Computation of unstable flows using system codes

The prediction of unstable flows by means of so-called systems-codes is discussed in this paper. In the nuclear field, verification and validation (V and V) of codes applicable to predict the behavior of complex nuclear installations is a matter of huge collective effort presently. Multi-Physics-Multiple-Scales models, coupled with the needed wall laws is just one example if length scales go into wall layers or minute geometrical details. The validity of physical models as applicable to situations of interest in developing nuclear technology systems (e.g. natural circulation systems, super-critical conditions flows, very high temperature gas-cooled reactors) is now under consideration in the technical literature. This even applies to very simple problems, like cavity flows or one-dimensional (1D) systems. Some basic problems are presented for V and V that must correctly be solved by any systems code. Based on this, conclusions are drawn in relation to some present systems-codes, regarding the sensitivity to code options influencing its numerical approach and physical closure correlations.