Effect Of Velocity Gradient Research Articles

The effect of hot zone velocity and temperature gradient on the directional recrystallization of polycrystalline nickel

Cold-rolled polycrystalline nickel sheets were directionally annealed at 1000°C with either a large (1000°C/cm) or a small (50°C/cm) temperature gradient ahead of the hot zone. Columnar grains were produced at hot zone velocities from 2 to 100 mm/h for the large temperature gradient with only two ~6 mm wide grains across the width at 5, 10 and 30 mm/h. With increasing hot zone velocity above 30 mm/h, the columnar grains became narrower. For the same range of hot zone velocities with a small temperature gradient, equiaxed grains were produced, with the grain size decreasing with increasing hot zone velocity. This difference arises because for the low temperature gradient recrystallization followed by substantial grain growth occurs ahead of the hot zone. Both interrupted directional annealing runs and runs (with the large temperature gradient) of nickel in which the nickel had been primary recrystallized at low temperature showed that the columnar grain structure developed by secondary recrystallization.

On the Nature of Angular Momentum Transport in Nonradiative Accretion Flows

The principles underlying a proposed class of black hole accretion models are examined. The flows are generally referred to as and are characterized by inward transport of angular momentum by thermal convection and outward viscous transport, vanishing mass accretion, and vanishing local energy dissipation. In this paper, we examine the viability of these ideas by explicitly calculating the leading-order angular momentum transport of axisymmetric modes in magnetized, differentially rotating, stratified flows. The modes are destabilized by the generalized magnetorotational instability, including the effects of angular velocity and entropy gradients. It is explicitly shown that modes that would be stable in the absence of a destabilizing entropy gradient transport angular momentum outward. There are no inward-transporting modes at all, unless the magnitude of the (imaginary) Brunt-Vaisala frequency is comparable to the epicyclic frequency, a condition requiring substantial levels of dissipation. When inward-transporting modes do exist, they appear at long wavelengths, unencumbered by magnetic tension. Moreover, very general thermodynamic principles prohibit the complete recovery of irreversible dissipative energy losses, a central feature of convection-dominated models. Dissipationless flow is incompatible with the increasing inward entropy gradient needed for the existence of inward-transporting modes. Indeed, under steady conditions, dissipation of the free energy of differential rotation inevitably requires outward angular momentum transport. Our results are in good agreement with global MHD simulations, which find significant levels of outward transport and energy dissipation, whether or not destabilizing entropy gradients are present.

Extended time-term method for identifying lateral structural variations from seismic refraction data

A time-term method is a simple travel time inversion for seismic refraction crustal studies. Although this method is based on simple travel time calculation, it is robust in determining a basement velocity under the complicated surface structure. In the present paper, this method is extended to detect lateral velocity variation in the crystalline crust. We consider a model where a basement is composed of segments with a different velocity. Defining a likelihood function for unknown time-terms and refractor velocities, their best estimates are obtained by minimizing Akaike’s Bayesian Information Criterion (ABIC). To realize accurate velocity estimation, we also incorporate an effect of vertical velocity gradient within the refractor, which was ignored in the conventional algorithms. Numerical tests undertaken for realistic crustal structures show that our method successfully detects lateral velocity variation in a basement even for a case with undulated surface layers. It is particularly stable and reliable if the surface layers are not thick and their velocities are more than 30—40% smaller than the refractor velocities. Then, the refractor velocities are almost decoupled to the time-terms in the process of inversion. The incorporation of the effect of the velocity gradient is inevitably important when travel time data at far distances are included in the inversion.

The effect of air-induced velocity gradient and dissolved oxygen on bioflocculation in the trickling filter/solids contact process

The trickling filter/solids contact process (TF/SC) was developed in the late 1970s to improve the final effluent quality of existing trickling filters, to be able to meet stricter effluent EPA requirements. Although the TF/SC process has been successful in achieving this goal, it is still not fully understood, and there is limited information regarding the factors affecting the flocculation phenomena occurring in the solids contact chamber (SCC), such as the effect of bubble size and dissolved oxygen on bioflocculation efficiency. To better understand the kinetics of bioflocculation in a continuous flow solids contact chamber, a long-term experimental program was conducted using a TF/SC pilot plant constructed at the Marrero, Louisiana wastewater treatment plant. This program, developed from January 1998 through October 2000, included a detailed investigation of the kinetics of the bioflocculation process taking place in the SCC. This paper deals with the effect of air-induced velocity gradient on bioflocculation. Analysis of the observed data demonstrates that excellent final effluent quality can be obtained with a wide range of air-induced velocity gradients ( G) and with dissolved oxygen levels as low as 0.5 mg/l. However, from the point of view of SCC design, a value of G=25/s, achieved using a fine-bubble aeration system, and dissolved oxygen (DO) levels between 1.0 and 1.4 mg/l, are recommended to consistently produce a very good final effluent. This study also demonstrates that additional mechanical flocculation units would not be needed if a fine-bubble aeration system were used in the SCC and the floc structure could be preserved while transferring the floc from the SCC to the secondary sedimentation tank. However, considering that floc breakup frequently occurs during mixed liquor transfer from the SCC to the secondary clarifier, flocculator center wells should be used as a precaution in final clarification units to re-flocculate the mixed liquor.

Effect of Microlift Force on the Performance of Ultralight Aircraft

Microlift force, which is the aerodynamic force developed in the presence of a wind gradient, can be responsible for a sizeable improvement of gliding performance of ultralight aircraft. A mathematical formulation based on Lagrange's equation method for the analysis of microlift is presented, where the effects of vertical velocity gradients on aircraft motion are taken into account. After giving some details on the modeled vehicle, the behavior of an ultralight sailplane during flight in a wind gradient is investigated. Optimal maneuvers are computed to interpret and discuss the performance of ultralight sailplanes soaring in a wind gradient. The proposed approach provides an effective means for understanding and improving the piloting techniques for sailplane and ultralight aircraft in nonuniform wind

Influence of Equivalence Ratio, Velocity Gradient, and Surface Roughness of Substrate on the Combustion Synthesis of Diamond Films Using a Flat Flame Burner.

An experimental study to synthesize diamond films has been conducted under a well-defined flow field to make clear the mechanism of combustion synthesis of diamond films. Using a flat flame burner, the homogeneous diamond films are synthesized on substrates, above which flat, premixed acetylene/hydrogen/oxygen flames are established. The effect of equivalence ratio and velocity gradient is examined on the growth rates of diamond films, crystal sizes, and their morphology. It is found that not only the maximum growth rate but also the maximum crystal size can be obtained when the equivalence ratio is about 2.50, and that the growth rate, crystal size, and/or morphology are nearly the same when the velocity gradient is kept constant. These results are confirmed by a conical flame with a welding torch. In addition, the scratching treatment for the substrates by diamond powder is unnecessary, the maximum growth rate of diamond films can be obtained when the surface roughness is about 0.11 μm, and the nuclei of diamond appear in scratches on the substrates.

Transport phenomena

Transport phenomena

Escape factors for laser-plasmas

A method of parameterizing escape factors (transmission factors and net radiative brackets) for conditions typical of laser-produced plasmas is introduced. The assumptions of planar geometry, exponentially decreasing emissivity and absorption coefficient with distance with a step rise at a particular point, and spatially constant Doppler broadened line profiles have been made. The effect of velocity gradients in spectrally shifting the absorption and emission line profiles relative to each other is taken into account assuming linear velocity gradients with distance. A parameter R representing the ratio of the spatial scale-length of the absorption coefficient to the Doppler decoupling length is introduced. Fitting formulae for transmission factors and net radiative brackets are given which are valid for all R and all optical depths. In the limit of small R (large Doppler decoupling length), the escape factors asymptotically approach formulae developed originally by, for example, Holstein assuming negligible plasma velocities. For large R (small Doppler decoupling length), the escape factors have been shown to asymptotically approach the Sobolev approximation. The parameterized net radiative bracket has been used in the hydrodynamic and atomic physics code ‘EHYBRID’ for the calculation of the effect of radiation trapping on population densities in laser-produced plasmas. The output of the modified EHYBRID code has then been post processed using the parameterised transmission factors to simulate 123 Ne-like and 399 F-like germanium resonance line intensities emitted in typical X-ray laser experiments. We obtain an agreement between the simulated and experimental spectra.

Modelling the effect of seismic velocity gradients on the change in geometrical spreading across a boundary

This paper begins with a review on how rays may be traced though isotropic inhomogeneous media separated by curved interfaces. The results are applied to the case of a layer over a half-space in which the seismic velocity varies linearly with depth and the interface is a horizontal plane. The presence of this type of velocity inhomogeneity will affect the observed amplitudes through its influence on the discontinuity in geometrical spreading at an interface. Analytical results will be compared to an approximate method for evaluation of geometrical spreading to ensure their accuracy. We show that the focusing effects due to the presence of velocity gradients become significant in our examples only for large angles of incidence.

Pseudo-thermal cooling in a bounded system of viscoelastic hard-spheres

Using computer simulations, the cooling rate of a system of inelastic hard spheres in the Couette geometry has been investigated after stopping the relative motion of boundaries. The energy of the system is initially balanced between the incoming kinetic energy from the moving boundaries and the dissipated kinetic energy due to the successive collisions of inelastic particles. When the relative motion of moving boundaries is suddenly stopped the pseudo-thermal energy of the system decays. However, the rate of decay is found to be slower than that introduced for a shear-free ensemble. Moreover, the simulations clearly show that the existence of pseudo-thermal energy is a necessary condition for diffusion in a particulate system. The simulation results may elucidate the physical origin of some phenomena such as clustering and freezing in hard-sphere systems, and the effect of velocity gradients on these phenomena.

Effects of ambient pressure on flame structure of CO/H2/N2 counterflow diffusion flame

A numerical study has been conducted for the understanding of flame structure with the fuel composition of 40 per cent CO, 30 per cent H2, 30 per cent N2 and an oxidizer composition of 79 per cent N2 and 21 per cent O2 in a counterflow diffusion flame. Numerical results are obtained for flames over wide-ranged conditions of axial velocity gradients and ambient pressures. Main concern is specially given to effects of axial velocity gradient and ambient pressure on flame structure. It is seen that the role of axial velocity gradient on combustion processes is globally opposite to that of ambient pressure, on the basis of the Damköhler number considering the balance of convection, diffusion, and chemical reaction. That is, chemical non-equilibrium effects become dominant with increasing axial velocity gradient, but are suppressed with increasing ambient pressure. Flame strength is globally weakened by the increase of axial velocity gradient but is augmented by the increase of ambient pressure. However, flame extinction is described better on the basis of only chemical reaction and in this case axial velocity gradient and ambient pressure play a similar role conceptually such that the increase of axial velocity gradient and ambient pressure cause the flame not to be extinguished and extend the extinction limit, respectively. It is suggested that a combustion process like flame extinction is mainly influenced by the competition between the radical formation reaction and the third-body recombination reaction. Copyright © 2001 John Wiley & Sons, Ltd.

Development of SGS Models for the Dynamic Procedure Using Finite Difference Method. 2nd Report, Modeling of SGS Scalar Flux.

Two different subgrid-scale scalar flux models, namely isotropic and anistropic representation, for the dynamic procedure using finite difference method are derived following the procedure proposed in the previous reports. The proposed models have been assessed on plane turbulent channel flow with passive scalar transport at various grid resolutions and are found to show better agreement with DNS results than those of Smagorinsky's model. The isotropic model is improved to be anisotropic representation by considering the effect of GS velocity gradient on the SGS scalar flux and using the results of a priori test obtained by the LES of a finer grid resolution. The anisotropic model mitigates the isotropic one's overestimation of scalar intensity and streamwise scalar flux near the wall. The notable feature of the proposed models is their insensitivity to the discrete form of the test filtering operation applied during the implementation of the dynamic procedure, which clearly indicates the validity of the proposed models for use in the context of finite difference method.

Effect of vertical velocity gradient on ground motion in a sediment-filled basin due to incident SV wave

The natural sedimental deposits in basins show strong vertical heterogeneity in their material parameters. The aim of this paper is to investigate the effects of such vertical heterogeneity, especially vertical velocity gradient, inside basin on the seismic ground motion through the parametric study on the response of a two-dimensional semi-cylindrical sediment-filled basin to a vertical incidence of plane SV wave using the pseudospectral method. This numerical study has tried to find the effects caused by vertical velocity gradient through the use of synthetic seismograms, wavefield snapshots and surface amplitude distribution. Simulation results clearly demonstrate the detailed character of wave propagation phenomena in basins with vertical velocity gradient, which produces characteristic amplification pattern of the surface motion caused mainly by the generation of strong Rayleigh wave induced at the basin edge associated with large lateral velocity change across the basin edge. Amplification pattern at the surface strongly depends on both the vertical velocity gradient in the basin and the predominant frequency of the incident wave. Although similar phenomena on wave propagation and surface motion found in previous studies for homogeneous basin models have also been observed in this study, it has been found that the vertical velocity gradient enhances such phenomena. The results suggest that it is important to represent the vertical velocity profiles accurately when we construct a structural model for realistic modelling of ground motion.

Amplitude of S diff across Asia: effects of velocity gradient and Qs in the D” region and the asphericity of the mantle

The amplitudes of diffracted SH (S diff) normalized to SKS, together with the S diff–SKS times, were analyzed to constrain the structure of the D” region beneath Asia and the northernmost Indian Ocean. While the S diff–SKS residuals (δt; relative to the Preliminary Reference Earth model, or PREM) are consistently negative from 95° to 120°, the amplitude residuals of S diff/SKS (δ A) show two trends of distance dependence, corresponding to distinct seismic structures in two adjacent zones in D”. In zone A, δ A increases significantly with distance, suggesting the presence of a negative velocity gradient in the base of the mantle. The travel time residuals independently require that the average velocity of zone A be faster than that of PREM. One-dimensional structures that reconcile both sets of constraints were sought through systematic forwarding modeling. Models with negative gradients that satisfy δt's match δ A's to an acceptable degree only if a high-quality factor ( Q s) is assumed. The preferred model for zone A has a 400–500 km thick negative gradient layer, with a ~4% velocity discontinuity at the top and Q s = 1000, an about three-fold increase from the PREM value. In zone B, the amplitude–distance curve is virtually flat, and a 200–300 km thick high-velocity layer with PREM-like gradient and Q s explains both observations well. To assess the role of mantle asphericity in δ A, we estimate the strength of focusing of the S waves into the Fresnel zone at the onset of diffraction in vertical cross-sections of 3-D tomographic models SAW12D and SKS12WM13. Both models predict stronger focusing in zone A than in zone B. The focusing effect is translated to a positive base-line shift in δ A, which, if applied to the model predictions, alleviates the need for an extremely high Qs in zone A. The simple 2-D experiment suggests that velocity gradient and the anelastic attenuation of the D” layer as well as the mantle heterogeneity all probably contribute to the decay characteristics and the level of amplitude of S diff. The slab subducted in the Mesozoic may be responsible for the structure depicted in this study.

Seven-Hole Pressure Probe Calibration Method Utilizing Look-Up Error Tables

A two-step calibration and measurement procedure for seven-hole pressure probes has been developed that allows simple and accurate real-time processing. The method adds an extra step to traditional least-squares calibration schemes such as Gallington’ s method (Gallington, R. W., “ Measurement of Very Large Flow Angles with Non-Nulling Seven Hole Probes,”Proceedings of the 27th International Instrumentation Symposium , Instrumentation Society of America, Triangle Park, NC, 1981, pp. 115 ‐130)involving the interpolation of error look-up tables. A measurement system employing the two-step calibration scheme is described. Measurements are made in pipe and wind-tunnel e ows, and a detailed uncertainty analysis is performed, to demonstrate the accuracy of the new scheme. Independent of the new scheme, the measurements also show some effects of Reynolds number, velocity gradient, and turbulenceon theprobeaccuracy. TheReynoldsnumbereffect was notanticipated and may indicate the need for multiple calibrations in some circumstances.

Effects of strong sound velocity gradients on propagation of low-frequency impulse sound: Comparison of fast field program predictions and experimental data

As part of a large outdoor sound propagation experiment series, propagation of low-frequency impluse sound at ranges from 100 to 1400 m was studied at Haslemoen, Norway in February 1995. Sound sources were charges of 1 and 8 kg C4 explosives. Experiments were carried out in both a uniform forest and above a flat and uniform open field. Extensive meteorological measurements were carried out. Both automatic weather stations mounted in permanent towers and a tethered balloon were employed. The measurements resulted in a manifold set of sound velocity profiles. Occasionally, profiles were pure downwind or upwind, but mostly they were complex, often with large vertical gradients. One conclusion is that one should not uncritically use simple profile parameterizations, e.g., logarithmic, in sound propagation models. A fast field program (FFP), CAPROS, handles the complex sound-speed profiles well, predicting propagation of sound at 8, 16, 32.5, and 63 Hz. In this model, ground is considered a viscoelastic medium. At 63 Hz, the forest seems to cause an excess attenuation. At the lower frequencies, distinct effects of forest are not observed. There is no evidence that FFP predictions are less accurate in periods with high turbulence levels, which is here characterized with dynamical instability. However, variability of meteorological profiles causes predictions to be less accurate. This investigation suggests that it is sufficient to sample atmospheric variables up to 300 m above ground to predict sound propagation at the ranges studied here.

Coupled analysis of injection molding filling and fiber orientation, including in‐plane velocity gradient effect

AbstractOn injection molding of short fiber reinforced plastics, fiber orientation during mold filling is determined by the flow field and the interactions between the fibers. The flow field is, in turn, affected by the orientation of fibers. The Dinh and Armstrong rheological equation of state for semiconcentrated fiber suspensions was incorporated into the coupled analysis of mold filling flow and fiber orientation. The viscous shear stress and extra shear stress due to fibers dominate the momentum balance in the coupled Hele‐Shaw flow approximation, but the extra in‐plane stretching stress terms could be of the same order as those shear stress terms, for large in‐plane stretching of suspensions of large particle number. Therefore, a new pressure equation, governing the mold filling process, was derived, including the stresses due to the in‐plane velocity gradients. The mold filling simulation was then performed by solving the new pressure equation and the energy equation via a finite element/finite difference method, as well as evolution equations for the second‐order orientation tensor via the fourth‐order Runge‐Kutta method. The effects of stresses due to the in‐plane velocity gradient on pressure, velocity, and fiber orientation fields were investigated in the center‐gated radial diverging flow in the cases of both an isothermal Newtonian fluid matrix and a nonisothermal polymeric matrix. In particular, the in‐plane velocity gradient effect on the fiber orientation was found to be significant near the gate, and more notably for the case of a nonisothermal polymer matrix.

On velocity-conditioned scalar mixing in homogeneous turbulence

Scalar mixing models are required to model turbulent molecular mixing in full probability density function (pdf) simulations of turbulent reacting flows. Despite the existence of direct numerical simulation (DNS) data suggesting the contrary, most scalar mixing models assume that molecular mixing is independent of the instantaneous velocity, i.e., 〈D∇2φ|V,ψ〉=〈D∇2φ|ψ〉. Since in a joint velocity, composition pdf calculation the velocity is known, this assumption is unnecessary and leads to a lack of local isotropy in the scalar field. Moreover, since velocity conditioning offers a numerically tractable approach for including the effects of local anisotropy and mean velocity gradients on scalar mixing, it should be of considerable interest for the numerical simulation of scalar mixing in inhomogeneous turbulent flows. An efficient numerical implementation of velocity-conditioned scalar mixing for full pdf simulations is proposed and verified against DNS data for homogeneous turbulence (isotropic and shear flow) with a uniform mean scalar gradient. A second-moment closure relating the velocity-conditioned scalar dissipation to the scalar fluxes and Reynolds stresses that is exact in the limit of a joint Gaussian pdf is also derived for use with moment closure models.

OH Masers as Diagnostics of Physical Conditions in Star-forming Regions. II. Effects of Large Velocity Gradients and Infrared Radiation

We have studied the pumping of OH masers in star-forming regions by introducing four pumping mechanisms one at a time. The four mechanisms are collisions, local line overlap, nonlocal line overlap, and external infrared radiation field. The first two have been investigated in a previous paper (Paper I). Here we study the effects of nonlocal line overlap and of an external infrared radiation field. As in Paper I, we have used the most accurate collision rates of OH with ortho- and para-H<SUB>2</SUB>, which became available recently, and we have taken the effects of line overlap into account as accurately as possible using a recently developed formalism. We have found that, despite the significant change in the collision rates, the effects of nonlocal line overlap and of an external infrared radiation field are necessary for the explanation of the main lines in the ground state of OH. The same is true for the explanation of the general qualitative behavior of H II/OH masers. Our calculations have revealed a number of diagnostics of the physical conditions in star-forming regions. These diagnostics are enumerated in our conclusions.

MATRIZANT APPROACH TO ACOUSTIC ANALYSIS OF PERFORATED MULTIPLE PIPE MUFFLERS CARRYING MEAN FLOW

A new method based on the matrizant theory is developed for acoustic analysis of perforated pipe muffler components. The analysis is presented in a generality encompassing any number of parallel pipes that communicate along a common length. The utility and potential of the method is demonstrated on a plug muffler and a reverse flow muffler. The analysis shows, for the first time, that the distributed parameter theory of plane wave propagation in a perforated pipe provides a more satisfactory setting than the segmentation method for the analysis of the effects of axially varying quantities such as the mean flow velocity. The effect of the mean flow velocity gradient on the transmission loss of the plug and reversed flow mufflers are computed and are shown to be insignificant for practical purposes.