Showing papers on "Pressure gradient published in 1969"

Calculation of Turbulent Boundary Layers on Rough Surfaces in Pressure Gradient

Abstract: Rough surface skin-friction relations previously described in the literature are expanded in scope and range of applicability . The resulting equations are combined with the momemtum integral equation and an auxiliary equation for the shape factor to provide a calculation method for predicting the growth of the turbulent boundary layer over a rough surface in pressure gradient. Several comparisons have been made between the calculation method and available experimental data in zero, favorable, and adverse pressure gradients. The agreement in general is good.

Behavior of turbulent flow near a porous wall with pressure gradient

Abstract: : Van Driest's theory, which provides a continuous velocity and shear distribution for turbulent flow near a nonporous wall, is extended to turbulent flow near a porous wall. The new, modified theory enables the theoretical calculation of velocity profiles to be performed for a wider range of mass- transfer rates, and it gives good agreement with experimental data.

The measurement of skin friction in turbulent boundary layers with adverse pressure gradients

Abstract: This paper describes a floating-element skin friction meter which has been designed for use in adverse pressure gradients. The effects of secondary forces on the element, which arise from the pressure gradient, are examined in some detail. The limitations of various methods of measuring wall shear stress are discussed and the results from the floating element device are compared with measurements taken in a two-dimensional boundary layer using Preston tubes and velocity profiles. As it is planned to use the instrument later for direct measurements of the shear stress in three-dimensional boundary layers, the relevance of the instrument to this situation is also discussed.

Prediction of Pressure Gradients in Pipeline Systems during Two-Phase Flow:

Abstract: Simple procedures using charts for predicting friction/pressure gradients during turbulent-turbulent flow in pipes and the pressure changes over bends, changes of section, and valves are presented. For friction/pressure gradients comparison is made with the procedures of Baroczy, Lockhart and Martinelli, Martinelli and Nelson, Collier, Becker et al., and Thom.

The effect of pressure gradient on the law of the wall in turbulent flow

Abstract: The effect of a streamwise pressure gradient on the velocity profile in the viscous sublayer of a turbulent flow along a smooth wall in two-dimensional flow is estimated. In the analysis, a similarity argument is used and the necessary empirical information obtained from a constant pressure flow. An allowance is made for the departure from the wall value of the gradient of total shear stress normal to the wall. The results of analysis were used to generate new additive constants for use with Townsend's modified law of the wall velocity profile and subsequently Townsend's profile is found to be in good agreement with the measured velocity profiles in an adverse pressure gradient.

Flow of Non-Newtonian Liquids under a Varying Pressure Gradient

Abstract: IN papers presented at the 1968 autumn meeting of the British Society of Rheology and the Fifth International Congress on Rheology held at Kyoto, Japan, in 1968, we discussed the flow behaviour of non-Newtonian liquids in straight pipes of circular cross-section under the action of a pressure gradient P which varied sinusoidally about a non-zero mean. We considered a pressure gradient of the form where R denotes the “real part”. We were particularly interested in the relation between the mean pressure gradient and the mean flow-rate. In particular we showed theoretically that, for a fixed frequency n, one should expect an increase in mean flow-rate (for a given mean pressure p) for low mean pressures and a decrease in mean flow-rate at higher values of p. The theoretical predictions also indicated a “resonance” effect where quite large percentage increases in mean flow-rate could be expected in the low mean-pressure region.

A study of the composition of the lower solar corona.

Abstract: Some trends in the composition of the lower solar corona due to mixing and diffusion have been examined. Mixing has been treated through plausible inference from the thermal gradient and through analogy with the neutral atmosphere of the earth. These indicate that diffusion may be important in the lower corona. Changes in composition due to pressure and thermal gradients have been calculated with multi-component diffusion equations. Results indicate that (i) large enhancements of heavy elements are possible and (ii) the composition of the corona can show large variations with values of the thermal gradient in the lower corona.

A Computer Study of an Analytical Model of Boundary-Layer Transition

Abstract: The results of a computer study of a new analytical model of transition from laminar to turbulent flow are presented. The model is two-dimension al in the mean flow but threedimensional in the disturbance flow. In particular, an onset of transition boundary is derived for the case of fairly large disturbances introduced into a laminar flat-opiate boundary layer at approximately the critical layer. The nature of the upper and lower branches of this onset of transition map is related to the physical processes going on in the modeled boundary layer. It is found that the upper branch of this curve is defined by a balance of dissipative effects and tendency-towards-isotropy effects as they affect the normal component of turbulent kinetic energy in the boundary layer. The lower branch is denned by a balance of diffusive effects and tendency-towards-isotropy effects on the same normal velocity correlation. Within the framework of these observations, the effect of a "favorable" pressure gradient on transition is discussed. It is suggested that the conventional approach to computing the stability boundaries of laminar flows in the presence of pressure gradient is incorrect in that it does not account for stream tube stretching effects that are shown to be important in a region near the stagnation point of blunt bodies. Finally the nature of a complete transition, as defined by the model, is discussed, and several suggestions for future research are put forth.

Atmospheric pressure background fluctuations in the mesoscale range

Abstract: A study of mesoscale-range pressure fluctuations on a large (250-km) array of microbarographs has shown a correlation of seasonal pressure spectrum levels with horizontal distance to the core of jet-stream winds. On a long-term basis, the greatest pressure variance results from occasional synoptic-scale weather disturbances that are concentrated into short intervals of time (at most a few tens of hours). A lower-level but more continuous source of pressure background results from waves generated directly by jet-stream perturbations. These waves correlate in velocity and direction with the jet-stream winds over the array.

The turbulent wall jet in an arbitrary pressure gradient

Abstract: : A method for calculating the growth of a turbulent wall jet in streaming flow was developed. The flow is assumed to be two-dimensional, incompressible and over a plane, smooth wall. Downstream variations of pressure are permitted and separation in an adverse pressure gradient may be predicted. The method incorporates procedures for matching the flow to that at the blowing slot, although it is postulated that the upstream boundary layer there is thin enough that the wall jet develops without an unmixed wake (i.e. there is not a minimum in the mean-velocity profile). The method incorporates four integral momentum equations taken from the wall to various points in the flow. The calculation of the outer shearing stress, although empirical, is based on the large-eddy equilibrium hypothesis and therefore has some foundation. The remaining empiricism in the method is based on measurements in self-preserving wall jets. The method was used to predict the jet-momentum coefficient required to suppress separation over a trailing-edge flap attached to a thin aerofoil. Plausible curves have been obtained using assumed values of upstream boundary layer at the slot. (Author)

Velocity profiles in laminar oscillatory flow in tubes

Abstract: The oscillatory laminar flow of a Newtonian fluid in a circular tube has been investigated when the fluid is subjected to a periodic pressure gradient. A technique has been developed which allows the velocity amplitude to be determined as a function of radius and the results agree closely with theoretical predictions. The work is being continued with a view to employing the technique for the determination of the rheological properties of viscoelastic fluids.

Control systems for mining equipment

Abstract: THE SPECIFICATION DESCRIBES A HYDRAULIC-PNEUMATIC ANALOG TRANSDUCER FOR PROVIDING A PNEUMATIC PRESSURE INDICATION OF THE HYDRAULIC PRESSURE EXISTING IN A HYDRAULIC RAM. THE TRANSDUCER MAKES USE OF A MEMBRANE OF WHICH ONE SIDE IS SUBJECTED TO THE HYDAULIC PRESSURE WHILE THE OTHER SIDE, IN ACCORDANCE WITH THE HYDRAULIC PRESSURE OBTAINING, VARIES IN ITS DISTANCE FROM THE END OF A TUBE. AIR IS CAUSED TO PASS BETWEEN THE RESULTING ANNULAR GAP BETWEEN THE END OF THE TUBE AND THE MEMBRANE AND THE RESULTING PRESSURE GRADIENT IS MEASURED BY MEANS OF A PENUMATIC PRESSURE GAUGE.

Ridge surface system for maintaining laminar flow

Abstract: IN MANY APPLICATIONS WHERE FLUID FLOWS OVER A SOLID SURFACE, OR WHERE A BODY MOVES THROUGH A FLUID MEDIUM, LAMINAR, RATHER THAN TURBULENT OR SPERATED, FLOW IS DESIRABLE. FRICTION IN THE FLUID BOUNDARY LAYER IN COMBINATION WITH AN ADVERSE PRESSURE GRADIENT CAN CAUSE TURBULENCE OR SEPARATION EVEN ON A VERY SMOOTH SURFACE. AN ADVERSE PRESSURE GRADIENT EXISTS ON THE AFT PORTION OF MOST BODIES DUE TO THE DECELERATION OF FLUID WHICH HAS BEEN ACCELERATED OVER A CONVEX FORWARD PORTION. THE PRESENT INVENTION CREATES PATHS OF RELATIVELY FAVORABLE PRESSURE GRADIENT BY PROVIDING A SYSTEM OF SMOOTH RIDGES SHAPED AND LOCATED SUCH THAT THEY ACCELERATE CROSS FLOW COMPONENTS AS WELL AS THE FLOW IN THE FREE STREAM DIRECTION. THE RIDGES SWEEP SOME OF THE OUTER FLOW INTO THE BOUNDARY LAYER TO ENERGIZE THE LAYER IN AN ORDERLY FASHION. THE FLOW IN THE BOUNDARY LAYER IS ACCELERATED OVER THE RIDGES BECAUSE OF BOUNDARY LAYER THINNING AND BECAUSE THE RIDGES HAVE LESS CURVATURE THAN THE SURFACE. THEREFORE, THE RIDGES CAUSE LESS DECERLERATION OF THE FREE STREAM COMPONENTS THAN THE SURFACE, CREATING PATHS FOR RELATIVE ACCELERATION OF FLOW UP THE RIDGES. IN ADDITION, THE RIDGES MAY BE SHAPED IN LATERAL CROSS SECTION TO CREATE A VENTURI BETWEEN TWO RIDGES. IF THE RIDGES ARE EXTENDED BEYOND THE TRAILING EDGE OF THE SURFACE RELATIVELY FAVORABLE PRESSURE GRADIENTS CAN BE MAINTAINED TO THE SURFACE TRAILING EDGE. THE RIDGE SYSTEM PERFORMS THE FUNCTION OF ENERGIZING THE BOUNDARY LAYER IN THE MANNER OF VORTEX GENERATORS WITHOUT INTRODUCING TURBULENCE. THE RIDGE SYSTEM ALSO PERFORMS THE FUNCTION OF A BOUNDARY LAYER BLEED SYSTEM WITHOUT MECHANICAL COMPLEXITY.

Effect of wall conductances on convective magnetohydrodynamic channel flow

Abstract: The problem of convective magnetohydrodynamic channel flow in a vertical channel subjected simultaneously to an axial temperature gradient and a pressure gradient is examined when the thermal and the electrical conductance of the channel walls are arbitrary. The effects of wall conductances on the flow rate and heat transfer are found and discussed. When the vertical temperature gradient is negative, which is the case of heating from below, there exists a critical Rayleigh number at which the fluid becomes unstable. The critical Rayleigh number is also found as a function of the wall conductances.

The Turbulent Boundary Layer - Experimental Heat Transfer with Blowing, Suction, and Favorable Pressure Gradient

Abstract: Experimental heat transfer data from turbulent boundary layers with transpiration and favorable pressure gradients

A theoretical study on diffusion in pulsating flow

Abstract: A theoretical study has been made on the effect of pulsations in the flow field on interphase mass transfer The phenomenon is of interest in studies of the cardiovascular system as well as in traditional engineering applications The physical situation studied corresponds to fully developed flow in a long conduit with a periodic pressure gradient The mass transfer problem was solved analytically for low amplitude pulsations for the two limiting cases of very small and very large frequencies In addition, several numerical solutions were developed in the intermediate region where the asymptotic solutions are least accurate The solutions taken together give a good quantitative overall view of the phenomenon One of the most interesting and unexpected results is that at very low frequencies it is possible for a pulsatile flow to yield a lower interphase mass flux than a steady Poiseuille flow with the same velocity

Measurements of the Longitudinal Pressure Gradient in Direct-Current Discharges

Abstract: Measurements have been made of the longitudinal pressure gradient in dc discharges in helium, neon, and argon. Studies have been conducted at current densities ∼ 1 A/cm2, over a pressure range of approximately two orders of magnitude with an accuracy of ± 5μ. Present results when compared at high pressures with calculations based on theory indicate good agreement with Leiby and Oskam's theory by comparison with previous theories of the effect. The studies also indicate an anomalous behavior of the gradient at higher discharge currents in neon and argon.

Reverse Flow Solutions for Compressible Laminar Boundary‐Layer Equations

Abstract: Several new similarity solutions are presented for the laminar compressible boundary layer with reverse flow for Prandtl numbers of 1.0 and 0.723. New solutions without reverse flow are presented for Prandtl number equal 0.723. Wall‐to‐stagnation temperature ratios from 0 to 2 are considered. These solutions quantitatively show the effects of pressure gradient, and wall heat transfer on the wall shearing stress and enthalpy gradient.

Approximate calculation of Reynolds analogy for turbulent boundary layer with pressure gradient.

Abstract: An approximate method is developed to calculate the Reynolds analogy factor for a turbulent boundary layer with pressure gradient. The inverse of the Reynolds analogy factor is found to equal the integral across the boundary layer of the product of the slope of the nondimensional velocity profile and the ratio of the nondimensional energy transfer to the nonmensional shear. This result requires a Prandtl number of unity. Approximations for the shear stress, energy transfer, and velocity distribution across the boundary layer make the Reynolds analogy factor depend on a nondimensional pressure gradient parameter and on a velocity profile shape parameter. An adverse pressure gradient is predicted to increase and a favorable pressure gradient to decrease the Reynolds analogy factor. Comparison with experimental data indicates this result to be qualitatively correct, but more data are needed for a quantitative test. Also obtained is a generalization of Crocco's relation between velocity and total enthalpy for a nonadiabatic surface without pressure gradient. As the pressure gradient departs from zero, the relation between the nondimensional total enthalpy and the nondimensional velocity varies widely from Crocco's relation.

Analysis of suddenly started laminar flow in the entrance region of a circular tube

Abstract: Suddenly started laminar flow in the entrance region of a circular tube, with constant inlet velocity, is investigated analytically by using integral momentum approach. A closed form solution to the integral momentum equation is obtained by the method of characteristics to determine boundary layer thickness, entrance length, velocity profile, and pressure gradient.

Transmission of fluids through a pipeline

Abstract: A volatile fluid is transmitted through a pipeline containing a series of check valves by differential heating along contiguous sections of the pipeline to create a decreasing temperature gradient and a consequent decreasing pressure gradient in the direction of desired transmission to thereby cause fluid flow in a desired direction, the check valves preventing the reversal of fluid transmission upon reversal or discontinuance of the temperature gradient. In long pipelines, the differential heating is supplied by the sun moving along the pipeline. Multicomponent fluids can be transmitted by a selection of a volatile mixture of fluid components in such proportions as to create desired pressure differentials and rates of transmission.

Some Experiments on Sonic Velocity in Two-Phase One-Component Mixtures and Some Thoughts on the Nature of Two-Phase Critical Flow:

Abstract: The pressure drop characteristics and flow rates encountered in measurements of flashing water flow in parallel-sided pipes cannot be simply related to the thermodynamic equilibrium theory of the velocity of sound in homogeneous two-phase one-component mixtures. It is shown in the paper that the velocity of propagation through such a mixture is different in magnitude, both to the thermodynamic equilibrium theory and to the theory postulating no phase change. Measurements of the velocity of propagation of compression and rarefaction waves in atmospheric pressure stagnant boiling water are described and interpreted. Some aspects of bubble motion in a pressure gradient are discussed. Measurements in stagnant water are not precisely relevant to the case of flowing liquid in which the vapour component is slipping positively with respect to the liquid. The effect of slip is discussed.It seems to the authors that some relationship between the sonic velocity obtained from their tests and the critical velocity cor...