Hi,

Now I am simulating gas flow in a tube

I use couple solver implicit ,ideal gas model, second upwind.I define the courant number as 0.5. I find the residuals sometims increase,sometimes decrease.When I change to first upwind,the same problem is still unsolved.

How can I solve this problem? Would you give me some advice?

Thank you. Any answer will be welcome.

Devy 2001.5.25

Hi,Mr Sundar:

Thank you for your reply.

Now I am simulating gas flow in a tube

I use couple solver implicit ,ideal gas model, second upwind,three mass-inlets(subsonic),one pressure-out(supersonic),and other wall boundary.I define the courant number as 0.5. I find the residuals sometims increase,sometimes decrease.When I change to first upwind,the same problem is still unsolved.

When I check mass flow rate,I find the value of massinet is bigger the value I input and all of mass flow rate of the inlet and outlet is less then 0.The value of residuals is about 1.2e-1.

Thank you.

Devy 2001.5.26

You should check mass flux instead of mass flow-rate. Flux calculation is based on face values instead of cell value, therefore gives you exactly what was input at the inlet boundary.

How have you adjusted your under-relaxation factors? I start them low (maybe 0.1) and then when I notice steady convergence I increase the factors a little bit. What about your initial conditions and boundary conditions. I find that if one thing isn't defined improperly (improperly in respect to Fluent), the solution blows up.

Hi,

Thank you for your reply.

I am using couple implicit model.there is notunder-relaxation factors.I think I define the correct initial conditions and boundary conditions.

Thank you

Devy 2001.6.9

You can still adjust the courant number with the implicit coupled solver. The default value is 5. Make to 0.1, for example, and run it for a while, then try to increase the value to 1.0 and so on.