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991.
如何激发学生的创新意识、培养创新能力是当前教学的中心任务之一。本文笔者根据在流体力学课程教学中将简单易行、灵活多样的演示实验带进课堂,引发学生兴趣,启发创新思维方面的教学实践,总结了几点经验和体会。 相似文献
992.
Effects of the method of identification of the diffusion coefficient on accuracy of modeling bound water transfer in wood 总被引:2,自引:0,他引:2
An alternative approach to determining the bound water diffusion coefficient is proposed. It comprises a method for solving
the inverse diffusion problem, an improved algorithm for the bound-constrained optimization as well as an alternative submodel
for the diffusion coefficient’s dependency on the bound water content. Identification of the diffusion coefficient for Scots
pine wood (Pinus sylvestris L.) using the proposed inverse approach is presented. The accuracy of predicting the diffusion process with the use of the
coefficient values determined by traditional sorption methods as well as by the inverse modeling approach is quantified. The
similarity approach is used and the local and global relative errors are calculated. The results show that the inverse method
provides valuable data on the bound water diffusion coefficient as well as on the boundary condition. The results of the identification
can significantly improve the accuracy of mass transfer modeling as studied for drying processes in wood. 相似文献
993.
各向同性弹性损伤的双标量描述 总被引:24,自引:1,他引:24
损伤状态的描述是损伤力学中仍未完善解决的基本问题.我们旨在对此问题就最简单的一种情形——各向同性弹性损伤,进行较为全面的研究.首先,指出了古典各向同性损伤理论中,基于应变等效假设,用单个标量损伤变量描述损伤状态的局限性.然后,建立了一个用两个标量损伤变量描述的各向同性弹性损伤模型.此模型解除了古典理论的局限,能完全描述各向同性弹性损伤,并且得到本文数值实验的验证.最后,将本文模型与现有细观力学结果连接,给出了宏细观损伤变量之间的关系,使得细观量可以通过宏观量来反映,建立了一个用细观损伤材料常数描述细观缺陷特征的损伤本构模型 相似文献
994.
995.
Oscillatory shear measurements on a series of branched graft polystyrenes (PS) synthesized by the macromonomer technique are presented. The graft PS have similar molar masses (M
w
between 1.3×105 g/mol and 2.4×105 g/mol) and a polydispersity M
w
/M
n
around 2. The molar masses of the grafted side chains M
w,br range from 6.8×103 g/mol to 5.8×104 g/mol, which are well below and above the critical entanglement molar mass M
c of linear polystyrene. The average number
of side chains per molecule ranges from 0.6 to 6.7. The oscillatory measurements follow the time–temperature superposition principle. The shift factors do not depend on the number of branches. The zero-shear viscosities of all graft PS are lower than those of linear PS with the same molar mass, which can be attributed to the smaller coil size of the branched molecules. It is shown that the influence of branching on the frequency dependence of the dynamic moduli is weak for all graft PS that were investigated, which can be explained by the low entanglement density.Electronic Supplementary Material Supplementary material is available for this article at
This article has already been published online first (DOI: ). Due to an oversight at the publisher's, this version contained several mistakes. The article is herewith republished in its entirety as a "publisher's erratum". 相似文献
996.
997.
Numerical and experimental investigation of wave dynamic processes in high-speed train/tunnels 总被引:1,自引:0,他引:1
Numerical and experimental investigation on wave dynamic processes induced by high-speed trains entering railway tunnels are
presented. Experiments were conducted by using a 1:250 scaled train-tunnel simulator. Numerical simulations were carried out
by solving the axisymmetric Euler equations with the dispersion-controlled scheme implemented with moving boundary conditions.
Pressure histories at various positions inside the train-tunnel simulator at different distance measured from the entrance
of the simulator are recorded both numerically and experimentally, and then compared with each other for two train speeds.
After the validation of nonlinear wave phenomena, detailed numerical simulations were then conducted to account for the generation
of compression waves near the entrance, the propagation of these waves along the train tunnel, and their gradual development
into a weak shock wave. Four wave dynamic processes observed are interpreted by combining numerical results with experiments.
They are: high-speed trains moving over a free terrain before entering railway tunnels; the abrupt-entering of high-speed
trains into railway tunnels; the abrupt-entering of the tail of high-speed trains into railway tunnels; and the interaction
of compression and expansion waves ahead of high-speed trains. The effects of train-tunnel configuration, such as the train
length and the train-tunnel blockage ratio, on these wave processes have been investigated as well. 相似文献
998.
Sedat F. Tardu 《Applied Scientific Research》1995,54(4):349-385
This work deals with the effect of the riblets on the coherent structures near the wall. The emphasis is put on the genesis of the quasi-streamwise vortices in the presence of the riblets. The quasi-streamwise vortices regenerate by the tilting of wall normal vorticity induced by prevailing structures. This requires a mechanism which leads to a temporal streamwise dependence near the elongated flow structures and to a subsequent formation of new wall normal vorticity. It is suggested here that the action of existing quasi-streamwise vortices on the sidewalls of wall normal vorticity may create a local, streamwise dependent spanwise velocity and therefore, a secondary wall normal vorticity field. A preliminary analysis of the set-up and the time and space development of this secondary three-dimensional flow associated with the regeneration mechanism, is given. An attempt is made, in order to explain the drag reduction performed by the riblets through an intermittent model, based on the protrusion height. Logical estimates of the amount of drag reduction are obtained. The differences between the mechanism suggested here and those based on forced control experiments are also discussed. 相似文献
999.
In this work, we make use of numerical experiments to explore our original theoretical analysis of two-phase flow in heterogeneous porous media (Quintard and Whitaker, 1988). The calculations were carried out with a two-region model of a stratified system, and the parameters were chosen be consistent with practical problems associated with groundwater flows and petroleum reservoir recovery processes. The comparison between theory (the large-scaled averaged equations) and experiment (numerical solution of the local volume averaged equations) has allowed us to identify conditions for which the quasi-static theory is acceptable and conditions for which a dynamic theory must be used. Byquasi-static we mean the following: (1) The local capillary pressure,everywhere in the averaging volume, can be set equal to the large-scale capillary pressure evaluated at the centroid of the averaging volume and (2) the large-scale capillary pressure is given by the difference between the large-scale pressures in the two immiscible phases, and is therefore independent of gravitational effects, flow effects and transient effects. Bydynamic, we simply mean a significant departure from the quasi-static condition, thus dynamic effects can be associated with gravitational effects, flow effects and transient effects. To be more precise about the quasi-static condition we need to refer to the relation between the local capillary pressure and the large-scale capillary pressure derived in Part I (Quintard and Whitaker, 1990).
Herep
c
¦y represents the local capillary pressure evaluated at a positiony relative to the centroid of the large-scale averaging volume, and {p
c
}¦x represents the large-scale capillary pressure evaluated at the centroid.In addition to{p
c
}
c
being evaluated at the centroid, all averaged terms on the right-hand side of Equation (1) are evaluated at the centroid. We can now write the equations describing the quasi-static condition as
,
, This means that the fluids within an averaging volume are distributed according to the capillary pressure-saturation relationwith the capillary pressure held constant. It also means that the large-scale capillary pressure is devoid of any dynamic effects. Both of these conditions represent approximations (see Section 6 in Part I) and one of our main objectives in this paper is to learn something about the efficacy of these approximations. As a secondary objective we want to explore the influence of dynamic effects in terms of our original theory. In that development only the first four terms on the right hand side of Equation (1) appeared in the representation for the local capillary pressure. However, those terms will provide an indication of the influence of dynamic effects on the large-scale capillary pressure and the large-scale permeability tensor, and that information provides valuable guidance for future studies based on the theory presented in Part I.Roman Letters
A
scalar that maps {}*/t onto
-
A
scalar that maps {}*/t onto
-
A
interfacial area between the -region and the -region contained within, m2
-
A
interfacial area between the -region and the -region contained within, m2
-
A
interfacial area between the -region and the -region contained within, m2
-
a
vector that maps ({}*/t) onto
, m
-
a
vector that maps ({}*/t) onto
, m
-
b
vector that maps ({p}–
g) onto
, m
-
b
vector that maps ({p}–
g) onto
, m
-
B
second order tensor that maps ({p}–
g) onto
, m2
-
B
second order tensor that maps ({p}–
g) onto
, m2
-
c
vector that maps ({}*/t) onto
, m
-
c
vector that maps ({}*/t) onto
, m
-
C
second order tensor that maps ({}*/t) onto
, m2
-
C
second order tensor that maps ({}*/t) onto
. m2
-
D
third order tensor that maps (
) onto
, m
-
D
third order tensor that maps (
) onto
, m
-
D
second order tensor that maps (
) onto
, m2
-
D
second order tensor that maps (
) onto
, m2
-
E
third order tensor that maps () onto
, m
-
E
third order tensor that maps () onto
, m
-
E
second order tensor that maps () onto
-
E
second order tensor that maps () onto
-
p
c
=(), capillary pressure relationship in the-region
-
p
c
=(), capillary pressure relationship in the-region
-
g
gravitational vector, m/s2
-
largest of either or
-
-
-
i
unit base vector in thex-direction
-
I
unit tensor
-
K
local volume-averaged-phase permeability, m2
-
K
local volume-averaged-phase permeability in the-region, m2
-
K
local volume-averaged-phase permeability in the-region, m2
- {K
}
large-scale intrinsic phase average permeability for the-phase, m2
-
K
–{K
}, large-scale spatial deviation for the-phase permeability, m2
-
K
–{K
}, large-scale spatial deviation for the-phase permeability in the-region, m2
-
K
–{K
}, large-scale spatial deviation for the-phase permeability in the-region, m2
-
K
*
large-scale permeability for the-phase, m2
-
L
characteristic length associated with local volume-averaged quantities, m
-
characteristic length associated with large-scale averaged quantities, m
-
I
i
i = 1, 2, 3, lattice vectors for a unit cell, m
-
l
characteristic length associated with the-region, m
-
;
characteristic length associated with the-region, m
-
l
H
characteristic length associated with a local heterogeneity, m
-
-
n
unit normal vector pointing from the-region toward the-region (n
=–n
)
-
n
unit normal vector pointing from the-region toward the-region (n
=–n
)
-
p
pressure in the-phase, N/m2
- p
local volume-averaged intrinsic phase average pressure in the-phase, N/m2
- {p
}
large-scale intrinsic phase average pressure in the capillary region of the-phase, N/m2
- p
local volume-averaged intrinsic phase average pressure for the-phase in the-region, N/m2
- p
local volume-averaged intrinsic phase average pressure for the-phase in the-region, N/m2
-
p
–{p
}, large scale spatial deviation for the-phase pressure, N/m2
-
p
–{p
}, large scale spatial deviation for the-phase pressure in the-region, N/m2
-
p
–{p
}, large scale spatial deviation for the-phase pressure in the-region, N/m2
-
P
c
p
–{p
}, capillary pressure, N/m2
- {pc}c
large-scale capillary pressure, N/m2
-
r
0
radius of the local averaging volume, m
-
R
0
radius of the large-scale averaging volume, m
-
r
position vector, m
-
, m
-
S
/, local volume-averaged saturation for the-phase
-
S
*
{}*{}*, large-scale average saturation for the-phaset time, s
-
t
time, s
-
u
, m
-
U
, m2
-
v
-phase velocity vector, m/s
- v
local volume-averaged phase average velocity for the-phase in the-region, m/s
- v
local volume-averaged phase average velocity for the-phase in the-region, m/s
- {v
}
large-scale intrinsic phase average velocity for the-phase in the capillary region of the-phase, m/s
- {v
}
large-scale phase average velocity for the-phase in the capillary region of the-phase, m/s
-
v
–{v
}, large-scale spatial deviation for the-phase velocity, m/s
-
v
–{v
}, large-scale spatial deviation for the-phase velocity in the-region, m/s
-
v
–{v
}, large-scale spatial deviation for the-phase velocity in the-region, m/s
-
V
local averaging volume, m3
-
V
volume of the-phase in, m3
-
V
large-scale averaging volume, m3
-
V
capillary region for the-phase within, m3
-
V
capillary region for the-phase within, m3
-
V
c
intersection of m3
-
V
volume of the-region within, m3
-
V
volume of the-region within, m3
-
V
()
capillary region for the-phase within the-region, m3
-
V
()
capillary region for the-phase within the-region, m3
-
V
()
, region in which the-phase is trapped at the irreducible saturation, m3
-
y
position vector relative to the centroid of the large-scale averaging volume, m
Greek Letters
local volume-averaged porosity
-
local volume-averaged volume fraction for the-phase
-
local volume-averaged volume fraction for the-phase in the-region
-
local volume-averaged volume fraction for the-phase in the-region
-
local volume-averaged volume fraction for the-phase in the-region (This is directly related to the irreducible saturation.)
- {}
large-scale intrinsic phase average volume fraction for the-phase
- {}
large-scale phase average volume fraction for the-phase
- {}*
large-scale spatial average volume fraction for the-phase
-
–{}, large-scale spatial deviation for the-phase volume fraction
-
–{}, large-scale spatial deviation for the-phase volume fraction in the-region
-
–{}, large-scale spatial deviation for the-phase volume fraction in the-region
-
a generic local volume-averaged quantity associated with the-phase
-
mass density of the-phase, kg/m3
-
mass density of the-phase, kg/m3
-
viscosity of the-phase, N s/m2
-
viscosity of the-phase, N s/m2
-
interfacial tension of the - phase system, N/m
-
, N/m
-
, volume fraction of the-phase capillary (active) region
-
, volume fraction of the-phase capillary (active) region
-
, volume fraction of the-region (
+
=1)
-
, volume fraction of the-region (
+
=1)
-
{p
}–
g, N/m3
-
{p
}–
g, N/m3 相似文献
1000.
O. Wünsch 《Rheologica Acta》1990,29(2):163-169
An experiment is described to determine the two Bingham material constants (yield stress
f and differential viscosity
) of viscoplastic fluids. The principle of the experiment is based on the falling-ball technique, where the stationary velocities of balls with different diameters and densities are measured. The required theory to calculate the Bingham material constants is illustrated. Experimental results of aqueous Carbopol 941-solutions are reported. These are listed in dependence of concentration in tables and diagrams.
Zusammenfassung Es wird ein Versuchsaufbau beschrieben, der es ermöglicht, die beiden Binghamschen Stoffparameter (Fließspannung f und differentielle Viskosität ) einer viskoplastischen Flüssigkeit zu bestimmen. Der Versuch basiert auf dem Kugelfallprinzip, bei dem in einem Zylinder die stationäre Sinkgeschwindigkeit von Kugeln im Schwerefeld gemessen werden. Neben der Geschwindigkeit gehen das spezifische Gewicht der Flüssigkeit sowie die Geometrie und das spezifische Gewicht verschieden großer Kugeln in die Berechnung der Stoffparameter ein. Die zugehörige Theorie wird kurz erläutert. Im experimentellen Teil werden wäßrige Carbopol 941-Lösungen untersucht. Die Ergebnisse sind in Abhängigkeit der Konzentration tabellarisch angegeben und graphisch dargestellt.相似文献