Solving a stationary problem can take over 24 hours even within these constraints. For highly rarefied flows it can also be necessary to adjust the solver tolerances. For a stationary problem the default solver relative tolerance of 10−7 is reasonable for flows with Knudsen numbers of approximately 1. For a highly rarefied flow the solver tolerance might need to be tightened significantly (if the Neglect Scattering (Free Molecular Flow) checkbox is selected under Flow Properties, the relative tolerance is automatically changed to 10−10).

To create a probe to monitor the maximum velocity in the solution, first set up a Maximum coupling on the model domains (for example, maxop1) and add a Global Variable Probe for maxop1(tran.U). Adjust the solver settings to display the probe output at a step taken by the solver during convergence (this requires changing the Results While Solving settings on the Segregated solver node in the solver sequence).

To do this, in the Model Builder under the Study > Solver Configurations > Solver node, click Dependent Variables. In the Initial Values of Variables Solved For section, choose Solution from the Method list and select the solver itself in the Solution list.