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Questions about "bouyancy water" model
Posted Nov 24, 2014, 2:36 a.m. EST Fluid & Heat, Heat Transfer & Phase Change Version 5.0 6 Replies
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Hi, guys
I have been learning the "bouyancy water" model of the "model libraries" these days , it's a simple model——temperature differential produces density variation that drives the bouyancy flow in water. But there are several questions bothers me.
1. In this case, the volume force of laminar flow is bouyancy caused by density variation. The model specifies volume force as F=-nitf1.rho*g_const. what's the meaning of nitf1.rho? does it mean Δρ? so the fomule above means F=-Δρg?
2. In the "material library", some properties of certain material are predefined functions. For example, Density rho is a function of temperature T,expresses as rho=rho(T).I want to know the exact fomula of rho(T), How to get that ?
3.About laminar flow(I'm not quite familier with hydromechanics)
In this case, two boundary conditions(Volume Force & Pressure Point Constraint) are necessary. So we need at least two boundary conditions to solve a fluid flow problem, is that correct? And what's the physical meaning of "Pressure Point Constraint"? It seems I can set pressure point constraint to either point of the geometry, and it would not affect the result. So what's the physical meaning of "Pressure Point Constraint"?
Thank you very much for your time !
I have been learning the "bouyancy water" model of the "model libraries" these days , it's a simple model——temperature differential produces density variation that drives the bouyancy flow in water. But there are several questions bothers me.
1. In this case, the volume force of laminar flow is bouyancy caused by density variation. The model specifies volume force as F=-nitf1.rho*g_const. what's the meaning of nitf1.rho? does it mean Δρ? so the fomule above means F=-Δρg?
2. In the "material library", some properties of certain material are predefined functions. For example, Density rho is a function of temperature T,expresses as rho=rho(T).I want to know the exact fomula of rho(T), How to get that ?
3.About laminar flow(I'm not quite familier with hydromechanics)
In this case, two boundary conditions(Volume Force & Pressure Point Constraint) are necessary. So we need at least two boundary conditions to solve a fluid flow problem, is that correct? And what's the physical meaning of "Pressure Point Constraint"? It seems I can set pressure point constraint to either point of the geometry, and it would not affect the result. So what's the physical meaning of "Pressure Point Constraint"?
Thank you very much for your time !
6 Replies Last Post Nov 26, 2014, 2:54 a.m. EST
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Posted:
1 decade ago
1. In this case, the volume force of laminar flow is bouyancy caused by density variation. The model specifies volume force as F=-nitf1.rho*g_const. what's the meaning of nitf1.rho? does it mean Δρ? so the fomule above means F=-Δρg?
Question 1 and 2 are related as the density ρ is a function of T thus resulting in a density difference in areas of temperature varities thus producing the force F=-Δρg. The variable nitf1.rho refers to the chosen material which is defined as a function of T. nitf1.rho is used so one automatically uses the correct density for the given temperature. You could also specify the relationship yourself if the desired material is not present in the COMSOL material library.
2. In the "material library", some properties of certain material are predefined functions. For example, Density rho is a function of temperature T,expresses as rho=rho(T).I want to know the exact fomula of rho(T), How to get that ?
Under the "Materials" node in the "Model Builder" click "Water, liquid", click "Basic", click "Piecewise 3", click "Plot"
The function is given as:
838.466135+1.40050603*T^1-0.0030112376*T^2+3.71822313E-7*T^3
Start: 273.15 [K]
End: 553.75 [K]
3.About laminar flow(I'm not quite familier with hydromechanics)
In this case, two boundary conditions(Volume Force & Pressure Point Constraint) are necessary. So we need at least two boundary conditions to solve a fluid flow problem, is that correct? And what's the physical meaning of "Pressure Point Constraint"? It seems I can set pressure point constraint to either point of the geometry, and it would not affect the result. So what's the physical meaning of "Pressure Point Constraint"?
You are solving the Navier-Stokes equations (see the equation view of the Laminar Flow physics). Note that the gradient of the pressure is present and not the pressure itself. A pressure point constraint is therefore needed to ensure a unique solution.
You need at least one Dirichlet BC for the flow and one for the pressure. In this case your Dirichlet boundary conditions for the velocity field are the no slip conditions. As said before, the pressure is specified through the pressure point constraint.
I hope this helps
Best Regards
Mikael
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Posted:
10 years ago
Thank you very very much!!!!!!!
it is really saved my life! : )
it is really saved my life! : )
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Posted:
10 years ago
Question 1 and 2 are related as the density ρ is a function of T thus resulting in a density difference in areas of temperature varities thus producing the force F=-Δρg. The variable nitf1.rho refers to the chosen material which is defined as a function of T. nitf1.rho is used so one automatically uses the correct density for the given temperature. You could also specify the relationship yourself if the desired material is not present in the COMSOL material library.
Another question: If the variable nitf1.rho refers to ρ(T), isn't Δρ supposed to be (nitf1.rho-rho)？
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Posted:
10 years ago
Question 1 and 2 are related as the density ρ is a function of T thus resulting in a density difference in areas of temperature varities thus producing the force F=-Δρg. The variable nitf1.rho refers to the chosen material which is defined as a function of T. nitf1.rho is used so one automatically uses the correct density for the given temperature. You could also specify the relationship yourself if the desired material is not present in the COMSOL material library.
Another question: If the variable nitf1.rho refers to ρ(T), isn't Δρ supposed to be (nitf1.rho-rho)？
No because the model has a volume force added:
-nitf1.rho*g_const
So as soon as the buoyancy force exceeds the volumetric force one has upward motion so the volumetric force is crucial for natural convection modeling.
Best regards
Mikael
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Posted:
10 years ago
Got it.Thanks again!
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Posted:
10 years ago
Got it.Thanks again!
No problem and good luck!
Best regards
Mikael