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Gravity and 2D axisymmetric horizontal flow

Posted 2 juin 2011, 12:25 UTC−4 Fluid & Heat, Computational Fluid Dynamics (CFD), Geometry Version 4.2 3 Replies

Rui Silva
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Hi folks.

A colleague and myself are having a discussion about Gravity Force in Comsol.

I have a feeling I have read somewhere that the gravity force is defined by default in 2D axisymmetric horizontal flow,
however my colleague disagrees with me.

So I was wondering if anyone can shed some light on the subject.

And If it isn't defined by default how do you define it in 2D axisymmetric horizontal flow to be perpendicular
to the flow?

Hope someone can help.

Cheers.
3 Replies Last Post 27 févr. 2012, 08:18 UTC−5
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 2 juin 2011, 15:16 UTC−4
Hi

a gravity force appears in COMSOL as a "body force" that applies a force density (per element) of "g_const*solid.rho" in units [N/m^3]. But it's not present "by default" you must add it explicitely.

In 2D-axi it's more restrictif, as you have only 1 direction possible: along the axis of symmetry (up or down along "z"). So you have no way to model a "horizontal" pipe w,r,t gravity in 2D axi, bacause in 2D axi that would represents a centrifugal force along the radial direction "r"

For me you have only 3D to correctly model a flow in a cylindrical horizontal pipe with gravity applied vertically

--
Good luck
Ivar
Hi a gravity force appears in COMSOL as a "body force" that applies a force density (per element) of "g_const*solid.rho" in units [N/m^3]. But it's not present "by default" you must add it explicitely. In 2D-axi it's more restrictif, as you have only 1 direction possible: along the axis of symmetry (up or down along "z"). So you have no way to model a "horizontal" pipe w,r,t gravity in 2D axi, bacause in 2D axi that would represents a centrifugal force along the radial direction "r" For me you have only 3D to correctly model a flow in a cylindrical horizontal pipe with gravity applied vertically -- Good luck Ivar
Jeff Hiller COMSOL Employee

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Posted: 1 decade ago 2 juin 2011, 15:22 UTC−4
If there's a force per unit volume that applies perpendicularly to your actual pipe, then the situation is not axisymmetric.
If there's a force per unit volume that applies perpendicularly to your actual pipe, then the situation is not axisymmetric.
Brian Clarkson
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Posted: 1 decade ago 27 févr. 2012, 08:18 UTC−5
When using 4.2A the body force for gravity can be expressed as: -nits.rho*g, where rho is the temperature dependant density from a library material, and nits in the case is the None-IsoThermal Flow module.

However, this does not appear to work in 3.5A (which I still use also for a number of reasons).

How can I use library definitions of density (temperature dependance) in the body force in version 3.5A?
When using 4.2A the body force for gravity can be expressed as: -nits.rho*g, where rho is the temperature dependant density from a library material, and nits in the case is the None-IsoThermal Flow module. However, this does not appear to work in 3.5A (which I still use also for a number of reasons). How can I use library definitions of density (temperature dependance) in the body force in version 3.5A?

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