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[FAQ]

When defining a simulation involving contact, POLYDATA informs the user that the prediction on the velocity field is disabled. What does it mean actually? For solving a transient simulation, a predictor-corrector time marching scheme is used. In such a scheme, the predictor is a first guess of the solution at the next time step (n+1) and is usually obtained by extrapolating the unknown fields from past time, the corrector is the actual solution at that step (n+1) resulting from the non-linear solver. The deviation between predictor and corrector is a measure of the accuracy and is used for selecting subsequent time steps. In simulations involving contact, this procedure cannot be applied for the velocity field. Indeed, at a contact occurrence, the velocity is suddenly assigned another value controlled by the contact parameters e.g. it jumps down to zero at the contact occurrence with a fixed mold. In such a circumstance, the predictor-corrector scheme would fail since the velocity would apparently exhibit a high inaccuracy. In order to circumvent this, the velocity is simply removed from the list of fields used for the accuracy evaluation. The predictor is not evaluated (or the prediction on the velocity field is disabled). However, the velocity field is kept for the evaluation of convergence. In most contact simulations, the prediction of the velocity field is disabled (as explained above). The accuracy of the transient scheme is evaluated on the basis of other fields.