about residuals in fluent
 

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

Re: about residuals in fluent
 

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

Re: about residuals in fluent
 

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.

Re: about residuals in fluent
 

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.

Re: about residuals in fluent
 

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

Re: about residuals in fluent
 

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.