Highly negative pressure value for outlet with specified pressure

CFD-fellow · April 26, 2014, 16:49

Hi
Im modeling a free surface flow( air over water ).
>>>>>>>
Although Ive set the static pressure at outlet,after the third time step the cfx-post shows highly negative pressure for outlet (time step 1 and 2 was correct) but correct velocity as expected to be vise versa.

Any help would be appreciated

ghorrocks · April 27, 2014, 06:34

Your simulation is probably numerically unstable and is about the diverge. You need to improve numerical stability - do that by improving mesh quality, double precision, better initial conditions or other means.

CFD-fellow · April 27, 2014, 09:31

Thanks Glenn
My results are completely wrong but my residuals have a logical behavior (I mean its not near divergency).
According to my experience with Fluent, i think the solver doesnt have the right to change the specified variable in boundary condition under any condition (even divergency) and any step of the solution.
Is it possible that CFX-SOLVER has this right, to improve convergency or prevent divergency in early iterations? Or maybe i havent enough study on CFX boundary conditions.
Regards

ghorrocks · April 27, 2014, 18:32

What have you set the boundary as? An outlet or opening? And which option of outlet or opening?

CFD-fellow · April 28, 2014, 05:17

I use outlet with static pressure. Can opening boundary solve my problem?
I have attached the pressure contour for first and 10th time steps.My reference pressure is 1atm.

ghorrocks · April 28, 2014, 07:01

The pressure spots are spurious flows from the free surface model. These ae very common and hard to avoid. but careful choice of free surface model parameters and high mesh quality with tight convergence can reduce them.

But regarding your question on why the boundary is not fixed to the value you defined it to:

I think you will find the boundary face will be fixed to the value you defined. The values you are showing are the conservative values which represent the control volume inside the domain, and therefore are free to vary their value.

April 26, 2014, 16:49	Highly negative pressure value for outlet with specified pressure	#1
CFD-fellow Senior Member Behrooz Jamshidi Join Date: Apr 2013 Posts: 110 Rep Power: 13	Hi Im modeling a free surface flow( air over water ). >>>>>>> Although Ive set the static pressure at outlet,after the third time step the cfx-post shows highly negative pressure for outlet (time step 1 and 2 was correct) but correct velocity as expected to be vise versa. Any help would be appreciated

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April 27, 2014, 06:34		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Your simulation is probably numerically unstable and is about the diverge. You need to improve numerical stability - do that by improving mesh quality, double precision, better initial conditions or other means.

April 27, 2014, 09:31		#3
CFD-fellow Senior Member Behrooz Jamshidi Join Date: Apr 2013 Posts: 110 Rep Power: 13	Thanks Glenn My results are completely wrong but my residuals have a logical behavior (I mean its not near divergency). According to my experience with Fluent, i think the solver doesnt have the right to change the specified variable in boundary condition under any condition (even divergency) and any step of the solution. Is it possible that CFX-SOLVER has this right, to improve convergency or prevent divergency in early iterations? Or maybe i havent enough study on CFX boundary conditions. Regards

April 27, 2014, 18:32		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	What have you set the boundary as? An outlet or opening? And which option of outlet or opening?

April 28, 2014, 07:01		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	The pressure spots are spurious flows from the free surface model. These ae very common and hard to avoid. but careful choice of free surface model parameters and high mesh quality with tight convergence can reduce them. But regarding your question on why the boundary is not fixed to the value you defined it to: I think you will find the boundary face will be fixed to the value you defined. The values you are showing are the conservative values which represent the control volume inside the domain, and therefore are free to vary their value.