FLUENT changes my boundary conditions setup (2D transonic compressor airfoil)

the_knife · May 27, 2012, 14:14

Hi,
I'm working on my thesis and I have to simulate an airfoil of a transonic compressor. I have got all the thermodynamic parameters at the inlet and outlet.
I set up the boundary conditions, but after the run this values are different.
I try to explain better. For example I set up a pressure inlet condition with a P_tot = 101350 Pa and P_stat = 56000 Pa (Ma around 0.99).
After the simulation the P_stat at the inlet is 96000 and do the Mach is different.
I've tried all the possible combination of boundary conditions, and always something change.

I don't know how to solve this problem.

Thank you

Andrea

pete · May 27, 2012, 15:09

You should set Ptot on the inlet and Pstat on the outlet, not both at the inlet.

the_knife · May 27, 2012, 16:08

Thank you for the sunday answer :-).

The thing is that fixing P_tot and P_stat I fix also the Mach number.
The value that I have in both inlet and outlet are P_tot P_stat and Mach number. I don't know which is the best combination of boundary condition for my simulation.
I fixed both P_tot and P_stat because in this way I also fixed the Mach number.

pete · May 27, 2012, 16:13

Defining Ptot and Ma number on the inlet only is not a complete boundary condition. What do you specify at the outlet?

the_knife · May 27, 2012, 16:35

Ok, I'm surprised to see that there are a lot of people available on sunday :-).

This is what I have:
INLET:
- Ma,Ptot,Pstat,Direction of flow,Ttot
OUTLET:
-Ma,Ptot,Pstat,Ttot

I've tried to use for example this configuration:
INLET: pressure inlet with Ptot, Pstat and direction of the flow
OUTLET: pressure outlet with Pstat.
The problem is that after the simulation the Pstat is totally different from my initial Pstat set up.

pete · May 27, 2012, 16:49

That is way too many boundary conditions. The simulation is over-determined. The most common and most stable boundary condition for compressible flows is to set Ptot, Ttot and direction of flow at the inlet and Pstat at the outlet.

the_knife · May 27, 2012, 17:20

Ok, thank you very much. I will try this evening and I will tell you.
Thank you very much.

Andrea

pete · May 27, 2012, 18:30

If you do not get the desired inlet Ma number you should adjust the outlet Pstat. The change needed can be estimated with simple compressible isentropic flow relations for Ma, Ptot and Pstat. Look them up in a textbook if you do not know them or check out CFD-Wiki which has a bit here: http://www.cfd-online.com/Wiki/Isent...flow_relations

the_knife · May 27, 2012, 20:37

I know the isentropic relationship. In fact, I thought that setting both the P_stat and P_tot I was able to fix also the Ma.

I'm tring to change a bit the value of the outlet pressure to see what happens.

Thank you

Andrea

May 27, 2012, 14:14	FLUENT changes my boundary conditions setup (2D transonic compressor airfoil)	#1
the_knife New Member Andrea Join Date: May 2012 Posts: 5 Rep Power: 14	Hi, I'm working on my thesis and I have to simulate an airfoil of a transonic compressor. I have got all the thermodynamic parameters at the inlet and outlet. I set up the boundary conditions, but after the run this values are different. I try to explain better. For example I set up a pressure inlet condition with a P_tot = 101350 Pa and P_stat = 56000 Pa (Ma around 0.99). After the simulation the P_stat at the inlet is 96000 and do the Mach is different. I've tried all the possible combination of boundary conditions, and always something change. I don't know how to solve this problem. Thank you Andrea

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May 27, 2012, 15:09		#2
pete Administrator Peter Jones Join Date: Jan 2009 Posts: 682 Rep Power: 10	You should set Ptot on the inlet and Pstat on the outlet, not both at the inlet.

May 27, 2012, 16:08		#3
the_knife New Member Andrea Join Date: May 2012 Posts: 5 Rep Power: 14	Thank you for the sunday answer :-). The thing is that fixing P_tot and P_stat I fix also the Mach number. The value that I have in both inlet and outlet are P_tot P_stat and Mach number. I don't know which is the best combination of boundary condition for my simulation. I fixed both P_tot and P_stat because in this way I also fixed the Mach number.

May 27, 2012, 16:13		#4
pete Administrator Peter Jones Join Date: Jan 2009 Posts: 682 Rep Power: 10	Defining Ptot and Ma number on the inlet only is not a complete boundary condition. What do you specify at the outlet?

May 27, 2012, 16:35		#5
the_knife New Member Andrea Join Date: May 2012 Posts: 5 Rep Power: 14	Ok, I'm surprised to see that there are a lot of people available on sunday :-). This is what I have: INLET: - Ma,Ptot,Pstat,Direction of flow,Ttot OUTLET: -Ma,Ptot,Pstat,Ttot I've tried to use for example this configuration: INLET: pressure inlet with Ptot, Pstat and direction of the flow OUTLET: pressure outlet with Pstat. The problem is that after the simulation the Pstat is totally different from my initial Pstat set up.

May 27, 2012, 16:49		#6
pete Administrator Peter Jones Join Date: Jan 2009 Posts: 682 Rep Power: 10	That is way too many boundary conditions. The simulation is over-determined. The most common and most stable boundary condition for compressible flows is to set Ptot, Ttot and direction of flow at the inlet and Pstat at the outlet.

May 27, 2012, 17:20		#7
the_knife New Member Andrea Join Date: May 2012 Posts: 5 Rep Power: 14	Ok, thank you very much. I will try this evening and I will tell you. Thank you very much. Andrea

May 27, 2012, 18:30		#8
pete Administrator Peter Jones Join Date: Jan 2009 Posts: 682 Rep Power: 10	If you do not get the desired inlet Ma number you should adjust the outlet Pstat. The change needed can be estimated with simple compressible isentropic flow relations for Ma, Ptot and Pstat. Look them up in a textbook if you do not know them or check out CFD-Wiki which has a bit here: http://www.cfd-online.com/Wiki/Isent...flow_relations

May 27, 2012, 20:37		#9
the_knife New Member Andrea Join Date: May 2012 Posts: 5 Rep Power: 14	I know the isentropic relationship. In fact, I thought that setting both the P_stat and P_tot I was able to fix also the Ma. I'm tring to change a bit the value of the outlet pressure to see what happens. Thank you Andrea