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排序方式: 共有9966条查询结果,搜索用时 187 毫秒
991.
992.
R. S. Neve 《International Journal of Heat and Fluid Flow》1993,14(4):341-348
A computational fluid dynamics technique has been used to predict the likely concentration levels along a pipe wall of chemicals injected radially from a nozzle. No published empirical data appear to be available, despite the importance of this subject in protecting pipe walls in the vicinity of the dosing point if the chemicals and pipe materials are incompatible. Validation of predictions is by comparison with experimental data for other parameters related to the flow. Where possible, results have been analyzed and presented in dimensionless form so that the article can act as a more generally useful design guide. 相似文献
993.
In this study, we have investigated the influence of shape of planar contractions on the orientation distribution of stiff fibers suspended in turbulent flow. To do this, we have employed a model for the orientational diffusion coefficient based on the data obtained by high-speed imaging of suspension flow at the centerline of a contraction with flat walls. This orientational diffusion coefficient depends only on the contraction ratio and turbulence intensity. Our measurements show that the turbulence intensity decays exponentially independent of the contraction angle. This implies that the turbulence variation in the contraction is independent of the shape, consistent with the results by the rapid distortion theory and the experimental results of axisymmetric contractions. In order to determine the orientation anisotropy, we have solved a Fokker–Planck type equation governing the orientation distribution of fibers in turbulent flow. Although the turbulence variation and the orientational diffusion are independent of the contraction shape, the results show that the variation of the orientation anisotropy is dependent on shape. This can be explained by the variation of the rotational Péclet number, Per, inside the contractions. This quantity is a measure of the importance of the mean rate of the strain relative to the orientational diffusion. We have shown that when Per < 10 turbulence can significantly influence the evolution of the orientation anisotropy. Since in contractions with identical inlet conditions the streamwise position where Per = 10 depends on the shape, the orientation anisotropy is dependent on the variation of rate of strain in a given contraction. We demonstrate the shape effect by considering contraction with flat walls as well as three contractions with different mean rate of strain variation. 相似文献
994.
I. M. Bayanov M. Z. Gil'mullin V. Sh. Shagapov 《Journal of Applied Mechanics and Technical Physics》2003,44(6):858-865
A mathematical model is constructed for motion of a heavy gas along the underlying terrain on the basis of equations of gas dynamics with allowance for the force of gravity, transfer by air masses, and turbulent diffusion. With the use of the method of coarse particles, the time dependences of the coordinates of the fore front, upper boundary, and volume of the cloud of a heavy gas in the presence and absence of wind are analyzed. It is shown that turbulent diffusion leads to a linear increase in cloud volume in time. Three-dimensional shapes of the heavy gas cloud are obtained for various ambient conditions. It is shown that, despite diffusion, the heavy gas spreads predominantly along the underlying terrain. 相似文献
995.
In the paper anomalous diffusion appearing in a porous medium composed of two porous components of considerably different diffusion characteristics is examined. The differences in diffusivities are supposed to result either from two medium types being present or from variations in pore size (double porosity media). The long-tail effect is predicted using the homogenization approach based on the application of multiple scale asymptotic developments. It is shown that, if the ratio of effective diffusion coefficients of two porous media is of the order of magnitude smaller or equal O(
2), where is a homogenization parameter, then the macroscopic behaviour of the composite may be affected by the presence of tail-effect. The results of the theoretical analysis were applied to a problem of diffusion in a bilaminate composite. Analytical calculations were performed to show the presence of the long-tail effect in two particular cases.Notations
c
i
the concentration of chemical species in water within the medium i
-
D
i
the effective diffusion coefficient for the medium i
-
D
ij
eff
the macroscopic (or effective) diffusion tensor in the composite
-
ERV
the elementary representative volume
-
h
the thickness of the period
-
l
a chracteristic length of the ERV or the periodic cell
-
L
a characteristic macroscopic length
-
n
the volumetric fraction of the material 2
- 1–n
the volumetric fraction of the material 1
-
N
the unit vector normal to
-
t
the time variable
-
x
the macroscopic (or slow) space variable
-
y
the microscopic (or fast) space variable
-
c
1c
,C
2c
,D
1c
,D
2c
the characteristic quantities
-
T,T
1L
,T
2L
,T
1l
,T
2l
the characteristic times
-
c
1
*
,c
2
*
,D
1
*
,D
2
*
,t
*
the non-dimensional variables
-
the homogenization parameter
- 1
the domain occupied by the material 1
- 2
the domain occupied by the material 2
-
the interface between the domains 1 and 2
-
the total volume of the periodic cell
-
/xi
the gradient operator
-
the gradient operator 相似文献
996.
Observation time-dependent self-diffusion coefficients can be used to obtain microstructural information of porous media. This paper presents two different kinds of Monte Carlo simulations of the self diffusion process of fluids like water in porous systems, a lattice-free method and a lattice-based method. The results for simple porous media model geometries agree well with each other and with published analytical as well as semi-analytical equations. The use of these equations, which are important for the interpretation of Pulsed Field Gradient-Nuclear Magnetic Resonance (PFG-NMR) time-dependent diffusion data with respect to properties of porous media, is discussed. 相似文献
997.
This paper presents a study on the deformation of anisotropic fibrous porous media subjected to moistening by water in the liquid phase. The deformation of the medium is studied by applying the concept of effective stress. Given the structure of the medium, the displacement of the solid matrix is not taken into account with respect to the displacement of the liquid phase. The transport equations are derived from the model proposed by Narasimhan. The transport coefficients and the relation between the variation in apparent density and effective stress are obtained by test measurements. A numerical model has been established and applied for studying drip moistening of mineral wool samples capable or incapable of deformation.Nomenclature
D
mass diffusion coefficient [L2t–1]
-
e
void fraction
-
g
gravity acceleration [Lt–2]
-
J
mass transfer density [ML–2t–1]
-
K
hydraulic conductivity [Lt–1]
-
K
s
hydraulic conductivity of the solid phase [Lt–1]
-
K
*
hydraulic conductivity of the deformable porous medium [Lt–1]
-
P
pressure of moistening liquid [ML–1 t–2]
-
S
degree of saturation
-
t
time [t]
-
V
speed [Lt–1]
-
X
horizontal coordinate [L]
-
Z
vertical coordinate measured from the bottom of porous medium [L]
-
z
z-coordinate [L]
Greek Letters
porosity
- 1
total hydric potential [L]
-
g
gas density [ML–3]
-
1
liquid density [ML–3]
-
0
apparent density [ML–3]
-
s
density of the solid phase [ML–3]
-
density of the moist porous medium [ML–3]
-
external load [ML–1t–2]
-
effective stress [ML–1t–2]
-
bishop's parameter
-
matrix potential or capillary suction [L]
Indices g
gas
- 1
moistening liquid
- p
direction perpendicular to fiber planes
- s
solid matrix
- t
direction parallel to fiber planes
- v
pore
Exponent *
movement of solid particles taken into account 相似文献
998.
We consider a quasistatic problem of frictional contact between a viscoelastic body and a moving foundation. The contact is with wear and is modeled by normal compliance and a law of dry friction. The novelty in the model is that it allows for the diffusion of the wear debris over the potential contact surface. Such kind of phenomena arise in orthopaedic biomechanics and influence the properties of joint prosthesis. We derive a weak formulation of the problem and state that, under a smallness assumption on the problem data, there exists a unique weak solution for the model. To cite this article: M. Shillor et al., C. R. Mecanique 331 (2003). 相似文献
999.
Different from the approaches used in the earlier papers, in this paper, the Halanay inequality technique, in combination
with the Lyapunov method, is exploited to establish a delay-independent sufficient condition for the exponential stability
of stochastic Cohen–Grossberg neural networks with time-varying delays and reaction–diffusion terms. Moreover, for the deterministic
delayed Cohen–Grossberg neural networks, with or without reaction–diffusion terms, sufficient criteria for their global exponential
stability are also obtained. The proposed results improve and extend those in the earlier literature and are easier to verify.
An example is also given to illustrate the correctness of our results. 相似文献
1000.
A boundary layer analysis is used to investigate the effect of lateral mass flux on mixed convection heat and mass transfer over inclined permeable surfaces in porous media. The conservation equations that govern the problem are reduced to a system of non-linear ordinary differential equations and then the resulting equations is solved by numerical method. The numerical results for heat and mass transfer in terms of Nusselt and Sherwood number are presented in x-y plots for the buoyancy ratio (N) and Lewis number (Le) with mass flux pammeter (Fw). The obtained results are validated against previously published results with on special case of the problem. 相似文献