Influence of boundary conditions

The measurement ofC potential is in practice related to the particular set of boundary conditions used. 

The liquid voliune transported by time unit for a cylinder of diameter 0 = 2rand 

In a porous medium the determination of the electric field is not easy and E can be expressed in a different way. Taking the current intensity one obtains 

This expression allows for the determination of C neither knowing the radius of the pores nor that of the capillaries. The measurement of a flow combined to that of intensity and electric conductance, allows for the determination of Cpotential in the porous medium with a charged double layer. 

The experimental device is here very analogous to an osmosis apparatus, consisting in two open compartments communicating by a porous material. In the present case, an electric field is applied to the porous material and this substance is permeable to both solute and solvent particles. Moreover the porous material is supposed to be charged on surface. When a current passes through the porous medium, a difference A/i in liquid height takes place between the two compartments. This phenomenon ceases when the current stops. A difference Ap in pressure between the two compartments corresponds to A/i 

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Neumann M and Steinhauser O 1980 The influence of boundary conditions used in machine simulations on the structure of polar systems Mol. Phys. 39 437-54... 

Such conditions are inherent in the model of a real reactor (impermeability to gas and quasi-adiabaticity of heat-insulated reactor walls). In the case of a thread-shaped element, a special technique should be employed in practice for the conditions of Eq. (2) to be realized. However, a detailed study has shown that at large enough values of L/d the influence of boundary condition types on the characteristic properties of DS becomes insignificant (as a rule, in experimental runs Ljd > 10 lO" ). 

Figure 6 shows the numerical model geometry for the simulation. The model includes the two heaters, bentonite, plug, tunnel and host rock. Numerical area is decided considering influence of boundary conditions. Since the site can be assumed line symmetric with the xz-plane, the model is made only in the positive part of y direction. The number of elements is 5,760. The number of nodes is 26,401. The host rock is assumed to be a homogenous media. 

Influence of boundary condition Two kinds of hydraulic boundary conditions are tested. The first one supposes an impermeable tunnel wall, the second a prescribed relative humidity (HR). Figure 8 shows that the assumption is very influent close to the wall (first 15 cm). The mechanical consequence of this low saturation level is an important increase of suction so that plasticity never occurs. In conclusion, imposed HR is not suitable for elastoplastic model using Bishop s effective stresses. Thus, no flow condition has been adopted. 

Figure 8. Influence of boundary condition (no flow or imposed relative humidity)...

Since the two limiting cases of open and closed ends have been shown to lead, respectively, to an enhancement and a reduction of the mean square displacement in comparison to an infinite single-file system, it may be anticipated that, imder the influence of boundary conditions intermediate between these two hmiting cases, molecular propagation in a finite single-file system may even proceed as in a single-file system of infinite extension. 

Examples of the influence of boundary conditions and constraints on capillary forces are reported in detail in section 9.3.I.I. 

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