Problem with Momentum Imbalance

Nerdybat · February 15, 2021, 02:20

Hi,
I am a beginner in both CFD and CFX. I am trying to run a wind tunnel case (flow over an airfoil) with the following BC's:
Inlet - Total Pressure, Total Temperature
Outlet - Avg Static Pressure
I am using air ideal gas as the material
While most similar runs with the same BC's have converged, this particular case has a problem. the H-energy and P-mass imbalances do reduce over time(but not below a particular value say 0.05)
But then the W-Momentum imbalance increases and keeps increasing causing other imbalances to increase too and my velocity and static pressure(I've placed monitors) at inlet oscillate continuously because of this. I do not know what the problem is and I would appreciate it a lot if someone helped me out here.
Please reply soon as I am in a bit of a hurry. Thanks

Gert-Jan · February 15, 2021, 03:53

Sounds like you have found a transient solution. Then don't try to solve it in steady state, but using a transient setup. That won't give you a single value so you need to determine transient average values.

Nerdybat · February 15, 2021, 04:19

Hi Gert-Jan,
Thank you for replying.
What if I intend the problem to be a steady state one? How do I find out where this transient nature is coming from and how do I eliminate it?

Gert-Jan · February 15, 2021, 04:26

During steady state, you can make backups at multiple iterations. In Post you can analyse these to find out what the source is.
It the solution has a transient nature, then mostly there is not much you can do. Sometimes it is possible to increase or decrease the Autotime Scale Factor so that the solver doesn't find the instabilities. Just give it a try.

ghorrocks · February 15, 2021, 05:13

There are a few things you should try before you do a transient simulation. The FAQ describes a few things to look at: https://www.cfd-online.com/Wiki/Ansy...gence_criteria

But if it ends up that the simulation is transient then you have to forget about a steady solution because it does not exist.

Nerdybat · February 15, 2021, 07:45

Hi Gert-Jan and Ghorrocks,
Thank you for providing clarity through your replies.

February 15, 2021, 02:20	Problem with Momentum Imbalance	#1
Nerdybat New Member JD Join Date: Jan 2021 Posts: 3 Rep Power: 5	Hi, I am a beginner in both CFD and CFX. I am trying to run a wind tunnel case (flow over an airfoil) with the following BC's: Inlet - Total Pressure, Total Temperature Outlet - Avg Static Pressure I am using air ideal gas as the material While most similar runs with the same BC's have converged, this particular case has a problem. the H-energy and P-mass imbalances do reduce over time(but not below a particular value say 0.05) But then the W-Momentum imbalance increases and keeps increasing causing other imbalances to increase too and my velocity and static pressure(I've placed monitors) at inlet oscillate continuously because of this. I do not know what the problem is and I would appreciate it a lot if someone helped me out here. Please reply soon as I am in a bit of a hurry. Thanks

February 15, 2021, 03:53		#2
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,911 Rep Power: 28	Sounds like you have found a transient solution. Then don't try to solve it in steady state, but using a transient setup. That won't give you a single value so you need to determine transient average values. aero_head likes this.

February 15, 2021, 04:26		#4
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,911 Rep Power: 28	During steady state, you can make backups at multiple iterations. In Post you can analyse these to find out what the source is. It the solution has a transient nature, then mostly there is not much you can do. Sometimes it is possible to increase or decrease the Autotime Scale Factor so that the solver doesn't find the instabilities. Just give it a try. aero_head and Nerdybat like this.

February 15, 2021, 05:13		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,853 Rep Power: 144	There are a few things you should try before you do a transient simulation. The FAQ describes a few things to look at: https://www.cfd-online.com/Wiki/Ansy...gence_criteria But if it ends up that the simulation is transient then you have to forget about a steady solution because it does not exist. aero_head and Nerdybat like this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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February 15, 2021, 04:19		#3
Nerdybat New Member JD Join Date: Jan 2021 Posts: 3 Rep Power: 5	Hi Gert-Jan, Thank you for replying. What if I intend the problem to be a steady state one? How do I find out where this transient nature is coming from and how do I eliminate it?

February 15, 2021, 07:45		#6
Nerdybat New Member JD Join Date: Jan 2021 Posts: 3 Rep Power: 5	Hi Gert-Jan and Ghorrocks, Thank you for providing clarity through your replies.