Showing papers on "Pressure gradient published in 1977"

Pulsating bubble technique for evaluating pulmonary surfactant.

Abstract: Surface tension is determined with an apparatus which records pressure across the surface of a bubble, expanded in the sample liquid and communicating with ambient air. The disposable sample chamber, with a volume of 20 microliter, communicates with a pulsator and a pressure transducer. The volume displacement of the pulsator's moving piston is hydraulically geared down 1,000 times, which gives the pulsator a stroke volume of 0.43 microliter. When this volume is moving into the sample chamber, it causes the bubble radius to change from a maximum of 0.55 mm, accurately measured through a microscope, to a minimum of 0.4 mm. The pulsator speed is usually 20 rpm, but it can be changed from 0.02 to 80 rpm. From the known pressure gradient across bubble surface, and bubble radius, surface tension is calculated with the law of Laplace.

The role of hot plasma in magnetospheric convection

Abstract: Several authors have shown that the presence of hot plasma in the magnetosphere can severely modify an imposed convection pattern and even prevent its penetration to low latitudes. Here with the aid of simple theoretical models we argue that the important parameters governing such effects are the hot plasma pressure, its number density, and the ionospheric conductivity. A result of this is that a fluid description of the phenomenon can be given. Changes in convection naturally give rise to east-west pressure gradients near the ring current inner edge, and these give rise to east-west flows, which cause the shielding effect. We show that plasma compressional energy is lost in Joule dissipation in the ionosphere in these flows which are rapidly set up on the nightside. On the dayside the process is slower, but we conclude that the dayside conductivity is the parameter controlling final ring current penetration. In the long time limit the fluid approximation breaks down, and we qualitatively discuss the development of steady state flow patterns and the effects of collisionless plasma behavior in this limit.

On the Rotation Rate of the Direction of Sea and Land Breezes

Abstract: Attention is drawn to the observational fact that the rate of turning of the direction of sea and land breezes is far from uniform over the diurnal cycle. A theoretical analysis of the problem is then undertaken for a two-dimensional sea and land breeze model. It is shown that the rate of local turning equals the sum of three principal terms. The first term is −kf (f = Coriolis parameter, k = vertical unit vector), a term known from previous theoretical work; the second is the cross product of the horizontal mesoscale pressure gradient (approximately equivalent to the diurnal heating/cooUng of the land relative to the sea) and the velocity of the breezes; the third involves the cross product of the horizontal large-scale pressure gradient, assumed not be affected by the diurnal beating, and the aforementioned velocity. All three terms represent rotation about the vertical but, while the first term is a constant, the other two are variable both in magnitude and sign. These two variable terms modul...

Nonparallel stability of boundary layers with pressure gradients and suction

Abstract: An analysis is presented for the linear nonparallel stability of boundary layer flows with pressure gradients and suction. The effect of the boundary layer growth is included by using the method of multiple scales. The present analysis is compared with those of Bouthier and Gaster and the roles of the different definitions of the amplification rates are discussed. The results of these theories are compared with experimental data for the Blasius boundary layer. Calculations are presented for stability characteristics of boundary layers with pressure gradients and nonsimilar suction distributions.

Experiments on the fine structure of turbulence

Abstract: Investigations have been carried out of some aspects of the fine-scale structure of turbulence in grid flows, in boundary layers in a zero pressure gradient and in a boundary layer in a strong favourable pressure gradient leading to relaminarization. Using a narrow-band filter with suitable mid-band frequencies, the properties of the fine-scale structure (appearing as high frequency pulses in the filtered signal) were analysed using the variable discriminator level technique employed earlier by Rao, Narasimha & Badri Narayanan (1971). It was found that, irrespective of the type of flow, the characteristic pulse frequency (say Np) defined by Rao et al. was about 0·6 times the frequency of the zero crossings. It was also found that, over the small range of Reynolds numbers tested, the ratio of the width of the fine-scale regions to the Kolmogorov scale increased linearly with Reynolds number in grid turbulence as well as in flat-plate boundarylayer flow. Nearly lognormal distributions were exhibited by this ratio as well as by the interval between successive zero crossings. The values of Np and of the zero-crossing rate were found to be nearly constant across the boundary layer, except towards its outer edge and very near the wall. In the zero-pressure-gradient boundary-layer flow, very near the wall the high frequency pulses were found to occur mostly when the longitudinal velocity fluctuation u was positive (i.e. above the mean), whereas in the outer part of the boundary layer the pulses more often occurred when u was negative. During acceleration this correlation between the fine-scale motion and the sign of u was less marked.

Stable arrays of resonant bubbles in a 1‐MHz standing‐wave acoustic field

Abstract: As observed microscopically, bubbles in a standing‐wave acoustic field move to the pressure nulls and oscillate, in elliptical orbits, about a common axis. The orbits of individual bubbles are equally spaced about 100 μm apart along the axis, forming a linear array. These arrays form in tap water for peak pressure amplitudes of 10–20 bar, but persist at levels as low as 1.5 bar. The 7‐μm diameter of the bubbles is approximately the theoretical size for resonant air bubbles in water driven at the 986 kHz frequency of the field. The radius of the orbits is approximately inversely proportional to the pressure gradient at the pressure null, with proportionality constant 0.25 bar. In a simplified model of the orbit phenomenon the expected proportionality constant is given by (2)1/2ps, where ps is the threshold pressure amplitude for rectified diffusion and has the value 0.18 bar. Calculations of ps, based on current theories for rectified diffusion, are in substantial agreement with the this value. The period ...

The Evolution of Low-Mass Close Binary Systems. V. Transport Processes in the Envelopes of Contact Components: Erratum

Abstract: Starting from an adiabatic, isopotential, irrotational approximation to the hydrodynamic equations, the nature of mass flow in the envelopes of contact components of semi-detached and contact systems is discussed. The local pressure reduction near L/sub 1/ results in mass outflow from the interior as well as surface flow. A component with a deep convective envelope can couple through the inner Lagrangian point well before its photosphere reaches its nominal Roche lobe. The horizontal pressure gradients characterizing the flow have scale heights only a fraction of the geometrical depth of contact. Eddy formation by Coriolis forces is inhibited by the turbulent viscosity of a convective medium, but the net effect of convection is further to impede net mass flow. The thermal diffusion time scale of the common envelope is found to be of the order of the sound travel time between components. Therefore flow between components can significanty perturb the vertical thermal structure of the common envelope only in the region near the inner Lagrangian point. A model of luminothat envelopeity transfer in contact binaries is proposed in which large-scale heat transport is accomplished by the coupling of Eddington-Sweet-type circulation with the dynamical flow near L/sub 1/. The large-scale circulation betweenmore » components absorbs or releases energy in the envelope of each star according to whether or not it is in the same sense as the static vertical entropy gradient in that envelope. An approximate criterion is obtained for good thermal contact in a common envelope, and shown to accord well with observed depths of contact.« less

Magnetofluiddynamic flow with a pressure gradient and fluid injection

Abstract: The nonlinear partial differential equation of motion for an incompressible fluid flowing over a flat plate under the influence of a magnetic field and a pressure gradient, and with or without fluid injection (or ejection) through the plate is transformed to a nonlinear, third order ordinary differential equation by using a stream function and a similarity transformation. The necessary boundary conditions are developed for flow with and without fluid injection (or ejection), and an example is presented to illustrate the solution to the flow problem. The controlling equation reduces to the well known Falkner-Skan equation when the magnetic field is zero, and if additionally the pressure gradient is zero, the equation reduces to the Blasius equation.

Self-generated magnetic fields in plasmas

Abstract: The generation of magnetic fields by currents driven by the electron pressure gradient in non-uniform plasmas is considered. The equations for field generation and energy balance are derived under the MHD approximation. It is shown that there exists a class of waves, thermal magnetic waves, which propagate along surfaces of constant density. The properties of such waves are examined. In addition, a class of self-similar solutions are presented. These represent an unstable growth of magnetic field in localized plasma regions, i.e. magnetic hot spots.

A model for a stable coronal loop.

Abstract: We present here a new plasma-physics model of a stable active-region arch which corresponds to the structure observed in the EUV. Pressure gradients are seen, so that the equilibrium magnetic field must depart from the force-free form valid in the surrounding corona. We take advantage of the data and of the approximate cylindrical symmetry to develop a modified form of the commonly assumed sheared-spiral structure. The dynamic MHD behavior of this new pressure/field model is then evaluated by the Newcomb criterion, taken from controlled-fusion physics, and the results show short-wavelength stability in a specific parameter range. Thus we demonstrate the possibility, for pressure profiles with widths of the order of the magnetic-field scale, that such arches can persist for reasonable periods. Finally, the spatial proportions and magnetic fields of a characteristic stable coronal loop are described.

Waterspout Wind, Temperature and Pressure Structure Deduced from Aircraft Measurements

Abstract: During September 1974 in the Lower Florida Keys, the first successful penetrations of mature waterspouts were accomplished by a specially instrumented research aircraft. Throughout the course of each penetration, the measurement system recorded the temperature, the pressure and the three-dimensional velocity field near and within the visible funnel. Multiple penetrations of both cyclonic and anticyclonic waterspouts in various life-cycle stages were achieved. The results indicate that the waterspout funnel structure exhibits 1) a warm central core region, 2) positive vertical velocities of 5–10 m s−1 outside of the warm core, and 3) tangential velocities and horizontal pressure gradients with characteristics similar to but with magnitudes greater than those of the dust devil. A scale analysis of each term in the governing equations of motion suggests a simplified set of modeling equations. The simple Rankine-combined vortex model with cyclostrophic flow explains approximately 75% of the total mea...

Thick Axisymmetric Turbulent Boundary Layer and Near Wake of a Low-Drag Body of Revolution,

Abstract: : Detailed measurements of pressure distributions, mean velocity profiles and Reynolds stresses were made in the thick, axisymmetric boundary layer and the near wake of a low-drag body of revolution. The data are presented in graphical as well as tabular form for convenience in later analysis. These measurements shed some light on the joint influence of transverse and longitudinal surface curvatures and pressure gradients on the boundary-layer development and on the manner in which an axisymmetric boundary layer becomes a fully-developed wake. Apart from giving a complete set of data on such an important flow configuration, the measurements should provide a fairly rigorous test case for some of the recent turbulence closure models which claim a level of generality not achieved by the older phenomenological models. By inclusion of recently proposed modifications to account for the effects of the extra rates of strain on the turbulence length scale arising from longitudinal and transverse surface curvatures, it is shown that the boundary layer in the tail region of a body of revolution is dominated by the extra strain rates and that more research is needed to account for them properly even in the most recent calculation procedures.

Steady motion through a branching tube

Abstract: The fluid motion through a straight pipe that bifurcates symmetrically into two semi-circular pipes is considered at high Reynolds numbers, with the aim of deriving some of the effects of the three-dimensionality per se. In a corresponding planar problem the long-scale boundary layer approach yields the initial development of the induced pressure gradient fairly directly, the problem being linear. By contrast, in the three-dimensional situation the favourable pressure gradient is determined by a nonlinear eigenvalue relation, because the pressure gradient is much stronger here and interacts critically with the inviscid coreflow to produce nonlinear behaviour near the outer walls. The proposed structure of the initial part of the boundary layer is apparently self-consistent, with a two-tiered development near the outer walls where the secondary motion is of a relatively fast vortex type and the axial skin friction suffers a sharp increase. The secondary coreflow is a displacement-induced streaming combined with a vortex provoked by the pressure gradient. The analysis applies for any realistic oncoming flow in the single tube and can be extended to treat other tube cross-sections.

On the Oil Film Behaviour of Piston Rings : Correction of Effective Pressure Region of Oil Film

Abstract: This paper reports on a theoretical calculating procedure of the oil film behaviour between the piston ring and the cylinder wall. The analysis assumes: (1) that the sliding surface of the ring may be approximated by a parabola; (2) that the oil film viscosity is constant; and (3) that the trailing end of oil film pressure varies according to the Reynolds boundary condition, namely the pressure profile ends at apposition where the pressure becomes a downstream pressure and also the pressure gradient becomes zero. The paper gives also an example of oil film behaviour between the circular-faced piston ring and the cylinder wall which has been calculated under the conditions corresponding to a small sized 4-stroke cycle kerosene engine.

A New Passive Boundary-Layer Control Device

Abstract: Experimental results show that a thin cylinder mounted near solid flow boundaries is capable of suppressing turbulent boundary-layer separation caused by adverse pressure gradients in subsonic flows. The cylinder is located parallel to the wall, normal to the flow, and several boundary-layer thicknesses away from the surface. Flowfield measurements suggest that the cylinder-wake/wall-boundary-layer interaction is responsible for the effects found. The ranges of geometrical and boundary-layer variables leading to improved performance were established for axisymmetric diffusers equipped with such devices. Empirical design rules are given. A comparison with vortex generators is made.

Turbulence Model for Nonequilibrium Adverse Pressure Gradient Flows

Abstract: This paper presents a method for calculating compressible adverse pressure gradient boundary layers by using an algebraic eddy viscosity turbulence model that has been modified for variable pressure gradient and turbulence memory effects. The pressure gradient corrections are based on previous incompressible data correlations. Several methods for including the effects of turbulence memory are evaluated. A new lag model, which gives good agreement with available experimental data, is developed. Finally, a correlation is developed for the lag length parameter employed in the model as a function of the known experimental flow variables.

Pipeline leakage detection and location - by measuring pressure gradients on section ends and pressure differential using stated formula

Abstract: In a system for the detection and location of leaks in pipelines for oil, water and gas, the pressure gradients are measured at the start and end of the section concerned, together with the total pressure differential After subtraction, a cross correlation function is produced and the location of the leak is determined from a quoted formula This results in a reliable location of a leak even at small leakage rates of 2% and when the fluid flow is not stationary

Epidural Measurement of Intracranial Pressure by A Newly-developed Pressure Transducer

Abstract: A new pressure transducer for epidural application which the authors developed to reduce the following four factors generally affecting epidural pressure (EDP) values registered on a transducer; 1) transducer characteristics, 2) effects of dural tension, 3) stress concentration effect due to minute irregularities on the dural surface, and 4) adhesion effect between the dura and transducer, is reported. Thus, if the theoretical coplanar measurement is obtained, the values registered by this pressure transducer closely approximates those of the cerebrospinal pressure directly beneath the dura. Even with these improvements, some discrepancies are observed between EDP and intraventricular pressure (IVP). When EDP and IVP are compared, it is found that the difference in both values increases as IVP is higher in the same case, and EDP is usually higher than IVP. These discrepancies are mainly attributed to the fact that stress concentration effects could not be entirely eliminated. It was considered that a certain leeway percentage should be established as a safeguard in reading EDP values. No conclusion could be made as to the pressure gradient between EDP and IVP.

Shear stress and sediment transport in unsteady turbulent flows

Abstract: Predictions of sediment transport rates in coastal waters often display large errors. The source of the error is in part the result of insufficient attention to the character of the shear stress field in unsteady-nonuniform flows. A review of available laboratory and field data is used to show that shear stress magnitude and distribution will vary in response to the sense and amplitude of the horizontal pressure gradient. The response appears sufficient to alter both bed and suspended load transport. The consistency of the data indicates that present predictive techniques, based on uniform flow data, should be modified so as to permit inclusion of probable pressure gradient effects.

Fuel injection pump for internal combustion engines

Abstract: A fuel injection pump is driven by the engine via a drive shaft that also rotates a cam ring. The cam ring reciprocates radial pistons which deliver fuel under high pressure to fuel lines located around an axially sliding distribution member. The amount of fuel delivered is determined in part by the timing of the opening of a pressure relief channel which is opened by a sliding annular ring surrounding the distribution member. The position of this ring and hence the fuel quantity delivered is adjusted by the equilibrium between spring forces and fluid pressure from a regulating valve. A separate pressure control valve adjusts the pressure gradient across an arbitrarily settable throttle which also affects the regulating pressure. Separate mechanisms adjust an engine starting excess quantity and a fuel bypass which is thermostatically controlled to maintain the pump operating temperature within prescribed limits.

The effect of rotation on a gas-operated free-piston pressure gauge

Abstract: It is shown that when a gas-operated free-piston pressure gauge is used for measuring pressure relative to atmospheric pressure, the usual relative rotation of the piston and cylinder with added masses can produce a pressure effect up to ten times greater than the accuracy of measurements relative to vacuum. The magnitude of this pressure effect depends on the rotational frequency and on whether a bell-jar is in place or not.