Time-dependent flow problems are characterized by the presence of one (or more) time derivative in the basic equations, and are distinct from steady-state and evolution problems. Time-dependent flow problems can have time-dependent inlet flow rates, boundary conditions, material parameters, and so on. In other respects, the problems are similar to steady-state problems, so only the time-dependent aspects will be discussed in this chapter.

Time-dependent flows may or may not reach a steady-state solution, depending on flow parameters and boundary conditions. Because of the intrinsic nonlinearity of most flow problems, it is not possible, in general, to predict whether a transient flow will or will not lead to a steady-state flow regime.

In Ansys Polyflow the time dependence of material data and boundary conditions is defined in terms of a time parameter t, in much the same way as evolution problems (see Evolution) are defined in terms of S. The numerical parameters for controlling the time-marching scheme are similar to those used in evolution. The fundamental difference between time-dependent and evolution problems is that the evolution scheme is controlled by convergence of the iterative scheme, while the time-marching scheme is controlled by the convergence and the accuracy of the time-integration technique.