Chemical reactions and fluid material properties often induce strongly nonlinear couplings among velocity, temperature, and concentration fields. Although source terms arising from chemical reactions and equations of state are sometimes complex, it is important to note that these relationships are always algebraic (though nonlinear) and never differential.

Other nonlinearities may originate from the constitutive equations and/or problems involving free surfaces, which introduce a coupling between the velocity and the coordinates of the mesh.

For chemically reacting flows, evolution on both the pre-exponential factor and the enthalpy is recommended. The chemically neutral "Stokes" solution is chosen as an initial guess. Since the Stokes solution is structurally different from the final result, a careful continuation procedure is needed to obtain convergence of the iterative scheme.

See Evolution for more details on using evolution.