Anyone ever result in a different inlet pressure after iteration?

serene6390 · March 21, 2013, 16:28

Hi, I'm doing simulation of a compresible flow in a pipe with some bumps on the wall.
I set the boundary condition of inlet pressure to be 10MPa and outlet pressure 7.4MPa, but it's so wierd that after long time iteration and the result comes converged( residual lower than 10e-4), the xy plot of static pressure shows the inlet pressure is different from the boundary condition I set (about 9.2MPa).

Can anyone ever experienced that? Please help me. Thank you so much.

and by the way, anyone knows how to set the friction factor in a pipe?

Thanks again!

Far · March 22, 2013, 00:01

Quote:

Originally Posted by serene6390

Hi, I'm doing simulation of a compresible flow in a pipe with some bumps on the wall.
I set the boundary condition of inlet pressure to be 10MPa and outlet pressure 7.4MPa, but it's so wierd that after long time iteration and the result comes converged( residual lower than 10e-4), the xy plot of static pressure shows the inlet pressure is different from the boundary condition I set (about 9.2MPa).

Can anyone ever experienced that? Please help me. Thank you so much.

and by the way, anyone knows how to set the friction factor in a pipe?

Thanks again!

Inlet static pressure is function of outlet static pressure. But inlet total pressure should remain the same.

jthiakz · March 22, 2013, 06:34

You mean to say that you imposed only two pressure BC's and solver has adjusted the pressure at inlet.
# What is the massflow/velocity/ Temperature at inlet or outlet
# Any interface inbetween ?

serene6390 · March 23, 2013, 20:58

Quote:

Originally Posted by jthiakz

You mean to say that you imposed only two pressure BC's and solver has adjusted the pressure at inlet.
# What is the massflow/velocity/ Temperature at inlet or outlet
# Any interface inbetween ?

Yes, I set them Pressure inlet and Pressure outlet, the inlet temperature is 300K and the model is adiabatic, I didn't input the mass flow.
the model is just a pipe with a bump on the wall, I'm looking for the condition when the flow is choked. So I fixed the inlet pressure and modify the outlet pressure to check whether the flow is choked at the bump. Is there anything wrong with my operation? Thanks a lot for your help.

serene6390 · March 23, 2013, 21:15

Quote:

Originally Posted by Far

Inlet static pressure is function of outlet static pressure. But inlet total pressure should remain the same.

Thank you so much for your reply. I checked my model and the total pressure remain the same but outlet total pressure doesn't.

I'm quite confused. In my model, I set the inlet pressure and outlet pressure as boundary condition, why do they change at last? should I change it to be velocity inlet ? What's your advise if I want to fix the inlet pressure?

Thank you so much for any of your help!

jthiakz · March 24, 2013, 04:32

You need to specify one total quantity at intet. If the pressure ratio is approx 1.8, you will be able to demonstrate choked flow. by the way where did you get this inlet pr values. is it a measured quantity or scaled model.

# operating pressure also plays a role to predcit velocity

serene6390 · March 24, 2013, 15:43

Quote:

Originally Posted by jthiakz

You need to specify one total quantity at intet. If the pressure ratio is approx 1.8, you will be able to demonstrate choked flow. by the way where did you get this inlet pr values. is it a measured quantity or scaled model.

# operating pressure also plays a role to predcit velocity

Thank you very much for your concern. What I'm trying to do is to find the proper boundary condition for my model, there's a bump on the wall in the middle of the pipe, so the 1.8 ratio you mentioned is not suitable for my model. I did the 1D computation using Matlab and using the solution from Matlab as the boundary condition to run in the Fluent.

Do you have an idea how to make the inlet static pressure fixed as 10MPa?

Far · March 24, 2013, 15:52

What should be inlet total pressure? That has to be larger than static pressure !

To get inlet static pressure of 10 Mpa, vary outlet static pressure and see where you get the required static pressure at inlet.

jthiakz · March 24, 2013, 16:38

how did you calculate the total quantities in mat lab.In any case u'll need total P or T or velocity or mass-flow.
If I don't any of this data. I'll impose assumed (higher than Ps inlet)total pressure inlet and static pressure outlet and monitor mass in ,mass out , Static Pressure (inlet,before the bump, after the bump). so based on the monitor plot, I'll adjust the Ptot inlet, till it matches with static,pressure of 10mpa. This way i can check is possible.
Pressure ratio 1.8 is applicable not from the inlet to outlet rather applicable from inlet to the local downstream pressure near the bump.

March 21, 2013, 16:28	Anyone ever result in a different inlet pressure after iteration?	#1
serene6390 New Member Jing Join Date: Feb 2013 Posts: 25 Rep Power: 13	Hi, I'm doing simulation of a compresible flow in a pipe with some bumps on the wall. I set the boundary condition of inlet pressure to be 10MPa and outlet pressure 7.4MPa, but it's so wierd that after long time iteration and the result comes converged( residual lower than 10e-4), the xy plot of static pressure shows the inlet pressure is different from the boundary condition I set (about 9.2MPa). Can anyone ever experienced that? Please help me. Thank you so much. and by the way, anyone knows how to set the friction factor in a pipe? Thanks again!

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March 22, 2013, 06:34		#3
jthiakz Member Thiagu Join Date: Oct 2012 Location: India Posts: 60 Rep Power: 14	You mean to say that you imposed only two pressure BC's and solver has adjusted the pressure at inlet. # What is the massflow/velocity/ Temperature at inlet or outlet # Any interface inbetween ?

March 24, 2013, 04:32		#6
jthiakz Member Thiagu Join Date: Oct 2012 Location: India Posts: 60 Rep Power: 14	You need to specify one total quantity at intet. If the pressure ratio is approx 1.8, you will be able to demonstrate choked flow. by the way where did you get this inlet pr values. is it a measured quantity or scaled model. # operating pressure also plays a role to predcit velocity

March 24, 2013, 15:52		#8
Far Senior Member Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,558 Blog Entries: 6 Rep Power: 54	What should be inlet total pressure? That has to be larger than static pressure ! To get inlet static pressure of 10 Mpa, vary outlet static pressure and see where you get the required static pressure at inlet.

March 24, 2013, 16:38		#9
jthiakz Member Thiagu Join Date: Oct 2012 Location: India Posts: 60 Rep Power: 14	how did you calculate the total quantities in mat lab.In any case u'll need total P or T or velocity or mass-flow. If I don't any of this data. I'll impose assumed (higher than Ps inlet)total pressure inlet and static pressure outlet and monitor mass in ,mass out , Static Pressure (inlet,before the bump, after the bump). so based on the monitor plot, I'll adjust the Ptot inlet, till it matches with static,pressure of 10mpa. This way i can check is possible. Pressure ratio 1.8 is applicable not from the inlet to outlet rather applicable from inlet to the local downstream pressure near the bump.