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Pressures - gauge v. absolute and Fluid properties
Posted Sep 6, 2011, 4:32 p.m. EDT Fluid & Heat, Computational Fluid Dynamics (CFD) Version 4.2 7 Replies
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Hello,
I have a couple of basic questions relating to pressure in my model, and I can't seem to find the answers in either the user manual or online discussion forums:
- Is the default pressure setting gauge or absolute? Or is either acceptable as long are you are consistent (although it seems that allowing this to vary would cause problems with material properties)? I noticed that if I try to use pressure from one of the fluid modes in my heat transfer modes I have to know whether or not that pressure was absolute in order to set pa = p or pa = p + pref.
- Why do fluid (laminar, turbulent) modules that would presumably calculate pressure and have spatially varying pressures have "absolute pressure" as an input under "Fluid Properties"? In this case, how do you decide what pressure to input, since pressure changes spatially? It makes sense to me that you need to set the initial pressure and I am using pressure boundary conditions for a Darcy's law mode (although I am not sure if these should be gauge or absolute as mentioned above), but I don't understand why there is another pressure input (and what it is used for) under the general "Fluid Properties."
Thanks in advance for your help!
I have a couple of basic questions relating to pressure in my model, and I can't seem to find the answers in either the user manual or online discussion forums:
- Is the default pressure setting gauge or absolute? Or is either acceptable as long are you are consistent (although it seems that allowing this to vary would cause problems with material properties)? I noticed that if I try to use pressure from one of the fluid modes in my heat transfer modes I have to know whether or not that pressure was absolute in order to set pa = p or pa = p + pref.
- Why do fluid (laminar, turbulent) modules that would presumably calculate pressure and have spatially varying pressures have "absolute pressure" as an input under "Fluid Properties"? In this case, how do you decide what pressure to input, since pressure changes spatially? It makes sense to me that you need to set the initial pressure and I am using pressure boundary conditions for a Darcy's law mode (although I am not sure if these should be gauge or absolute as mentioned above), but I don't understand why there is another pressure input (and what it is used for) under the general "Fluid Properties."
Thanks in advance for your help!
7 Replies Last Post Nov 13, 2011, 1:09 p.m. EST
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Posted:
1 decade ago
Hi Nancy
You have a good point there, I too would like to better understand even by reading the docs, I always though it was my lack of CFD experience ;)
I have more or less the same understanding as you: the pressure "p" in the physics are the gauge pressure, to which you must add (COMSOL does it normally for you) the absolute pressure to use the material properties depending on the Total pressure. hence there are several pressure definitions in your physics nodes.
The only thing is when you are mixing physics and are writing your own equations: I tend to get lost about is it "p" or "pa" or ...?
That is also why often you need to set (fix) the pressure somewhere (i.e. at a point in your geoemtry) to get a good conversion, or just to get a first conversion
I would too appreciat that it was explained in a simpler unanbiguous way in the doc. So far I test it out on simple cases, but I find myself repeating the same tests several times, as I move in and out of the different physics
--
Good luck
Ivar
You have a good point there, I too would like to better understand even by reading the docs, I always though it was my lack of CFD experience ;)
I have more or less the same understanding as you: the pressure "p" in the physics are the gauge pressure, to which you must add (COMSOL does it normally for you) the absolute pressure to use the material properties depending on the Total pressure. hence there are several pressure definitions in your physics nodes.
The only thing is when you are mixing physics and are writing your own equations: I tend to get lost about is it "p" or "pa" or ...?
That is also why often you need to set (fix) the pressure somewhere (i.e. at a point in your geoemtry) to get a good conversion, or just to get a first conversion
I would too appreciat that it was explained in a simpler unanbiguous way in the doc. So far I test it out on simple cases, but I find myself repeating the same tests several times, as I move in and out of the different physics
--
Good luck
Ivar
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Posted:
1 decade ago
I am also looking for the clarification on this issue!. In my system the pressure at the entrance is 800 Torr (800/760=1.052 atm) and at exit is 1 atm. Assuming Pref=1 atm, I set 0 pressure at outlet and 40 Torr (800-760=40) at the inlet. The results do not make sense. Velocity is too high. Can anybody see the problem?
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Posted:
1 decade ago
Hi
From my understanding you are right, so there might be something else wrong.
When everything is at 1[atm] common pressure, you solve for the difference, with "0[Pa]" at the output.
But you must be sure any other pressure dependet physics is using the correct pressure value, the absolute (sum of both) or differencial pressure. This depends on the physcis cases. Most material properties are based on asolute pressure, hence the reminder to set the absolute pressurein the main physics node (generally preset to defaut 1[atm]). Note: always check ALL BC ad IC
--
Good luck
Ivar
From my understanding you are right, so there might be something else wrong.
When everything is at 1[atm] common pressure, you solve for the difference, with "0[Pa]" at the output.
But you must be sure any other pressure dependet physics is using the correct pressure value, the absolute (sum of both) or differencial pressure. This depends on the physcis cases. Most material properties are based on asolute pressure, hence the reminder to set the absolute pressurein the main physics node (generally preset to defaut 1[atm]). Note: always check ALL BC ad IC
--
Good luck
Ivar
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Posted:
1 decade ago
Thank you, Ivar. It seems that my calculations are right. High velocities are due to short length. I think it is time to review my undergraduate books!.
I am sorry Nancy for thread hijacking.
I am sorry Nancy for thread hijacking.
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Posted:
1 decade ago
I think it has to do with setting of boundary conditions. Typically, with a fluid problem you set the exit pressure. Numerically, the code works better if you set this boundary condition to zero instead of the actual pressure you intend. The reference pressure is the actual pressure which added to zero gives you the true pressure that is then used to evaluate the equation of state.
I have realized lately that COMSOL works so well now that it is not necessary to do this, and I have been setting my exit pressure to the actual desired pressure and not zero. Therefore, my reference pressure is zero, i.e., this additive feature is not used and the pressure field is directly used in the equation of state evaluation.
I have realized lately that COMSOL works so well now that it is not necessary to do this, and I have been setting my exit pressure to the actual desired pressure and not zero. Therefore, my reference pressure is zero, i.e., this additive feature is not used and the pressure field is directly used in the equation of state evaluation.
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Posted:
1 decade ago
Hi James
Thanks for the precisions. Nice to get positivee feedback from the experts.
I would just stress that, for your case, one must be sure ALL reference pressures are set to "0" (there might be several places where p_abs is defined, depending on how the model is set up) otherwise one can easily have some bad surprises.
For critical models, I add a probe on all such critical values (using the variable name), to be sure I have a trace of the values used during solving
--
Good luck
Ivar
Thanks for the precisions. Nice to get positivee feedback from the experts.
I would just stress that, for your case, one must be sure ALL reference pressures are set to "0" (there might be several places where p_abs is defined, depending on how the model is set up) otherwise one can easily have some bad surprises.
For critical models, I add a probe on all such critical values (using the variable name), to be sure I have a trace of the values used during solving
--
Good luck
Ivar
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Posted:
1 decade ago
Yes, i agree Ivar. Typically, the way I handle that is to specify a variable in the global sections that is my exit pressure. Then everywhere in the model that the exit pressure is used, I will insert that variable. Indeed, it is a good idea to do this with all your data so that there are no "hard wired" numbers in the model input, but only in your global section at the top of the model tree. This is not necessary of course, but just good practice to prevent problems.