Internal Pressure Gradient Research Articles

Tidal motions and tidally induced fluxes through La Línea submarine canyon, western Alboran Sea

Detailed observations from two mooring lines deployed in La Línea submarine canyon, western Alboran Sea, are presented. This is a narrow canyon in the sense that its width is always less than the internal radius of deformation. Tidal currents within the canyon are polarized in the along‐canyon direction according to its narrow nature. They have considerable amplitude (values of around 0.5 m/s are often observed) and are forced by the internal pressure gradients associated with the baroclinic tide that is generated in the surroundings. Subsequent amplification of onshore baroclinic currents within the canyon accounts for the large amplitude observed. Cross‐shelf exchange through the canyon due to tidal motions is different from zero despite the close to zero mean of tidal currents. The explanation is based on the asymmetry of water properties flowing up‐canyon and down‐canyon (some sort of tidal rectification). Regarding the energy flux, the canyon seems to be an adequate conduit to carry energy to the shore. Estimations made from our observations indicate that energy input onto the shelf per unit length parallel to the shore at the canyon head is enough to maintain mixing on the shelf at intermediate depths.

A simple model of the large-scale circulation of Mediterranean Water and Labrador Sea Water

A steady, 2 1 2 layer planetary geostrophic model is used to study the circulation and mixing of Mediterranean Water (MW) and Labrador Sea Water (LSW). The model includes parameterizions of salt fingering, mesoscale variability, and meddies. A close relationship between the vertical density ratio (and expected strength of salt fingering) and the potential vorticity anomaly of the upper salt tongue is identified and used to parameterize salt fingering as a spatially nonhomogeneous diapycnal mass flux. A balance between baroclinic Rossby wave propagation and salt fingering produces an asymmetric tongue of potential vorticity extending into the interior from the eastern boundary, consistent with the observed Mediterranean salt tongue. The resulting internal pressure gradients give rise to a large-scale anticyclonic circulation in the upper layer of the salt tongue with northward flow at mid-latitudes, consistent with geostrophic flow estimates based on hydrographic data. Salt fingering that attains maximum strength at mid-latitudes forces a large-scale circulation in the lower MW/LSW depth range that advects water from the western boundary into the eastern basin near 50°N, where it then flows to the south under the upper MW, and finally returns to the western boundary. This pattern is generally consistent with a variety of estimates for the circulation of LSW, and suggests that the eastward flow of LSW off the western boundary may be, at least in part, forced by salt fingering in the interior. The addition of mixing due to mesoscale eddies modifies the details of the potential vorticity distribution, but does not change the qualitative behavior. Representing meddies as a distributed source of mass to the upper layer results in a further westward extension of the salt tongue signature and enhances the strength of the deep recirculation.

Model of laser-driven mass transport in thin films of dye-functionalized polymers

Based on Newtonian fluid dynamic relations, a model is constructed to describe laser-induced mass transport in thin films of polymers containing isomerizable azobenzene chromophores, in which surface profile diffraction gratings can be inscribed with an interference pattern of coherent light. The Navier–Stokes equations for laminar flow of a viscous fluid are developed to relate velocity components in the film to pressure gradients in the polymer film, by definition of boundary layer conditions. This general laminar flow model is applicable to the formation of surface gratings through a variety of mechanisms. Considering the mechanism of an isomerization-driven free volume expansion to produce internal pressure gradients, a specific model is developed to describe polymer flow resulting from laser-induced isomerization of the bulky chromophores. This yields an expression relating the time evolution of the surface gratings to properties which could be varied experimentally, such as those of the irradiating light, inscription geometry, and bulk polymer, which incorporates no arbitrary fitting parameters or integration constants. In the general model, the rate of grating inscription is predicted to vary directly with the intensity of the inscription laser, to vary inversely with the molecular weight of the polymer below the limit of entanglement, and to scale with the third power of the initial thickness of the film. Considering an isomerization pressure mechanism, the model predicts the rate of grating inscription to further vary with the free volume requirements of the induced geometric transformation of the dye molecules, and the polarization state of the inscription laser. Predictions from the model were tested against the results of experiments to vary these parameters, and are shown to be in good agreement.

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

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

On the mechanism of microbial cell disruption in high-pressure homogenisation

The tensions produced in the wall of a rigid, thin-walled, liquid-filled sphere as it moves with an axisymmetric straining flow are examined. This problem has not been previously addressed. A generalised correlation for the maximum wall tension, expressed in dimensionless form as a Weber number ( We), is developed in terms of the acceleration number ( Ac) and Reynolds number ( Re) of the straining flow. At low Reynolds number We is dominated by viscous forces, while inertial forces due to internal pressure gradients caused by sphere acceleration dominate at higher Re. The generalised correlation has been used to examine the case of a typical yeast cell (a thin-walled, liquid-filled sphere) passing through a typical high-pressure homogeniser (a straining-flow device). At 56 MPa homogenising pressure, a 6 μm yeast cell experiences tensions in the inertially dominated regime ( Re=100). The correlation gives We=0.206, corresponding to a maximum wall tension of 8 Nm -1. This is equivalent to an applied compressive force of 150 μN and compares favourably with the force required to break yeast cells under compressive micromanipulation (40–90 μN). Inertial forces may therefore be an important and previously unrecognised mechanism of microbial cell disruption during high-pressure homogenisation. Further work is required to examine the likelihood of cell deformation in the high-strain-rate short-residence-time environment of the homogeniser, and the effect that such deformation may have on the contribution of inertial forces to disruption.

Linear elastic vs elastic-plastic fracture mechanics methods in nuclear vessel integrity assessments

This paper compares analytical failure predictions for a flawed PWR vessel from linear elastic and elastic-plastic fracture mechanics, as currently permissible by ASME code. The significant conservatism under upper-shelf conditions provided by the first approach has been quantified in terms of internal pressure and wall-thickness strain gradient. Monotonic crack growth withstood by the assessed component has been evaluated through several elastic-plastic criteria, and leak-before-break and related crack-arrest events have been inferred to be likely the deeper the postulated pre-crack is. Research results indicate that logarithmic J-R curve data fitting is more appropriate and conservative than the conventionally used power law in regard to extensive crack propagation. Conservative linear elastic and elastic-plastic predictions have also been confirmed by testing small-scale and/or deeply sidegrooved testpieces.

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

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

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

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.

COMBINED INTERNAL CAROTID AND HYPOGLOSSAL ARTERY ENDARTERECTOMIES IN A SYMPTOM-FREE PATIENT WITH CONTRALATERAL INTERNAL CAROTID ARTERY OCCLUSION

COMBINED INTERNAL CAROTID AND HYPOGLOSSAL ARTERY ENDARTERECTOMIES IN A SYMPTOM-FREE PATIENT WITH CONTRALATERAL INTERNAL CAROTID ARTERY OCCLUSION

On the Net Cyclonic Circulation in Large Stratified Lakes

Abstract This paper proposes a possible explanation for the mean cyclonic circulation in large stratified lakes The condition of no heat flux through the bottom boundary causes the isotherms to dip near the shores to intersect the sloping bottom orthogonally. This “doming” of the thermocline causes an internal pressure gradient in the surface layer with higher pressure nearshore and results in a geostrophic cyclonic circulation.

A MECHANISM FOR TRANSPORT OF SUBSTANCES BY TEMPERATURE-INDUCED PRESSURE GRADIENTS

Diurnal changes in air and soil temperatures lead to temperature gradients between air and soil, between roots and shoots, and within plant organs. In response to these gradients, fluctuations in gas pressures may develop in organs that are resistant to exchange of gases. These fluctuations may regulate mass flow of gases or solutions within plants. Patterns of diurnal temperature changes were generated to illustrate temperature gradients between roots and shoots. Experimental confirmation of pressure changes induced by temperature differences between roots and shoots were measured with water manometers attached to stumps of detopped tomato plants. When roots were maintained 8 C lower than shoots, internal pressure decreased by 22 cm H2O. Reversing the direction of the temperature gradient led to an approximately equal and opposite pressure change and to sap movement. These results support a hypothesis that internal pressure gradients resulting from temperature gradients contribute to transport of substances in plants.

Oxygen transport in the submerged freshwater macrophyte Egeria densa planch. II. Role of lacunar gas pressures

A pressure transducer was used to measure internal gas pressures ( p I) in the lacumar system of Egeria densa Planch. Under illumination, p I values up to 25 kPa above atmospheric developed in shoots in unstirred water. Upon circulation of the water, p I values decreased to 50–90% of the maximal values in unstirred water. Positive pressures, indicating partitioning of photosynthetic O 2 into the lacunae, were recorded in both re-circulating and open-circuit flow. When the shoot was darkened, negative internal pressures (5–10 kPa below atmospheric) developed. The pressure gradient ( Δp) required to drive O 2 transport in the lacunar system was calculated from the Hagen-Poiseuille equation. This gradient was 9.15 × 10 −4 kPa m −1. Manometric measurements of p I and pressure transfer rates showed that transient gradients of approximately 0.9 kPa m −1 developed following dark to light changes. However, pressure equilibration (mean rate = 0.02 m s −1) resulted in rapid attainment of constant p I under steady-state conditions. It is proposed that O 2 transport in Egeria is sustained by simple diffusion, assisted by advective movements under internal pressure gradients that develop following changes in external conditions.

Water Movements at a Mid-Central Basin Site: Time and Space Scales, Relation to Wind, and Internal Pressure Gradients

Current meter and thermistor string records made at a mid-basin site from May, 1979, through February, 1980, and during August, 1980, are used to determine the time and space scales of horizontal motion in mid lake and to relate these to the major forcing variables. Motions at frequencies larger than 0.125 cycles per hour are horizontally coherent over a few km only, whereas lower frequency motions may cohere significantly over tens of km in the stratified season. Of the four depths sampled, 10 m, 15 m, 20 m, and 21 m, highest current speeds are associated with circularly polarized clockwise rotating motion at the inertial period at 15-m depth. Surveillance cruise data show the mid central basin array to be located in a zone of relatively flat thermocline topography but suggested that internal pressure gradients might at times be large enough to influence bottom flow. Limited small-scale sampling in the vicinity of the mid-basin array reveals the existence of intense but short scale (5 km) internal pressure gradients that are undersampled by the array of thermistor strings (separated by 10 km). Multivariate statistical techniques in the frequency domain indicate that the internal pressure gradients estimated by means of the array of thermistor strings are of marginal dynamical significance (although the smaller scale gradients may in fact contribute substantially to the observed variance). A simple model of locally driven currents appropriate to a region of constant depth is proposed. This model and the statistical analyses together help to interpret the effects of wind stress and Coriolis forces. Many of the observed features of circulation, including seasonal evolution, can be related to the role of stratification in governing the vertical distribution of turbulent mixing.

An experimental study of transient effects in the breakup of viscous drops

A computer-controlled four-roll mill is used to examine two transient modes of deformation of a liquid drop: elongation in a steady flow and interfacial-tension-driven motion which occurs after the flow is stopped abruptly. For modest extensions, drop breakup does not occur with the flow on, but may occur following cessation of the flow as a result of deterministic motions associated with internal pressure gradients established by capillary forces. These relaxation and breakup phenomena depend on the initial drop shape and the relative viscosities of the two fluids. Capillary-wave instabilities at the fluid-fluid interface are observed only for highly elongated drops. This study is a natural extension of G. I. Taylor's original studies of the deformation and burst of droplets in well-defined flow fields.

A coupled ice‐ocean model of a wind‐driven coastal flow

Ice movement driven by winds along the coast is studied using a two‐dimensional (vertical seaward), ice‐ocean coupled model. Right‐hand alongshore winds are given when internal ice stresses are important to determine the ice movement. These winds induce right‐hand coastal currents and ice movement. A shoreward Ekman flow beneath the ice is a major mechanism to constrain the ice over the shelf, balancing the internal ice pressure gradient. When the wind ceases, the ice decreases its alongshore velocity, and the ice‐covered band becomes wider. The alongshore ice velocity is sensitive to ice shear strength, which determines the shear stress at the coast; i.e., weak (strong) shear strength allows a large (small) ice velocity at the coast, resulting in intense (weak) Ekman flow and downwelling, which induces a fast (slow) coastal current. The alongshore velocity is also sensitive to the relatively small cross‐shore component of the wind; i.e., the seaward (shoreward) component reduces (induces) the shear stress and allows large (small) ice velocity. The results are applied to the ice over the Labrador shelf.

Hypolimnion Flow Between the Central and Eastern Basins of Lake Erie During 1977 (Interbasin Hypolimnion Flows)

The shallow central basin of Lake Erie (mean depth 19 m) is separated from the deeper eastern basin (mean depth 28.5 m) by the Pennsylvania Ridge which extends southward from the base of Long Point to Erie, Pennsylvania. The crest of the ridge lies at a depth of 15 m except for a narrow notch near Erie, Pennsylvania, where communication between the two basins extends to 24 m depth. Both basins are stratified in summer, and the thin (2 m) hypolimnion of the central basin becomes depleted of oxygen by late summer. Flows to the hypolimnion of the central basin from the mid thermocline water of the eastern basin have been thought to be important sources of dissolved oxygen to the central basin. Data from ship cruises and current meter moorings made in 1977 have been used to form an estimate of the hypolimnion flux across the sill area. A strong correlation between winds and mean flux is observed and the dynamic balance appears to be one where surface pressure gradient created by wind stress is opposed by internal pressure gradients and by bottom friction. While the total quantity of oxygen transported by the subsurface flow is significant in terms of the later summer oxygen consumption of the central basin hypolimnion, its effects are confined to the eastern half of that basin due to the relatively weak horizontal diffusion in the mid-basin hypolimnion.

Activated Prominences; Mechanisms for Activation and Eruption

A possible relationship between the hot prominence transition sheath, increased internal turbulent and/or helical motion prior to prominence eruption and the prominence eruption (“disparition brusque”) is discussed. The associated darkening of the filament or brightening of the prominence is interpreted as a change in the prominence’s internal pressure gradient which, if of the correct sign, can lead to short wavelength turbulent convection within the prominence. Associated with such a pressure gradient change may be the alteration of the current density gradient within the prominence. Such a change in the current density gradient may also be due to the relative motion of the neighbouring plages thereby increasing the magnetic shear within the prominence, i.e., steepening the current density gradient. Depending on the magnitude of the current density gradient, i.e., magnetic shear, disruption of the prominence can occur by either a long wavelength ideal MHD helical (“kink”) convective instability and/or a long wavelength resistive helical (“kink”) convective instability (tearing mode). The long wavelength ideal MHD helical instability will lead to helical rotation and thus unwinding due to diamagnetic effects and plasma ejections due to convection. The long wavelength resistive helical instability will lead to both unwinding and plasma ejections, but also to accelerated plasma flow, long wavelength magnetic field filamentation, accelerated particles and long wavelength heating internal to the prominence.

Calculations of gas phase diffusion and reaction in heterogeneous catalysts the importance of viscous flow

AbstractThe problem of multicomponent gas phase diffusion fora single overall reaction in porous catalysts is analyzed using the dusty‐gas theory. Recent authors have shown that neglecting internal pressure gradients results in modeling inconsistencies and errors in computation in small pore catalysts. However, calculations based upon kinetic theory estimates of the transport coefficients reveal that the viscous flow contribution to the component i molar flux can be neglected with excellent justification. The resulting equations are internally consistent and greatly simplified. For illustrative purposes the multicomponent example of P. Schneider7 is reformulated to take into account nonuniform pressure (but negligible viscous flux), and as a consequence the inconsistency noted in the original reference is removed. Two methods for computing the effectiveness factor are discussed.

Failure probability analysis of an elastic orthotropic brittle cylinder subjected to axisymmetric thermal and pressure loading

Procedures for the calculation of the principal stresses in a long hollow circular cylinder of a brittle, orthotropically anisotropic material, subjected to a uniform internal pressure and axisymmetric radial temperature gradient are developed; the Runge-Kutta method is used in the solution, after an initial iterative stage. An equation for the failure probability of the cylinder, derived from the Weibull distribution, is presented, in which allowance is made for the anisotropy in mechanical strength.

Coastal Upwelling/Downwelling Cycles in Southern Lake Superior

Extensive current meter, hydrographic and wind data were collected in the region of the Keweenaw Current, a strong coastal jet, during the late spring and early summer regime of Lake Superior. These data were compared with surface wind and pressure patterns over the region with the following conclusions: 1) Longshore winds associated with migrating low-pressure systems generate onshore Ekman transport and coastal downwelling. The coastal downwelling results in an intensification of the onshore internal pressure gradient, and consequently the coastal jet. Speeds of over 75 cm s−1 are both observed and calculated, suggesting the Keweenaw Current, at these times, is in geostrophic equilibrium. 2) Longshore winds associated with migrating high-pressure systems generate offshore Ekman transport and coastal upwelling. Upwelling velocities of 13-50 m day−1, an offshore surface transport layer of 10 m, a near-shore deceleration of the Keweenaw Current and current direction fluctuations of near- and sub-inertial period are found associated with the upwelling. The period of passage of highs (upwelling) and lows (downwelling) in July is about 4–6 days. This gives the Keweenaw Current the appearance of a pulsating coastal jet of period 4–6 days and amplitude about 60 cm s−1.