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Comsol momentum balance
Posted May 13, 2010, 11:54 p.m. EDT 2 Replies
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I'm solving a coupled system of mass , energy and momentum balances.
I got a question regarding the momentum balance in Comsol (weakly compressible flow). In the boundary condition section. If you choose to have velocity as the inlet condition and a 1 bar pressure as the outlet. The Comsol solution shows a constant pressure throughout, while the inlet velocity and the dimension of the geometry and the temperature dictates implicitely an inlet pressure based on the gas law PV=nRT in which V=velocity *area . Why Comsol does not consider this in the flow analysis.?
This occurs unless you choose to enter a pressure as the inlet condition.
2 Replies Last Post May 14, 2010, 7:05 p.m. EDT
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Posted:
1 decade ago
Hi
have you read through carefully the multiphysics doc on these boundary conditions?
as I was just now studying my new V4 doc (mph_ref.pdf) I agree that inlet velocity is the only one not mentioning inlet pressure. But if I remember right in fluid you have also the "gauge pressure" uncertainty, one should fix the pressure somewhere inside the model, its not just enough to start with initial pressure != 0.
Its true that some of these simple examples for each boundary condition in COMSOL (and there are many in there) are missing, a good student job for some years, would be nice to have them as a list of choices in the model forum
Sorry I cannot add much more, anyhow I have just the books here, V4 running is for next week ;)
Good luck
Ivar
have you read through carefully the multiphysics doc on these boundary conditions?
as I was just now studying my new V4 doc (mph_ref.pdf) I agree that inlet velocity is the only one not mentioning inlet pressure. But if I remember right in fluid you have also the "gauge pressure" uncertainty, one should fix the pressure somewhere inside the model, its not just enough to start with initial pressure != 0.
Its true that some of these simple examples for each boundary condition in COMSOL (and there are many in there) are missing, a good student job for some years, would be nice to have them as a list of choices in the model forum
Sorry I cannot add much more, anyhow I have just the books here, V4 running is for next week ;)
Good luck
Ivar
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Posted:
1 decade ago
I don't understand your issue exactly but here is something that comes to mind. It sounds like you are modelling gas flow. In gas flows the pressure drops can be rather small (on the order of a few Pascals) - this compared to your reference pressure 1 bar ( ~ 100,000 Pa) may be totally negligible making your pressure look as if it was uniform everywhere. If you are after pressure drop, try a an incompressible model with constant physicals similar to that of your gas at 1 bar and set exit pressure to 0 and see what happens, just for the fun of it.
I'm solving a coupled system of mass , energy and momentum balances.
I got a question regarding the momentum balance in Comsol (weakly compressible flow). In the boundary condition section. If you choose to have velocity as the inlet condition and a 1 bar pressure as the outlet. The Comsol solution shows a constant pressure throughout, while the inlet velocity and the dimension of the geometry and the temperature dictates implicitely an inlet pressure based on the gas law PV=nRT in which V=velocity *area . Why Comsol does not consider this in the flow analysis.?
This occurs unless you choose to enter a pressure as the inlet condition.