In Ansys Polyflow, contact is implemented through a penalty technique. Once a node slightly penetrates a solid domain, the discrete momentum equation of the node is modified by a penalty term, which can be different in the normal and tangential directions in order to model the physics accurately.

In the normal direction, the condition that the fluid velocity must be equal to the wall velocity is enforced with the penalty coefficient :

(17–1)

where is the normal to the free surface. In the above equation, is the velocity of the mold. In most cases, a translation velocity is sufficient for describing the mold motion. However, in some situations, it is necessary to describe a more complex motion which combines rotation and translation.

The coefficient has physical dimensions, which should be consistent with the system of units used for the simulation.

The product of and a typical value for the relative parison velocity counterbalances the pressure applied to inflate the parison. The larger the value of , the closer the velocity of the fluid will be to the velocity of the wall (in the normal direction for Equation 17–1). On the other hand, if is not large enough, the fluid will progressively penetrate further into the mold and contact detection at the surface will be less accurate.

A large value of will not, in general, result in excessive stiffness of the equation system, provided that numerical round-off errors are not encountered (that is, provided that all numbers can be accurately represented in the computer). By default, Ansys Polydata uses a value of , which is appropriate for most cases. When contact detection is modeled, the time-marching parameters must be modified as described in Time Dependence and Contact Handling and Inputs for 2D and 3D Contact Detection.

The technique is similar in the tangential direction. The only difference is that a prescribed slip may be required for physical reasons. The tangential velocity is prescribed through a linear law of the "slip" type, given by

(17–2)

Note that, like all slip coefficients, has physical dimensions that must be checked. The default value of in Ansys Polydata is , which usually corresponds to a zero relative velocity condition in the tangential direction. Possible slipping is considered if is lower than the penalty coefficient in the normal direction. As mentioned above for the value of , a large value of does not cause excessive stiffness of the equation system, provided that numerical round-off errors are not encountered.

Note that Equation 17–2 is the only available slipping law in contact applications, and that temperature and pressure dependence (as described in Shear Force Calculation) are not considered.