Showing papers on "Pressure gradient published in 1981"

An Energy and Angular-Momentum Conserving Vertical Finite-Difference Scheme and Hybrid Vertical Coordinates

Abstract: An energy and angular-momentum conserving vertical finite-difference scheme is introduced for a general terrain-following vertical coordinate which is a function of pressure and its surface value. A corresponding semi-implicit time scheme is also defined. These schemes am used to compare the usual sigma coordinate with the hybrid coordinate which reduces to pressure above a fixed level and with a modified hybrid coordinate which tends uniformly to pressure at upper levels. Error in the representation of the stratospheric pressure gradient over steep orography can be significantly reduced by use of the hybrid coordinate but the semi-implicit scheme is less stable. The modified hybrid coordinate offers a useful compromise.

Toroidal drift modes driven by ion pressure gradients

Abstract: Ion pressure gradient‐driven drift modes are analyzed for their parametric dependence on the shear, the toroidal aspect ratio, and the pressure gradient using the ballooning toroidal mode theory. An approximate formula for the anomalous ion thermal conductivity is derived for the turbulent regime.

On the growth of turbulent regions in laminar boundary layers

Abstract: Turbulent spots evolving in a laminar boundary layer on a nominally zero pressure gradient flat plate are investigated. The plate is towed through an 18 m water channel, using a carriage that rides on a continuously replenished oil film giving a vibrationless tow. Turbulent spots are initiated using a solenoid valve that ejects a small amount of fluid through a minute hole on the working surface. A novel visualization technique that utilizes fluorescent dye excited by a sheet of laser light is employed. Some new aspects of the growth and entrainment of turbulent spots, especially with regard to lateral growth, are inferred from the present experiments. To supplement the information on lateral spreading, a turbulent wedge created by placing a roughness element in the laminar boundary layer is also studied both visually and with probe measurements. The present results show that, in addition to entrainment, another mechanism is needed to explain the lateral growth characteristics of a turbulent region in a laminar boundary layer. This mechanism, termed growth by destabilization, appears to be a result of the turbulence destabilizing the unstable laminar boundary layer in its vicinity. To further understand the growth mechanisms, the turbulence in the spot is modulated using drag-reducing additives and salinity stratification.

Hemodynamic determinants of pulmonary valve motion during systole in experimental pulmonary hypertension.

Abstract: To clarify the determinants of pulmonary valve (PV) motion in pulmonary hypertension, we examined the correlations among PV echo patterns, the pulmonary artery (PA) flow curve just above the PA orifice and the pulmonary artery-right ventricle (PA-RV) pressure gradient. By constricting the PA, we could produce a variety of PV echo patterns, including midsystolic semiclosure in open-chest dogs. Throughout the experiments, the PV echo pattern and PA flow curve were similar in pattern and timing. When the PV echo showed midsystolic semiclosure with reopening. The PA flow curve showed a transient decrease followed by a transient increase during midsystole. The PA-RV pressure gradient became transiently positive (PA pressure greater than RV pressure) and then negative in midsystole only when the PV echo showed midsystolic semiclosure with reopening. In conclusion, PV motion during systole may be instantaneously determined by PA flow change and the PA-RV pressure gradient during the cardiac cycle in experimental pulmonary hypertension.

Equilibrium boundary layers in moderate to strong adverse pressure gradients

Abstract: An analysis of equilibrium boundary layers based on the Schofield–Perry defect law, which applies to flow in a moderate to strong adverse pressure gradient, is presented. The conditions derived for self-preserving or equilibrium boundary layers differ from those given by previous analyses based on the usual velocity-defect law. It is shown that twelve observed boundary layers on smooth walls conform to these new conditions for precise equilibrium flow. As the analytical expression for the Schofield–Perry defect law does not vary with pressure gradient, a specific expression for the shear-stress profile in any equilibrium layer can be derived. The predicted shear-stress profiles show good agreement with experimental data. Limits for the flow parameters within which equilibrium layers can exist are derived, and it is shown that observed equilibrium layers fall within these limits. A prediction method for layers in smoothly changing adverse-pressure gradients is outlined and demonstrated using equilibrium data. The unified account of equilibrium flow in adverse pressure gradients presented here is used to resolve some disagreements in the literature concerning existence conditions for equilibrium boundary layers.

Anti-solar acceleration of ionospheric plasma across the Venus terminator

Abstract: It is demonstrated that the horizontal ionospheric particle pressure gradient across the Venus terminator is the principal body force accelerating the plasm to the observed anti-solar velocity. The large scale horizontal electromagnetic body force is typically an order of magnitude smaller than the particle pressure gradient. The viscous body forces above 300 km - drag or acceleration - are also an order of magnitude smaller than the pressure gradient body force. In the immediate vicinity of the ionopause where the ionospheric plasma density decreases below 1000 ions/cu cm and the magnetic field strength increases, the electromagnetic body force may become significant. The electromagnetic body force may also be significant in the nightside ionosphere. The bulk of the ionospheric flow momentum is not derived from the ionosheath momentum.

Turbulence in plane channel flows

Abstract: This paper complements an earlier study of the mean velocities in turbulent flows in a flat channel, one of whose walls can move relative to the other, so that the role of the stress gradient within the wall layers can be varied widely and in a controlled manner.Measurements of longitudinal, normal and lateral velocity fluctuation intensities (u′,v′,w′) and of shear stresses have been made in essentially fully developed flows established by various combinations of pressure gradient and wall velocity The channel aspect ratio (breadth/height) has been varied between 12 and 28 and the development ratio (development length/height) between 20 and 45. The introduction of a turbulence-generating grid at the entrance to the duct increases the effective development length.The study has considered twenty-six flows that are two-dimensional in the mean, which have been established by blowing and relative motion either in the same direction or directly opposed. Empirical descriptions, based on similarity laws incorporating either the wall stress or the local stress, are developed for the turbulence near the walls and in the core. The profiles of u′, v′ and w′ coalesce, to a reasonable approximation, when normalized with appropriate length and velocity scales. Extensive ‘plateau’ regions are identified, in which the scaled intensities are sensibly constant.A number of quantities characteristic of the structure of the turbulence are considered, in order to elucidate the effect of the stress gradient on the wall layer, and stages in the erosion of the constant-stress layer are identified.

Turbulence measurements in an adverse-pressure-gradient three-dimensional turbulent boundary layer along a circular cylinder

Abstract: Measurements, including the six components of the Reynolds-stress tensor, have been made along three generators of a centre-mounted circular cylinder with an elliptical nose cone. The pressure distribution was axisymmetric upstream and asymmetric downstream. The streamwise adverse pressure gradient led to almost zero skin friction in the direction of the limiting streamline, and the circumferential pressure gradient led to skew angles up to 30° in the vicinity of the wall. Special emphasis was laid on measurements in the wall region (y+ > 4), on the repeatability of these measurements and on an estimate of error bounds. The turbulence level encountered (up to 60% close to the wall) was much higher than in other threedimensional boundary-layer measurements. It has been shown that available measuring techniques have to be improved considerably or even abandoned if used under these conditions. Previous measurements of collateral velocity profiles in three-dimensional boundary layers will probably now have to be corrected for severe aerodynamic interference effects.It has been shown that the normal stresses , both characteristic of three-dimensional flow and caused by the circumferential pressure gradient, are influenced in different ways by the streamwise and circumferential pressure gradients. Spectra of u′-fluctuations are again similar to those obtained from two-dimensional boundary layers.Mean velocity profiles obey the linear and logarithmic law of the wall known from two-dimensional boundary layers both along a line of symmetry and in the threedimensional boundary layer. This may be because the streamwise pressure gradient dominates over the circumferential pressure gradient in this experiment.Finally it has been found that the skew angle γ of the Reynolds shear stress vector leads the skew angle ζ of the resultant velocity gradient or ‘mean shear’, both having the opposite sign of the skew angle β of the mean velocity vector except close to the wall. The ratio of Reynolds shear stress and turbulent kinetic energy is no longer ‘approximately’ constant as is assumed for two-dimensional boundary layers.

Spatial structure of flare‐associated perturbations in the solar wind simulated by a two‐dimensional numerical MHD model

Abstract: The dynamic behavior of flare-associated disturbances has been investigated using a time-dependent two-dimensional MHD numerical model for a one-fluid solar wind with adiabatic expansion. Simulations of the development and propagation of perturbations have been performed between 18 R/sub S/ and 226 R/sub S/ (where R/sub S/ = solar radius) in an angular sector of the equatorial plane of the sun 90/sup 0/ wide. Several test computations have been carried out with different initial pulse characteristics. These pulses are set arbitrarily at the inner boundary assuming that a shock wave is already formed. The parameters are the velocity of the shock front, the angular width of the perturbation, and its time duration at 18 R/sub S/. It is shown that in every case the time delay between 18 R/sub S/ and 226 R/sub S/ depends on the total amount of energy released by the prototype flare (hence in the pulse). This dependence is stronger with the initial velocity than with the angular width. Also it appears that the shock wave propagates according to a power law of time: R = at/sup b/. After a relatively short time the expansion of the wave is dominated by the internal energy so that themore » longitudinal extent of the perturbation at 1 AU seems to be only a function of the time elapsed after the arrival of the front shock at this distance. The existence of a reverse shock which is formed after a few hours is shown to last a time long enough to reach 1 AU. Its longitudinal extension is limited to the area around the flare central meridian where the pressure gradient induced by the initial condition is strong enough.« less

A Phenomenological Model for Wind Speed and Shear Stress Profiles in Vegetation Cover Layers

Abstract: A phenomenological model for the mean wind speed and Reynolds shear stress profiles with height in a vegetation cover layer is derived from forms suggested by truncation of the equations of turbulent fluid motion at second order in fluctuating velocity products. The initial formulation is unique in that the force per unit volume resisting fluid motion is treated as a body force having a height-dependent character. The body force is assumed to be proportional to the instantaneous speed squared and in the opposite direction from the instantaneous velocity. Viscous forces are ignored as are all pressure forces except for a steady vertical pressure gradient. Closure of the, equations is effected by a phenomenological assumption linking the static pressure and the square of the mean wind speed. The mean wind speed profile predicted by the model is an exponential in the cumulative drag area per unit planform area as a function of height, which is a simple exponential in height for cover with uniform pl...

Similarity and stability of the laminar boundary layer in a streamwise corner

Abstract: The study of laminar viscous flow along the line of intersection of two solid surfaces at right angles is examined in its present state, and outstanding differences between various experimental and theoretical results are analysed. New experimental results are presented in which the stability of the corner boundary layer is examined in terms of the degree of streamwise similarity of its velocity profiles. Conclusive evidence is found that the layer does not exist in stable laminar form when the streamwise pressure gradient is zero and the Reynolds number much above about 10$^4$. The new results also help to explain the differences between various experimental results, and between theory and experiment, which have characterized the corner boundary layer problem for several years. By extrapolation, an approximate prediction is obtained of what the velocity profile of the corner boundary layer would be in the limiting case of zero pressure gradient, if the layer were stable in that state. The predicted profile is compared with the results of current theories.

Mechanisms of Natural Fracturing in a Sedimentary Basin

Abstract: Mechanisms of natural fracture generation in a sedimentary basin may be studied by the use of physical properties of sedimentary rocks and of pore fluid pressure. The physical properties of rocks, such as Poisson's ratio and Young's modulus, under laboratory conditions are available, but only a few of these are reported under subsurface conditions. In this paper, in-situ values of Poisson's ratio derived from analysis of artificial hydrofracturing data in the Gulf Coast are used for estimating horizontal stresses in both hydrostatically and abnormally pressured zones there. In a tectonically relaxed basin, such as the Gulf Coast, the horizontal stress can be a function of vertical (overburden) stress, fluid pressure, and Poisson's ratio. The horizontal stress increases with depth in the hydrostatic pressure zone, but is abnormally low in the abnormally pressured zone. Fracture pressure (and its gradient) given as a sum of (least) horizontal stress and fluid pressure is, however, not abnormally low in the abnormally pressured zone because the fluid pressure there is abnormally high. This paper describes the in-situ fracture pressure gradient estimated for the Gulf Coast area. On the basis of models of vertical fluid charging through an aquifer and a fault system during continuous burial, the depth at which the charged fluid pressure gradient exceeds the fracture gradient (or the point of natural hydrofracturing) is estimated. After an erosional event, the most significant decline of fracture pressure gradient is expected in the uppermost part of the original abnormal pressure zone. Hydrofracturing there could occur if an effective fluid is charging from deep to shallow through an aquifer or a fault system. If the erosion continues to a point at which approximately two-thirds of the total sediment thickness is removed, the horizontal stress may become almost zero. If such a stress condition is developed, the sedimentary rocks become greatly relaxed, possibly causing fractures.

Efficient Time Integration Schemes for Atmosphere and Ocean Models

Abstract: As atmospheric numerical models increased in complexity, the need for computationally efficient time integration schemes became apparent. Early efforts along this line began with Marchuk (1974) and Robert et al. (1972). They noted that atmospheric motions can be separated into two classes; the Rossby modes, where nonlinearity and quasigeostrophic balance (i.e., balance between Coriolis and pressure gradient forces) play an important role, and gravity modes, for which the pressure gradient terms are balanced by the inertial terms. It was also observed that the high-frequency gravity modes, which propagate faster than the Rossby modes, are quasilinear and carry only a small portion of the total energy.

Nongradient Theory for Oblique Turbulent Flames with Premixed Reactants

Abstract: An earlier analysis based on second-order closure and the Bray-Moss-Libby model of premixed combustion is applied to infinite, planar turbulent flames which are oblique to the oncoming reactants and which have undeflected mean streamlines. In such flames all three mechanisms, dilatation, Reynolds shear stresses, and mean pressure gradient, contribute to the balance of turbulent kinetic energy. Examination of the appropriate conservation equations in primitive form indicates that the intensity of the fluctuations of the velocity component normal to the flame and the mean flux of product in that direction are independent of obliquity. Thus earlier findings regarding countergradient diffusion and production of turbulence due to the mean pressure gradient prevail in oblique flames. The mean flux of product in the tangential direction and the intensity of the fluctuations of the velocity component in that direction are calculated. It is found that the mean streamlines of parcels of reactants and products are significantly different from one another and from the mean streamline. The intensities downstream of highly oblique turbulent flames are predicted to lead to two-dimension al turbulence in the plane containing the normal and tangential coordinates.

Transient hydromagnetic convective flow in a rotating channel with porous boundaries

Abstract: The paper studies the combined free and forced convection in a rotating, viscous, incompressible fluid confined between two parallel porous plates. Assuming that the temperature varies linearly along the plates and the pressure gradient maintained uniform over the planes parallel to the plates, the velocity, temperature and the stresses are calculated analytically. Their behaviours for different values of the parameters viz., the Hartmann number, the Grashoff number and the suction parameter etc., are discussed graphically giving out the interplay between the various forces.

Isostasy, magma chambers, and plate driving forces on the East Pacific Rise

Abstract: The density distribution within a cooling plate is calculated, which incorporates temperature and pressure effects. From this density distribution the pressure field within the plate and the gravity field at sea level are computed for various degrees of isostatic compensation. In this model the pressure field within the plate has a horizontal gradient at shallow depths away from zero age and a horizontal gradient toward zero age at greater depths caused by the loading of the ocean. Isostatic equilibrium is approached if one allows the loading due to the water to depress the seafloor and at the same time allows mass conservation by flow at depth toward zero age. A viscosity model based on a Newtonian rheology which included temperature and pressure effects has a high gradient close to the plane separating positive and negative pressure gradients which would facilitate the return flow and decouple the lithosphere from the asthenosphere. Addition of a crust to the homogeneous model does not substantially change these conclusions. Comparison of this model with examples of East Pacific Rise data suggests that some areas may not be in complete isostatic equilibrium, implying the existence of horizontal pressure gradients toward zero age in the asthenosphere. This model can be made to fit the general features of the East Pacific Rise but not the detailed gravity and topography near zero age. If one allows convective cooling of the crust by water, partial melting of the upper mantle, and intrusion of this partial melt into the crust, the water depths increase more rapidly near zero age, and an increased positive gravity anomaly is produced over the rise axis, both of which produce a better fit to East Pacific Rise data at 12°N. This study suggests that crustal magma chambers on the East Pacific Rise may be associated with anomalous positive gravity anomalies caused by a positive density contrast between magma and fractured porous crustal rocks.

Apparatus for detecting the fuel quantity delivered to the combustion chambers of a diesel engine

Abstract: A fuel quantity measuring apparatus, preferably for a Diesel engine, is proposed in which the course of the fuel pressure is evaluated. A specialized pressure gradient of a first polarity marks the onset of supply or injection, and the maximum of the pressure gradient of the other polarity represents the end of injection or supply. The question of which designation prevails, whether injection onset and end or supply onset and end, is determined by the measurement location, that is, whether it is in the direction of the injection valve or in the direction of the supply element of the fuel supply pump. The injection quantity, for example, may be determined via the integral of the pressure during the metering period. It is recommended, when a piezoelectric pressure receptor is used, that a resistor-capacitor element be switched parallel thereto.

On the Pressure Losses Due to the Tip Clearance of Centrifugal Blowers

Abstract: The pressure distribution along the shroud is measured for three types of centrifugal impeller at seven different values of tip clearance each. The change of input power due to a change of tip clearance is related to the effective blockage at the impeller tip. Since the change of input power is little for the test cases, the variation of local pressure gradient along the shroud is mostly attributed to the change of local pressure loss. The local pressure loss is related to the local tip clearance ratio and to the local pressure gradient based on the deceleration of relative velocity in the impeller. Since the local pressure gradient is largest near the throat of the impeller, for many impellers the clearance ratio at the throat is used as the representative value. The tip clearance loss is related to the clearance ratio and the pressure rise based on the deceleration of relative velocity in the impeller. A good correlation is observed in all cases at various flow rate.

Correlation Between Macroscopic Porosity Location and Liquid Metal Pressure in Centrifugal Casting Technique

Abstract: Radiographic analysis of uniform cylindrical castings fabricated by the centrifugal casting technique has revealed that the macroscopic porosity is dependent on the location of the sprue attachment to the casting. This is attributed to the significant pressure gradient associated with the centrifugal casting technique. The pressure gradient results in different heat transfer rates at portions of the castings near and away from the free surface of the button. Consequently, the macroscopic porosity is invariably at portions of the casting close to the free surface of the button. In addition, some optimized sprue-reservoir combinations could be predicted and proved, based on this pressure gradient concept.

Structure of the transient wall-friction law in one-dimensional models of laminar pipe flows

Abstract: The problem of describing an unsteady cylindrical pipe flow with one-dimensional equations is investigated, and an exact method for obtaining a closure relationship is proposed for the transient shear stress in a laminar flow submitted to an arbitrary transient pressure gradient. Extensive comparisons are given for a step or a harmonic pressure gradient between the approximate solution derived from this method, some results of the literature and exact solutions of the Navier–Stokes equations.

Interchange-Ballooning Mode in l = 2 Helical Systems

Abstract: A high shear toroidal helical system such as Heliotron E gives a limiting beta comparable to tokamaks, by examining stability against interchange-ballooning modes with a low toroidal mode number. The limiting beta depends on equilibrium pressure profiles and a high beta value is found for the pressure having a flat profile in the central region and a large pressure gradient at the high shear peripheral region of the plasma column.

Rheological aspects of thermoplastic foam extrusion

Abstract: Several rheological aspects of thermoplastic foam extrusion are described by a phenomenological model of the flow in an extrusion die. The macroscopic effects of the phase change, from a homogeneous polymer melt that contains a blowing agent, to a foam, are described in terms of two dimensionless parameters. α is defined as the ratio of the pressure gradient in the melt phase to the average pressure gradient in the foam phase. Θ represents the ratio of the pressure drop in the melt phase to the pressure drop in the foam phase. The position at which the phase change initiates, the exit contribution to the Bagley ends pressure correction, and the true wall shear stress in the melt phase are related to α and Θ. The quality of the foam produced is discussed in terms of the extrusion conditions and their effect on the foaming position in the die. It is demonstrated that the ends pressure correction cannot be neglected on the basis of a long die alone. Asymptotic conditions for which the phase change has negligible effect on the melt flow rate are predicted. α is calculated from the data of Han and Villamizar, who measured the pressure distribution in the die and observed the phase change directly. The prediction of α from theory is complicated by the lack of suitable constitutive relations for the foam phase. Since Θ only requires knowledge of the pressure at which the phase change initiates, it is relatively easy to evaluate.

Magnetohydrodynamic Taylor vortex flow under a transverse pressure gradient

Abstract: Electromagnetically driven flows between infinite conducting cylinders subjected to a constant axial magnetic field B0 are considered. The linear stability problem for axisymmetric stationary disturbances is treated. In contrast with Chandrasekhar’s conjecture, the present numerical results indicate that the critical wave‐number tends to infinity with the magnetic field. The nonlinear state of the flow, slighly above transition, is investigated by means of Stuart’s energy approach. Further theoretical results are compared with experiment. At criticality, excellent agreement is found between linear stability theory and experiment. Above transition, only the shape of the experimental curves is correctly predicted by Stuart’s energy method.

Natural Convective Boundary-Layer on Two-Dimensional and Axisymmetric Surfaces in High-Pr Fluids or in Fluid-Saturated Porous Media

Abstract: In natural convective boundary layers on inclined surfaces, the surface-normal component of the buoyancy force induces a pressure gradient across the boundary layer. For the class of flows in which inertial effects are unimportant (including flows at high Prandtl number as well as flow through fluid-saturated porous media), a local nonsimilarity analysis indicates that the effects of the surface-normal pressure gradient on the temperature profile can be characterized by a single local configuration-parameter which depends on the local geometry and on the Rayleigh number. Under Mangler's transformation the reported computational results become applicable to axisymmetric as well as 2-dimensional geometries of arbitrary contour. In contrast to the single-parameter dependence of the temperature profiles, the velocity profiles depend upon 2 local geometric parameters, as illustrated for the example of an inclined flat plate.

Method and apparatus for refracting a laser beam

Abstract: This invention is a method and apparatus for refracting a laser beam. The beam can be collimated, focused, or expanded by passing it along the longitudinal axis of a volume of gas which has a radial pressure gradient. The pressure gradient causes a corresponding gradient in density and refractive index. Such a gradient can conveniently be established by the use of a gas vortex chamber. A vortex chamber will act as a negative lens. It can be located at or near the focal point of a focused laser beam as a collimating element. A gas vortex lens is useable at power densities above those which conventional optical materials can withstand.

Influence of wall waviness on friction and pressure drop in channels

Abstract: An attention has been given to investigate the flow behavior of an incompressible viscous fluid confined in horizontal wavy channels and set in motion due to the movement of the upper waand the pressure differences. The governing equations have been solved analytically as well as numerically subject to the relevant boundary conditions by assuming that the solution consists of two parts: a mean part and a disturbance or perturbed part. For small and mod- erate Reynolds numbers, the analytical solution for the perturbed part has been found to be in good agreement with the numerical one. The effects of Reynolds number, the pressure gradient parameter, and the undulation wavenumber on fric- tion and pressure drop are found to be quite significant. In addition to the flow behavior for both long and short waves and for large Reynolds numbers, the effect of the wall waviness on friction and pressure drop has been examined for any arbitrary amplitude of the wavy wall.