Local Shear Flow Research Articles

Transport coefficients for an inelastic gas around uniform shear flow: Linear stability analysis

The inelastic Boltzmann equation for a granular gas is applied to spatially inhomogeneous states close to uniform shear flow. A normal solution is obtained via a Chapman-Enskog-like expansion around a local shear flow distribution. The heat and momentum fluxes are determined to first order in the deviations of the hydrodynamic field gradients from their values in the reference state. The corresponding transport coefficients are determined from a set of coupled linear integral equations which are approximately solved by using a kinetic model of the Boltzmann equation. The main new ingredient in this expansion is that the reference state f(0) (zeroth-order approximation) retains all the hydrodynamic orders in the shear rate. In addition, since the collisional cooling cannot be compensated locally for viscous heating, the distribution f(0) depends on time through its dependence on temperature. This means that in general, for a given degree of inelasticity, the complete nonlinear dependence of the transport coefficients on the shear rate requires analysis of the unsteady hydrodynamic behavior. To simplify the analysis, the steady-state conditions have been considered here in order to perform a linear stability analysis of the hydrodynamic equations with respect to the uniform shear flow state. Conditions for instabilities at long wavelengths are identified and discussed.

Indentation Creep Behavior in Ce-Based Bulk Metallic Glasses at Room Temperature

The room temperature creep behaviors of Ce-based bulk metallic glasses were examined by the use of nanoindentation. The creep rate and creep rate sensitivity of Ce-based BMGs were derived from indentation creep curves. The low creep rate sensitivity of Ce-based BMGs indicates that the room temperature creep is dominated by localized shear flow. The experimental creep curves can be described by a generalized Kelvin model. Furthermore, the creep retardation spectrum is calculated for the Ce-based metallic glasses. The results showed that creep retardation spectrum consists of two relatively separated peaks with the well defined characteristic relaxation times.

Shear band formation in granular media as a variational problem.

Strain in sheared dense granular material is often localized in a narrow region called the shear band. Recent experiments in a modified Couette cell provided localized shear flow in the bulk away from the confining walls. The nontrivial shape of the shear band was measured as the function of the cell geometry. First, we present a geometric argument for narrow shear bands that connects the function of their surface position with the shape in the bulk. Assuming a simple dissipation mechanism, we show that the principle of minimum dissipation of energy provides a good description of the shape function. Furthermore, we discuss the possibility and behavior of shear bands that are detached from the free surface and are entirely covered in the bulk.

From Fluid Flows to Crystallization: New Results from Complex Plasmas

The diversity of complex plasma systems is discussed and the various systems are quantified in terms of the “state variables” Γ and κ (see text). It is shown that complex plasmas may exist as crystals, one-phase fluids, two-phase fluids and granular media. The kinetic details and origins of localized shear flows during the liquid–solid phase transition are investigated, as well as plane and curved shears—both on the basis of measurements and simulations. The origins of kinetic instabilities are then briefly discussed.

Study of mechanical deformation in bulk metallic glass through instrumented indentation

Instrumented sharp indentation experiments at the nano- and micro-length scales were carried out in an attempt to quantify the deformation characteristics of Vitreloy 1™ bulk metallic glass. The experiments were accompanied by detailed three-dimensional finite element simulations of instrumented indentation to formulate an overall constitutive response. By matching the experimentally observed continuous indentation results with the finite element predictions, a general Mohr–Coulomb type constitutive description was extracted to capture the dependence of multiaxial deformation on both shear stresses and normal stresses. This constitutive response is able to provide accurate predictions of the evolution of shear bands seen in uniaxial compression tests. Constrained deformation of the material around the indenter results in incomplete circular patterns of shear bands whose location, shape and size are also captured well by the numerical simulations. The analysis is also able to predict the extent of material pile-up observed around the indenter. The surface deformation features are also consistent with mechanisms such as localized shear flow, serrated yielding and adiabatic heating, which are observed during macroscopic mechanical tests.

Shear flow induced interface instability

A system of two stratified layers at a free surface, consisting of distilled water above a layer of salty water separated by an interface, is studied under laboratory conditions involving uniform temperature heating from below. Shadowgraph and particle images have been used with temperature and salt concentration measurements to investigate the interface instability induced by convection when it is developing in the upper and lower layer. It is found that the interface is governed by local shear flow that induces a Kelvin–Helmholtz instability. Moreover, the entrainment interface is subject to a combination of two closely related effects: (1) double diffusion and convective motion and (2) double diffusion and Kelvin–Helmholtz instability.

Kinematic dynamo action in large magnetic Reynolds number flows driven by shear and convection

A numerical investigation is presented of kinematic dynamo action in a dynamically driven fluid flow. The model isolates basic dynamo processes relevant to field generation in the Solar tachocline. The horizontal plane layer geometry adopted is chosen as the local representation of a differentially rotating spherical fluid shell at co-latitude ϑ; the unit vectors [xcirc ], ŷ and zˆ point east, north and vertically upwards respectively. Relative to axes moving easterly with the local bulk motion of the fluid the rotation vector Ω lies in the (y, z)-plane inclined at an angle ϑ to the z-axis, while the base of the layer moves with constant velocity in the x-direction. An Ekman layer is formed on the lower boundary characterized by a strong localized spiralling shear flow. This basic state is destabilized by a convective instability through uniform heating at the base of the layer, or by a purely hydrodynamic instability of the Ekman layer shear flow. The onset of instability is characterized by a horizontal wave vector inclined at some angle ∈ to the x-axis. Such motion is two-dimensional, dependent only on two spatial coordinates together with time. It is supposed that this two-dimensionality persists into the various fully nonlinear regimes in which we study large magnetic Reynolds number kinematic dynamo action.When the Ekman layer flow is destabilized hydrodynamically, the fluid flow that results is steady in an appropriately chosen moving frame, and takes the form of a row of cat's eyes. Kinematic magnetic field growth is characterized by modes of two types. One is akin to the Ponomarenko dynamo mechanism and located close to some closed stream surface; the other appears to be associated with stagnation points and heteroclinic separatrices.When the Ekman layer flow is destabilized thermally, the well-developed convective instability far from onset is characterized by a flow that is intrinsically time-dependent in the sense that it is unsteady in any moving frame. The magnetic field is concentrated in magnetic sheets situated around the convective cells in regions where chaotic particle paths are likely to exist; evidence for fast dynamo action is obtained. The presence of the Ekman layer close to the bottom boundary breaks the up–down symmetry of the layer and localizes the magnetic field near the lower boundary.

Nonlinear α2Ω-dynamo waves in stellar shells

Nonlinear α2Ω-dynamo waves are considered in a thin turbulent, differentially rotating convective stellar shell. Nonlinearity arises from α-quenching, while an asymptotic solution is based on the small aspect ratio of the shell. Wave modulation is linked to a latitudinal-dependent local α-effect and zonal shear flow magnetic Reynolds numbers Rαf(θ) and R Ω g(θ) respectively; here θ is the latitude. The study is a direct extension of that of Meunier et al. (1997) for αΩ-dynamo waves which corresponds to finite dynamo number R α R Ω in the limit R α → 0. The essential picture developed is that of a modulated dynamo wave whose amplitude varies spatially with θ. The linear solution is controlled by the properties of the double turning point θc of the ordinary differential equation for the mode amplitude. Significantly, though θc is real and is located at the local dynamo number maximum in the αΩ-dynamo limit R α → 0, it migrates into the complex θ-plane once R α ≠ 0. Linear and weakly nonlinear solutions are found over a limited range of R α and their qualitative properties are found to be largely similar to those for the αΩ-dynamo limit. One significant astrophysical difference is the fact that the frequency generally decreases with increasing R α. Thus α2Ω-stellar dynamos may occur with αΩ-dynamo wave characteristics but exhibit significantly longer cycle times increased by a factor roughly two or more. Finite amplitude dynamo waves, like those when R α→0, are modulated by an envelope which evaporates smoothly at some low latitude but is terminated abruptly by a front at a high latitude θF. Significantly, for given non-zero R α, these frontal solutions are subcritical (a property linked to the complex-value taken by θc). For sufficiently large Rα, however, new low frequency modes emerge that are more closely related to steady α2-dynamos localised near the pole θ = π/2. In these circumstances, up to four distinct finite amplitude states are identified; they may be loosely characterised as αΩ-high frequency, α2Ω-medium frequency, α2-low frequency and α2-steady modes. In view of the possible mode competition, we comment on the likely realised physical state.

Stability Analysis for Noise-Source Modeling of a Part-Span Flap

The relevant and important flow features of the local mean flowfleld In the vicinity of a flap side edge are extracted from an extensive computational and experimental database. Based on the local flow features, possible mechanisms for flow-induced noise sources that are responsible for sound generation at flap side edges are conjectured. Relatively simple flow models for the dominant noise sources are proposed and developed. The models are based on the most amplified instability modes of the local shear flow. Stability analysis of the modeled flowfleld provides an estimate of the frequencies associated with the local flow unsteadiness. Computed frequencies show the proper trend and are in good agreement with NASA Langley Research Center's acoustic microphone array and unsteady surface-pressure measurements.

Melt memory effects in polymer crystallization

AbstractPolymer melts submitted to local shear flow fields, as well as isotropic solid samples, were treated in the melt at different temperatures and for various duration of time. The effects of thermal history on kinetics and morphological characteristics of subsequent isothermal crystallization was investigated. Results suggest that subcritical pseudocrystalline aggregates, behaving as predetermined athermal nuclei when temperature is brought below melting point, can survive in the melt for long time. Their concentration decays exponentially with a single characteristic relaxation time. For all investigated polymers, temperature dependence of relaxation times can be fitted by an Arrhenius law with high apparent activation energy. Experiments performed with narrow molecular weight fractions of poly(ethylene oxide) suggest that relaxation times relevant to destruction of oriented nucleation precursors are proportional to M1,5.

Brillouin scattering in the isotropic phase of liquid crystals

An optical beating technique was applied to the study of thermal fluctuations in isotropic phases of nematogens (those liquids changing into nematic liquid crystals in cooling) by Brillouin scattering. Wide-range and high-resolution spectroscopy was made. With this method, the power spectra of the light scattered in 6 CB (p-n-hexyl p′-Cyanobiphenil) was observed over the temperature range from 28.8 to 65.0 °C. Not only the fluctuation of density, which causes the Brillouin triplet, but also the fluctuation of orientation, a kind of the local order, appears in these liquids. The light-scattering spectrum reflects the reorientational relaxation. The model suggested by de Gennes theoretically predicts the spectra of both polarized (VV) and depolarized (VH) scattering. By fitting these equations to the experimental results, characteristic values were obtained which describe the dynamics of nematogens: Γ, the relaxation frequency of the orientational fluctuation, and C, the coupling constant between the orientational fluctuation and the local shear flow. The physical meanings of these parameters are discussed in the framework of critical phenomena.

Do physiological concentrations of IgG induce a direct aggregation of red blood cells: comparison with fibrinogen

Under physiological conditions, red blood cells (RBCs) form aggregates that allow blood flow in all the circulatory system. RBC aggregation is the result of local flow shear stress, erythrocyte properties and macromolecular interactions between adjacent cells. Plasma proteins like fibrinogen or IgG are considered to promote RBC aggregation by a mechanism that remains to be explained. In the present study, we have examined the precise role of IgG on RBC fast-phase aggregation, in comparison with that of fibrinogen. Under our experimental conditions, we observed no fast-phase aggregating effect for IgG, at either physiological or supraphysiological concentrations, while fibrinogen induces strong aggregation of RBC. We also suspect the other plasma proteins to play a role in the RBC aggregating process.

Magnetic viscosity by localized shear flow instability in magnetized accretion disks

view Abstract Citations (165) References (62) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Magnetic Viscosity by Localized Shear Flow Instability in Magnetized Accretion Disks Matsumoto, R. ; Tajima, T. Abstract Differentially rotating disks are subject to the axisymmetric instability for perfectly conducting plasma in the presence of poloidal magnetic fields (Balbus & Hawley 1991). For nonaxisymmetric perturbations, we find localized unstable eigenmodes whose eigenfunction is confined between two Alfven singularities at omegaD = +/- omegaA, where omegaD is the Doppler-shifted wave frequency and omegaA = kparallel nuA is the Alfven frequency. The radial width of the unstable eigenfunction is Delta x is approximately omegaA/(AKy), where A is Oort's constant and ky is the azimuthal wavenumber. The growth rate of the fundamental mode is larger for smaller values of ky/kz. The maximum growth rate when ky/kz is approximately 0.1 is approximately 0.2 Omega for the Keplerian disk with local angular velocity Omega. It is found that the purely growing mode disappears when ky/kz is greater than 0.12. In a perfectly conducting disk, the instability grows even when the seed magnetic field is infitesimal. Inclusion of the resistivity, however, leads to the appearance of an instability threshold. When the resistivity eta depends on the instability-induced turbulent magnetic fields delta B as eta (mean value of delta B squared), the marginal stability condition self consistently determines the alpha-parameter of the angular momentum transport due to magnetic stress. For fully ionized disks, the magnetic viscosity parameter alphaB is between 0.001 and 1. Our three-dimensional MHD simulation confirms these unstable eigenmodes. It also shows that the alpha-parameter observed in simulation is between 0.01 and 1, in agreement with theory. The observationally required smaller alpha in the quiescent phase of accretion disks in dwarf novae may be explained by the decreased inoization due to the temperature drop. Publication: The Astrophysical Journal Pub Date: June 1995 DOI: 10.1086/175739 Bibcode: 1995ApJ...445..767M Keywords: Accretion Disks; Flow Stability; Magnetization; Magnetohydrodynamic Flow; Shear Flow; Three Dimensional Models; Viscosity; Angular Velocity; Eigenvectors; Magnetic Field Configurations; Poloidal Flux; Stress Tensors; Toroidal Plasmas; Astrophysics; ACCRETION; ACCRETION DISKS; INSTABILITIES; MAGNETOHYDRODYNAMICS: MHD; PLASMAS full text sources ADS |

Plastic deformation in glassy polymers by atomistic and mesoscopic simulations

We present here a short summary of two rather different, but complementary, simulations of plastic deformation in a flexible chain glassy polymer. In the first, atomistic simulation, a molecular structure model is used in which well‐established force fields between atoms and atom groups on a typical chain polymer are introduced to account for the most relevant molecular degrees of freedom that govern both structural and shear relaxations. The principal result of this simulation which has been described in great detail elsewhere [Mott et al., Philos. Mag. 67, 931–978 (1993a)] is that shear relaxations are in the form of abrupt shear transformations occurring in volume elements of ∼10 nm size and resulting in transformation shear strains of ∼2%. This simulation has established that the chemically specific conformational constraints are internal to the volume elements and that the interaction of the elements with their surroundings is elastic, suggesting that the phenomenon can be meaningfully simulated by a two‐dimensional mesoscopic model. In the second part of the present communication we summarize the most important findings of the extensive two‐dimensional mesoscopic simulation which we [Bulatov and Argon, Model. Sim. Mater. Sci. Eng. 2, 167–184 (1994a); 185–202 (1994b); 203–222 (1994c)] have performed. These findings include the important effects of disorder related misfit stresses in governing both quasihomogeneous flow at elevated temperatures and localized shear flow at lower temperatures. In addition, we demonstrate that these misfit stresses are the key ingredient that governs the initial deformation transients, as well as the well‐known distributed kinetics of the deformation process in amorphous media in general.

Workability of aluminium-based metal-matrix composites in cold compression

Cold compression tests have been carried out on aluminium-based metal-matrix composite (MMC) materials. Metallographic examinations have revealed very large local plastic flow in the matrix material subjected to compressive loading. It is found that localized shear flow of the matrix may control the failure of the composite under compression. A new processing technique has been developed and the cold workability of the MMCs has been greatly improved. The new processing technique is of significance for industrial application in that it shows the possibility of an alternative route for the forming of metal-matrix composites: hot forming to near net-shape, proper heat treatment and precision cold forging to final dimensions.

Effect of an eccentric severe stenosis on fibrin(ogen) deposition on severely damaged vessel wall in arterial thrombosis. Relative contribution of fibrin(ogen) and platelets.

Coronary thrombosis is a dynamic process dependent on the pathological substrate, the local shear forces, and blood factors. We investigated the effect of a severe (80%) eccentric stenosis on fibrin(ogen) interaction with a deeply damaged vessel wall, its relation to platelet deposition in thrombus formation, and the influence of time on thrombus growth. Porcine 125I-fibrinogen and autologous 111In-platelets were injected into pigs instrumented for extracorporeal circulation and treated with low-dose heparin (aPTT ratio < 1.5) that has been previously shown and herein confirmed not to affect platelet and/or fibrin(ogen) attachment. Tunica media, as a model of severely injured vessel wall, was mounted in a tubular perfusion chamber containing an eccentric axisymmetric sinusoidal stenosis obstructing the lumen and exposed for 1, 5, and 10 minutes to perfusing blood. A shear rate of 424 s-1 at the laminar, parallel parabolic local flow perfused segments one to two orders of magnitude greater at the apex of the stenosis. Fibrin(ogen) deposition, its axial distribution with respect to the apex, and its relation to platelet deposition were determined by an ex vivo analysis of the test substrates. Fibrin(ogen) and platelet deposition were both significantly higher at the apex of the stenosis than at either the prestenotic or poststenotic area at all the studied perfusion times (P < .02). However, fibrin(ogen) deposition demonstrated a significantly smaller degree of increase from the prestenotic area to the apex as well as a smaller degree of decrease from the latter to the poststenotic region, compared with platelet deposition (P < .05). Although both fibrin(ogen) and platelet deposition increased over time, the ratio of fibrin(ogen) to platelets showed a progressive decrease that became significant from 5 to 10 minutes (P < .03) at either low or high shear rate. The rate of platelet deposition was relatively constant; however, fibrin(ogen) deposition progressively decreased, especially at the apex. On severely damaged vessel wall, fibrin(ogen) and platelet deposition is maximal at the apex of the stenosis where shear rate is extremely high and parallel streamlines are deformed. Nevertheless, fibrin(ogen) deposition is significantly less dependent on high shear rate than is platelet deposition, and the pattern is not influenced by time. Finally, fibrin(ogen) deposition appears to be predominant in the thrombus layers adjacent to a severely damaged vessel wall regardless of the local shear stress levels and flow conditions.

Platelet Deposition Induced by Severely Damaged Vessel Wall Is Inhibited by a Boroarginine Synthetic Peptide with Antithrombin Activity

Thrombin plays a key role in platelet activation and thrombosis. Specific inhibition of thrombin appears to be one of the best approaches to prevent thrombus formation. We have studied the effects of a synthetic alpha-aminoboronic acid derivative- [Ac, (D) Phe-Pro-Boro-Arg-Hydrochloric acid] - on platelet deposition on severely damaged arterial wall. Platelet deposition was evaluated under well characterized rheological conditions in an original perfusion chamber and detected by autologous 111In-labeled platelets. The study was performed "in vivo" in a porcine model of arterial thrombosis triggered by severely damaged vessel wall at blood flow conditions mimicking mild stenosis (1690 s-1) and patent (212 s-1) vessels. In addition, ex-vivo platelet aggregation activity was evaluated by whole blood impedance aggregometry using collagen, ADP and thrombin as agonists. The synthetic alpha-aminoboronic peptide was intravenously administered as a bolus followed by continuous infusion. Ex vivo thrombin-induced whole blood platelet aggregation was totally abolished, while ADP- and Collagen-induced whole blood platelet aggregation was not modified. The effects of the synthetic antithrombin on platelet deposition were evaluated in native blood (non-anticoagulated) conditions and in combination with heparin. Under both experimental conditions, the synthetic peptide significantly inhibited platelet deposition at local flow conditions of both high (1690 s-1) and low (212s-1) shear rates. Our results suggest that specific inhibition of locally generated thrombin might be a good strategy to prevent platelet dependent arterial thrombus formation independently of the local flow shear rate of the area at risk.

The effects of Kelvin-Helmholtz instability on resonance absorption layers in coronal loops

One of the long-standing uncertainties in the wave-resonance theory of coronal heating is the stability of the resonance layer. The wave motions in the resonance layer produce highly localized shear flows which vary sinusoidally in time with the resonance period. This configuration is potentially susceptible to the Kelvin-Helmholtz instability (KHI), which can enhance small-scale structure and turbulent broadening of shear layers on relatively rapid ideal timescales. We have investigated numerically the response of a characteristic velocity profile, derived from resonance absorption models, to finite fluid perturbations comparable to photospheric fluctuations. We find that the KHI primarily should affect long (approximately greater than 6 x 10(exp 4) km) loops where higher velocity flows (M approximately greater than 0.2) exist in resonance layers of order 100 km wide. There, the Kelvin-Helmholtz growth time is comparable to or less than the resonance quarter-period, and the potentially stabilizing magnetic effects are not felt until the instability is well past the linear growth stage. Not only is the resonance layer broadened by the KHI, but also the convective energy transport out of the resonance layer is increased, thus adding to the efficiency of the wave-resonance heating process. In shorter loops, e.g., those in bright points and compact flares, the stabilization due to the magnetic field and the high resonance frequency inhibit the growth of the Kelvin-Helmholtz instability beyond a minimal level.

On a local predictor of shear instability and the initiation of local turbulence

The stability of a local laminar shear flow and its transition into turbulent flow is considered as a local phenomenon. This transition may remain local, in which case the flow field is partially laminar and partially turbulent, or it may spread and make the whole field turbulent. One of the applications of this analysis is the prediction of local heat-convection rates, which are enhanced by local turbulence. Another application is in heart-lung blood pumps, where excessive shear rates are detrimental to red blood cells.

The rheology and mold flow of polyester sheet molding compound

AbstractFlow properties of sheet molding compounds (SMC) were measured by several rhemeters. Shear viscosities of SMC paste can be fitted by the Carreau viscosity equation and reduced to a single master curve by plotting the reduced shear viscosity (η/η0) against reduced shear rate (\documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} η0/T). The deformation fo glass filled compounds under extensional forces. When the shear forced was applied, the compound defomed like a deck of cards being slid. It behaved essentially like a laminate of many layers. On the other hand, SMC compounds elongated much less before sample failure occurred under the extensional flow. The extensional viscosity was much higher than the shear viscosity. This phenomenon was explained by a solid composite theory and theories developed from local shear flows between adjacent fibers. The flow of SMC compounds during molding showed that the surface layers flowed further than the inner layers when the mold surfaces were hot. This casued inner plies to show through at the surface and resulting in some wavy glass orientation. This phenomenon was explained equalitatively by a heat penetration model.