Showing papers on "Velocity gradient published in 1975"
TL;DR: In this article, the authors examined the effect that appreciable molecular extension has on the crystallization of long chain molecules and showed that to achieve high molecular extensions in solution a longitudinal velocity gradient of strain rate about 103s-1 is required.
Abstract: This work examines the effect that appreciable molecular extension has on the crystallization of long chain molecules. Elementary theoretical considerations presented indicate that to achieve high molecular extensions in solution a longitudinal velocity gradient of strain rate about 103s-1 is required. A method of generating such a velocity gradient, involving flow between opposed jets, is reported and the nature of this flow pattern is examined and quantitatively analysed. The behaviour of polyethylene-xylene solutions in the flow field is presented, notably birefringence observations and measurements indicate that a high degree of molecular alinement can be achieved in specific localized areas of the flow field; also concentration effects are observed which are discussed in terms of entanglement concepts. The effect chain alinement has on crystallization is examined in detail, in particular the ‘shish kebab ’ morphology of the crystals so produced is examined in relation to the hydrodynamic conditions in which they were grown.
123 citations
TL;DR: In this article, the authors present the results of an unreversed seismic refraction profile on the East Pacific Rise near the Siqueiros Fracture Zone recorded using a digital ocean bottom seismograph (OBS)1.
Abstract: WE present the results of an unreversed seismic refraction profile on the East Pacific Rise near the Siqueiros Fracture Zone recorded using a digital ocean bottom seismograph (OBS)1. An analysis of P wave arrival times and amplitudes indicates a velocity gradient in the top 2 km, with the velocity reaching 6.7 km s−1. This is underlain by a low velocity channel some 1.4 km thick in which the velocity decreases to around 4.8 km s−1. Below this low velocity region there is a velocity gradient from 6.2 to 6.8 km s−1 and mantle velocities of 7.7 km s−1 are reached at a depth of 6 km below the sea bed.
86 citations
TL;DR: In this paper, an examination of the Jordan form of the matrix of the velocity gradient of a steady, homogeneous, isochoric flow is made, along with the eigenvalues of such a matrix, to discover when such a flow is strong or weak.
Abstract: There are some flows in which certain strain components grow exponentially in time, while there are other flows in which the components depend otherwise on the time. In this paper the former type are called strong flows and the latter weak. An examination of theJordan form of the matrix of the velocity gradient of a steady, homogeneous, isochoric flow is made, along with the eigenvalues of such a matrix, to discover when such a flow is strong or weak. It is shown that if the eigenvalues are all zero or if one is zero and the other two purely imaginary, then the flow is weak, with the remaining cases leading to strong flows.
68 citations
TL;DR: In this article, it was shown that the externally applied velocity field is responsible for the nucleation of the fibrous crystals, subsequent growth is then influenced by both the local streaming of polymer melt around the growing tip of the fibre and the external velocity field.
Abstract: In situ optical observations have been made of the way polyethylene melt can crystallize whilst subject to certain longitudinal velocity gradients. In general crystallization is seen to occur as the generation of fibres 5 to 50 μm in diameter. Hydrodynamic considerations lead to the conclusion that the externally applied velocity field is responsible for the nucleation of the fibrous crystals, subsequent growth is then influenced by both the local streaming of polymer melt around the growing tip of the fibre and the external velocity field. The effect enhanced pressure has on flow induced crystallization is also examined.
62 citations
01 Jan 1975
TL;DR: The beginning of the phenomenological studies of viscosity goes back to the ancient Greeks and later the Romans characteristically applied what they had learned in practical ingenious ways [1].
Abstract: Viscosity is a transport phenomenon. Viscosity is the transport of momentum due to a velocity gradient. The beginning of the phenomenological studies of viscosity goes back to the ancient Greeks and later the Romans characteristically applied what they had learned in practical ingenious ways [1]. Modern theories of viscosity of liquids are based on continuum mechanics and molecular theory.
39 citations
TL;DR: In this paper, the authors evaluated the minimum threshold speeds above which turbulence always occurs in jar-testing, using various jar-test configurations and found that the resulting data should prove useful for application of laboratory data to water-treatment-plant design.
Abstract: The jar-test procedure is widely used to simulate the water-pretreatment process in the laboratory to produce data for process control, yet few carefully controlled jar-test techniques are found in related literature. Jar-testing has depended upon the approach of each investigator.1 3 However, the interpretation of jar-test data must be founded on unvarying and well-calibrated techniques if they are to be quantitatively meaningful. One of the important variables in the procedure is the mixing intensity, which is related to the rotational speed and the configuration of the agitator as well as the geometry of the mixing vessel. The purpose of this study was to determine the mixing intensity, expressed as the mean velocity gradient "<7," throughout the applicable speed range, using various jar-test configurations. The resulting data should prove useful for application of laboratory data to water-treatment-plant design. Camp1 has called attention to the facts that (1) the fluid condition in full-scale plant mixing and flocculation basins is always turbulent, even when G values are relatively low; and (2) at speeds commonly used in jar-test machines, laminar flow conditions may occur. One object of this study was to evaluate the minimum threshold speeds above which turbulence always occurs in jar-testing. Camp and Stein5 applied Stokes' theory6 to relate the total energy input to what they called a root-mean-square velocity gradient G (Stokes' theory states that the velocity gradient equals the square root of energy dissipation at a point, divided by the absolute viscosity of the fluid): G= du = [W~ dz V fl (1)
34 citations
TL;DR: The effects of high and variable shear stress on the platelet release reaction and aggregation were investigated and the thrombin-induced release of serotonin was appreciably greater than in the corresponding controls incubated at rest.
Abstract: Platelet thrombi in suspensions stirred in an aggregometer or subjected to shear flow in a Couette viscometer or circular tube can only form as a result of collisions between the cells. In whole blood, the presence of the red cells results in pronounced lateral displacements of the paths of the platelets through the vessel, which in turn increases the frequency of collisions between platelets and also makes possible platelet-wall interactions. In addition, blood cells in a suspension undergoing shear flow are subjected to fluid stresses which can result in their deformation. Such stresses may locally attain high values in the arterial circulation at bifurcations, sharp bends and stenoses where the flow is disturbed. Since platelet thrombi have been found downstream of sites of such geometry, the effects of high and variable shear stress on the platelet release reaction and aggregation were investigated. Washed platelets containing C14-serotonin in Tyrodes-albumin solution were subjected to oscillatory flow at 37 degrees C and provided the time average wall velocity gradient was above 2,000 sec-1, the thrombin-induced release of serotonin was appreciably greater than in the corresponding controls incubated at rest. No measurable serotonin was released as a result of flow alone. This synergistic effect of oscillatory shear flow was also noted in the presence of inhibitors of the release reaction.
34 citations
TL;DR: In this paper, the growth of anchored "shish kebab" type fibrous crystals of polyethylene is examined and the problem is reduced to its simplest form of uniform streaming parallel to the major axis of a high aspect ratio ellipsoid.
Abstract: Hydrodynamic factors affecting the growth of anchored ‘shish kebab’ type fibrous crystals of polyethylene are examined. The problem is reduced to its simplest form of uniform streaming parallel to the major axis of a high aspect ratio ellipsoid. TheStokes flow solution shows that local velocity gradients near the tip and the side surface are very much greater than typically applied external velocity gradients; also the magnitude and nature of the velocity gradient at the tip strongly suggests a controlling mechanism by which the chain extended backbone of the fibre forms.
32 citations
TL;DR: In this article, a simple momentum advection effect is considered in a current aligned with the y axis on which there is superimposed a "cross" flow in the x-z plane.
Abstract: Some simple momentum advection effects are considered in a current aligned with the y axis on whichthere is superimposed a "cross" flow in the x-z plane. The cross flow coupled with horizontal shear in thecurrent tends to generate differences along the vertical in the longshore velocity, while vertical mixingtends to even out such differences. As in the scalar diffusion problem considered by Taylor, a balanceis possible between the two tendencies. The equilibrium velocity distribution may support considerablelateral momentum flux, which, in the case of zero rotation, is directed down the velocity gradient, allowingthe definition of an effective horizontal viscosity. When rotational effects are significant, both the senseand the magnitude of the momentum flux come to depend in a complex way on the total vorticity f + S,where f is Coriolis parameter and S the current shear. Some illustrative examples are calculated for cross flow produced by frictional effects in a boundary current. These show that...
26 citations
TL;DR: In this paper, the start-up response of pipe flow to a step input of constant flow rate given by an automatic solenoid valve has been studied experimentally by the use of electrochemical technique.
Abstract: The start-up response of pipe flow to a step input of constant flow rate given by an automatic solenoid valve has been studied experimentally by the use of electrochemical technique. The variation of velocity distribution and velocity gradient at the pipe wall with the passage of time has been measured far downstream from the inlet section, where flow becomes fully developed in the steady state. The velocity profiles of the start-up flow development show a trend essentially different from those of steady-state flow development at different distances in the entrance region of a circular pipe: they show a minimum at the axis and a maximum in the intermediate region between the axis and the wall as the result of non-uniformity of acceleration in the central core (annular jet effect). The development of the laminar boundary layer with time could be regarded as that of the constant-stress layer near the wall. Still, the velocity profiles in the laminar boundary layer at different times are similar to each other.
24 citations
TL;DR: In this article, the magnetic splitting peculiarities of the absorption lines in the sunspot spectrum are considered and the connection between the zero line displacement of a longitudinal magnetic field and the crossover effect is discussed.
Abstract: The magnetic splitting peculiarities of the absorption lines in the sunspot spectrum are considered. The most common and typical of them is breaking of all Stokes parameter symmetry in regard to the line center. The possible reason of this effect is the macroscopic gas motion with inhomogeneous velocity. Computed contours are given for the line Fe i λ5250 A with various combinations of magnetic and velocity fields. Magneto-optical effects within the line which are connected with the magnetic and velocity field inhomogeneity are discussed. The observation results are discussed for longitudinal magnetic field zero lines. These observations were carried out for the sunspot and photosphere in two spectral lines Fe i λλ 5250 and 5233 A. In the sunspot the regular displacement of one zero line with respect to the other zero line takes place whereas in the photosphere the displacements are random. The possible reason of the regular displacement is the change of the magnetic field direction in the different optical layers wherein corresponding spectral lines are formed effectively. The connection between the zero line displacement of a longitudinal magnetic field and the crossover effect is discussed. The computation results are given which agree with observations and illustrate the above-mentioned relationship. The influence of the Stokes parameter asymmetry on the measured magnetic field signals is considered.
TL;DR: In this article, the velocity gradient has been measured on vertical length scales down to ≈ 30 cm using a free fall probe with a neutrally buoyant vane, and it is shown that the shear modulus s 2 is concentrated into narrow regions of high gradient in the same way as the density structure.
TL;DR: In this article, two co-axial cylinders were designed for Couette apparatuses with inflow and outflow connections allowing flocculation to be studied on a flowthrough basis, rather than as a batch process.
TL;DR: In this paper, the authors extended the study of shear flow instabilities in nematics, aligned perpendicular to the velocity and velocity gradient, to the plane Poiseuille flow case.
Abstract: We have extended our study of shear flow instabilities in nematics, aligned perpendicular to the velocity and velocity gradient, to the plane Poiseuille flow case. We recover the homogeneous as well as the roll instability modes. The non-uniform anisotropic stress tensor gives rise to body forces which deflect the flow lines and couple with the distortions. Novel structures are met due to the existence of distortions near both the upper and lower plates. A new variable-wavelenght linear mode is found.
01 Jan 1975
TL;DR: In this article, the authors describe the development of the boundary layer in a region of an adverse pressure gradient leading to increased cross flows and finally terminating in a 3D. separation.
Abstract: An experiment is described in which a three-dimensional boundary layer flow under infinite swept wing conditions is simulated. A description is given of the development of the boundary layer in a region of an adverse pressure gradient leading to increased cross flows and finally terminating in a three-dimensional. separation. Measurements are reported of the mean velocity profiles, the wall shear stress and the components of the Reynolds stress tensor. These measurements reveal a decreasing mixing length with increased cross flow and a substantial difference between the direction of the shear stress and the velocity gradient. After the separation line a region with an almost spanwise flow is observed. Calculations are presented of the boundary layer development with a finite difference method using semi-empirical shear stress relations based on the turbulent energy equation. This method fails to predict separation. When empirical modifications, based on the experimental results are introduced, the agreement improves. Close to separation, however, the calculations are very sensitive to the pressure distribution and this might be related to a Goldstein-type singularity at separation.
TL;DR: In this paper, a model was used to analyze the effect of injection or suction on the eddy diffusivity of momentum, friction factor and pressure gradient in turbulent pipe flow.
Abstract: A theoretical and experimental study was conducted on a fully developed turbulent pipe flow with uniform fluid injection or suction through the wall. A simple model was used to analyze the effect of injection or suction on the eddy diffusivity of momentum, friction factor and pressure gradient. The model predicts that both the normalized eddy diffusivity and local friction factor are linear functions of m/(-dp*/dx*), that is), eM/vRe(-dp*/dx*)=F1(y/R)•m/(-dp*/dx*)+F2(y/R) f•(-dp*/dx*)=1/2+4m/(-dp*/dx*)and that dp*/dx*=fn (m). Functions F1(y/R), F2(y/R) and fn(m) were determined by experiment. The experimental runs were executed at Re=15, 200-95, 300 with wall flux ratio m=vW/ =-0.0042-0.00847.
TL;DR: In this paper, an analysis of turbulent boundary layer flow characteristics and the computational procedure used are discussed, and the integrated mass and momentum flux profiles and differentials of the integral quantities are used in the computations so that local evaluation of the streamwise velocity gradient is not necessary.
Abstract: An analysis of turbulent boundary layer flow characteristics and the computational procedure used are discussed. The integrated mass and momentum flux profiles and differentials of the integral quantities are used in the computations so that local evaluation of the streamwise velocity gradient is not necessary. The computed results are compared with measured shear stress data obtained by using hot wire anemometer and laser velocimeter techniques. The flow measurements were made upstream and downstream of an adiabatic unseparated interaction of an oblique shock wave with the turbulent boundary layer on the flat wall of a two dimensional wind tunnel. A comparison of the numerical analysis and actual measurements is made and the effects of small differences in mean flow profiles on the computed shear stress distributions are discussed.
01 Jan 1975
TL;DR: The behavior of turbulence can be greatly affected by fluctuating body forces if the latter are correlated with the velocity fluctuations as discussed by the authors, and the most common example is the large effect of gravity on flows with density fluctuations.
Abstract: The behavior of turbulence can be greatly affected by fluctuating body forces if the latter are correlated with the velocity fluctuations. The most common example is the large effect of gravity on flows with density fluctuations. If the density fluctuations arise because there is a mean density gradient in the same direction as the mean velocity gradient (as in a boundary layer on a heated or cooled horizontal surface) or if the flow is actually driven by the mean density differences (as in buoyant plumes in still air) then the density and velocity fluctuations are highly correlated and buoyancy can have a large effect. If the density increases upward (heavy fluid on top of light fluid) the flow is “unstable” and the density-velocity correlation can convert potential energy into turbulent kinetic energy. Conversely if the density decreases upward at a faster rate than is expected for fluid in hydrostatic equilibrium, existing turbulent energy can be converted into potential energy (because turbulent mixing tends to reduce the density gradient and thus raise the center of gravity of the fluid). We shall see below that a convenient parameter for correlating the effects of a density difference difference Δϱ across a fluid layer of thickness h with a typical velocity U is
$$ \frac{{\Delta \varrho gh}}{{\varrho {U^2}}} $$
which is the ratio of the hydrostatic pressure difference across the layer to (twice) a typical dynamic pressure.
TL;DR: In this paper, Bekritskaya et al. made an analysis of the development of lattice turbulence in a stream with a constant velocity gradient and determined the constants in the equations under the assumption that the structure of the turbulence tends toward a limiting state with constant values of the correlation coefficient, the degree of anisotropy, and the dimensionless velocity gradient.
Abstract: On the basis of the equations for the Reynolds stresses and the equation for the scale of the turbulence, an analysis is made of the development of lattice turbulence in a stream with a constant velocity gradient. The constants in the equations are determined under the assumption that, far from the lattice and with large Reynolds numbers, the structure of the turbulence tends toward a limiting state with constant values of the correlation coefficient, the degree of anisotropy, and the dimensionless velocity gradient. The constants in terms containing the viscosity are determined from a consideration of the flow beyond the lattice without a velocity gradient in the final stage of decay of the turbulence. The equations obtained were solved in an electronic computer. The calculation is in satisfactory agreement with the existing experimental data. For calculating flows with a variable velocity gradient, instead of the equation of the scale, it is proposed to use an equation for the frequency of the turbulent pulsations obtained in the present work. The computer calculations were made by S. I. Bekritskaya.
TL;DR: In this paper, it was shown that using expansions with respect to the average velocity gradient, retaining only the first two terms of the expansion, one can satisfactorily describe the turbulence in a pipe in the region of the core of the stream.
Abstract: It is shown that using expansions with respect to the average velocity gradient, retaining only the first two terms of the expansion, one can satisfactorily describe the turbulence in a pipe in the region of the core of the stream.
TL;DR: In this paper, a velocity gradient of the carrier gas flow along the sorbent bed is used to obtain chromarheographic enrichment of weakly-sorbable components, and it is shown that with an immobile field of velocities, a concentration of weaker components takes place, while with a moving field of velocity the concentration of the components occurs near their characteristic velocity.
Abstract: A new version of chromatography — chromarheography in described, in which a velocity gradient of the carrier gas flow along the sorbent bed is used. It is shown that with an immobile field of velocities, a concentration of weakly sorbable components takes place, while with a moving field of velocity the concentration of the components occurs near their characteristic velocities. The existence of chromarheographic enrichment of weakly sorbable substances is demonstrated for three systems.