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Modelling gravity

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Hi All,

I am trying to model water exiting a hose at a given velocity. I want to model the water trajectory and find out how far it travels before hitting the ground. Can anyone tell me where to start in Comsol? I am familiar with CFD, just not with the gravity part.

Thanks

2 Replies Last Post 22 oct. 2015, 01:59 UTC−4

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Posted: 9 years ago 22 oct. 2015, 01:47 UTC−4
Hi

In the flow node, add Volume force and define the y component as -g*rho (gravity acceleration*fluid density). Assuming that y axis is vertical.

Lasse
Hi In the flow node, add Volume force and define the y component as -g*rho (gravity acceleration*fluid density). Assuming that y axis is vertical. Lasse

Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)

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Posted: 9 years ago 22 oct. 2015, 01:59 UTC−4
Hi

Lasse gave you the correct way.

To easier remember it you should think over the units.
Gravity is a "domain load" (or Volume load) hence it applies to each element of volume dx*dy*dz (this dx*dy*dz element is not mentioned in the COMSOL GUI entries, but is always there in an implicit way).
In macroscopic physics we always say force is mass time acceleration

F[N]=m[kg]*a[m/s^2]

But in the microscopic continuum way one says:

F[N]= integral_over_volume(rho[kg/m^3]*a[m/s^2]*dx[m]*dy[m]*dz[m])

This is the default entry method of all GUI fields in COMSOL but COMSOL omits (implicitely) the "integral_over_volume(" and the ending "*dx*dy*dz)" and keep only the "rho_density*acceleration"

--
Good luck
Ivar
Hi Lasse gave you the correct way. To easier remember it you should think over the units. Gravity is a "domain load" (or Volume load) hence it applies to each element of volume dx*dy*dz (this dx*dy*dz element is not mentioned in the COMSOL GUI entries, but is always there in an implicit way). In macroscopic physics we always say force is mass time acceleration F[N]=m[kg]*a[m/s^2] But in the microscopic continuum way one says: F[N]= integral_over_volume(rho[kg/m^3]*a[m/s^2]*dx[m]*dy[m]*dz[m]) This is the default entry method of all GUI fields in COMSOL but COMSOL omits (implicitely) the "integral_over_volume(" and the ending "*dx*dy*dz)" and keep only the "rho_density*acceleration" -- Good luck Ivar

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