Showing papers on "Pressure gradient published in 1972"

A nonlinear analysis of pulsatile flow in arteries.

Abstract: An approximate numerical method for calculating flow profiles in arteries is developed. The theory takes into account the nonlinear terms of the Navier-Stokes equations as well as the nonlinear behaviour and large deformations of the arterial wall. Through the locally measured values of the pressure, pressure gradient, and pressure-radius function, the velocity distribution and wall shear at a given location along the artery can be determined. The computed results agree well with the corresponding experimental data.

The propagation and transfer properties of steady convective overturning in shear

Abstract: A conservative quantity is found as an integral of a component of the vorticity equation and used to formulate a non-linear theory of steady, two-dimensional convection in shear. The steering-level and propagation speed are determined in terms of a Richardson number and a density-scaling parameter. Case studies indicate a favourable agreement between theory and observation, especially where the Richardson number is of order unity. The parcel theory of convection is extended and the importance of the horizontal pressure gradient as a control on the updraught intensity is discussed quantitatively. Heat and momentum transfer laws are obtained in terms of the mean flow parameters, and define a non-Fickian transfer.

An Implicit Time Integration Scheme for Baroclinic Models of the Atmosphere

Abstract: A semi-implicit time integration algorithm developed earlier for a barotropic model resulted in an appreciable economy of computing time. An extension of this method to baroclinic models is formulated, including a description of the various steps in the calculations. In the proposed scheme, the temperature is separated into a basic part dependent only on the vertical coordinate and a corresponding perturbation part. All terms involving the perturbation temperature are calculated from current values of the variables, while a centered finite-difference time average is applied to the horizontal pressure gradient, the divergence, and the vertical motion in the remaining terms. This method gives computationally stable integrations with relatively large time steps. The model used to test the semi-implicit scheme does not include topography, precipitation, diabatic heating, and other important physical processes. Five-day hemispheric integrations from real data with time steps of 60 and 30 min show diff...

Peristaltic motion of a non-Newtonian fluid

Abstract: To understand theoretically the flow properties of physiological fluids, we have considered as a model the peristaltic motion of a power law fluid in a tube, with a sinusoidal wave of small amplitude travelling down its wall. The solution for the stream function is obtained as a power series in terms of the amplitude of the wave. The stream function and the velocity components are evaluated by solving numerically two point boundary value problems with a singular point at the origin. The influence of the applied pressure gradient along with non-Newtonian parameters on the streamlines and velocity profiles are discussed in detail.

On the balance of stresses in the plasma sheet

Abstract: The stress resulting from magnetic tension on the neutral sheet must, in a steady state, be balanced by any one or a combination of (1) a pressure gradient in the direction along the axis of the tail, (2) a similar gradient of plasma flow kinetic energy, and (3) the tension resulting from a pressure anisotropy within the plasma sheet. Stress balance in the first two cases requires that the ratios h/LX and BZ/BX be of the same order of magnitude, where h is the half-thickness of the neutral sheet, LX is the length scale for variations along the axis of the tail, and BZ and BX are the magnetic field components in the plasma sheet just outside the neutral sheet. The second case requires, in addition, that the plasma flow speed within the neutral sheet be of the order of or larger than the Alfven speed outside the neutral sheet. Stress balance in the third case requires that just outside the neutral sheet the plasma pressure obey the marginal firehose stability condition.

Large-scale motion of a turbulent boundary layer during relaminarization

Abstract: A fully developed turbulent boundary layer was subjected to a strongly favourable pressure gradient in order to investigate the role of the large eddy structure during ‘relaminarization’. Measurements of%he mean velocity profiles indicated that the ‘law of the wall’ disappeared in the region of the maximum pressure gradient. The three fluctuating velocity components and the tangential Reynolds stress were obtained to determine more precisely the nature of the decay of the turbulent structure. These measurements indicated that the absolute level of the velocities and stress were approximately constant along a mean streamline except near the wall. However, the relative levels were decreasing, as reported previously by several authors. The intermittency factor γ decreased along the mean streamlines until most of the boundary layer had only a negligible turbulence level. Space-time auto- and cross-correlations of u, v and I (the intermittency function) of the large-scale structure were obtained in the region of maximum pressure gradient and are compared with those measured in a zero pressure gradient flow.

Equilibrium and relaxation in turbulent wakes

Abstract: In order to study the memory of the larger eddies in turbulent shear flow, experiments have been conducted on plane turbulent wakes undergoing transition from an initial (carefully prepared) equilibrium state to a different final one, as a result of a nearly impulsive pressure gradient. It is shown that under the conditions of the experiments the equations of motion possess self-preserving solutions in the sense of Townsend (1956), but the observed behaviour of the wake is appreciably different when the pressure gradient is not very small, as the flow goes through a slow relaxation process before reaching final equilibrium. Measurements of the Reynolds stresse show that the approach to a new equilibrium state is exponential, with a relaxation length of the order of 103 momentum thicknesses. It is suggested that a flow satisfying the conditions required by a self-preservation analysis will exhibit equilibrium only if the relaxation length is small compared with a characteristic streamwise length scale of the flow.

Structure and orientations of solar-wind interaction fronts: Pioneer 6

Abstract: An analysis is given of five stream-stream interaction events observed in the Pioneer 6 plasma and field data. That the time profiles of all the parameters are consistent among the events and with previous descriptions given in the literature substantiates the notion of a common interaction type. The velocity variations in four of the events tend to lie in planes approximately parallel to the corotation spiral; thus it is implied that the events are reasonably stationary in time in the corotating frame. The data are interpreted in terms of a corotating interaction front, which is a ridge of high pressure parallel to a corotating spiral. The increased pressure is due to radial compression of the streams. The east-west (zonal) flow directions also occur in the front and are shown to be produced by the zonal pressure gradient of the pressure ridge.

One‐Dimensional Model of Shallow‐Mantle Convection

Abstract: One of the models proposed for mantle convection is a counterflow in the asthenosphere to balance the motion of the lithospheric plates. An analysis of this hypothesis has been made, using a model in which the variables depend only on depth. Velocity and temperature profiles are coupled by a temperature- and depth-dependent viscosity. The velocity of the crustal plate and the actual viscosity function are the only inputs to the model. Pressure must increase with distance from the ridge for there to be a return flow at depth and no net mass flow across a vertical section. Horizontal pressure gradients between about 0.1 and 1.0 b/km and shear stresses at the crustal plate between 0.1 and 0.4 kb have been obtained for wide variations in the plate velocity and the viscosity function. However, for these same examples, the surface heat flux is remarkably insensitive to parameter variations; it is between about 0.2 and 0.3 μcal/cm2 sec. Heating by viscous dissipation provides a self-lubricating mechanism. The higher the plate speed, the lower is the drag on the plate.

An Absolute High-Pressure Microviscometer Based on Refractive Index

Abstract: A new method of measuring refractive index in a lubricant point contact is described which allows the density, pressure and viscosity in the fluid to be determined. The viscosity measurements are absolute, only the elastic and optical constants of the glass are needed. Due to the very high pressures developed (1 GPa) in the 0.3 mm diameter contact very large pressure gradients are produced, hence viscosities four orders higher than conventional high-pressure viscometers normally reach, can be measured. Four lubricants tested all showed that the pressure viscosity coefficient dropped sharply above 10$^{3}$ Pa s (10$^{4}$ poise); the limit of the normal viscometer. One fluid - a polyphenylether - apparently vitrified at high pressures to a limiting viscosity of 10$^{6}$ Pa s. Ball bounce is shown to limit the range of fluids that can be tested with this apparatus.

The turbulent boundary layer on a porous plate: An experimental study of the heat transfer behavior with adverse pressure gradients

Abstract: An experimental investigation of the heat transfer behavior of the near equilibrium transpired turbulent boundary layer with adverse pressure gradient has been carried out. Stanton numbers were measured by an energy balance on electrically heated plates that form the bottom wall of the wind tunnel. Two adverse pressure gradients were studied. Two types of transpiration boundary conditions were investigated. The concept of an equilibrium thermal boundary layer was introduced. It was found that Stanton number as a function of enthalpy thickness Reynolds number is essentially unaffected by adverse pressure gradient with no transpiration. Shear stress, heat flux, and turbulent Prandtl number profiles were computed from mean temperature and velocity profiles. It was concluded that the turbulent Prandtl number is greater than unity in near the wall and decreases continuously to approximately 0.5 at the free stream.

An experiment on the adiabatic compressible turbulent boundary layer in adverse and favourable pressure gradients

Abstract: A wind-tunnel model was developed to study the two-dimensional turbulent boundary layer in adverse and favourable pressure gradients with out the effects of streamwise surface curvature. Experiments were performed at Mach 4 with an adiabatic wall, and mean flow measurements within the boundary layer were obtained. The data, when viewed in the velocity transformation suggested by Van Driest, show good general agreement with the composite boundary-layer profile developed for the low-speed turbulent boundary layer. Moreover, the pressure gradient parameter suggested by Alber & Coats was found to correlate the data with low-speed results.

Influence of Stream Turbulence and Pressure Gradient upon Boundary Layer Transition

Abstract: In considering boundary layer transition, the available experimental data and prediction methods are reviewed and further experimental results are presented. Some empirical rules are suggested for the separate effects of stream turbulence and of pressure gradient and of both combined. The approximate nature of these rules is described and the causes indicated.

Thermal Creep in Gases

Abstract: The development of a pressure gradient in a gas‐filled capillary due to the application of a temperature gradient is known as thermal transpiration. In the near‐continuum limit (small Knudsen number) the mechanism for the transpiration effect is a creeping motion of the gas in a thin layer adjacent to the surface. Measurements performed in the near‐continuum limit are compared with a number of theoretical calculations for the thermally induced creep velocity. The experimental data for He, Ne, Ar, Kr, and N2 agree best with the recent theoretical work of Loyalka and indicate little or no dependence on gas type, in contrast with another recent theory which indicates a marked dependence on the thermal accommodation coefficients. In addition, the effect of the thermal creep coefficient upon the evaluation of rotational collision numbers from thermal transpiration measurements is discussed.

Comparative Clinical Studies of Epidural and Ventricular Pressure

Abstract: Monitoring of ventricular pressure (VP) has been widely used in neurosurgical practice. Because of certain disadvantages, other techniques of recording intracranial pressure (ICP) have been developed. Miniature transducers have been placed both subdurally and epidurally, and recordings made either directly or telemetrically. Since pressure gradients may exist within the intracranial space [1] and since elastic forces from the dura and the brain tissue act on a transducer placed on the brain surface [2], the correlation between ventricular pressure and pressure recorded by any such extracerebral device must be determined before the information can be useful to the clinician.

Non-Newtonian behavior of a suspension of liquid drops in tube flow

Abstract: This paper present an analysis and computed results of the flow of deformable droplets of one Newtonian flow suspended in another Newtonian fluid. Surface tension is assumed to act at the interface of the two fluids. Non-Newtonian characteristics of the suspension are demonstrated and are qualitatively similar to flow characteristics of blood in capillaries. The results show that the deformation of the drops may result in a significant reduction in the pressure gradient compared with that necessary for a suspension of rigid spheres or spherical (nondeformable) liquid drops. The decrease in the pressure gradient is velocity dependent. This constitutes a mechanism of non-Newtonain behavior of the suspension as a whole, and is attributable to the flexibility of the suspended elements. The resultant shapes of the liquid drops are similar to the shapes of red blood cells that have been observed in narrow glass capillaries as well as in blood vessels.

Mean wind-direction shear through a forest canopy

Abstract: The equations of motion applying to the wind field in a forest canopy are simplified to a balance between the shearing stress gradient and either the form-drag of the leaves in the upper dense canopy, or the overall horizontal pressure gradient in the more open space beneath. The equations imply that, in descending through the forest, the stress and wind vectors turn through an angle which depends on the forest characteristics and on the stability and the speed of the airflow above the forest. The turning is roughly confirmed by an overall average measured on a very flat site near Thetford, Norfolk, covered by an extensive uniform pine forest.

Stability of slurry trenches

Abstract: THE NUMERICAL EVALUATION OF THE STABILITY OF THE SLURRY TRENCHES MOST COMMONLY USED UP TO NOW IS BASED ON THE ASSUMPTION OF THE EXISTANCE OF A "MEMBRANE" ALONG THE WALL ON WHICH THE LIQUID PRESSURE IS ACTING. THEORETICAL CONSIDERATIONS AS WELL AS EXPERIMENTAL TESTS ON MODELS AND IN-SITU REVEALED, HOWEVER, THAT THE ASSUMPTION MENTIONED ABOVE IS NOT ALWAYS CORRECT. IF CAN BE OBSERVED THT A PENETRATION OF THE SLURRY INTO THE VOIDS OF THE SOIL TAKES PLACE. THE PENETRATION IS GOVERNED MOSTLY BY THE MUTUAL CONDITIONS OF GRAIN SIZE DISTRIBUTION AND THE STAGNATION PRESSURE GRADIENT OF THE SUSPENSION. THESE CONDITIONS IMPLY THAT HYDROSTATIC PRESSURE IS ACTING IN THE VOIDS OF THE SOIL ADJACENT TO THE TRENCH. THIS PAPER ATTEMPTS TO PROVE THIS. INVESTIGATIONS INTO THE STABILITY OF THE SINGLE GRAIN INSIDE THE EARTH WALL LEAD TO ADDITIONAL CONSIDERATIONS FOR THE SAFETY OF SLURRY TRENCHES. SOME METHODS ARE PRESENTED FOR TESTING THE RELEVANT PARAMETERS IN THE LABORATORY AS WELL AS ON THE SITE. /TRRL/

A Study on the Measurement of the Local Void Fraction by the Optical Fibre Glass Probe : 1st Report, On the Method for the Measurement of the Local Void Fraction in a Liquid Metal Two-Phase Flow

Abstract: The local void fraction is one of the most essential parameters to clarify the precise structure of a two-phase flow. In the report, a new method is tried for the measurement of the local void fraction. The method is based on the principle that if there is a liquid metal at the top of the optical fibre glass probe, the signal light reflects and turns back the fibre glass. If not, the signal light scatters out. In the first report, the measurement is carried out in the case of a mercury-air two-phase flow in a vertical pipe. The present results show good agreement with those obtained by the method of pressure gradient.

Supersonic Turbulent Boundary Layer in Adverse Pressure Gradient. Part I: The Experiment

Abstract: Experimental measurements of the mean profile characteristics of the supersonic turbulent boundary layer in a region of moderate adverse pressure gradient along the curved surface of an isentropic ramp model are reported. Detailed surveys of impact pressure, static pressure and total temperature were made through the boundary layer and local values of wall shear stress were obtained using the Preston tube technique. The measurements were made for a nominal tunnel nozzle setting of Mach 3.5, closely adiabatic wall, and momentum thickness Reynolds number of 1.9 to 4.2 x 10. In addition to the mean profile data, fluctuation data were obtained using constant temperature hot wire anemometry at one station in the region of zero pressure gradient and at one station in the region of adverse pressure gradient.

Compression Permeability of Al2O3 Cakes Formed by Pressure Slip Casting

Abstract: The pressure gradient naturally effected across the cake on pressure casting can lead to pore-character gradients and dimensional distortion if the cake is significantly compressible. The compressibilities of cakes formed on pressure slip casting of aqueous Al2O3 slips defiocculated with an organic polyelectrolyte were determined for applied pressures ≤500 psi. In general the compressibility decreased with increasing pressure. Improved deflocculation of slips from which cakes were formed drastically reduced the level and pressure dependence of compressibility; cakes cast from well-deflocculated slips were insignificantly compressible. Specific resistances determined concurrently agreed well with results obtained previously in kinetic studies. Compression-permeability behavior can serve as a quality control test for the slip casting of technical ceramics.

Current Sheet Structure in an Inductive-Impulsive Plasma Accelerator

Abstract: Probe and laser light-scattering measurements of electron density have been made in the current sheet of a pulsed inductive accelerator. The discharge consists of a thin flat disk of pure je current. The only current carriers are electrons. The gas is ionized and swept up into a 4 mm thick current sheet. Ions are accelerated by an axial E field, ahead of the mass layer, produced both by electron current and negative electron pressure gradient. This field nearly vanishes at the rear of the mass layer where j x B force balances the positive electron pressure gradient.