For blow molding and thermoforming simulations that use the shell element, only the linear interpolation scheme is available for the temperature, velocity, and position variables. (This limitation is related to the contact detection algorithm.) The constant interpolation scheme is enabled by default for the thickness variable, and, since a Lagrangian representation is used for the thickness variable, each individual mesh element is handled as a material element.
With a Lagrangian representation, the mass equation for a shell no longer involves any space derivative of the thickness variable. It is therefore possible to use a discontinuous, constant interpolation on an element basis for the thickness variable.
On a reasonably dense finite-element mesh, the discontinuous interpolation for the thickness requires as many unknowns as a linear interpolation. However, it allows Ansys Polyflow to solve the mass equation at the level of the individual element, since the discontinuous interpolation does not require any inter-element communication. This can lead to an appreciable reduction of the computation time, typically by about 10 to 25% with respect to the linear interpolation. The constant interpolation reinforces mass conservation at the level of the individual element.
Note: Constant interpolation is available only when both the mold and the fluid domains are described as surfaces in the geometric space.
For both Newtonian and viscoelastic shell elements, you can change from the default constant (discontinuous) interpolation scheme for the thickness to a linear (continuous) scheme by selecting Linear thickness in the Interpolation menu for the shell sub-task.
Interpolation
When a multilayer system is defined, a thickness field is defined for each individual layer, but all thickness variables use the same interpolation (either constant discontinuous or linear continuous).