Changing turbulence model and getting error at outlet

ndabir · February 9, 2010, 11:20

Hi

I am simulating a turbine stator blade with different turbulence models. I have 1.3 million cells and the y+ is below 3. I get good results for k-e, SSTand K-omega and I don't have any convergence problem. But when I only switch the model to RSM and espesially RSM (BSL), I get this famous message after 10 iterations:

"A wall has been placed at portion(s) of an OUTLET
| boundary condition (at 10.0% of the faces, 1.4% of the area)
| to prevent fluid from flowing into the domain.
| The boundary condition name is: S1 Outlet.
| The fluid name is: Air Ideal Gas.
| If this situation persists, consider switching
to an Opening type boundary condition instead"

the percentage for area is smaller at first iterations but it reaches to 1.4% after 200 iterations.

Can anyone says whats wrong with my problem? I don't change any boundary conditions, only the turbulence model is changed.
Also MAX RMS doesn't reach even 10^-4 for mass. how can I force the residuals to come down more than 10^-5 is RSM models?

triple_r · February 9, 2010, 17:11

Hi,

RSM turbulence models are more sophisticated than the other models and they usually need a better initial condition to converge. Have you tried using one of your previous solutions as the initial guess?

Also consider making a mesh with slightly lower y+ or much larger y+. y+<1 is desired if you are not using wall functions, and if you are using one, then the desired y+ is (I think) more than 30.

ghorrocks · February 9, 2010, 17:56

It is common for convergence problems with RSM models. They are very tricky to use. They require much higher mesh quality than 2-eqn models. They also commonly resolve finer flow features than 2-eqn models, so you will probably find vortices being created which convect to the outlet and cause some backflow at an outlet as they pass. This is just a warning so you can still proceed, but convergence will be harder.

February 9, 2010, 11:20	Changing turbulence model and getting error at outlet	#1
ndabir Senior Member navid Join Date: Jan 2010 Posts: 110 Rep Power: 16	Hi I am simulating a turbine stator blade with different turbulence models. I have 1.3 million cells and the y+ is below 3. I get good results for k-e, SSTand K-omega and I don't have any convergence problem. But when I only switch the model to RSM and espesially RSM (BSL), I get this famous message after 10 iterations: "A wall has been placed at portion(s) of an OUTLET \| boundary condition (at 10.0% of the faces, 1.4% of the area) \| to prevent fluid from flowing into the domain. \| The boundary condition name is: S1 Outlet. \| The fluid name is: Air Ideal Gas. \| If this situation persists, consider switching to an Opening type boundary condition instead" the percentage for area is smaller at first iterations but it reaches to 1.4% after 200 iterations. Can anyone says whats wrong with my problem? I don't change any boundary conditions, only the turbulence model is changed. Also MAX RMS doesn't reach even 10^-4 for mass. how can I force the residuals to come down more than 10^-5 is RSM models?

February 9, 2010, 17:11		#2
triple_r Senior Member Reza Join Date: Mar 2009 Location: Appleton, WI Posts: 116 Rep Power: 17	Hi, RSM turbulence models are more sophisticated than the other models and they usually need a better initial condition to converge. Have you tried using one of your previous solutions as the initial guess? Also consider making a mesh with slightly lower y+ or much larger y+. y+<1 is desired if you are not using wall functions, and if you are using one, then the desired y+ is (I think) more than 30.

February 9, 2010, 17:56		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	It is common for convergence problems with RSM models. They are very tricky to use. They require much higher mesh quality than 2-eqn models. They also commonly resolve finer flow features than 2-eqn models, so you will probably find vortices being created which convect to the outlet and cause some backflow at an outlet as they pass. This is just a warning so you can still proceed, but convergence will be harder.