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Old   May 15, 2010, 02:07
Default Stable Boundary Conditions
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Hi,
i'm trying to simulate a 1-1/5 stage turbine.
I want to use a profile BC at inlet.

But when I use T_tot, velocity x, velocity r, velocity theta, k and epsilon as an inlet profile and average static pressure as an outlet, the solver crashes:

"Fatal bounds error detected
variable: absolute pressure"

how can i use the velocities? because when i use them, i cant specify total pressure at inlet (which i would also have as a profiel data) anymore...

what is the way if i want to use T_tot, Velocity -x,-r,-theta, k and epsilon as an inlet profile BC??

Thank you very much!!!
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Old   May 15, 2010, 07:01
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Have you set the correct reference pressure? Is your boundary pressure correct relative to the reference pressure?
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Old   May 15, 2010, 11:15
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I set the reference pressure of the domain to "0 atm".
For the inlet I can't specify any pressure (i'm using velocity u, r, theta). There is no option where I can specify for egsample total pressure at inlet.
If I use unit velocity direction, than I can specify the pressure at inlet. but not withe the absolut values of the velocity....

At outlet I set 1.1 bar average static pressure...

but the solver crashes due to the BC...

What can I do?
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Old   May 16, 2010, 12:37
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Hi,

the referece pressure should be set near your operating pressures. I think 0 atm is low for a turbine. You should set at least 1 atm. The relative pressure will be (1.1bar-1atm) of course.

And I think, you can not set total pressure and velocity at the inlet together, because the equations will be overconstrained.
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Old   May 17, 2010, 09:35
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so is this a correct way of setting BC:

inlet: velocity -u, -r, -theta, T_tot, k, epsilon
outlet: average static pressure

???
but when i use this, the solver crashes.....
what is wrong?
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Old   May 17, 2010, 09:38
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The crash can caused by many other problems! what is the error message?
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Old   May 17, 2010, 09:45
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i tried a lot of different BC setups with the same model, and they all worked..
now i wanted to use velocity components (from measurement) for the inlet.

when i use velocity components, no pressure at inlet can be deffined anymore...

1st try:

inlet: velocity components, t_tot, k, epsilon (profile BC)
outlet: massflow

-> error!


2nd try:
inlet: velocity components, t_tot, k, epsilon (profile BC)
outlet: average static pressure

-> same error message:

"Fatal bounds error detected
variable: absolute pressure"



what can i do if i want to use velocity componets at inlet?

thank you
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Old   May 17, 2010, 09:49
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The BC setting seems to be OK. Check your values, units etc. I have no more idea.
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Old   May 17, 2010, 18:46
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The error message is simply saying you are trying to get a negative absolute pressure somehow. Based on the sketchy information you have provided I have no idea where, but somewhere in your setup you are asking for a negative absolute pressure.
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Old   May 18, 2010, 13:03
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I set the reference pressure now to 1bar and the outlet average static pressure to 1.1-1=0.1 bar and it worked...

but don't really know why...

anyhow, thank you guys!
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Old   May 18, 2010, 19:14
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Numerical round-off. If you ran using double precision it probably would have worked. But the better solution is to set a reference pressure more representative of the flow average pressure, which is what you have done.
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Old   May 18, 2010, 19:23
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Trust me, the problem is caused by numerical round off leading to a convergence problem.
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