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1.
D. Y. Hsieh 《Acta Mechanica Sinica》1994,10(3):193-202
The elemental mechanisms for many hydrodynamic instabilities can be identified as negative damping, negative diffusion or
ellipticity. Identifications of some well-known hydrodynamic instabilities are made. Model equations in connection with the
instability associated with ellipticity should be studied more extensively. 相似文献
2.
On tensile instabilities and ellipticity loss in fiber-reinforced incompressible non-linearly elastic solids 总被引:1,自引:0,他引:1
Loss of ellipticity and associated failure in fiber-reinforced non-linearly elastic solids is examined for uniaxial plane deformations. We consider separately fiber reinforcement that either endows the material with additional stiffness only in the fiber direction or introduces additional stiffness under shear deformations. In the first case it is shown that loss of ellipticity under tensile loading in the fiber direction corresponds to a turning point of the nominal stress and requires concavity of the Cauchy stress–stretch curve. For the second example loss of ellipticity occurs after the nominal stress maximum and prior to a turning point of the Cauchy stress. 相似文献
3.
Instability of thermocapillary convection in liquid bridges of low Prandtl number fluids is investigated by direct three-dimensional and time-dependent simulation of the problem. The field equations are numerically solved explicit in time and with finite difference methods in a staggered cylindrical grid. The numerical results are analyzed and interpreted in the general context of the bifurcation's theory. According to recent stability analyses the computations show that for semiconductor melts the first bifurcation is characterized by the loss of spatial symmetry rather than by the onset of oscillatory flow. When the basic axisymmetric flow field becomes unstable, after a short transient, a three-dimensional supercritical steady state is obtained. It is shown that the flow field organization, depending on the critical wave number, is related to the geometrical aspect ratio of the liquid bridge and that lower is the aspect ratio, higher is the critical wave number and more complex the thermofluid-dynamic field structure. 相似文献
4.
Summary In this work, equations of the kinetics and kinematics are developed for heterogeneous materials containing inelastic discontinuities
with moving boundaries. From the derived free energy and the power of external forces one obtains the driving force acting
on the moving boundary. Introducing the interface operators and some hypothesis on inelastic fields, one gets the driving
force for the formation of an ellipsoidal domain. The theoretical model is illustrated by the derivation of nucleation and
growth conditions of a martensitic plate inside an inhomogeneous plastic strain field. The obtained results are combined with
a study of the kinetics and kinematics to derive the constitutive equation of an austenitic single crystal, from which the
overall behavior of polycrystalline TRIP steels is deduced using the self-consistent scale-transition method. Comparison with
experimental data shows a good agreement.
Received 7 May 1999; accepted for publication 14 June 1999 相似文献
5.
Bacterial flagellar filaments can undergo a polymorphic phase transition in both vitro and vivo environments. Each bacterial flagellar filament has 12 different helical forms which are macroscopically represented by different pitch lengths and helix radii. For external mechanical force induced filament phase transitions, there is so far only one experiment performed by Hotani in 1982, who showed a very beautiful cyclic phase transition phenomenon in his experiment on isolated flagellar filaments. In the present paper, we give a detailed mechanical analysis on Hotani's experiments. Through theoretical computations, we obtained a phase transition rule based on the phase transition mechanism. The theoretical analysis provides a foundation facilitating the establishment of phase transition theory for bacterial flagellar filaments. 相似文献
6.
J. J. Healey 《Applied Scientific Research》1993,51(1-2):49-53
Experiments have been performed on a laminar flat plate boundary layer undergoing transition to turbulence. Reproducible disturbances were introduced via a loudspeaker embedded at some upstream location and their evolution over the plate measured using hot-wire anemometry. A new technique has been used to estimate the number of nonlinearly independent modes in the reconstructed phase portraits. Nonlinear maps were then fitted that explicitly model the spatial evolution of disturbances. These maps are consistent with classical linear stability theory for small disturbances, and give rise to Smale horse-shoe like behaviour for larger amplitude distrubances. Thus a mechanism for generating chaos has been uncovered. 相似文献
7.
Flow-induced oscillations of rigid cylinders exposed to fully developed turbulent flow can be described by a fourth order autonomous system. Among the pertinent constants, the mass ratio is the control parameter governing the transition from limit cycle oscillations to chaotic vibrations. Particular attention is paid to the stability of the limit cycles: it has been found that they lose their stability at the point of appearance of quasi-periodic motion. The documentation of this transition is performed in terms of Lyapunov exponents, phase plots, Fourier spectra, bifurcation diagrams, and Poincaré maps. As opposed to the calculation of the Lyapunov exponents where remarkable numerical difficulties were encountered, the investigation of the remaining quantities shows clearly the passage of cylinder motions from limit cycle oscillations to more and more irregular vibrations, leading finally to chaos. 相似文献
8.
The method for controlling chaotic transition system was investigated using sampled- data . The output of chaotic transition system was sampled at a given sampling rate , then the sampled output was used by a feedbacks subsystem to construct a control signal for controlling chaotic transition system to the origin . Numerical simulations are presented to show the effectiveness and feasibility of the developed controller. 相似文献
9.
10.
The effect of internal surface modification on flow instabilities in forced convection boiling in a horizontal tube 总被引:1,自引:0,他引:1
This paper presents the experimental result of a study on the effects of heat transfer enhancement on two-phase flow instabilities in a horizontal in-tube flow boiling system. Five different heat transfer surface configurations and five different inlet temperatures are used to observe the effect of heat transfer enhancement and inlet subcooling. All experiments are carried out at constant heat input, system pressure and exit restriction. Dynamic instabilities, namely pressure-drop type, density-wave type and thermal oscillations are found to occur for all the investigated temperatures and enhancement configurations, and the boundaries for the appearance of these oscillations are found. The effect of the enhancement configurations on the characteristics of the boiling flow dynamic instabilities is studied in detail. The comparison between the bare tube and the enhanced tube configurations are made on the basis of boiling flow instabilities. Differences among the enhanced configurations are also determined to observe which of them is the most stable and unstable one. The amplitudes and periods of pressure-drop type oscillations and density-wave type oscillations for tubes with enhanced surfaces are found to be higher than those of the bare tube. The bare tube is found to be the most stable configuration, while tube with internal springs having bigger pitch is found to be the most unstable one among the tested tubes. It is found that system stability increases with decreasing equivalent diameter for the same type heater tube configurations; however, on the basis of effective diameter there is no single result such as stability increase/decrease with increasing/decreasing effective diameter. 相似文献
11.
The incremental constitutive relation and governing equations with combined stresses for phase transition wave propagation in a thin-walled tube are established based on the phase transition criterion considering both the hydrostatic pressure and the deviatoric stress. It is found that the centers of the initial and subsequent phase transition ellipses are shifted along the σ-axis in the στ-plane due to the tension-compression asymmetry induced by the hydrostatic pressure. The wave solution offers the "fast" and "slow" phase transition waves under combined longitudinal and torsional stresses in the phase transition region. The results show some new stress paths and wave structures in a thin-walled tube with phase transition, differing from those of conventional elastic-plastic materials. 相似文献
12.
Determining liquid–vapor phase equilibrium is often required in multiphase flow computations. Existing equilibrium solvers are either accurate but computationally expensive or cheap but inaccurate. The present paper aims at building a fast and accurate specific phase equilibrium solver, specifically devoted to unsteady multiphase flow computations. Moreover, the solver is efficient at phase diagram bounds, where non‐equilibrium pure liquid and pure gas are present. It is systematically validated against solutions based on an accurate (but expensive) solver. Its capability to deal with cavitating, evaporating, and condensing two‐phase flows is highlighted on severe test problems both 1D and 2D. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
13.
Dr. R. G. Larson 《Rheologica Acta》1992,31(6):497-520
In polymer solutions or blends, flow can strongly influence the degree of mixing of the components. In a shearing flow, droplets in a dispersion can be broken down to sizes comparable to the dimensions of the polymer molecules themselves, thereby inducing molecular-scale mixing. Demixing can also occur when the two components of the mixture differ greatly in viscoelastic properties. Shear or extensional flow can induce polymer migration in nonhomogeneous flows or in flows with curved streamlines, and can render turbid solutions or blends that are otherwise transparent. Flow can also induce polymer gelation, and can induce ordering transitions in liquid crystals or block copolymers. Here, we review these phenomena, discuss proposed mechanisms, and assess the degree to which recent theories can account for the observations. Because the phenomena are complex, multiple experimental probes and theoretical methods are required to study them. Successful theories must incorporate polymer/polymer or polymer/solvent thermodynamics, critical phenomena, and phase transitions, as well as polymer theology and the kinetics of diffusion or crystallization. The experimental techniques used to study these phenomena are equally wide ranging, and include turbidity measurements, light, x-ray, and neutron scattering, fluorescence quenching, microscopy, and theology. 相似文献
14.
J. Langenberg M. Heise G. Pfister J. Abshagen 《Theoretical and Computational Fluid Dynamics》2004,18(2-4):97-103
We present results of an experimental study on the stability of Taylor–Couette flow in case of counter-rotating cylinders and an imposed axial through flow. We are able to confirm results form recent numerical investigations done by Pinter et al. [24] by measuring the absolute and convective stability boundaries of both propagating Taylor vortices (PTV) and spiral vortices (SPI). Thus our work shows that these theoretical concepts from hydrodynamic stability in open flows apply to experimental counter-rotating Taylor–Couette systems with an imposed axial through flow. PACS 47.20.-k, 05.45.-a, 47.15.fe 相似文献
15.
The paper deals with a mathematical problem describing an exothermic chemical reaction in a diffusing substance possibly undergoing
a change of phase. Global well-posedness in the classical sense is proved for the corresponding system of PDEs. Moreover,
cases in which the phases are separated by sharp interphases or by transition regions are discussed. The limit case of negligible
diffusion is also considered.
Sommario Si studia il problema matematico che descrive una reazione chimica esotermica in una sostanza che diffonde e puo' subire cambiamenti di fase. Si dimostra esistenza globale in senso classico del relativo sistema di equazioni alle derivate parziali e si discute la possibilita' che le fasi siano separate da una regione di transizione e non da una netta superficie di interfase. Il caso limite di assenza di diffusione e' anche brevemente esaminato.相似文献
16.
A new coupled map lattic(CML)model is given by using some stability analysis for the related difference equations.Numerical results show that the new model is.an effective one of studying spatiotemporal chaos,especially for strongly coupled systems. 相似文献
17.
18.
Bacterial flagellar filament can undergo a stress-induced polymorphic phase transition in both vitro and vivo environments. The filament has 12 different helical forms (phases) characterized by different pitch lengths and helix radii. When subjected to the frictional force of flowing fluid, the filament changes between a left-handed normal phase and a right-handed semi-coiled phase via phase nucleation and growth. This paper develops non-local finite element method (FEM) to simulate the phase transition under a displacement-controlled loading condition (controlled helix-twist). The FEM formulation is based on the Ginzburg-Landau theory using a one-dimensional non-convex and non-local continuum model. To describe the processes of the phase nucleation and growth, viscosity-type kinetics is also used. The non-local FEM simulation captures the main features of the phase transition: two-phase coexistence with an interface of finite thickness, phase nucleation and phase growth with interface propagation. The non-local FEM model provides a tool to study the effects of the interfacial energy/thickness and loading conditions on the phase transition. 相似文献
19.
20.
Relaxation of the Navier–Stokes–Korteweg equations for compressible two‐phase flow with phase transition 下载免费PDF全文
The Navier–Stokes–Korteweg (NSK) system is a classical diffuse‐interface model for compressible two‐phase flow. However, the direct numerical simulation based on the NSK system is quite expensive and in some cases even not possible. We propose a lower‐order relaxation of the NSK system with hyperbolic first‐order part. This allows applying numerical methods for hyperbolic conservation laws and removing some of the difficulties of the original NSK system. To illustrate the new ansatz, we first present a local discontinuous Galerkin method in one and two spatial dimensions. It is shown that we can compute initial boundary value problems with realistic density ratios and perform stable computations for small interfacial widths. Second, we show that it is possible to construct a semi‐discrete finite‐volume scheme that satisfies a discrete entropy inequality. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献