Showing papers on "Pressure gradient published in 1979"

Pore Pressure in Heated Concrete Walls: Theoretical Prediction

Abstract: Synopsis Pore-water pressures in concrete can be calculated by a previously developed theory which is based on thermodynamic properties of water and takes into account the huge changes in permeability and sorption isotherm with temperature, as well as the changes of pore space due to temperature and pressure. After reviewing the theory, finite-element solutions are com-pared with weight-loss tests of Chapman and England, and theoretical predictions are made for rapid heating of thick walls, either sealed or unsealed. A two-dimensional axisymmetric finite-element solution is developed to analyse theffect of a hot spot on the wall. The pore-pressure peaks are found to be much higher than for slow heating (25 atm versus 8 atm), and still about 50% higher when the heating is confined to a hot spot. The moisture movement in regions where the pressure gradient is opposite to the temperature gradient is found to be rather irregular and to exhibit oscillations. The theory predicts the phenomenon of ‘moisture clog...

Axial flow in trailing line vortices

Abstract: Axial flow in the core of a laminar steady trailing vortex from the tip of a semi‐infinite wing is analyzed assuming small departure of the axial velocity from the free‐stream velocity. It is further assumed that the axial pressure gradient is determined by the swirl velocities of an ideal infinite line vortex in which the radial and the associated axial velocity variations are neglected in the equation for the angular momentum. The axial and lateral variations of the axial velocity depend on the strength of the vortex and initial axial velocity distribution which must be specified at some station behind the wing except at the virtual origin of the vortex where a nonintegrable singularity exists. Numerical solutions for the axial velocity are obtained using the axial pressure gradient given by the line vortex and analytical solutions are obtained using an equivalent axial pressure gradient with good agreement between the two sets of axial velocity distributions. Resolution of the previous uncertainties in this field is given which were due to the unrecognized singularity at the virtual origin of the vortex. Using the calculated axial velocity the neglected radial and the associated axial fluxes of angular momentum are determined and the limits of validity of the theory presented here in terms of a suitably defined vortex Reynolds number and a nondimensional distance measured from the virtual origin of the vortex are given.

Nighttime thermospheric winds at high latitudes

Abstract: Nighttime thermospheric winds have been measured from Ester Dome, near College, Alaska (64.8°N, 147.8°W), using the high-resolution Fabry-Perot interferometer of the University of Michigan Airglow Observatory. The winds are determined from the Doppler shifts of the (0I) 15867 K (6300 A) line emission. Measurements obtained during geomagnetic quiet times in January and February 1972 are averaged to obtain the characteristic nighttime behavior of both the zonal and the meridional neutral wind components at F-region heights. Ion drifts during this period were measured by the Chatanika incoherent scatter radar facility. The measurements of the average nighttime zonal and meridional neutral wind components and of the average ion drift components are used with a three-dimensional dynamic model of the neutral thermosphere to determine the effectiveness of various forces in controlling the winds at F-region heights in the high-latitude thermosphere. A least squares fit of the measured and calculated nighttime variations of the zonal and meridional winds, using the ion drag determined from the radar measurements, gives the Fourier coefficients of latitudinal and longitudinal pressure gradients that are necessary to drive the observed wind pattern. The results show that the ion drag force is essentially balanced by the pressure force in the meridional direction. However, in the zonal direction the pressure force does not balance the ion drag force. The pressure gradients, derived in terms of exospheric temperature variations, show a different nighttime variation from that predicted by the MSIS model for that latitude. The derived zonal winds are westward in the early evening hours, opposite of the winds calculated from a dynamic model that uses the pressure forces determined from the MSIS semiempirical model and ion drag.

Salinity gradient power: utilizing vapor pressure differences.

Abstract: By utilizing the vapor pressure difference between high-salinity and lowsalinity wvater, one can obtain power from the gradients of salinity. This scheme eliminates the major problems associated with conversion methods in which membranes are used. The method we tested gave higher conversion efficiencies than membrane methods. Furthermore, hardware and techniques being developed for ocean thermal energy conversion may be applied to this approach to salinity gradient energy conversion.

Hall and Ion-Slip Effects in MHD Couette Flow with Heat Transfer

Abstract: An analysis of the MHD Couette flow on taking into account the Hall and the ion-slip effects has been carried out for fully developed flow. Exact solutions to the velocity components, magnetic field components, axial and transverse components of the skin-friction, temperature and the rate of heat transfer have been derived. The numerical values of the transverse induced pressure gradient, the skin friction and the rate of heat transfer are entered in tables and the others have been shown on graphs. It has been observed that the flow may become unstable when M is small and se (Hall parameter) and si (ion-slip parameter) are large or at large value of M.

A Comparison of Some Shallow Wind-Driven Currents

Abstract: Four sets of current measurements made in water depths ranging between 28 and 38 m over periods ranging from three to five weeks are examined and compared. The response of the water column to wind forcing is examined by computing regression coefficients between the surface wind stress and two different parameterizations of bottom stress in terms of measured currents. Coefficients computed for the different data sets vary by as much as a factor of 4. While such variations might be due to instrumental differences, it seems more likely that the assumed dynamical balance between surface and bottom stress is incomplete, i.e., other forces such as the alongshore pressure gradient are quantitatively important even when the water depth is comparable to the turbulent Ekman layer thickness.

Survey and bibliography on attainment of laminar flow control in air using pressure gradient and suction, volume 1

Abstract: A survey was conducted and a bibliography compiled on attainment of laminar flow in air through the use of favorable pressure gradient and suction. This report contains the survey, summaries of data for both ground and flight experiments, and abstracts of referenced reports. Much early information is also included which may be of some immediate use as background material for LFC applications.

Uniform stress conditions in the diamond anvil cell at 200 kilobars.

Abstract: Pressure gradients in a diamond anvil cell have been measured with a 4:1 methanol:ethanol mixture as a pressure medium up to 350 kilobars. When pressure is applied rapidly, stress gradients are shown to be negligible up to 200 kilobars and probably above. With this procedure it is possible to significantly increase the precision of pressure measurements above 100 kilobars.

Influence of the pressure in the Plateau-Gibbs borders on the drainage and the foam stability

Abstract: The effect of the pressure in the Plateau-Gibbs borders of the foam upon its drainage and its stability has been investigated. The principle of the method and the apparatus for applying pressure gradient in the Plateau-Gibbs borders of the foam are described. The kinetics of establishing of the capillary pressure in the Plateau-Gibbs borders of the foam in dependence of the height of the foam column, the applied pressure difference and the type of the surfactant has been studied. The obtained results are qualitatively confirmed by an investigation of the foam water content change (calculated by measurements of its specific conductivity) at the same experimental conditions. The influence of the increased drainage rate and respectively the decreased foam water content on the foam stability when a pressure gradient is applied in its Plateau-Gibbs borders is discussed. It is shown that the time for the foam destruction when a pressure gradient is applied in the foam Plateau-Gibbs borders can be used for characterising the foam stability at definite conditions.

Near Field Velocity Measurements in a Fully Pulsed Subsonic Air Jet

Abstract: Measurements in the near field of streamwise mean and turbulent velocities are reported for a fully pulsed axisymmetric subsonic air jet exhausting into still air. The mean velocity field follows the same scaling laws as a steady jet, but the pulsing moves the effective origin well upstream from the jet exit. Entrainment rates were found to be significantly higher than for the steady jet and are a function of two length scales. Mean and turbulent velocity length scaling differ significantly. The measurements indicate that momentum is not conserved, which is attributed to the existence of a significant pressure gradient for which case a modified thin flow conservation equation is presented, Ensemble averaging of the signals permitted the separation of the pseudo and intrinsic turbulence components.

Nucleation Processes in Large Scale Vapor Explosions

Abstract: A spontaneous nucleation model is proposed for the mechanisms which lead to explosive boiling in the free contacting mode. The model considers that spontaneous nucleation cannot occur until the thermal boundary layer is sufficiently thick to support a critical size vapor cavity, and that significant bubble growth requires an established pressure gradient in the cold liquid. This results in a prediction that, for an interface temperature above the spontaneous nucleation limit, large cold liquid droplets will remain in film boiling due to coalescence of vapor nuclei, whereas smaller droplets will be captured by the hot liquid surface and rapidly vaporize, which agrees with the experimental observations. The model also predicts that explosions are eliminated by an elevated system pressure or a supercritical contact interface temperature, and this is also in agreement with experimental data.

Calculation of Turbulent-Inviscid Flow Interactions with Large Normal Pressure Gradients

Abstract: A simple and economical iterative scheme is presented for calculating turbulent shear layers with significant pressure gradients normal to the plane of the layer, such as occur on highly-curved surfaces or near the trailing edges of lifting airfoils, and for matching the shear-layer calculations to calculations of the inviscid external flow. The iteration required to solve the elliptic equations describing the shear layer is combined with the iteration needed for the matching, and the finite-difference solution of the normal-component momentum equation is a simple quadrature at each iteration. Therefore computing time is little greater than in conventional displacement-surface calculations that ignore normal pressure gradients. OUNDARY-layer (thin-shear-layer) equations are derived from the Navier-Stokes equations by assuming that streamwise (x) gradients of velocity are small compared to velocity gradients normal to the surface (y). The resulting simplifications1 include the disappearance of the pressure gradient normal to the surface, from which follows the smallness of the velocity gradient dU/dy in the "inviscid" flow just outside the shear layer. This in turn leads to the concept of a displacement thickness to represent the displacement, nominally independent of y, of the "inviscid" flow streamlines near the shear layer. It is important to note that once the basic assumption fails, all the simplifications disappear. If the shear layer changes rapidly in the x direction, so that dV/dx is large, normal pressure gradients are im- portant both within the layer and outside it; not only does the displacement thickness fail to represent the displacement of the external flow, but its definition, even as a mere shear-layer parameter, becomes ambiguous. It will be seen that correc- tions based on the surface radius of curvature may be highly inaccurate. All but the most rapidly changing viscous or turbulent flows will still be recognizable as fairly thin shear layers, and the terms in the Navier-Stokes equations that are neglected in the boundary-layer equations will still be v fairly small. Therefore, thin-shear-layer concepts are still useful in analytic and computational work. In particular, the effects of the normal pressure gradient on the shear layer will be small enough to be included by iterative improvement of a con- ventional marching calculation rather than by a fully elliptic calculation, and this more economical approach has been adopted in the present work.

Pressure Gradient and Leading-Edge Effects on the Corner Bounday Layer

Abstract: Results are presented of velocity and pressure measurements made in the initially laminar boundary layer in a streamwise corner formed by two flat plates at 90° to each other set at various incidences. The leading edges of the plates were sharp in contrast to earlier tests with an aerofoil type leading edge. It was found impossible to obtain a steady enough flow for useful measurements to be made at zero incidence and pressure gradient, a small incidence associated with a favourable pressure gradient was necessary. This is believed to be because of the development of separation bubbles at the sharp leading edge at very small incidences due to small variations of flow direction to be expected in a wind tunnel. The profiled nose used in earlier tests afforded flow conditions much closer to the ideal theoretical model involving zero pressure gradient, but it is argued that any nose however shaped may introduce disturbances in the form of characteristic secondary flows that may well determine the downstream response of the boundary layer. In any case the corner flow is highly unstable at all but very low Reynolds numbers, and in the absence of a region of favourable pressure gradient a Reynolds number in terms of distance downstream of the leading edge greater than about 105 is unlikely to be attained in practice with the flow remaining smooth and laminar.

Theoretical prediction of the pressure gradients in coextrusion of non‐newtonian fluids

Abstract: In previous publications we have reported our experimental observations that, when the less viscous component flows outside the more viscous component in sheath-core coextrusion through a cylindrical die, or in sandwich three layer film coextrusion through a slit die, the pressure gradients in a two-phase system can be lower than those in the two components flowing individually. The experimental observations suggest that the energy requirement for extrusion can be decreased, and the throughput increased, when one judiciously chooses proper combinations of materials and/or optimal processing conditions. In this paper, we present a theoretical analysis, which shows that the seemingly anomalous behavior of the experimentally observed pressure gradient reduction is indeed possible, depending on the rheological properties of the individual components involved in coextrusion.

Variability of Current and Bottom Pressure Across the Continental Shelf in the Northeast Gulf of Alaska

Abstract: The low-frequency (<0.25 cph) velocity and bottom pressure variability were studied in the northeast Gulf of Alaska from March to August 1976. Measurements of velocity in 100, 185 and 250 m of water showed a contrast between the flow at the shelf break and that on the shelf. The former circulation had a weak mean alongshore flow (5 cm s−1), but large anticyclonic low-frequency fluctuations. On the shelf the flow was almost entirely alined along isobaths. The anticyclonic shelf break fluctuations did not propagate onto the shelf. Bottom pressure variations measured at four locations showed little variation along the shelf and a linear decrease in bottom pressure variance across the shelf. Correlations of bottom pressure gradient with velocity indicate much of the alongshore flow was consistent with barotropic quasi-geostrophic dynamics. Cross-shelf flow could not be related to the pressure gradients. Examination of the pressure field response to the wind showed that the nearshore sea level setup a...

Computation of supersonic viscous flows over ogive-cylinders at angle of attack

Abstract: The parabolic Navier-Stokes (PNS) marching finite-difference method is applied to 3-D viscous flow over pointed ogive-cylinders, and to turbulent flow over a cone. Ogive computations were performed using the new technique recently reported by Vigneron, Rakich, and Tannehill. Comparison is made with experiment and inviscid computations. The present results show that this method, which neglects part of the pressure gradient in the x-momentum equation, is nevertheless valid for flows with a strong favorable pressure gradient. In addition, turbulent separated flow over a cone has been computed using the older PNS code due to Lubard and Helliwell. It is found that one must freeze the turbulent eddy-viscosity model upstream of 3-D separation to get agreement with experiment.

Reflection and transmission of fluid transients at an elbow

Abstract: A fundamental problem in the analysis of fluid-filled pipeline systems containing various types of joints and fittings (such as tees, elbows, U-joints and omega-bends) arises when the effects of such structural parts on the propagation of pressure transients through the system are considered. Often, all that is desired is a simple characterization of these discontinuities in terms of reflection and transmission pressure coefficients that can be incorporated into one-dimensional pressure transient algorithms or computer codes. In this context, the problem of characterizing elbow-like structural members (including elbows and U-joints) is considered. The theoretical analysis is explained. Theoretical results indicate that for high frequency waves, virtually all of the wave is transmitted through the elbow, regardless of elbow geometry. For low frequency waves, a partial negative pressure reflection occurs at the elbow and the strength of this reflection is of the order of 15% to 30% of that of the incoming wave; the amount of reflection depends strongly upon the ratio of the wall thickness to the mean tube radius, is somewhat dependent upon the total bend angle, but is relatively insensitive to the radius of curvature of the elbow. Some simple experiments with two straight sections of water-filled aluminum electricalmore » conduit welded to either end of a gentle 90/sup 0/-elbow tend to confirm certain aspects of the theory. A short, impact-generated positive pressure pulse in one of the straight sections disperses somewhat at the elbow, producing a broad partial negative reflection; the rest is transmitted. By varying the static pressure head in the system, it has been ascertained that cavitation at the inlet side of the elbow can occur if the static head is not large enough to overcome the partial negative reflection.« less

The coastal longshore pressure gradient: Temporal variations and driving mechanisms

Abstract: Along the northern coastal boundary of the Mid-Atlantic Bight, synthetic subsurface pressure records derived from 1975 tide gage and airway weather observations are examined for fluctuations having monthly time scales. Additional calculations are performed to determine the factors controlling variance in these pressure data. Similar analyses are applied to the longshore gradient in synthetic subsurface pressure. Results of the study indicate that synthetic subsurface pressures are controlled by three mechanisms, two each at either end of the coastal segment. Fluctuations in Gulf Stream position have a significant effect on monthly mean pressures, which are uniform along the coast. At the western end of the coastal segment, east-west winds are also important, while changes in water density contribute significantly to subsurface pressure variance at the eastern end. Coastal steric differences are found to be a minor contributor to monthly fluctuations of longshore pressure gradient, while the coastal mean east-west winds effect a linear response in the gradient. Using this relationship and upon removal of geodetic leveling errors, the annual mean longshore sea surface slope is calculated to be 1.6×10−7, forming a westward pressure gradient along the northern boundary of the Mid-Atlantic Bight.

Hydrostatic transmission lubrication

Abstract: A hydrostatic transmission having a pump and a motor of the radial piston type with each cylinder slipper bearing rotatably supported by a hybrid or combination hydrostatic and hydrodynamic bearing on an internal bearing. The hydrostatic bearing component has a restricted port connecting hydrostatic pressure fluid from the cylinder to supply a distribution recess in the slipper bearing surface for hydrostatic pressure feed of the fluid film providing hydrostatic balancing pressure gradient between the slipper and internal bearing to balance a high percentage of the hydrostatic pressure load and to meet flow requirements for low speed operation. The hydrodynamic bearing component has a low pressure source to spray fluid on the internal bearing, annular dams on each side retaining fluid on the internal bearing, and a taper at the leading edge of the slipper bearing to force fluid into the bearing clearance and provide a hydrodynamic balancing pressure component and fluid flow increasing with speed to balance the centrifugal load and effect balancing of the remaining hydrostatic load and to meet flow requirements which increase with speed. The total hydrostatic and hydrodynamic balancing pressure balances the total load.