共查询到20条相似文献,搜索用时 171 毫秒
1.
Hiroshi Watanabe Tomohiro Sato Motoyuki Hirose Kunihiro Osaki Ming-Long Yao 《Rheologica Acta》1999,38(2):100-107
Rheo-dielectric behavior was examined for 4−4′−n-octyl-cyanobiphenyl (8CB) having large dipoles parallel to its principal axis (in the direction of the C≡N bond). In the
quiescent state at all temperatures (T) examined, orientational fluctuation of the 8CB molecules was observed as dielectric dispersions at characteristic frequencies
ωc>106 s−1. In the isotropic state at high T, no detectable changes of the complex dielectric constant ɛ*(ω) were found under slow flow at shear rates ˙γ≫ωc. In the nematic state at intermediate T, the terminal relaxation intensity of ɛ*(ω) was decreased under such slow flow. In the smectic state at lower T, the flow effect became much less significant. These results were related to the flow-induced changes of the liquid crystalline
textures in the nematic and smectic states, and the differences of the rheo-dielectric behavior in these states are discussed
in relation to a difference of the symmetry of molecular arrangements in the nematic and smectic textures.
Received: 1 October 1998 Accepted: 13 January 1999 相似文献
2.
Leonid Brevdo 《Journal of Elasticity》1997,49(3):201-237
We solve the initial-boundary-value linear stability problem for small localised disturbances in a homogeneous elastic waveguide
formally by applying a combined Laplace – Fourier transform. An asymptotic evaluation of the solution, expressed as an inverse
Laplace – Fourier integral, is carried out by means of the mathematical formalism of absolute and convective instabilities.
Wave packets, triggered by perturbations localised in space and finite in time, as well as responses to sources localised
in space, with the time dependence satisfying e−iωt
+ O(e−ɛt
), for t → ∞, where Im ω0 = 0 and ω > 0 , that is, the signaling problem, are treated. For this purpose, we analyse the dispersion relation of the
problem analytically, and by solving numerically the eigenvalue stability problem. It is shown that due to double roots in
a wavenumber k of the dispersion relation function D(k, ω), for real frequencies ω, that satisfy a collision criterion, wave packets with an algebraic temporal decay and signaling
with an algebraic temporal growth, that is, temporal resonances, are present in a neutrally stable homogeneous waveguide.
Moreover, for any admissible combination of the physical parameters, a homogeneous waveguide possesses a countable set of
temporally resonant frequencies. Consequences of these results for modelling in seismology are discussed.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
3.
A closed-form model for the computation of temperature distribution in an infinitely extended isotropic body with a time-dependent
moving-heat sources is discussed. The temperature solutions are presented for the sources of the forms: (i) 01(t)=0
exp(−λt), (ii) 02(t) =0(t/t
*)exp(−λt), and 03(t)=0[1+a
cos(ωt)], where λ and ω are real parameters and t
* characterizes the limiting time. The reduced (or dimensionless) temperature solutions are presented in terms of the generalized
representation of an incomplete gamma function Γ(α,x;b) and its decomposition C
Γ and S
Γ. The solutions are presented for moving, -point, -line, and -plane heat sources. It is also demonstrated that the present
analysis covers the classical temperature solutions of a constant strength source under quasi-steady state situations.
Received on 13 June 1997 相似文献
4.
Nicolae Tarfulea 《应用数学和力学(英文版)》2000,21(3):283-290
IntroductionInthispaper,weconsidertheellipticsystem(1λ) -Δu=f(λ,x,u)-v (inΩ),-Δv=δu-γv(inΩ),u=v=0(onΩ),whereΩisasmoothboundeddomaininRN(N≥2)andλisarealparameter.Thesolutions(u,v)ofthissystemrepresentsteadystatesolutionsofreactiondiffusionsystemsderivedfromseveralap… 相似文献
5.
Fractalgeometryisapowerfultooltodescribecomplexphenomenon.Especiallyitisappropriatetoscalethenonuniformityandnonsequenceofporousmedia.Ifthemechanicsoffluidflowthroughporousmediaisstudiedbyusingfractal,thediscernibleandcognitiveabilityforporousmediaan… 相似文献
6.
Robert McKibbin 《Transport in Porous Media》1986,1(3):271-292
The theory describing the onset of convection in a homogeneous porous layer bounded above and below by isothermal surfaces
is extended to consider an upper boundary which is partly permeable. The general boundary condition p + λ ∂p/∂n = constant is applied at the top surface and the flow is investigated for various λ in the range 0 ⩽ λ < ∞. Estimates of the magnitude and horizontal distribution of the vertical mass and heat fluxes at the surface, the horizontally-averaged
heat flux (Nusselt number) and the fraction of the fluid which recirculates within the layer are found for slightly supercritical
conditions. Comparisons are made with the two limiting cases λ → ∞, where the surface is completely impermeable, and λ = 0, where the surface is at constant pressure. Also studied are the effects of anisotropy in permeability, ξ = K
H
/K
V
, and anisotropy is thermal conductivity, η = k
H
/k
V
, both parameters being ratios of horizontal to vertical quantities. Quantitative results are given for a wide variety of
the parameters λ, ξ and η. In the limit ξ/η → 0 there is no recirculation, all fluid being converted out of the top surface, while in the limit ξ/η → ∞ there is full recirculation. 相似文献
7.
The problem of the self-similar boundary flow of a “Darcy-Boussinesq fluid” on a vertical plate with temperature distribution
T
w(x) = T
∞+A·x
λ and lateral mass flux v
w(x) = a·x
(λ−1)/2, embedded in a saturated porous medium is revisited. For the parameter values λ = 1,−1/3 and −1/2 exact analytic solutions
are written down and the characteristics of the corresponding boundary layers are discussed as functions of the suction/ injection
parameter in detail. The results are compared with the numerical findings of previous authors.
Received on 8 March 1999 相似文献
8.
This paper presents the electromagnetic wave propagation characteristics in plasma and the attenuation coefficients of the microwave in terms of the parameters he, v, w, L, wb. The φ800 mm high temperature shock tube has been used to produce a uniform plasma. In order to get the attenuation of the electromagnetic wave through the plasma behind a shock wave, the microwave transmission has been used to measure the relative change of the wave power. The working frequency is f = (2-35)GHz (ω=2πf, wave length A =15cm-8mm). The electron density in the plasma is ne = (3×10^10-1×10^14) cm^-3. The collision frequency v = (1×10^8-6×10^10) Hz. The thickness of the plasma layer L = (2-80)cm. The electron circular frequency ωb=eBo/me, magnetic flux density B0 = (0-0.84)T. The experimental results show that when the plasma layer is thick (such as L/λ≥10), the correlation between the attenuation coefficients of the electromagnetic waves and the parameters ne,v,ω, L determined from the measurements are in good agreement with the theoretical predictions of electromagnetic wave propagations in the uniform infinite plasma. When the plasma layer is thin (such as when both L and A are of the same order), the theoretical results are only in a qualitative agreement with the experimental observations in the present parameter range, but the formula of the electromagnetic wave propagation theory in an uniform infinite plasma can not be used for quantitative computations of the correlation between the attenuation coefficients and the parameters ne,v,ω, L. In fact, if ω<ωp, v^2<<ω^2, the power attenuations K of the electromagnetic waves obtained from the measurements in the thin-layer plasma are much smaller than those of the theoretical predictions. On the other hand, if ω>ωp, v^2<<ω^2 (just v≈f), the measurements are much larger than the theoretical results. Also, we have measured the electromagnetic wave power attenuation value under the magnetic field and without a magnetic field. The result indicates that the value measured under the magnetic field shows a distinct improvement. 相似文献
9.
A linear stability analysis is used to study the conditions marking the onset of secondary flow in the form of longitudinal
vortices for plane Poiseuille flow of water in the thermal entrance region of a horizontal parallel-plate channel by a numerical
method. The water temperature range under consideration is 0∼30°C and the maximum density effect at 4°C is of primary interest.
The basic flow solution for temperature includes axial heat conduction effect and the entrance temperature is taken to be
uniform at far upstream location jackie=−∞ to allow for the upstream heat penetration through thermal entrance jackie=0. Numerical results for critical Rayleigh number are obtained for Peclet numbers 1, 10, 50 and thermal condition parameters
(λ
1, λ
2) in the range of −2.0≤λ
1≤−0.5 and −1.0≤λ
2≤1.4. The analysis is motivated by a desire to determine the free convection effect on freezing or thawing in channel flow
of water. 相似文献
10.
Hermano Frid 《Archive for Rational Mechanics and Analysis》2006,181(1):177-199
We prove the asymptotic stability of two-state nonplanar Riemann solutions for a class of multidimensional hyperbolic systems
of conservation laws when the initial data are perturbed and viscosity is added. The class considered here is those systems
whose flux functions in different directions share a common complete system of Riemann invariants, the level surfaces of which
are hyperplanes. In particular, we obtain the uniqueness of the self-similar L∞ entropy solution of the two-state nonplanar Riemann problem. The asymptotic stability to which the main result refers is
in the sense of the convergence as t→∞ in Lloc1 of the space of directions ξ = x/t. That is, the solution u(t, x) of the perturbed problem satisfies u(t, tξ)→R(ξ) as t→∞, in Lloc1(ℝn), where R(ξ) is the self-similar entropy solution of the corresponding two-state nonplanar Riemann problem. 相似文献
11.
Laminar mixed convection from a continuously moving vertical surface with suction or injection 总被引:1,自引:0,他引:1
The boundary layer flow over a uniformly moving vertical surface with suction or injection is studied when the buoyancy forces
assist or oppose the flow. Similarity solutions are obtained for the boundary layer equations subject to power law temperature
and velocity boundary conditions. The effect is of various governing parameters, such as Prandtl number Pr, temperature exponent
n, injection parameter d, and the mixed convection parameter λ=Gr/Re2, which determine the velocity and temperature distributions and the heat transfer coefficient, are studied. The heat transfer
coefficient increases as λ assisting the flow for all d at Pr=0.72 however, for n=−1 it decreases sharply with λ. On the other hand, increasing λ has no effect on heat transfer coefficient for Pr=10 at n=0, and 1 for almost all values of d studied. However, for n=−1 it has similar effect as for Pr=0.72. It is also found that Nusselt number increases as n increases for fixed λ and d.
Received on 26 March 1997 相似文献
12.
13.
The steady mixed convection boundary-layer flow over a vertical impermeable surface in a porous medium saturated with water
at 4°C (maximum density) when the surface heat flux varies as x
m
and the velocity outside the boundary layer varies as x
(1+2m)/2, where x measures the distance from the leading edge, is discussed. Assisting and opposing flows are considered with numerical solutions
of the governing equations being obtained for general values of the flow parameters. For opposing flows, there are dual solutions
when the mixed convection parameter λ is greater than some critical value λ
c
(dependent on the power-law index m). For assisting flows, solutions are possible for all values of λ. A lower bound on m is found, m > −1 being required for solutions. The nature of the critical point λ
c
is considered as well as various limiting forms; the forced convection limit (λ = 0), the free convection limit (λ → ∞) and
the limits as m → ∞ and as m → −1. 相似文献
14.
M. Banerjee 《Transport in Porous Media》2008,73(3):333-347
The field measurements and numerical results for intermittent flow regime in a sandy soil show that the time distributions
of the soil water flux q(z, t), and the soil water content θ(z, t)at various depths are periodic in nature, where t is time and z is the depth (i.e., at the surface z = 0 and at depths z = − 5, − 10, − 15 cm, etc). The period of q(z, t) and θ(z, t) variations are generally determined by the sum of the duration of pulse and the duration between the initiation of two consecutive
pulses of water at the soil surface. Fourier series models have been given for q(z, t) and θ(z, t) variations. The predicted Fourier results for these variations have been compared with the experimentally verified numerical
results—designated as observed values. The results show that the amplitudes of these variations were damped exponentially with depth, and the phase shift
increased linearly with depth. 相似文献
15.
C. -H. Chen 《Heat and Mass Transfer》2000,36(1):79-86
An numerical study of the flow and heat transfer characteristics associated with a heated, continuously stretching surface
being cooled by a mixed convection flow has been carried out. The relevant heat transfer mechanisms are of interest in a wide
variety of practical applications, such as hot rolling, continuous casting, extrusion, and drawing. The surface velocity of
the continuously stretching sheet was assumed to vary according to a power-law form, that is, u
w
(x)=Cx
p
. Two conditions of surface heating were considered, a variable wall temperature (VWT) in the form T
w
(x)−T
∞=Ax
n
and a variable surface heat flux (VHF) in the form q
w
(x)=Bx
m
. The governing differential equations are transformed by introducing proper nonsimilarity variables and solved numerically
using a procedure based on finite difference approximations. Results for the local Nusselt number and the local friction coefficient
are obtained for a wide range of governing parameters, such as the surface velocity parameter p, the wall temperature exponent n, the surface heat flux exponent m, the buoyancy force parameters (ξ for the VWT case and χ for the VHF case), and Prandtl number of the fluid. It is found
that the local Nusselt number is increased with increasing the velocity exponent parameter p for the VWT case, while the opposite trend is observed for the VHF case. The local friction coefficient is increased for
a decelerated stretching surface, while it is decreased for an accelerated stretching surface. Also, appreciable effects of
the buoyancy force on the local Nusselt number and the local friction coefficient are observed for both VWT and VHF cases,
as expected.
Received on 11 January 1999 相似文献
16.
The problem of non-Darcy natural convection adjacent to a vertical cylinder embedded in a thermally stratified porous medium
has been analyzed. Nonsimilarity solutions are obtained for the case that the ambient temperature increases linearly with
height of the cylinder. A generalized flow model was used in the present study to include the effects of the macroscopic viscous
term and the microscopic inertial force. Also, the thermal dispersion effect is considered in the energy equation. Thus, the
main aim of this work is to examine the effects of thermal stratification and non-Darcy flow phenomena on the free convection
flow and heat transfer characteristics. It was found that the present problem depends on six parameters, namely, the local
thermal stratification parameter ξ, the boundary effect parameter Bp, the modified Grashof number Gr*, wall temperature exponent m, the curvature parameter ω, and the modified Rayleigh number based on pore diameter Ra
d
. The impacts of these governing parameters on the local heat transfer parameter are discussed in great detail. Also, representative
velocity and temperature profiles are presented at selected values of the thermal stratification parameter. In general, the
local heat transfer parameter is increased with increasing the values of m, ω, and Ra
d
; while it is decreased with increasing the values of ξ, Bp, and Gr*.
Received on 19 May 1998 相似文献
17.
H. Watanabe Tomohiro Sato Motoyuki Hirose Kunihiro Osaki Ming-Long Yao 《Rheologica Acta》1998,37(6):519-527
Dielectric relaxation behavior was examined for 4-4′-n-pentyl-cyanobiphenyl (5CB) and 4-4′-n-heptyl-cyanobiphenyl (7CB) under flow. In quiescent states at all temperatures examined, both 5CB and 7CB exhibited dispersions
in their complex dielectric constant ε*(ω) at characteristic frequencies ω
c
above 106 rad s–1. This dispersion reflected orientational fluctuation of individual 5CB and 7CB molecules having large dipoles parallel to
their principal axis (in the direction of C≡N bond). In the isotropic state at high temperatures, these molecules exhibited no detectable changes of ε*(ω) under flow at
shear rates . In contrast, in the nematic state at lower temperatures the terminal relaxation intensity of ε*(ω) as well as the static
dielectric constant ε′(0) decreased under flow at . This rheo-dielectric change was discussed in relation to the flow effects on the nematic texture (director distribution)
and anisotropy in motion of individual molecules with respect to the director.
Received: 14 April 1998 Accepted: 29 July 1998 相似文献
18.
Hyperbolic heat conduction in the semi-infinite body with a time-dependent laser heat source 总被引:1,自引:0,他引:1
M. Lewandowska 《Heat and Mass Transfer》2001,37(4-5):333-342
The Cattaneo hyperbolic and classical parabolic models of heat conduction in the laser irradiated materials are compared.
Laser heating is modelled as an internal heat source, whose capacity is given by g(x,t)= I(t)(1−R)μexp(−μx). Analytical solution for the one-dimensional, semi-infinite body with the insulated boundary is obtained using Laplace transforms
and the discussion of solutions for different time characteristics of the heat source capacity (constant, instantaneous, exponential,
pulsed and periodic) is presented.
Received on 18 May 1999 相似文献
19.
Dual Porosity and Dual Permeability Modeling of Horizontal Well in Naturally Fractured Reservoir 总被引:1,自引:0,他引:1
Ren-Shi Nie Ying-Feng Meng Yong-Lu Jia Fu-Xiang Zhang Xiang-Tong Yang Xin-Nian Niu 《Transport in Porous Media》2012,92(1):213-235
This article is the first investigation on the dual permeability flow issue for horizontal well-production in a naturally
fractured dual-porosity reservoir. Based on the inter-porosity flow from matrix system to fracture system and treating the
media directly connected with horizontal wellbore as matrix and fracture systems, we established a model of horizontal well-production
and then solved the model using some modern mathematical methods, such as Laplace integral transformation, separation of variables,
eigenvalue, and eigenfunction. Later in the article, we obtained the standard log–log type curves using numerical simulation
and analyzed the transient flow behavior thoroughly, which showed it is dual porosity and dual permeability flow behavior.
The numerical simulation results showed that there are obvious differences between dual permeability and single permeability
models. The dual permeability flow behavior accelerates energy supplement during production and reduces the classical matrix-fracture
(V-shaped) response. We also showed that type curves characteristics are affected by external boundary conditions, the parameter
κ, ω
f and λ
mf, etc. The research results show that our model would be a good semi-analytical model supplied to users. Because the single
permeability modeling ignores the direct fluid supply from matrix to wellbore, we recommend using the dual permeability modeling
to make well testing and rate decline interpretation in real case studies. 相似文献
20.
Yoshihiro Ueda Tohru Nakamura Shuichi Kawashima 《Archive for Rational Mechanics and Analysis》2010,198(3):735-762
This paper is concerned with the asymptotic stability of degenerate stationary waves for viscous gases in the half space.
We discuss the following two cases: (1) viscous conservation laws and (2) damped wave equations with nonlinear convection.
In each case, we prove that the solution converges to the corresponding degenerate stationary wave at the rate t
−α/4 as t → ∞, provided that the initial perturbation is in the weighted space
L2a=L2(\mathbb R+; (1+x)a dx){L^2_\alpha=L^2({\mathbb R}_+;\,(1+x)^\alpha dx)} . This convergence rate t
−α/4 is weaker than the one for the non-degenerate case and requires the restriction α < α*(q), where α*(q) is the critical value depending only on the degeneracy exponent q. Such a restriction is reasonable because the corresponding linearized operator for viscous conservation laws cannot be dissipative
in L2a{L^2_\alpha} for α > α*(q) with another critical value α*(q). Our stability analysis is based on the space–time weighted energy method in which the spatial weight is chosen as a function
of the degenerate stationary wave. 相似文献