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PDE with algebraic equation
Posted May 29, 2015, 7:25 a.m. EDT 3 Replies
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I am trying to implement mass concervation myself, but I do not understand how to do this correctly. To keep it simple at first, the density rho is constant
Mass conservation PDE:
d(rho)/dt + d(u*rho)/dt = 0
Algebraic equation
rho = 8 kg/m^3
Later I will try to couple this with Transport of diluted species, then the density rho will be changing since a difference is gas composition (molecular mass of the mixture changes due to different in composition)
Any help on how to implement the algebraic equation, I think I need to take the derivative somehow, but how do I do that neatly in Comsol?
Mass conservation PDE:
d(rho)/dt + d(u*rho)/dt = 0
Algebraic equation
rho = 8 kg/m^3
Later I will try to couple this with Transport of diluted species, then the density rho will be changing since a difference is gas composition (molecular mass of the mixture changes due to different in composition)
Any help on how to implement the algebraic equation, I think I need to take the derivative somehow, but how do I do that neatly in Comsol?
3 Replies Last Post May 29, 2015, 2:50 p.m. EDT
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Posted:
9 years ago
Gilbert,
Did not check it by myself but you might try the following:
1. Define integration operator over the volume where you want impose constraint on density.
2. Use this integration operator to define variables “vol”, “m”, and “den”.
3. Add Global ODES and DAEs interface.
4. In Global Equations node, define variable name “u” and set equation f=den-8[kg/m^3]. This will impose constraint on density in volume.
Image for the above steps is attached.
Did not check it by myself but you might try the following:
1. Define integration operator over the volume where you want impose constraint on density.
2. Use this integration operator to define variables “vol”, “m”, and “den”.
3. Add Global ODES and DAEs interface.
4. In Global Equations node, define variable name “u” and set equation f=den-8[kg/m^3]. This will impose constraint on density in volume.
Image for the above steps is attached.
Attachments:
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Posted:
9 years ago
Thanks so much for your detailed reply! I will look into on monday right away. Any change you could add the model you made, so I can check how you set up everything?
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Posted:
9 years ago
Does anyone know a good link which explains how to transform some PDE's, ODE's and algebraic equations in a set that COMSOL could solve? As in, how to organise them