State equations and viscosity

Hello all, several questions about the navier stokes solver. couldnt find answers in the user’s guide

  1. What state equation is employed in solution of navier-stokes? Is it the ideal gas equation? What if I want to solve a problem with a different state equation?
  2. What units (if any) should the viscosity in “Navier-stokes” solver given in? Is it unitless? If so- what is it scaled by? What confuses me is that in the Couette flow example we were given a viscocity of ~0.4, which is huge in SI units.
  3. If I want to solve the navier stokes equation with the Sutherland’s law, which parameters should I give? is the given “mu” parameter now the reference viscosity, rather than the constant viscocity of the fluid?
  4. where would I find the answer to such questions? is this forum the only place? is there a more elaborate user’s guide then the one in the website?

1.) Ideal gas. If you want to add another one you will need to modify the source code in the appropriate places, since this is not currently configurable at the .ini file level.

2.) PyFR does not pre-suppose any units. You can set up everything using e.g. SI units if you wish. You can alternatively, for example, set your relevant length scale to have a value of 1 (when you make the mesh), your relevant velocity to have a value of 1, and your relevant density to have a value of 1, and then set mu = 1/Re where Re is your target Reynolds number.

3.) Yes, mu becomes the reference viscosity.

4.) Yes, this forum is the best place. Alternatively you can look in the source code.

Hope this helps. Let me know if you have any followups.



thank you for your answers peter. this solved my issues