To define slip conditions on a boundary section, do the following:
Select the Slip conditions menu item in the list of boundary condition choices.
Slip conditions
Set the value for the velocity of the wall (
).
Define v_wall, the velocity of the wall
To assign a translational velocity, specify the translation velocity.
To assign a rotational velocity in 2D, specify the rotation center and angular velocity.
To assign a rotational velocity in 3D, specify the 1st point of the axis, 2nd point of the axis, and angular velocity.
In 2D, the rotation axis is perpendicular to the plane of the computational domain. In 3D, it is defined by two points.
Select the slip law you want to use and specify the related parameters:
To use Equation 8–1 (the default), select F(v) = generalized Navier’s law and specify constant values for k (
) and e (
).
F(v) = generalized Navier’s law
To use Equation 8–2, select F(v) = threshold law and specify constant values for k (
), k2 (
), and yc (
).
F(v) = threshold law
To use Equation 8–3, select F(v) = asymptotic law and specify constant values for k (
) and vc (
).
F(v) = asymptotic law
To use Equation 8–4, select F(v) = generalized threshold law and specify constant values first for tau1 (
), then tau0 (
) Vslip1 (
), Vcrit (
), and the exponent
b
which must be
greater than or equal to 1.
F(v) = generalized threshold law
Select the temperature dependence law you want to use and specify the related parameters:
To use Equation 8–6, select H(T) = 1 (temperature independent).
H(T) = 1 (temperature independent)
To use Equation 8–7, select H(T) = Arrhenius approximate law and specify constant values for alfa (
) and Ta (
).
H(T) = Arrhenius approximate law
To use Equation 8–8, select H(T) = Arrhenius law and specify constant values for alfa (
), Ta (
), and T0 (
).
H(T) = Arrhenius law
Select the pressure dependence law you want to use and specify the related parameters:
To use Equation 8–9, select G(p) = 1 (pressure independent).
G(p) = 1 (pressure independent)
To use Equation 8–10, select G(p) = exponential dependence and enter a constant value for
in the panel that opens. When you choose
pressure-dependent slippage, it may be a good idea to keep the
default interpolation for the calculation of the unknown force
density field (see the description for the Advanced
options menu item in the step that follows).
G(p) = exponential dependence
To use Equation 8–11, select G(p) = linear dependence for p>=0 and enter a constant value for
in the panel that opens. When you choose
pressure-dependent slippage, it may be a good idea to keep the
default interpolation for the calculation of the unknown force
density field (see the description for the Advanced
options menu item in the step that follows).
G(p) = linear dependence for p>=0
Assign a constraint on geometric normal vector.
There can be a potential conflict of boundary conditions in some situations. You can have slipping boundary conditions on a boundary side and a free surface on an adjacent boundary side. The finite element contributions from both sides may result in a motion of the corresponding intersection point, which may receive an inappropriate velocity component.
For preventing possible difficulties, you may reinforce the constraint on the vanishing normal velocity component by canceling some component of the normal vector to the wall. This can be achieved by selecting:
Define constraints on wall normals
Specify the components of the geometric normal vector to the boundary to be canceled, to reinforce the constraint on vanishing velocity along the other directions.
Select one of the following as applicable to cancel the appropriate component of the geometric normal vector:
Cancel X-component of wall normals
Cancel Y-component of wall normals
Cancel Z-component of wall normals
The latter option is available in 3D flow. Such options have to be selected carefully, depending on the actual geometry. Also, at least one component should not vanish. For the above options, you can only cancel or restore the corresponding components of the wall normal vectors.
You can access advanced options for the slip condition definition by clicking Advanced options menu option.
Advanced options
The slipping formulation utilizes a stabilization parameter referred to as "epsilon", which can help resolve problems that arise from conflicting boundary conditions. You can modify the value of this parameter by clicking Modify epsilon and entering a new value in the dialog box that opens. Note that this epsilon parameter is only effective when a continuous interpolation is selected (see the information that follows).
Modify epsilon
Important: You should only modify the value of epsilon when recommended to do so by your support engineer.
The slipping boundary condition involves the evaluation of an unknown force density field. Like other fields, this force density field must be interpolated. You can specify that the interpolation is: program controlled, so that Polydata selects the interpolation according to the interpolation of the velocity field and slipping parameters (this is the default method); constant at the level of the elements; the same as that used for the pressure field; or the same as that used for the velocity field. To modify the method of interpolation for slipping, click Modify interpolation rule and enter a value that corresponds to your preferred method in the Specify the interpolation rule dialog box that opens.
Modify interpolation rule
In most cases, the default selection is suitable. When your calculation fails due to slipping, it is recommended that you try another interpolation. In any case, it is recommended that you carefully inspect the results upon the completion of the calculation.