Showing papers on "Pressure gradient published in 2000"

Observation of superfluid flow in a bose-einstein condensed Gas

Abstract: We have studied the hydrodynamic flow in a Bose-Einstein condensate stirred by a macroscopic object, a blue-detuned laser beam, using nondestructive in situ phase contrast imaging. A critical velocity for the onset of a pressure gradient has been observed, and shown to be density dependent. The technique has been compared to a calorimetric method used previously to measure the heating induced by the motion of the laser beam.

The Dynamics of a Partially Mixed Estuary

Abstract: Measurements of velocity, density, and pressure gradient in the lower Hudson River estuary were used to quantify the dominant terms in the momentum equation and to characterize their variations at tidal and spring‐ neap timescales. The vertical momentum flux (assumed to be due mainly to turbulent shear stress) was estimated indirectly, based on the residual from the acceleration and pressure gradient terms. The indirect estimates of stress compared favorably to bottom stress estimates using a quadratic drag law, supporting the hypothesis that the tidal momentum equation involves a local balance between tidal acceleration, pressure gradient, and stress divergence. Estimates of eddy viscosity indicated that there was significant tidal asymmetry, with flood tide values exceeding ebb values by a factor of 2. As a consequence of the asymmetry, the vertical structure of the tidally averaged stress bore no resemblance to the tidally averaged shear. In spite of the asymmetry of vertical mixing, the tidally averaged, estuarine circulation was found to depend simply on the intensity of bottom turbulence, which could be parameterized by a Rayleigh drag formulation based on the tidal velocity magnitude and the tidally averaged near-bottom flow. This seemingly paradoxical result indicates that the estuarine circulation can be modeled without detailed knowledge of the effective eddy viscosity, only requiring an estimate of the bottom drag coefficient, the tidal forcing conditions, and the baroclinic pressure gradient. A notable characteristic of this solution is an inverse dependence of the estuarine circulation on the amplitude of the tides.

Lithospheric pressure–depth relationship in compressive regions of thickened crust

Abstract: We investigate the pressure distribution with depth in regions undergoing horizontal shortening and experiencing crustal thickening both analytically and numerically. Our results show that, in a convergent tectonic setting, pressure can be considerably higher than lithostatic (the pressure resulting from the weight of the overburden). Increases in pressure with respect to lithostatic conditions result from both the contribution of horizontal stresses and the flexural vertical loads, the latter generated by the deflection of the upper crust and of the mantle because of the presence of topographic relief and a root, respectively. The contribution of horizontal stresses is particularly relevant to the upper crust and uppermost mantle, where rocks are thought to deform brittlely. In these domains, pressure gradients twice lithostatic can be achieved. The contribution of horizontal stresses is less important in the ductile domains as differential stresses are progressively relaxed; nevertheless, the effects are still noteworthy especially close to the brittle–ductile transition. Flexural vertical loads generated by the deflection of the upper crust and lithospheric mantle are relevant for rocks of the weaker lower crust. As a result of the combination of the two mechanisms, the pressure gradient varies vertically through the lithosphere, ranging from negative (inverted) gradients to gradients up to several times the lithostatic gradient. The pressure values range from one to two times the lithostatic values (1ρgz to 2ρgz).

Numerical simulation of pulsating turbulent channel flow

Abstract: Direct and large-eddy simulations of the Navier–Stokes equations are used to study the pulsating flow in a channel. The cases examined span a wide range of frequencies of the driving pressure gradient, and encompass different physical behaviors, from the quasi-Stokes flow observed at high frequencies, to a quasisteady behavior at the lowest ones. The validity of the dynamic Smagorinsky model to study this kind of unsteady flow is established by a posteriori comparison with direct simulations and experimental data. It is shown that the fluctuations generated in the near-wall region by the unsteady pressure gradient do not propagate beyond a certain distance lt from the wall, which can be estimated quite accurately by a simple eddy viscosity argument. No substantial departure from the Stokes regime at very high frequency (ω+ as high as 0.1) is observed. The time-dependent characteristics of the flow are examined in detail, as well as the topology of the coherent structures.

Optimal perturbations for boundary layers subject to stream-wise pressure gradient

Abstract: Configurations of perturbation velocity which optimally excite an algebraic growth mechanism in the Falkner–Skan boundary layer are studied using a direct–adjoint technique. The largest transient amplification is obtained by stream-wise oriented vortices, in agreement with previous results for the Blasius boundary layer. Adverse pressure gradient is found to increase the resulting growth, the reverse is true for accelerated flows. It is shown that optimally excited algebraic mechanisms are capable of competition with optimally excited Tollmien–Schlichting waves in super-critical flows before succumbing to viscous damping. Disturbances optimized for maximal amplification over shorter periods are generally oblique and can experience significant transient growth; it is argued that they should not be dismissed when searching for rapidly growing perturbations which may preferentially induce early transition. Optimal disturbances transform into streaks downstream of their inception, attesting to the ubiquity of these flow structures.

Two-dimensional blood flow through tapered arteries under stenotic conditions

Abstract: A mathematical model of non-linear two-dimensional blood flow in tapered arteries in the presence of stenosis is developed. An improved shape of the time-variant overlapping stenosis present in the tapered arterial lumen is given mathematically in order to update resemblance to the in vivo situation. The vascular wall deformability is taken to be elastic while the flowing blood contained in it is treated to be Newtonian. The non-linear terms appearing in the Navier–Stokes equations governing blood flow and the instantaneous taper angle are accounted for. The present analytical treatment bears the potential to calculate both the axial and the radial velocity profiles with low computational complexity by exploiting the appropriate boundary conditions and the input pressure gradient arising from the normal functioning of the heart. The computed results are found to converge at a high rate with the tolerance of ∼10−14 and agree well with the corresponding existing data. An extensive quantitative analysis is performed through numerical computations of the desired quantities presented graphically at the end of the paper which help estimating the effects of tapering, the wall motion, the stenosis and the pulsatile pressure gradient on the flow characteristics of blood and thereby the applicability of the present model is established.

The effect of plasma shape on H-mode pedestal characteristics on DIII-D

Abstract: The characteristics of the H-mode are studied in discharges with varying triangularity and squareness. The pressure at the top of the H-mode pedestal increases strongly with triangularity, primarily due to an increase in the margin by which the edge pressure gradient exceeds the ideal ballooning mode first stability limit. Two models are considered for how the edge may exceed the ballooning mode limit. In one model, access to the ballooning mode second stable regime allows the edge pressure gradient and associated bootstrap current to continue to increase until an edge localized, low toroidal mode number, ideal kink mode is destabilized. In the second model, the finite width of the H-mode transport barrier and diamagnetic effects raise the pressure gradient limit above the ballooning mode limit. We observe a weak inverse dependence of the width of the H-mode transport barrier, Δ, on triangularity relative to the previously obtained scaling Δ∝(βPEP)1/2. The energy loss for Type I ELMs increases with triangularity in proportion to the pedestal energy increase. At low density, the energy confinement of high-triangularity discharges is larger than discharges with low triangularity, as a result of an increase in the energy in the H-mode pedestal. At high density, both the change in pedestal pressure and the response of the density profile are found to play a role in setting the energy confinement. The highest energy confinement at high density was obtained in low-triangularity discharges.

Measurements in Separated and Transitional Boundary Layers Under Low-Pressure Turbine Airfoil Conditions

Abstract: Detailed velocity measurements were made along a flat plate subject to the same dimensionless pressure gradient as the suction side of a modern low-pressure turbine airfoil. Reynolds numbers based on wetted plate length and nominal exit velocity were varied from 50,000 to 300,000, covering cruise to takeoff conditions. Low and high inlet free-stream turbulence intensities (0.2% and 7%) were set using passive grids. The location of boundary-layer separation does not depend strongly on the free-stream turbulence level or Reynolds number, as long as the boundary layer remains non-turbulent prior to separation. Strong acceleration prevents transition on the upstream part of the plate in all cases. Both free-stream turbulence and Reynolds number have strong effects on transition in the adverse pressure gradient region. Under low free-stream turbulence conditions transition is induced by instability waves in the shear layer of the separation bubble. Reattachment generally occurs at the transition start. At Re = 50,000 the separation bubble does not close before the trailing edge of the modeled airfoil. At higher Re, transition moves upstream, and the boundary layer reattaches. With high free-stream turbulence levels, transition appears to occur in a bypass mode, similar to that in attached boundary layers. Transition moves upstream, resulting in shorter separation regions. At Re above 200,000, transition begins before separation. Mean velocity, turbulence and intermittency profiles are presented.

Influence of positive airway pressure on the pressure gradient for venous return in humans

Abstract: To study the effect of positive airway pressure (Paw) on the pressure gradient for venous return [the difference between mean systemic filling pressure (Pms) and right atrial pressure (Pra)], we in...

On the shape and velocity of fluid‐filled fractures in the Earth

Abstract: Summary In this paper I present a model to estimate the shape and velocity of slowly ascending buoyancy-driven fluid-filled fractures. The model considers elastic deformation, linear fracture mechanics and fluid flow. An attempt is made to incorporate the effect ofthe 2-D fluid pattern on the viscous pressure drop. Most other models assume that the viscous pressure drop can be approximated by flow through a channel with constant width, although the form of the fracture is known to deviate from such a simple shape. The 2-D flow in my model has an important consequence for the mechanism of buoyancy-driven fracture propagation—it predicts a large pressure gradient at the tail of the propagating fracture, indicating that the tail of the fracture is most important in hindering the fracture propagation. A singularity at the tail of the fracture can be avoided when a small amount of fluid trails in the channel left behind the propagating fracture. The trailing and decoupling of fluids at the tail seems to be accompanied by small flow and shape instabilities, which is indicated by the jerky movement of the tail observed in propagating air-filled fractures in solidified gelatine, and by numerical boundary element solutions of the coupled flow-deformation–fracturing problem. By comparing the predictions for propagation velocities with laboratory observations of buoyancy-driven fracture propagation in gelatine, I derive a non-dimensional effective thickness at the tail of the fracture for which trailing of fluids may occur. The model is applied to Earth-relevant problems such as oil- and water-filled fractures in pressurized sediments or magma-filled dykes in the lithosphere, which are discussed in the paper.

Scaling of Performance for Varying Density Ratio Coolants on an Airfoil With Strong Curvature and Pressure Gradient Effects

Abstract: An experimental study was conducted to investigate the film cooling performance on the suction side of a first-stage turbine vane. Tests were conducted on a nine times scale vane model at density ratios of DR = 1.1 and 1.6 over a range of blowing conditions, 0.2 ≤ M ≤ 1.5 and 0.05 ≤ I ≤ 1.2. Two different mainstream turbulence intensity levels, Tu∞ =0.5 and 20 percent, were also investigated. The row of coolant holes studied was located in a position of both strong curvature and strong favorable pressure gradient. In addition, its performance was isolated by blocking the leading edge showerhead coolant holes. Adiabatic effectiveness measurements were made using an infrared camera to map the surface temperature distribution. The results indicate that film cooling performance was greatly enhanced over holes with a similar 50 deg injection angle on a flat plate. Overall, adiabatic effectiveness scaled with mass flux ratio for low blowing conditions and with momentum flux ratio for high blowing conditions. However, for M < 0.5, there was a higher rate of decay for the low density ratio data. High mainstream turbulence had little effect at low blowing ratios, but degraded performance at higher blowing ratios.

Models of flux in porous media with memory

Abstract: Some data on the flow of fluids in rocks exhibit properties which may not be interpreted with the classic theory of propagation of pressure and of fluids in porous media [Bell and Nur, 1978; Roeloffs, 1988] based on Darcy's law, which states that the flux is proportional to the pressure gradient. Concerning the fluids, some may react chemically with the medium enlarging the pores; some carry solid particles, which may obstruct some of the pores; and finally, some may precipitate minerals in the pores diminishing their size or even closing them as in geothermal areas. These phenomena create a spatially variable pattern of mineralization and permeability changes that can be localized. In order to obtain a better representation of the flux and of the pressure of fluids, Darcy's law is modified, introducing general memory formalisms operating on the flow as well as on the pressure gradient, which imply a filtering of the pressure gradient without singularities. We also modify the second constitutive equation of diffusion, which relates the density variations of the fluid to the pressure, by introducing a rheology in the fluid also represented by memory formalisms operating on the pressure as well as on the density variations. The memory formalisms are then specified as derivatives of fractional order. The equations used here for the diffusion of fluids are different from the classic ones; however, the equation governing the diffusion of the pressure is the same as that of the flux, as in the classic case. For technical reasons the majority of the studies on diffusion is devoted to the diffusion of the pressure of the fluid rather than to the flux; in this paper we shall devote our attention to studying the flux and its spectral properties in a practical example seeing that the memory used implies a low-pass filtering of the flux or a band pass centered in the low-frequency range. A half space is considered where the boundary values are applied to the plane limiting it, and the problems solved are the computation of the Green function of the flux in the cases when (1) a pressure constant in time is applied on the boundary plane and (2) a periodic pressure is applied to the boundary plane while the half space is initially at zero pressure in both cases. We found closed form formulae for the flux and its spectrum. A discussion follows concerning the mode of determination of the parameters of memory formalisms ruling the diffusion using the observed pressure and/or the flux at several frequencies in problem 2. Concerning the flux, it is tentatively seen that when the medium is oil rock and the fluid is water, for a pressure of 105 Pa at the boundary and derivative of order 0.1, the flux at a distance of 0.1 km from the boundary plane is 1700 kg h−1.

Collisionless reconnection: Effects of Hall current and electron pressure gradient

Abstract: An analytical treatment is given of two-dimensional, quasi-steady collisionless reconnection on the basis of the generalized Ohm's law, including the effects of the Hall current and scalar electron pressure gradient. The equilibrium magnetic field configuration is of the form , containing a neutral line at z = 0 and a constant guide field BT. The dispersion relations of the waves in the ideal region as well as the reconnection layer are discussed, including the effects of plasma beta. When BT = 0, the reconnection layer supports obliquely propagating Alfven-whistler waves, and the reconnection dynamics is controlled by the Hall current. When BT/BP0 ≥ 1, the reconnection layer supports kinetic/inertial Alfven waves, and the reconnection dynamics is controlled by the electron pressure gradient. Analytical estimates are obtained for the nonlinear reconnection rate with and without the guide field from the generalized Ohm's law. A recent claim by Shay et al. (1999) that the reconnection rate is a “universal constant” is questioned. Although the leading-order reconnection rate is independent of the mechanism that breaks field lines (resistivity or electron inertia) and the system size as the system size becomes large, it does depend on global conditions such as the boundary conditions driving reconnection. The analytical predictions are tested by means of Hall magnetohydrodynamics simulations. While some of the geometric features of the reconnection layer and the weak dependence of the reconnection rate on resistivity are reminiscent of Petschek's classical model, the underlying wave and particle dynamics mediating reconnection in the presence of the Hall current and electron pressure gradient are qualitatively different.

Two-dimensional numerical modelling of a direct methanol fuel cell

Abstract: The results of a numerical simulation of a direct methanol fuel cell (DMFC) with liquid methanol feed are presented. A two-dimensional numerical model of a DMFC is developed based on mass and current conservation equations. The velocity of the liquid is governed by gradients of membrane phase potential (electroosmotic effect) and pressure. The results show that, near the fuel channel, transport of methanol is determined mainly by the pressure gradient, whereas in the active layers, and in the membrane, diffusion transport dominates. ‘Shaded’ zones, where there is a lack of methanol, are formed in front of the current collectors. The results reveal a strong influence of the hydraulic permeability of the backing layer K p BL on methanol crossover through the membrane. If the value of K p BL is comparable to that of the membrane and active layers, electroosmotic effects lead to the formation of an inverse pressure gradient. The flux of liquid driven by this pressure gradient is directed towards the anode and reduces methanol crossover.

Method and system for reducing effects of sea surface ghost contamination in seismic data

Abstract: An improved de-ghosting method and system that utilizes multi-component marine seismic data recorded in a fluid medium. The method makes use of two types of data: pressure data that represents the pressure in the fluid medium, such as sea water, at a number of locations; and vertical particle motion data that represents the vertical particle motion of the acoustic energy propagating in the fluid medium at a number of locations within the same spatial area as the pressure data. The vertical particle motion data can be in various forms, for example, velocity, pressure gradient, displacement, or acceleration. A spatial filter is designed so as to be effective at separating up and down propagating acoustic energy over substantially the entire range of non-horizontal incidence angles in the fluid medium. The spatial filter is applied to either the vertical particle motion data or to the pressure data, and then combined with the other data to generate pressure data that has its up and down propagating components separated.

Accelerating shear velocity in gravel-bed channels

Abstract: The behaviour of the shear velocity along a gravel-bed channel is investigated experimentally in the presence of a negative pressure gradient (accelerating flow). Different methods of estimation of the shear velocity, derived from vertical profiles of the mean longitudinal point velocity, are examined and a new method is proposed. Results show that the proposed method of estimation is comparable to the St Venant and Clauser's methods. At a specific cross section, for constant bottom slope and relative roughness, shear velocity increases with discharge.

Modification of high mode pedestal instabilities in the DIII-D tokamak

Abstract: The amplitude and frequency of modes driven in the edge region of tokamak high mode (H-mode) discharges [type I edge-localized modes (ELMs)] are shown to depend on the discharge shape. The measured pressure gradient threshold for instability and its scaling with discharge shape are compared with predictions from ideal magnetohydrodynamic theory for low toroidal mode number (n) instabilities driven by pressure gradient and current density and good agreement is found. Reductions in mode amplitude are observed in discharge shapes with either high squareness or low triangularity where the stability threshold in the edge pressure gradient is predicted to be reduced and the most unstable mode is expected to have higher values of n. The importance of access to the ballooning mode second stability regime is demonstrated through the changes in the ELM character that occur when second regime access is not available. An edge stability model is presented that predicts that there is a threshold value of n for second r...

Analysis of a pressure-stabilized finite element approximation of the stationary Navier-Stokes equations

Abstract: The purpose of this paper is to analyze a finite element approximation of the stationary Navier-Stokes equations that allows the use of equal velocity-pressure interpolation. The idea is to introduce as unknown of the discrete problem the projection of the pressure gradient (multiplied by suitable algorithmic parameters) onto the space of continuous vector fields. The difference between these two vectors (pressure gradient and projection) is introduced in the continuity equation. The resulting formulation is shown to be stable and optimally convergent, both in a norm associated to the problem and in the \(L^2\) norm for both velocities and pressure. This is proved first for the Stokes problem, and then it is extended to the nonlinear case. All the analysis relies on an inf-sup condition that is much weaker than for the standard Galerkin approximation, in spite of the fact that the present method is only a minor modification of this.

Multiple solutions and flow limitation in collapsible channel flows

Abstract: Steady and unsteady numerical simulations of two-dimensional flow in a collapsible channel were carried out to study the flow limitation which typically occurs when the upstream transmural pressure is held constant while flow rate and pressure gradient along the collapsible channel can vary independently. Multiple steady solutions are found for a range of upstream transmural pressures and Reynolds number using an arclength control method. The stability of these steady solutions is tested in order to check the correlation between flow limitation and self-excited oscillations (the latter being a consequence of unstable steady solutions). Both stable and unstable solutions are found when flow is limited. Self-excited oscillations and divergence instabilities are observed in certain solution branches. The instability of the steady solutions seems to depend on the unsteady boundary conditions used, i.e. on which parameters are allowed to vary. However, steady solutions associated with the solution branch before flow limitation where the membrane wall bulges are found to be stable for each of the three different boundary conditions employed. We conclude that there is no one to one correlation between the two phenomena in this two dimensional channel model.

Navier-Stokes simulations of gas flow in micro devices

Abstract: A numerical study of flow in micro channels and micro pipes is described. The simulations are performed by solving Navier-Stokes equations with a slip velocity boundary condition, using the LU-TVD implicit algorithm. A second-order slipping model incorporating pressure gradient is proposed and investigated. The numerical results obtained using the new slipping model are presented and found to compare well with available experimental data and numerical results from other references.Our computations also show that compressibility and the rarefied effects of gas flows are present in both micro channel and micro pipe flows. It is also found that the effect of rarefaction tends to mitigate the negative curvature of pressure distribution that can be attributed to compressibility.

3‐D numerical simulations of nonisothermal flow in co‐rotating twin screw extruders

Abstract: Three-dimensional nonisothermal flow simulations in the kneading disc regions of co-rotating twin screw extruders were performed using a finite element method. The standard Galerkin method and penalty function scheme were applied to the flow field. The streamline-upwind/Petrov-Galerkin scheme was used in the temperature field to reduce numerical oscillation. The simulations were carried out under the operational conditions of The Japan Steel Works TEX30 machine for various rotational speeds. The configuration was ten 2-lobe kneading discs with a 90° stagger angle. Experimental observations were also performed to validate the numerical simulations under the same operational conditions. The pressure in front of the tip in the rotation direction was higher than behind the tip, and the region behind the tip sometimes had a negative value. Since variation of the pressure gradient in the axial direction causes forward and backward flows in the disc gap regions, the disc gap regions play an important role for mixing. The temperature becomes higher with increasing rotation speed due to high viscous dissipation. A high temperature was observed on the disc surface, in the disc gap, and in the intermeshing regions. The numerical results of pressure profiles with the rotation and the temperature in the axial direction were in good agreement with the experimental observations.

Surface Pressure Fluctuations Beneath Two- and Three-Dimensional Turbulent Boundary Layers

Abstract: Surface pressure fluctuation measurements were made in two-dimensional turbulent boundary layers at two Reynolds numbers (Re θ = 7.3 × 10 3 and 2.34 × 104) and pressure-driven three-dimensional turbulent boundary layers at two Reynolds numbers (approach Re θ = 5.94 x 10 3 and 2.32 × 104). The collapse of spectral levels at middle and high frequencies and the effects of inner and outer boundary-layer scaling variables are shown for a wide range of Reynolds numbers (1.4 X 10 3 < 2.34 × 104) for the two-dimensional flows. Such scaling parameters do not collapse the pressure spectra beneath three-dimensional flows, which have a nearly constant, or flat, midfrequency range, and at some measurement stations and spectral levels within the flat- and high-frequency spectral ranges that significantly raise p'. Additionally, dimensional spectral levels within the flat-frequency range are independent of Reynolds number. Analysis based on the Poisson equation shows that the variation of the high-frequency spectral levels is related to the variation in near-wall mean velocity gradients and ν 2 structure due to the spanwise pressure gradient