Showing papers on "Velocity gradient published in 1982"
TL;DR: In this paper, the stability of sheared MHD flow of finite thickness in a compressible plasma was analyzed for the case of arbitrary orientation of the magnetic field, velocity flow, and wave vector in the plane perpendicular to the velocity gradient.
Abstract: A general stability analysis is given of the Kevin-Helmholtz instability, for the case of sheared MHD flow of finite thickness in a compressible plasma which allows for the arbitrary orientation of the magnetic field, velocity flow, and wave vector in the plane perpendicular to the velocity gradient. The stability problem is reduced to the solution of a single second-order differential equation including a gravitational term to represent the coupling between the Kelvin-Helmholtz mode and the interchange mode. Compressibility and a magnetic field component parallel to the flow are found to be stabilizing effects, with destabilization of only the fast magnetosonic mode in the transverse case, and the presence of both Alfven and slow magnetosonic components in the parallel case. Analysis results are used in a discussion of the stability of sheared plasma flow at the magnetopause boundary and in the solar wind.
433 citations
01 Jan 1982
TL;DR: In this article, the effects of the velocity gradient and the Lewis number of deficient reactants on the behavior and extinction of a premixed flame in a stagnation flow were studied experimentally.
Abstract: The combined effects of the velocity gradient and the Lewis number of deficient reactantsin the premixed gas on the behavior and extinction of a premixed flame in a stagnation flow were studied experimentally. The flat twin flames established in the stagnation region of the two dimensional opposed flow of the same mixture were used in order to make an adiabatic and noncatalytic stagnation surface. The flame extinction behavior observed is purely attributed to the flow-transport properties-chemical reaction interaction. The mixture employed was hydrogen, methane, propane, or butane with air. The flame temperature and the distance between the two flames were measured for the wide variations of the fuel concentration and the velocity gradient. As the stagnation velocity gradient was increased by increasing the opposed flow velocity, the flat two flames approached each other, and finally the extinction occurred abruptly. By observing the flame behavior, the extinction process of the flame was classified into two patterns corresponding to the ranges of the Lewis number Le of the deficient reactant in the premixed gas (fuel in the lean mixture and oxygen in the rich mixture). Its ranges are Le 1. Under the condition of Le 1 (lean hydrogen or methane-air and rich propane or butane-air), as the velocity gradient is increased, the flame temperature increases and reaches a maximum point at a certain value of the velocity gradient, beyond which the flame temperature decreases. The flame is extinguished close to the stagnation surface or in contact with the stagnation surface. The results of this experiment are in good agreement with the theoretical predictions.
85 citations
TL;DR: In this article, the structure of the East Pacific Rise in the vicinity of 12°N has been studied and it has been shown that the oceanic crust can be modeled with a rapid change in velocity in the top 2.5 km, a zone of low but uniform velocity gradient from 2 to about 5.5km and a transitional zone about 1 km thick to mantle material.
Abstract: University of Washington seismic data taken during ROSE phase 1 have been used to study the structure of the East Pacific Rise in the vicinity of 12°N. Off axis but within 50 km of the axis we find that the oceanic crust can be modeled with a rapid change in velocity in the top 2.5 km, a zone of low but uniform velocity gradient from 2.5 to about 5.5 km and a transitional zone about 1 km thick to mantle material. There is also evidence for substantial anisotropy in mantle velocity gradients, with a positive velocity gradient in the spreading direction and a negative velocity gradient parallel to the rise. Under the rise axis the data allow at most a very narrow zone of low velocity (less than about 2 km wide) in the crust.
60 citations
TL;DR: In this article, the velocity-gradient disturbances are not due to the usual interaction of Reynolds stress with the shear stress of the mean flow, but are due to interaction with the surface pressure fluctuations converted from the velocity fluctuations of the oncoming jet.
Abstract: Local measurement of the mass-transfer rate and velocity gradient when an axisymmetric jet impinges on a flat plate was carried out using an electrochemical technique. Local measurement of the surface pressure on the flat plate was carried out separately using piezoelectric pressure transducers. The stagnation-point mass-transfer coefficient reaches a maximum when the flat plate is placed at 6 nozzle diameters from a convergent nozzle. It has been confirmed that the mass transfer to the flat plate for a high Schmidt number is greatly enhanced owing to the velocity-gradient disturbances in the wall region of the boundary layer, while the momentum transfer is insensitive to such disturbances. The relative intensity of the velocity-gradient fluctuations on the wall has an extremely large value at and near to the stagnation point, and decreases downstream, approaching a large constant value.These velocity-gradient disturbances are not due to the usual interaction of Reynolds stress with the shear stress of the mean flow, but are due to the interaction with the surface-pressure fluctuations converted from the velocity fluctuations of the oncoming jet.The three co-ordinate dimensions of large-scale eddies are calculated from the auto- and spatial correlations of the surface-pressure fluctuations. It is considered that such large-scale eddies play an important role in the production of a velocity-gradient disturbance in the wall region of the boundary layer from the velocity turbulence of the approaching jet.
40 citations
01 Jan 1982
TL;DR: In this article, heat flux measurements in several specially designed turbulent flows with inhomogeneous temperature field are presented and analyzed with the purpose of evaluating the performance of the gradient transport models (GTM) as well as several of the generalizations, and of understanding the circumstances in which the GTM works in spite of the a priori conclusions to the contrary.
Abstract: Heat flux measurements in several specially designed turbulent flows with inhomogeneous temperature field are presented and analyzed with the purpose of evaluating the performance of the gradient transport models (GTM) as well as several of the generalizations, and of understanding the circumstances in which the GTM works in spite of the a priori conclusions to the contrary. One of the flows considered is a uniform grid-generated flow; two others are shear flows with transverse homogeneity and a constant mean velocity gradient. The fourth is the wake of a circular cylinder in which an asymmetric temperature field is created by heating a combination of thin wires located off-axis. A GTM with a constant turbulent diffusivity adequately describes the turbulent heat flux in all three homogeneous flows, while in the inhomogeneous flow, forcing gradient transport to the measured heat flux results in negative diffusivity over a part of the flow. Direct evaluation from the present and other measurements shows that none of the generalizations of the GTM is adequate. It is suggested that the large eddy transport in inhomogeneous shear flows is the possible cause for the failure of the GTM, and a simple criterion set forth here shows that its apparent success in symmetrically heated free shear flows is largely a happenstance caused by the imposed boundary conditions.
39 citations
TL;DR: In this paper, a velocity profile is measured through the depth of a 0.4 m deep flow of granular snow having a nominal speed of 7.0 m s −1 and fit by an exponential curve.
33 citations
TL;DR: In this paper, an experimental study of a laminar, two-dimensional inverted flame established in the wake of a thin plate was performed, and they were analyzed to obtain the stretching along the flame front at the base of the flame, namely, at a "stabilization point", and the flow velocity gradient along a streamline passing through that point.
29 citations
TL;DR: The theoretical analysis of dispersion with or without stress-induced diffusion and chemical reaction during laminar flow of viscoelastic fluids in capillaries is given in this paper.
Abstract: The theoretical analysis of dispersion with or without stress-induced diffusion and stress-induced chemical reaction during laminar flow of viscoelastic fluids in capillaries is given. The results are applied to diffusion and degradation in polymers under nonhomogeneous velocity gradient fields. It is shown that the concentration profiles during dispersion are strongly dependent on axial and radial positions in the presence of stress-induced diffusion. Stress-induced degradation rate of macromolecules during capillary flow decreases if the macromolecules are subject to stress-induced diffusion.
23 citations
TL;DR: In this paper, the authors used multiprobe techniques to study the coherent flow-oriented eddy pattern that exists close to a wall in a turbulent flow, and the results are consistent with the suggestion by Sirkar & Hanratty (1970a) that the floworiented eddies are approximately homogeneous in the flow direction and, on average, have inflows and outflows of equal magnitude coupled in the spanwise direction.
Abstract: Multiprobe techniques were used to study the coherent flow-oriented eddy pattern that exists close to a wall in a turbulent flow. Simultaneous measurements of the spanwise component sz of the fluctuating wall velocity gradient at a number of locations in the spanwise direction were used to detect different aspects of the secondary flows that are superimposed on the mean flow. Conditionally averaged measurements of the streamwise component of the wall velocity gradient and of the streamwise component of the velocity at a number of locations on a line perpendicular to the wall show how the streamwise velocity fluctuations are related to the eddy structure, as detected by the sz(z) variation.The results are consistent with the suggestion by Sirkar & Hanratty (1970a) that the flow-oriented eddies are approximately homogeneous in the flow direction and, on average, have inflows and outflows of equal magnitude coupled in the spanwise direction; i.e. they may be modelled, on average, by a sinusoidal sz(z) variation with dimensionless wavelength λ+ = 100. The results are also consistent with explanations by Fortuna and Hanratty (see Fortuna 1971; Hanratty, Chorn & Hatziavramidis 1977) and by Hatziavramidis & Hanratty (1979) of how these secondary flows are affecting momentum transport to the wall.
22 citations
TL;DR: In this paper, the flow behavior around two cylinders with the same diameters and the local mass transfer around the downstream one have been made clear experimentally in the range of Re=(1.5∼8)×10 and the clearance x*=(0∼5)d.
Abstract: Succeeding to the former, report on two circular cylinders with different diameters, the flow behavior around two cylinders with the same diameters and the local mass transfer around the downstream one have been made clear experimentally in the range of Re=(1.5∼8)×10 and the clearance x*=(0∼5)d. The main results are summerized as follows. (1) Flow in a closed vortex region formed between two cylinders can be classified. (2) The maximum Nu (Sh) up to the jump clearance closely depends on the velocity gradient at the reattachment point. (3) Overall Nusselt number is proportional to Re2/3 and shows an increase of 20-30% as compared with that of a single cylinder in spite of the change of the flow pattern due to the clearance.
22 citations
TL;DR: In this article, a path line tracking procedure is derived which determines the strain history of infinitesimal material elements in steady axisymmetric flow, and then interpolates spatial positions along path lines at residence times ( t − t ), magnitude of velocity, velocity gradient normal to the path line, and slope of path line at time t.
Abstract: A path line tracking procedure is derived which determines the strain history of infinitesimal material elements in steady axisymmetric flow. Path line tracking requires a pointwise knowledge of the kinematics as it is gained in numerical calculations (or may be available from measurements). Tabulated values of density as a function of position are also required. Path line tracking then interpolates spatial positions along path lines at residence times ( t — t ′), magnitude of velocity, velocity gradient normal to the path line, and slope of path line at time t . This knowledge is then condensed into the time dependent Finger strain components C −1 ( t ′, t ) for a given material element in steady axisymmetric flow. The tracking is applicable to compressible flows of arbitrarily large strains, since the strain history C −1 is determined for a material point (infinitesimal material element). Path line tracking avoinds differentials in space, except when determining the shear strain and when interpolating between the given point values of velocity. The method is tested in a specific example (planar stagnation flow) and it is applied to modelling of processing flows.
TL;DR: In this article, it was shown that the Clausius-Duhem inequality, together with the postulate that the Helmholtz free energy has a minimum in equilibrium, suffices to ensure that, except for a very special subclass, every Rivlin-Ericksen fluid of complexity 2 has a negative first normal stress difference for all small enough shearings in any viscometric flow.
Abstract: For any incompressible fluid whose stress is a frame indifferent function of the velocity gradient and the material time derivative of the velocity gradient, i.e., for any Rivlin‐Ericksen fluid of complexity 2, we show that thermodynamics implies that the first normal stress difference of viscometric flows must be nonpositive for small enough shearings unless a certain very special degeneracy occurs. More precisely, we show that the Clausius‐Duhem inequality, together with the postulate that the Helmholtz free energy has a minimum in equilibrium, suffices to ensure that, except for a very special subclass, every Rivlin‐Ericksen fluid of complexity 2 has a negative first normal stress difference for all small enough shearings in any viscometric flow. Our results significantly extend a similar analysis given by Dunn and Fosdick in 1974 for those special Rivlin‐Ericksen fluids of complexity 2 known as second grade fluids. In addition, they direct attention at a new class of complexity 2 fluids that have been little explored by theorists or experimenters. Furthermore, we study in detail the implications of our thermodynamic postulates for a certain subclass of these complexity 2 fluids that is more general than either second grade fluids or generalized Newtonian fluids. We find that for the fluids in this class the first normal stress difference may change sign as the shearing changes, and we find an interesting linkage between such sign alterations and potential local instabilities in the flow field. Finally, we examine the global stability of the rest state for our fluids and show that if the free energy has a strict, gobal minimum in equilibrium, then our fluids are better behaved than any Navier‐Stokes fluid, since not only does the kinetic energy of any disturbance decay in mean but so too does a certain positive definite function of the stretching tensor.
01 Jan 1982
TL;DR: In this paper, a model for predicting homogeneous anisotropic turbulence submitted to uniform mean velocity gradients is presented, which is closed in the frame of the E.D.N.M. theory.
Abstract: A model for predicting homogeneous anisotropic turbulence submitted to uniform mean velocity gradients is presented. Equations of second order correlations are used. They are closed in the frame of the E.D.Q.N.M. theory. Since the use of the fully three-dimensional expression for the tranfer results in cumbersome computations, a modeled form is proposed to interpolate this term. Then only a few complete calculations are required and computations can be carried out. Our results agree fairly well with the trends predicted by the experiments.
TL;DR: In this article, the flow of a laminar boundary layer into a suction slot has been examined experimentally in a low-turbulence water channel and it was shown that the slot Reynolds number and a parameter that describes the mean velocity gradient in the flow above the slot are the important variables in the problem.
Abstract: The flow of a laminar boundary layer into a suction slot has been examined experimentally in a low-turbulence water channel. Dimensional analysis shows that the slot Reynolds number and a parameter that describes the mean velocity gradient in the flow above the slot are the important variables in the problem. Measurements are reported of the length scales of the separation region within the slot and the slot-pressure loss. These data correlate in terms of the similarity variables and may be used as a predictive design tool. The mean-flow profiles above the slot, in both the upstream and downstream directions, were also measured using hot films, and the possibility of a Taylor-Gortler rotational instability occurring just downstream of the slot has been studied.
TL;DR: In this article, a double coaxial type burner with propane-air mixtures was used to investigate turbulences in open premixed jet flames by using shadow and schlieren graphs, and it was shown that low frequency oscillations of the flame occur in the vicinity of the boundary between the outer flame and the surrounding air stream.
Abstract: Experimental studies were carried out on turbulences in open premixed jet flames by using a double coaxial type burner with propane-air mixtures. Shadow and schlieren graphs showed that low frequency oscillations of the flame occur in the vicinity of the boundary between the outer flame and the surrounding air stream. The phenomena are surely caused by the hydrodynamic instability and it was also asserted from the fact that the phenomena could never be seen in the case of the surrounding flow velocity being large enough in measurement by a counter-type laser doppler velocimeter (LDV). Moreover, the organized motion with periodic vortices was entirely common in premixed, rich-premixed, and diffusion flames in the open atmosphere. The normalized frequencies had a tendency similar to the results obtained by some workers On the other hand, from the measurement of velocity fields by LDV, it was found that strong turbulences occurred on the flame front mainly by the velocity gradient, not |∂∪|∂r| but |...
TL;DR: In this paper, a velocity profile model for turbulent flow of non-Newtonian power-law fluids in smooth pipes has been proposed, following the approach of Stein et al. (1980) for Newtonian fluids.
Abstract: A new velocity profile model for turbulent flow of non-Newtonian power-law fluids in smooth pipes has been proposed, following the approach of Stein et al. (1980) for Newtonian fluids. The merit of the model, as different from all earlier works, lies in its ability to predict a zero velocity gradient at the centreline for all values of the pseudoplasticity index n. The predictions of the model when compared with the work of Bogue and Metzner (1963) have shown good agreement.
TL;DR: In this paper, the influence of velocity gradients, in a laminar approach flow, on the local burning velocity was investigated, to supplement and verify recent theoretical work, and the results, presented as burning velocity versus velocity gradient along the flame front, show that the local laminAR burning velocity decreases as the rate of stretching of the flame is increased.
TL;DR: Theoretical data on flow light scattering by prolate spheroids at ϑ = ω = 90°, ω being the equatorial angle with respect to the direction of flow is reported in this article.
Abstract: Theoretical data are reported on flow light scattering by prolate spheroids at ϑ = ω = 90°, ω being the equatorial angle with respect to the direction of flow. The primary variables considered are (1) the length and axial ratio of the spheroids within the range of dimensions accessible to the theory of Stevenson and (2) the velocity gradient. Simple analytical procedures are outlined for the determination of spheroid dimensions from the data obtained, combined, if necessary, with additional data on flow light scattering. It is shown that flow light scattering by means of the analytical technique outlined may allow one to obtain information not only on the dimensions of spheroids at rest but also on the potential reversible or irreversible dimensional changes produced by flow.
TL;DR: In this paper, the effects of relaxation and the initial wave front curvature on the growth and decay behavior of waves in the thermodynamical state of weak or strong equilibrium were investigated.
Abstract: The rate of amplification of a discontinuity in the velocity gradient is evaluated at the wave-head in a spatially uniform but time evolving flow of a relaxing gas. The paper investigates the effects of relaxation (present in the flow) and the initial wave front curvature on the growth and decay behaviour of waves in the thermodynamical state of weak or strong equilibrium.
TL;DR: A transient analysis was performed for extinction of the counter flow diffusion flame utilizing the assumptions of inviscid, incompressible, and laminar stagnation-point boundary layer flows.
Abstract: A transient analysis was performed for extinction of the counter flow diffusion flame utilizing the assumptions of inviscid, incompressible, and laminar stagnation-point boundary layer flows. The unsteadiness was induced via linear time variation of the stagnation point velocity gradient. The physical meaning of the middle solution of the quasi-steady theory was clarified. The effects of acceleration and deceleration of the flow were examined and it was found that strong acceleration tends to support the flame up to a small Damkohler number, which implies that the flame strength becomes large for flames under acceleration.
TL;DR: In this article, the blow-off limits of a premix industrial burner normally fired with natural gas, for natural gas and a low and a medium-heating value gas such as might be made from coal, are compared with the laminar flashback velocity gradient limits obtained in the classical manner.
Abstract: Data are presented on the blow-off limits of a premix industrial burner normally fired with natural gas, for natural gas, a low- and a medium-heating value gas such as might be made from coal. These limits, interpreted in terms of the wall velocity gradient, are compared with the laminar flashback velocity gradient limits obtained in the classical manner. The same relation is found for all three fuels. For one fuel, flashback-extinction limit is also observed and satisfactorily compared with the literature.
TL;DR: In this paper, it was shown that the action for boundary-free incompressible fluid flow is an absolute minimum with respect to all velocity-field transformations u→u* if u* is structured suitably in terms of u and an arbitrary solenoidal test field f.
Abstract: It is shown that the action for boundary‐free incompressible fluid flow (i.e., the time‐integral of the kinetic energy of the motion) is an absolute minimum with respect to all velocity‐field transformations u→u* if u* is structured suitably in terms of u and an arbitrary solenoidal test field f. As suggested by this physical minimum principle, inequality analysis is applied to obtain an upper bound on the time derivative of the dissipation integral, from which there follow sufficient conditions for a monotone‐decreasing dissipation integral and a monotone‐decreasing global Reynolds number. The latter result provides an experimentally consistent necessary condition for passage from laminar to turbulent flow. Finally, inequality analysis is employed to derive a time‐dependent lower bound on the maximum velocity gradient in a generic boundary‐free flow of finite energy.
Patent•
22 Apr 1982
TL;DR: In this article, the authors describe a titled apparatus with a fibrous path with a cross section reducing from the feed port of a fiber bundle to a discharge port and a feed port for feeding fibers separately in part of the path, and scarcely causing massive fibrous aggregates.
Abstract: PURPOSE:The titled apparatus, having a fibrous path with a cross section reducing from the feed port of a fibrous bundle to a discharge port and a feed port for feeding fibers separately in part of the path, and scarcely causing massive fibrous aggregates. CONSTITUTION:A path 5 is constricted in the forward direction fibers, and an air suction hole 6 is provided near a discharge port of a core yarn of fibrous bundle. The velocity gradient 8 of an air flow is produced by the change in the cross-sectional shape of the path 5 toward a false twisting nozzle 11 from near a line W-W of a fiber feeding port 7 toward a false twisting nozzle 11, and the flow velocity reaches a maximum in the bundling end of the path 5. A fibrous group (short fibrous group) 14 of definite length fed from the fiber feeding port 7 is made to proceed to the fiber forward direction by the velocity gradient of the air flow and transferred in a state of elongated individual short fibers. The occurrence of bent fibers is reduced, and the fiber density and the orientation are uniform due to the laminar flow in a fiber transferring pipe (P).
TL;DR: In this paper, the meridional velocity gradient at the wall of a rotating sphere was measured for aqueous polyox WSR-301 in the concentration range 100 to 3000 ppm.
Abstract: Extremely low normal stresses (of order 10 −4 Pa) are ascertainable by an electrochemical measurement of the meridional velocity gradient at the wall of a rotating sphere. To demonstrate the capabilities of the method, we present results for aqueous of Polyox WSR-301 in the concentration range 100 to 3000 ppm.
TL;DR: In this article, a naphthalene sphere in a circular tube under steady and pulsating airflow at room temperature is investigated by measuring the loss in weight or the decrease in dimension of the body after a given time.
Abstract: Mass transfer from a naphthalene sphere in a circular tube under steady and pulsating airflow at room temperature is investigated by measuring the loss in weight or the decrease in dimension of the body after a given time. Because of small mass-transfer rate, there is little difference between the results based on analyses with and without the resistance due to the rate of gas flow. Under the pulsating flow, however, the resistance is taken into consideration, because air and C10H8 gas may be mixed enough. Comparison of these results with theoretical ones reported previously gives the following semi-empirical expression: Sh=2+[(0.58)4+(O.38z0.4)4]1/4Sc1/3Re1/2p where, Sh=2rokf/D, z=(aω/U∞)3/2(a/ro)1/2, Sc=ν/D, Rep=2roU∞/ν (a: amplitude, D: diffusivity, kf: mass-transfer coefficient, ro: radius, U∞: free stream velocity, ν: kinematic viscosity, ω: angular frequency). As z=0, this expression agrees with the experimental one for steady flow mass-transfer, i.e., Sh=2+0.58Sc1/3Re1/2pThe experimental results for half naphthalene-coated sphere and of local mass-transfer show that the acceleration effect of pulsating flow on the mass transfer is remarkable in the dead-water region of the sphere, where the mass transfer is small under steady flow.Pressure and velocity in the pipe and around the sphere are measured by the use of a strain-gauge-type pressure transducer and a hot-wire anemometer, respectively, and the flow behavior is also examined.
TL;DR: In this paper, the velocity gradients of the contrastreaming electron beams observed in the Earth's magnetosphere can excite three types of ordinary mode instabilities, namely (i) ∇B-resonance electron instability, (ii) ion cyclotron instability, and (iii) unmagnetized ion instability.
Abstract: The velocity gradients of the contrastreaming electron beams observed in the Earth's magnetosphere can excite three types of ordinary mode instabilities, namely (i) ∇B-resonance electron instability, (ii) ion cyclotron instability, and (iii) unmagnetized ion instability. The ∇B-resonance electron instability occurs at small values of the shear parameter 10−4 10−2, the unmagnetized ion instability is excited which can generate electromagnetic waves having frequences from 5 to 50 Hz and typical wavelengthsλ⊥ ∼ (0.5–6)km. The growth rates of all the three velocity shear driven instabilities are reduced in the presence of cold background plasma. The turbulence generated by these instabilities may give rise to enhanced effective electron-electron and electron-ion collisions and broaden the bouncing electron beams.