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pressure in laminar flow

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I am running a CFD simulation using the laminar flow module. The module uses Navier Stokes equation in ahich the fluid velocity is the dynamical variable. The equations it is solving are clearly stated in the "equations" section of the module. However it also calculates pressure but I cannot se in the documentation how it is calculating pressure from velocity. Does anyone know what assumptions are being made when COMSOL calculates pressure in the laminar flow module?



4 Replies Last Post 17-Mar-2020, 12:21 pm GMT-4
Jeff Hiller COMSOL Employee

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Posted: 7 months ago 10-Mar-2020, 12:00 pm GMT-4
Updated: 7 months ago 17-Mar-2020, 3:13 pm GMT-4

Hi John,

No, no additional equation is needed. If you think about it, you have (in 3D), 3 scalar equations for momentum conservation (one for each direction) plus one equation for mass conservation, and 4 variables (3 velocities and 1 pressure). So there's just the right number of unknowns for the number of equations. One Finite Element textbook that covers how to formulate such finite elements is Bathe's Finite Element Procedures. In my copy (ISBN 0-13-301458-4), this is covered in Section 7.4. I am sure the same info can be found online as well.

Best,

Jeff

PS: If you introduced an equation of state, you'd be introducing an extra equation and an extra unknown (T), so that'd be OK too, but is not needed if the temperature is known.

Bernoulli is just an integral form of Navier Stokes, so it wouldn't add anything.

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Jeff Hiller
Hi John, No, no additional equation is needed. If you think about it, you have (in 3D), 3 scalar equations for momentum conservation (one for each direction) plus one equation for mass conservation, and 4 variables (3 velocities and 1 pressure). So there's just the right number of unknowns for the number of equations. One Finite Element textbook that covers how to formulate such finite elements is Bathe's Finite Element Procedures. In my copy (ISBN 0-13-301458-4), this is covered in Section 7.4. I am sure the same info can be found online as well. Best, Jeff PS: If you introduced an equation of state, you'd be introducing an extra equation and an extra unknown (T), so that'd be OK too, but is not needed if the temperature is known. Bernoulli is just an integral form of Navier Stokes, so it wouldn't add anything.

Jeff Hiller COMSOL Employee

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Posted: 7 months ago 10-Mar-2020, 2:06 pm GMT-4
Updated: 7 months ago 17-Mar-2020, 3:13 pm GMT-4

Hi John,

Pressure is not computed from velocity. Pressure is treated as a separate dependent variable, with its separate dof's, interpolation functions, etc. You can find more information on this physics interface in the COMSOL Multiphysics Reference Guide, around page 904 (for version 5.5).

Best,

Jeff

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Jeff Hiller
Hi John, Pressure is not computed from velocity. Pressure is treated as a separate dependent variable, with its separate dof's, interpolation functions, etc. You can find more information on this physics interface in the COMSOL Multiphysics Reference Guide, around page 904 (for version 5.5). Best, Jeff

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Posted: 7 months ago 17-Mar-2020, 9:39 am GMT-4

Hi Jeff, thanks for the reply. I feel there is still some assumption that must be missing? Is there an assumed equation of state that is being implemented something that relates pressure to volume and temperature maybe? Or perhaps Bornoulli's principle is being applied. I just dont see anything explicitly stated in the manual. Thanks for the help.

Hi Jeff, thanks for the reply. I feel there is still some assumption that must be missing? Is there an assumed equation of state that is being implemented something that relates pressure to volume and temperature maybe? Or perhaps Bornoulli's principle is being applied. I just dont see anything explicitly stated in the manual. Thanks for the help.

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Posted: 7 months ago 17-Mar-2020, 12:21 pm GMT-4

Jeff, thanks for the help and the explanation.

  • JB
Jeff, thanks for the help and the explanation. - JB

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