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Model Library: Gravity and Boundary Conditions

Posted 2012年10月2日 GMT-4 09:33 Fluid & Heat, Computational Fluid Dynamics (CFD), Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.3 1 Reply

Atit Koonsrisuk
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I have practiced a tutorial from Model Library called 'Gravity and Boundary Conditions'. There are 3 cases there.
1. Add 'Volume Force = -g_const*spf.rho' and specify 'Outlet' boundary condition as 'Pressure, no viscous stress' and then set 'p0 = -g_const*spf.rho*y'
2. Add 'Volume Force = -g_const*spf.rho' and specify 'Outlet' boundary condition as 'No viscous stress' and add 'Pressure Point Constraint' then set 'p0 = 0' at the lowest point of the outlet boundary
3. No volume force and specify 'Outlet' boundary condition as 'No viscous stress'

I found that the first 2 cases provide a very similar solution while the third one provide a very different. However, the tutorial does not say explicitly about the circumstances that we should use these settings. So I guess that the first two settings are for the case that the buoyancy effect is dominant and the last one is for the case that has no buoyancy effect. Is my understanding correct?

By the way, what are the differences between cases 1 and 2? What circumstances will we use case 1?

Thank you.


Atit
1 Reply Last Post 2012年10月2日 GMT-4 15:30
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 2012年10月2日 GMT-4 15:30
Hi

for me it sounds like you re modelling either a water tube outlit into free air, or into a water filled tank, with or without boance forces. And do not forget an outlet is also an inlet, that means nothing is prohiiting the fluid to flow in via an outlet, if the local flow direction allows to and the pressiure is defined such that it pushes the fluid back in ...

I'm not by my WS so I cannot open the model to see what it is really proposing ;)

--
Good luck
Ivar
Hi for me it sounds like you re modelling either a water tube outlit into free air, or into a water filled tank, with or without boance forces. And do not forget an outlet is also an inlet, that means nothing is prohiiting the fluid to flow in via an outlet, if the local flow direction allows to and the pressiure is defined such that it pushes the fluid back in ... I'm not by my WS so I cannot open the model to see what it is really proposing ;) -- Good luck Ivar

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