two new (as compared with a static transition region model) possibilities, namely that either the flow remains supersonic or slows down abruptly by shock formation, which then yields substantial coronal heating up to the canonical 106 K for the proton temperature.

Navier-Stokes Simulations of a Novel Viscous Pump

Abstract: A numerical study of flow in a novel viscous-based pumping device appropriate for microscale applications is described. The device, essentially consisting of a rotating cylinder eccentrically placed in a channel, is shown to be capable of generating a net flow against an externally imposed pressure gradient. Navier-Stokes simulations at low Reynolds numbers are carried out using a finite-volume approach to study the influence of various geometric parameters. Slip effects for gas flows are also briefly investigated. The numerical results indicate that the generated flow rate is a maximum when the cylinder is in contact with a channel wall and that an optimum plate spacing exists. These observations are in excellent agreement, both qualitatively and quantitatively, with a previous experimental study. Furthermore, it is shown that effective pumping is obtained even for considerably higher Reynolds numbers, thereby extending the performance envelope of the proposed device to non-microscale applications as well. Finally, slip-flow effects appear to be significant only for Knudsen numbers greater than 0.1, which is important from the point of view of microscale applications.

Existence of solutions to the transonic pressure gradient equations of the compressible euler equations in elliptic Regions

Abstract: We establish the existence of a smooth solution in tis elliptic region in the self—similar plane to the pressure—gradient system arisen from the wave—particle splitting of the two—dimensional compressible Euler system of equations. The pressure—gradient system takes the form Here (u,v) is the velocity, ρ is the density which is independent of time resulted from the splitting procedurep is the pressure, and is the energy. The natural (parabolically degenerate) boundary value is used.

Wave-intensity analysis: a new approach to left ventricular filling dynamics.

Abstract: In order to explore a new approach to the analysis of diastolic dysfunction, we adapted wave-intensity analysis (WIA), a time-domain analysis that provides information regarding both upstream and downstream events, to left ventricular (LV) filling WIA considers the pressure and flow waves as summations of successive wavelets, characterised by the direction they travel and by the sign of the pressure gradient associated with them Wave intensity is the product, dPdU, calculated from the incremental differences in LV pressure (dP) and mitral velocity (dU) and, during the diastolic filling interval, yields up to five dPdU peaks Peak 1 is caused by backward-travelling expansion waves that accelerate the blood while LV pressure falls, and may be related to "diastolic suction" Peak 2 is caused by forward-travelling compression waves which occur if acceleration continues after LV pressure begins to increase Peak 3 is caused by backward compression waves and is associated with rising LV pressure and deceleration Peak 4 is caused by forward compression waves and is associated with the increasing LV pressure and acceleration caused by atrial contraction Peak 5 is caused by backward compression waves and is associated with increasing pressure and deceleration These preliminary observations suggest that WIA can be useful in describing the mechanics of LV filling and, after much further work has been accomplished, it might prove useful in the detection and characterization of diastolic dysfunction

SEMI‐IMPLICIT FINITE VOLUME SHALLOW‐WATER FLOW AND SOLUTE TRANSPORT SOLVER WITH k–ε TURBULENCE MODEL

Abstract: A 3D semi-implicit finite volume scheme for shallow-water flow with the hydrostatic pressure assumption has been developed using the σ-co-ordinate system, incorporating a standard κ-e turbulence transport model and variable density solute transport with the Boussinesq approximation for the resulting horizontal pressure gradients. The mesh spacing in the vertical direction varies parabolically to give fine resolution near the bed and free surface to resolve high gradients of velocity, k and e. In this study, wall functions are used at the bed (defined by the bed roughness) and wind stress at the surface is not considered. Surface elevation gradient terms and vertical diffusion terms are handled implicitly and horizontal diffusion and source terms explicitly, including the Boussinesq pressure gradient term due to the horizontal density gradient. The advection terms are handled in explicit (conservative) form using linear upwind interpolation giving second-order accuracy. A fully coupled solution for the flow field is obtained by substituting for velocity in the depth-integrated continuity equation and solving for surface elevation using a conjugate gradient equation solver.

Model for subharmonic waves in granular materials

Abstract: Here, we present a model explaining the instability onset of granular surface waves. Our model is based on the fact that lateral motion of grains at large scale takes place when the layer is in free flight, whereas, thickness relaxation occurs only during the layer-plate collision. A master equation is introduced to take into account the mass motion while simple arguments allow us to write down a diffusion equation for the layer thickness. Numerical simulation shows that square patterns are selected by the nonlinearities introduced in this model and localized structures are observed when the local dilation in the layer becomes small with respect to the particle diameter. Figure 1 is a side view of the experimental cell showing the time evolution of the waves close to the layer-container collison. During the collision, thickness modulations relax until the layer takes off. Each layer modulation splits into two sand packets which move laterally. They subsequently collide and form new modulations shifted in space by half the pattern wavelength. The pattern is then subharmonic, oscillating at one half the drive frequency. Lateral motion of grains takes place during every cycle of the external excitation. This motion is similar to that of a fluid particle in a standing surface wave. However, there is one important difference: In the fluid case, the lateral transfer of mass is induced by the gradient of hydrostatic pressure which exists at all times (6). In the granular case, the pressure gradient exists only during the collision and is a result of the transfer of momentum from the plate to the grain network. In our analysis, we, therefore, consider the effective drive to be composed of a series of impulsive accelerations that only occur when the layer collides with the plate. The dynamics of the layer described in the continuum limit using two variables: , the local layer thickness, and , the horizontal component of the grain velocity ( represents the horizontal coordinates and is the time). We consider to be independent of the vertical coordinate. With these variables, the

Aspects of Vane Film Cooling With High Turbulence: Part II — Adiabatic Effectiveness

Abstract: A four-vane subsonic cascade was used to investigate the influence of turbulence on vane film cooling distributions. The influence of film injection on vane heat transfer distributions in the presence of high turbulence was examined in part 1 of this paper. Vane effectiveness distributions were documented in the presence of a low level of turbulence (1 percent) and were used to contrast results taken at a high level (12 percent) of large-scale turbulence. All data were taken at a density ratio of about 1. The three geometries chosen to study included one row and two staggered rows of downstream film cooling on both the suction and pressure surfaces as well as a showerhead array. Turbulence was found to have a moderate influence on one and two rows of suction surface film cooling but had a dramatic influence on pressure surface film cooling, particularly at the lower velocity ratios. The strong pressure gradients on the pressure surface of the vane were also found to alter film cooling distributions substantially. At lower velocity ratios, effectiveness distributions for two staggered rows of holes could be predicted well using data from one row superposed. At higher velocity ratios the two staggered rows produced significantlymore » higher levels of effectiveness than values estimated from single row data superposed. Turbulence was also found to reduce effectiveness levels produced by showerhead film cooling substantially.« less

A Review of Hypersonic Boundary Layer Stability Experiments in a Quiet Mach 6 Wind Tunnel

Abstract: Three recent experimental studies of transition on cones with adverse pressure gradient produced by a flared afterbody and with the additive stability modifiers of wall cooling, angle of attack and bluntness are reviewed. All tests were conducted in a quiet Mach 6 wind tunnel. The dominant instability was found to be the second mode. For the cases examined with linear stability theory, the N factors at mode saturation were in the range of 8.5 to 11. Evidence of a combined second-mode/Gortler transition process was found. Mean, rms and spectral freestream data for the quiet facility is presented and the role of low frequency freestream noise is discussed.

In vivo hydraulic conductivity of muscle: effects of hydrostatic pressure.

Abstract: We and others have shown that the loss of fluid and macromolecules from the peritoneal cavity is directly dependent on intraperitoneal hydrostatic pressure (Pip). Measurements of the interstitial pressure gradient in the abdominal wall demonstrated minimal change when Pip was increased from 0 to 8 mmHg. Because flow through tissue is governed by both interstitial pressure gradient and hydraulic conductivity (K), we hypothesized that K of these tissues varies with Pip. To test this hypothesis, we dialyzed rats with Krebs-Ringer solution at constant Pip of 0.7, 1.5, 2.2, 3, 4.4, 6, or 8 mmHg. Tracer amounts of 125I-labeled immunoglobulin G were added to the dialysis fluid as a marker of fluid movement into the abdominal wall. Tracer deposition was corrected for adsorption to the tissue surface and for local loss into lymphatics. The hydrostatic pressure gradient in the wall was measured using a micropipette and a servo-null system. The technique requires immobilization of the tissue by a porous Plexiglas plate, and therefore a portion of the tissue is supported. In agreement with previous results, fluid flux into the unrestrained abdominal wall was directly related to the overall hydrostatic pressure difference across the abdominal wall (Pip = 0), but the interstitial pressure gradient near the peritoneum increased only approximately 40% over the range of Pip = 1.5-8 mmHg (20-28 mmHg/cm). K of the abdominal wall varied from 0.90 +/- 0.1 x 10(-5) cm2.min-1.mmHg-1 at Pip = 1.5 mmHg to 4.7 +/- 0.43 x 10(-5) cm2.min-1.mmHg-1 on elevation of Pip to 8 mmHg. In contrast, for the same change in Pip, abdominal muscle supported on the skin side had a significantly lower range of fluid flux (0.89-1.7 x 10(-4) vs. 1.9-10.1 x 10(-4) ml.min-1.cm-2 in unsupported tissue). The differences between supported and unsupported tissues are likely explained in part by a reduced pressure gradient across the supported tissue. In conclusion, the in vivo hydraulic conductivity of the unsupported abdominal wall muscle in anesthetized rats varies with the superimposed hydrostatic pressure within the peritoneal cavity.

Radial artery diameter decreases with increased femoral to radial arterial pressure gradient during cardiopulmonary bypass.

Abstract: A clinically significant femoral to radial artery pressure gradient sometimes develops during cardiopulmonary bypass (CPB), but the mechanism responsible is not clear.We investigated when the pressure gradient developed and what mechanism could be responsible by comparing mean femoral to mean radial

Hybridization between σ- and z-co-ordinates for improving the internal pressure gradient calculation in marine models with steep bottom slopes

Abstract: It is discussed in this paper how the pressure gradient error in general vertical co-ordinate models (in which the σ-transformation is a special case) can be reduced by means of hybrid models. For a better understanding, the derivation of such a general vertical co-ordinate model from the Cartesian co-ordinate model is given. Two types of hybridization between σ- and z-co-ordinate models, each using one parameter specifying the degree of hydridization, are presented: (i) the mixed layer transformation with a constant number of layers which are refined near the surface and (ii) the z/σ-transformation which introduces steps near the bottom. In order to achieve good results with the models using other than σ-co-ordinates, a profile-conserving momentum advection discretization is developed. The different co-ordinate transformations are tested with 2D barotropic and baroclinic flows over a topographic bump. Those models with nearly horizontal co-ordinate surfaces in the stratified area give the best correspondence with an isopycnal reference solution. © 1997 John Wiley & Sons, Ltd.

Onset of Mobilization and the Fraction of Trapped Foam in Porous Media

Abstract: Usually, foam in a porous medium flows through a small and spatially varying fraction of available pores, while the bulk of it remains trapped. The trapped foam is under a pressure gradient corresponding to the pressure gradient imposed by the flowing foam and continuous wetting liquid. The imposed pressure gradient and coalescence of the stationary foam lamellae periodically open flow channels in the trapped foam region. Foam lamellae in each of these channels flow briefly, but channels are eventually plugged by smaller bubbles entering into the trapped region. The result is a cycling of flow channels that open and close throughout the trapped foam, leading to intermittent pulsing of foam flow in that region. The dynamic behavior of foam trapped in porous media is modeled here with a pore network simulator. We predict the magnitude of the pressure drop leading to the onset of flow of foam lamellae in the region containing trapped foam. This mobilization pressure drop depends only on the number of lamellae in the flow path and on the geometry of the pores that make up this path. The principles learned in this study allow us to predict the fraction of foam that is trapped in a porous medium under given flow conditions. We present here the first analytic expression for the trapped foam fraction as a function of the pressure gradient, and of the mean and standard deviation of the pore size distribution. This expression provides a missing piece for the continuum foam flow models based on the moments of the volume-averaged population balance of foam bubbles.

Experimental evaluation of acceleration correlations for locally isotropic turbulence

Abstract: The two-point correlation of the fluid-particle acceleration is the sum of the pressure gradient and viscous force correlations. The pressure-gradient correlation is related to the fourth-order velocity structure function. The acceleration correlation caused by viscous forces is formulated in terms of the third-order velocity structure function. Velocity data from grid-generated turbulence in a wind tunnel are used to evaluate these quantities. The evaluated relationships require only the Navier-Stokes equation, incompressibility, local homogeneity, and local isotropy. The relationships are valid for any Reynolds number. For the moderate Reynolds number of the wind-tunnel turbulence, the acceleration correlation is roughly three times larger than if it is evaluated on the basis of the assumption that velocities at several points are joint Gaussian random variables. The correlation of components of acceleration parallel to the separation vector of the two points is negative near its minimum at spacings close to 17 times the microscale. Its value near this minimum implies that fluid particles at those spacings have typical relative accelerations of one-half that of gravity in the directions toward and away from one another. For large Reynolds numbers, the two-point correlation of acceleration is dominated by the two-point correlation of the pressure gradient. The data verify that the acceleration correlation caused by viscous forces is much smaller than that caused by the pressure gradient.