# How to set the wall condition with shadow?

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 April 25, 2012, 13:17 How to set the wall condition with shadow? #1 New Member   Moser Join Date: Dec 2010 Posts: 12 Rep Power: 8 I am dealing with a solid liquid coupled model, like A||B, where A represents Solid while B represents liquid. There's a constant heat flux(e.g. 1000w/m2) from A conducting to B. || represents the interface between solid and liquid. I don't know how to define the wall condition of the interface. If both surfaces set with constant heat flux, actually only interface adjacent to B conducts heat. If the interfaces are set as coupled, I tried with heat generation rate or energy source term of cell zone of B to induce the constant heat flux from B. However, no temperature changes in A are observed. Could anyone heartful teach me how to deal with such a interface problem?

April 25, 2012, 13:34
#2
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Lucky Tran
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 Originally Posted by ryanmoser I am dealing with a solid liquid coupled model, like A||B, where A represents Solid while B represents liquid. There's a constant heat flux(e.g. 1000w/m2) from A conducting to B. || represents the interface between solid and liquid. I don't know how to define the wall condition of the interface. If both surfaces set with constant heat flux, actually only interface adjacent to B conducts heat. If the interfaces are set as coupled, I tried with heat generation rate or energy source term of cell zone of B to induce the constant heat flux from B. However, no temperature changes in A are observed. Could anyone heartful teach me how to deal with such a interface problem?
Leave the interfaces coupled and apply your boundary conditions where they would normally be applied, at the boundaries. How do you know that there is a constant heat flux at the interface? Your heat flux is dictated by the temperature distribution inside the solid region and the corresponding interaction with the fluid, you cannot arbitrarily force it to be constant heat flux.

Can you make a sketch of your setup? I am not sure you are doing this coupled problem correctly. If you are trying to simulate a constant heat flux to the fluid, then you do not need a coupled solid-fluid interface, you only need to be working with a fluid in that case.

April 25, 2012, 16:27
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Moser
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 Originally Posted by LuckyTran Leave the interfaces coupled and apply your boundary conditions where they would normally be applied, at the boundaries. How do you know that there is a constant heat flux at the interface? Your heat flux is dictated by the temperature distribution inside the solid region and the corresponding interaction with the fluid, you cannot arbitrarily force it to be constant heat flux. Can you make a sketch of your setup? I am not sure you are doing this coupled problem correctly. If you are trying to simulate a constant heat flux to the fluid, then you do not need a coupled solid-fluid interface, you only need to be working with a fluid in that case.
The boundary condition requires a constant heat flux through the interface. I am sorry that I'm unable to show you the sketch coz I don't know how to upload images.
My setup, as shown before, like A||B, where in A, 25W is constantly generated. the heat released from A conducts through the interface ||, to B.
I have tried imposing a heat source in A, but it seems that B is not affected by the heat source since no temperature changes are observed. Regarding the transient problem, someone suggest that not enough computation time may lead to this problem.

April 25, 2012, 16:53
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 Originally Posted by ryanmoser The boundary condition requires a constant heat flux through the interface. I am sorry that I'm unable to show you the sketch coz I don't know how to upload images. My setup, as shown before, like A||B, where in A, 25W is constantly generated. the heat released from A conducts through the interface ||, to B. I have tried imposing a heat source in A, but it seems that B is not affected by the heat source since no temperature changes are observed. Regarding the transient problem, someone suggest that not enough computation time may lead to this problem.
The interface is between A and B. You cannot impose any boundary conditions on the interface as it is not a boundary!
If you want to treat the interface as a boundary then you are equivalently solving
A by itself with b.c.'s or b.c.'s with B by itself
You cannot solve A||B and apply conditions to || as the interface is no longer a boundary.

April 25, 2012, 17:16
#5
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Moser
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 Originally Posted by LuckyTran The interface is between A and B. You cannot impose any boundary conditions on the interface as it is not a boundary! If you want to treat the interface as a boundary then you are equivalently solving A by itself with b.c.'s or b.c.'s with B by itself You cannot solve A||B and apply conditions to || as the interface is no longer a boundary.

Sorry not to make it clear. I actually treat the || as wall and need the wall boundary setting

April 25, 2012, 17:35
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 Originally Posted by ryanmoser Sorry not to make it clear. I actually treat the || as wall and need the wall boundary setting
There still is not any boundary there regardless of whether or not it is a wall or interface! It is not a boundary in either case!

April 25, 2012, 21:44
#7
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 Originally Posted by LuckyTran Leave the interfaces coupled and apply your boundary conditions where they would normally be applied, at the boundaries. How do you know that there is a constant heat flux at the interface? Your heat flux is dictated by the temperature distribution inside the solid region and the corresponding interaction with the fluid, you cannot arbitrarily force it to be constant heat flux. Can you make a sketch of your setup? I am not sure you are doing this coupled problem correctly. If you are trying to simulate a constant heat flux to the fluid, then you do not need a coupled solid-fluid interface, you only need to be working with a fluid in that case.

If I'm tying to simulate a constant heat flux to the fluid, only the b.c conatacting the fluid needs to be defined?

April 25, 2012, 22:08
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 Originally Posted by ryanmoser If I'm tying to simulate a constant heat flux to the fluid, only the b.c conatacting the fluid needs to be defined?
Yes, you even stated it exactly. The fluid has a constant heat flux applied to it. Notice that there is no notion of a wall or solid in that sentence, merely a boundary where the heat flux is applied.

April 25, 2012, 22:54
#9
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 Originally Posted by LuckyTran Yes, you even stated it exactly. The fluid has a constant heat flux applied to it. Notice that there is no notion of a wall or solid in that sentence, merely a boundary where the heat flux is applied.
Thank you so much.
I have tried, but a new problem comes out. The thermal conductivity of fluid is very low causing big energy convergence difficulty. To eliminate the influence of flow, I let the fluid be solid temporarily to compute pure thermal conduct problem. With low thermal conductivity, only 1e-01 of energy residuals can be reached, which leads to a bad temperature fied.
If thermal conductivity is increased to more than 70, this model can be easily converged.
How can I improve convergence performance at this stage?

April 25, 2012, 23:22
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 Originally Posted by ryanmoser Thank you so much. I have tried, but a new problem comes out. The thermal conductivity of fluid is very low causing big energy convergence difficulty. To eliminate the influence of flow, I let the fluid be solid temporarily to compute pure thermal conduct problem. With low thermal conductivity, only 1e-01 of energy residuals can be reached, which leads to a bad temperature fied. If thermal conductivity is increased to more than 70, this model can be easily converged. How can I improve convergence performance at this stage?
You're doing it all wrong. You should try to do some tutorials before attempting a problem on your own. You clearly lack basic knowledge of CFD in general.

You don't need to turn the fluid into a solid, you can simply disable solving the equations for fluid flow off temporarily. Also, the energy equations are by far the easiest to solve. You should have no trouble getting a converged solution.

It also makes absolutely no sense to change your thermal conductivity. If you already understand that with poor convergence leads to a bad temperature field, how is changing the thermal conductivity going to help!? It can only make it worse!

Next, your energy equation solution will not make any sense until there is a well converged flow solution. If anything you need to solve the flow well enough before trying energy and not the other way around. You cannot eliminate the influence of flow as you say, if you could, why are you doing CFD in the first place? The whole point is to be able to capture the flow influence. It doesn't matter how well converged your temperature distribution is, it is meaningless and outright incorrect until the flow is correct.

I would not say that a new problem came out, just that you really have no clue what is going on. You need to correct this first and not just turn random knobs. You are shooting a bullet through a car engine in hopes of making it run better!

April 25, 2012, 23:54
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Moser
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Quote:
 Originally Posted by LuckyTran You're doing it all wrong. You should try to do some tutorials before attempting a problem on your own. You clearly lack basic knowledge of CFD in general. You don't need to turn the fluid into a solid, you can simply disable solving the equations for fluid flow off temporarily. Also, the energy equations are by far the easiest to solve. You should have no trouble getting a converged solution. It also makes absolutely no sense to change your thermal conductivity. If you already understand that with poor convergence leads to a bad temperature field, how is changing the thermal conductivity going to help!? It can only make it worse! Next, your energy equation solution will not make any sense until there is a well converged flow solution. If anything you need to solve the flow well enough before trying energy and not the other way around. You cannot eliminate the influence of flow as you say, if you could, why are you doing CFD in the first place? The whole point is to be able to capture the flow influence. It doesn't matter how well converged your temperature distribution is, it is meaningless and outright incorrect until the flow is correct. I would not say that a new problem came out, just that you really have no clue what is going on. You need to correct this first and not just turn random knobs. You are shooting a bullet through a car engine in hopes of making it run better!
It might be the poor expression of mine. It is a natural convection problem. Energy has a significant influence on flow so that flow and energy must be solved simutaneously. Since big difficulty occurs during the compuation I want to do is to test if it is the low thermal conductivity that causes the disconvergence. I tried a material that has the same thermal property as the fluid and convergence really improves with increase in thermal conductivity which leads to higher thermal diffusion.

 Tags constant heat flux, fluent, interface

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