Internal flow is a very general category which describes the flow of a fluid that is contained by and passes through a solid structure. There may be one or several openings through which fluid enters and leaves the device. The solutions to internal flow problems are among the most difficult to achieve in “typical” CFD (Autodesk Simulation CFD is hardly typical!), particularly for turbulent and compressible flows with complex geometry. The reason is that there are often several flow regimes throughout different regions of the device, and hence the mathematical characteristics vary widely through the calculation domain.
Autodesk Simulation CFD has several tools to aid convergence for a wide range of internal flow problems. These tools include the Automatic Turbulent Start-Up algorithm, Auto-Convergence Control, and Auto-Stop. These algorithms work to prevent solution instability or divergence, particularly in the early iterations.