pressure boundary condition in AVL FIRE

doki · January 16, 2011, 02:39

I am performing numerical modeling of gas injection Through a single hole nozzle into a closed chamber with avl FIRE. I intend to define a Total Pressure at inlet of Injector, but the solution does not show any sensitivity to the boundary condition set. It appears that the boundary condition does not "penetrate" into the domain. I have checked almost everything and all the solution setting seems to be appropriately set.
The flow velocity at nozzle exit for pressure ratio of 20 between inlet and chamber is only 2 m/s in first time step!!!!! so definitely sth basic is wrong.
I would be grateful if anyone help me through the staff.

morshed_alm · January 20, 2011, 08:20

You can try with stagnation pressure boundary condition at the inlet and static pressure at the outlet. .

doki · January 30, 2011, 05:53

there is no outlet in the domain.
Stagnation pressure boundary condition needs Mach Number and Dynamic pressure be set at inlet. these values are actually the solution results! i.e. one can not determine them at first.
anyway, I have checked almost everything in the solution and got nothing!

neu · December 21, 2011, 16:39

Hi doki

I also have similar problem to you. I want to inject gas from 1mm diameter nozzle to a constant volume chamber.
What boundary conditions should I use?
Would you please tell me about important adjustment for getting good results?

Thanks,

Quote:

Originally Posted by doki

I am performing numerical modeling of gas injection Through a single hole nozzle into a closed chamber with avl FIRE. I intend to define a Total Pressure at inlet of Injector, but the solution does not show any sensitivity to the boundary condition set. It appears that the boundary condition does not "penetrate" into the domain. I have checked almost everything and all the solution setting seems to be appropriately set.
The flow velocity at nozzle exit for pressure ratio of 20 between inlet and chamber is only 2 m/s in first time step!!!!! so definitely sth basic is wrong.
I would be grateful if anyone help me through the staff.

doki · December 22, 2011, 12:07

Generally the pressure boundary conditions result in slow convergence. you SHOULD adjust the time step size, the URFs and even the number of iterations per time step.
other settings depend on the type of flow. I mean if the flow is under-expanded or not.

neu · December 22, 2011, 12:17

Hi Doki

Thanks so much.
You mean that the total pressure boundary condition is not good.
What boundary condition do you suggest?

Yes, my flow is under expanded. I mean that the injection pressure is nearly 100 atm and the chamber initial pressure is under 10 atm.

1- I use from simple method.
2- My time step in intial stage of calculation is 1e-07 and the gradually reduces to 2e-05.

Would you Please help me?

doki · December 27, 2011, 01:27

It is meaningless that a boundary condition type is not "good". it just might not be appropriate. in some cases you SHOULD use total pressure boundary condition. in each case you should adjust the required settings to get the best convergence.
Anyway, the pressure boundary conditions are a kind of "slow" in convergence.

neu · December 27, 2011, 12:52

Hi doki

Thanks. I have some question:

1- How many Min. number of iterations are need for good convergence?
2- Is it need that I active Second order option in Run mode section?
3- What about for activation of two stage pressure correction?
4- What should I choose in compressibility section between compressible, subsonic and supersonic?

Best,

January 16, 2011, 02:39	pressure boundary condition in AVL FIRE	#1
doki Member Join Date: Apr 2009 Posts: 41 Rep Power: 17	I am performing numerical modeling of gas injection Through a single hole nozzle into a closed chamber with avl FIRE. I intend to define a Total Pressure at inlet of Injector, but the solution does not show any sensitivity to the boundary condition set. It appears that the boundary condition does not "penetrate" into the domain. I have checked almost everything and all the solution setting seems to be appropriately set. The flow velocity at nozzle exit for pressure ratio of 20 between inlet and chamber is only 2 m/s in first time step!!!!! so definitely sth basic is wrong. I would be grateful if anyone help me through the staff. neu likes this.

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January 20, 2011, 08:20		#2
morshed_alm New Member Morshed Alam Join Date: Oct 2009 Posts: 6 Rep Power: 16	You can try with stagnation pressure boundary condition at the inlet and static pressure at the outlet. .

January 30, 2011, 05:53		#3
doki Member Join Date: Apr 2009 Posts: 41 Rep Power: 17	there is no outlet in the domain. Stagnation pressure boundary condition needs Mach Number and Dynamic pressure be set at inlet. these values are actually the solution results! i.e. one can not determine them at first. anyway, I have checked almost everything in the solution and got nothing!

December 22, 2011, 12:07		#5
doki Member Join Date: Apr 2009 Posts: 41 Rep Power: 17	Generally the pressure boundary conditions result in slow convergence. you SHOULD adjust the time step size, the URFs and even the number of iterations per time step. other settings depend on the type of flow. I mean if the flow is under-expanded or not.

December 22, 2011, 12:17		#6
neu New Member Join Date: Dec 2011 Posts: 6 Rep Power: 14	Hi Doki Thanks so much. You mean that the total pressure boundary condition is not good. What boundary condition do you suggest? Yes, my flow is under expanded. I mean that the injection pressure is nearly 100 atm and the chamber initial pressure is under 10 atm. 1- I use from simple method. 2- My time step in intial stage of calculation is 1e-07 and the gradually reduces to 2e-05. Would you Please help me?

December 27, 2011, 01:27		#7
doki Member Join Date: Apr 2009 Posts: 41 Rep Power: 17	It is meaningless that a boundary condition type is not "good". it just might not be appropriate. in some cases you SHOULD use total pressure boundary condition. in each case you should adjust the required settings to get the best convergence. Anyway, the pressure boundary conditions are a kind of "slow" in convergence.

December 27, 2011, 12:52		#8
neu New Member Join Date: Dec 2011 Posts: 6 Rep Power: 14	Hi doki Thanks. I have some question: 1- How many Min. number of iterations are need for good convergence? 2- Is it need that I active Second order option in Run mode section? 3- What about for activation of two stage pressure correction? 4- What should I choose in compressibility section between compressible, subsonic and supersonic? Best,