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Natural Convection Problem

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Hello,

I am having a lot of trouble trying to develop my model. First off, I am a beginner to COMSOL. I have used ABAQUS CEA before but this software package is so different. Anyhow, what I'm trying to model is the natural convection in a cooling glass fiber. The actual scenario involves a glass fiber that was heated from the top then allowed to cool to room temperature in air. What I want to see is the velocity field inside the fiber while its cooling (the flow pattern, velocity values, etc). I want to know this because we believe that the natural convection is causing crystal growth in the cooling glass fiber and we want to correlate the velocity field/location/amount to where the crystals are actually growing inside the fiber.

I am following the free convection in a water glass tutorial from COMSOL and simply manipulating it to my scenario, witch simply removes the glass, heat transfer through the glass, and the temperatures, but I can't get it to give me what I want. Right now I have my material set up as water, simply because we are running experiments to determine the density as a function of temperature etc, and the built in water material already has that. So, once I know the actual values that will be a quick change. What I would love is to see something like in the youtube video from COMSOL shown in the link below.

www.youtube.com/watch?v=xfyR8bomX_g

Any help would be great. I uploaded my file so anyone can take a look. I'm also using an older version of COMSOL, I hope it doesn't make too much of a difference.


3 Replies Last Post 22 giu 2015, 18:11 GMT-4
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Posted: 1 decade ago 29 mag 2012, 03:33 GMT-4
Hi all
i must say this is like the 5th thread i see dealing with natural convection and problems with the analysis.
this is also not the first time i have seen reference to "natural convection must be solved under transient solver".

well, the solution to the whole natural convection problem is complex but possible in both STATIONARY and TRANSIENT (i personally prefer for engineering application to use the stationary solution unless i am interested in the "Ramp-up" process) if only you define the physics properly , see below:

the system:
1. you need to create a large enough volume of fluid around the object to make sure that you don't get artifacts in the computation field due to the boundaries (usually 2-5 times the nominal dimension of the object in length in all directions)
2. define the fluid field: under 'absolute pressure' choose "pressure(nitf/fluid1)" , and as reference pressure enter the ambient pressure.

Mesh:
1. choose a "fine" mesh for most cases where the geometry is more elaborate then cubic presentations (you need a fine enough mesh to capture the fine temperature gradients in the near wall region, as well as the sharp directional acceleration gradients of the velocity field from the entrainment into the plum)

Boundary conditions:
1. the face where the plum (the heat plum raising from the object) is to be defined as "outflow" (under heat transfer BC) AND "outlet" (under laminar BC - set P0=P, ambient)
2. the faces where the fluid enters the system (to satisfy the entrainment of the plum) is to be defined as "open boundary" (with "no viscus stress" condition and T0=T, ambient)
3. Volume force, is to be defined as "-g_const*(nitf.rho-rho_ref)"
4. heat source, what ever is relevant to you system.

summation of BC:
1. volume force
2. open boundary
3. outflow
4. outlet
5. heat source

Solver:
1. choose the stationary solver and change
under study>solver configurations>solver1>fully coupled 1 , change the 'damping and termination' definition to being "automatic highly non-linear"

regarding convergence in Stationary studies:
as we all know and like you can find in many places in the literature, indeed natural convection has a transient element to it, as the plum and flow field fluctuates along the heated surface, HOWEVER, there is still the possibility to achieve a Stationary study that complies with the system.

the thing is, should you run a stationary solver, you will see that after some iterations as the convergence 'green bar' goes all the way to 90% it will then drop sharply to approximately 50%... this phenomena will fluctuate back and forth ENDLESSLY!
what you need to understand it that this is happening because the solver set on one solution then loss stability as it "moves" to another.

What you need to do: let the solution fluctuates back and forth 1-2 times, once you see the pattern - stop the solver, and there you have it!!! - your solution (you will see that should you stop the solver at a different fluctuation the flow field will look slightly different 'Plum-wise' but the temperature will be identical)

i have attached a working example, that has been lab tested to make sure the solver results are accurate.

Best regards to all
M.sc Yoav matia.
Hi all i must say this is like the 5th thread i see dealing with natural convection and problems with the analysis. this is also not the first time i have seen reference to "natural convection must be solved under transient solver". well, the solution to the whole natural convection problem is complex but possible in both STATIONARY and TRANSIENT (i personally prefer for engineering application to use the stationary solution unless i am interested in the "Ramp-up" process) if only you define the physics properly , see below: the system: 1. you need to create a large enough volume of fluid around the object to make sure that you don't get artifacts in the computation field due to the boundaries (usually 2-5 times the nominal dimension of the object in length in all directions) 2. define the fluid field: under 'absolute pressure' choose "pressure(nitf/fluid1)" , and as reference pressure enter the ambient pressure. Mesh: 1. choose a "fine" mesh for most cases where the geometry is more elaborate then cubic presentations (you need a fine enough mesh to capture the fine temperature gradients in the near wall region, as well as the sharp directional acceleration gradients of the velocity field from the entrainment into the plum) Boundary conditions: 1. the face where the plum (the heat plum raising from the object) is to be defined as "outflow" (under heat transfer BC) AND "outlet" (under laminar BC - set P0=P, ambient) 2. the faces where the fluid enters the system (to satisfy the entrainment of the plum) is to be defined as "open boundary" (with "no viscus stress" condition and T0=T, ambient) 3. Volume force, is to be defined as "-g_const*(nitf.rho-rho_ref)" 4. heat source, what ever is relevant to you system. summation of BC: 1. volume force 2. open boundary 3. outflow 4. outlet 5. heat source Solver: 1. choose the stationary solver and change under study>solver configurations>solver1>fully coupled 1 , change the 'damping and termination' definition to being "automatic highly non-linear" regarding convergence in Stationary studies: as we all know and like you can find in many places in the literature, indeed natural convection has a transient element to it, as the plum and flow field fluctuates along the heated surface, HOWEVER, there is still the possibility to achieve a Stationary study that complies with the system. the thing is, should you run a stationary solver, you will see that after some iterations as the convergence 'green bar' goes all the way to 90% it will then drop sharply to approximately 50%... this phenomena will fluctuate back and forth ENDLESSLY! what you need to understand it that this is happening because the solver set on one solution then loss stability as it "moves" to another. What you need to do: let the solution fluctuates back and forth 1-2 times, once you see the pattern - stop the solver, and there you have it!!! - your solution (you will see that should you stop the solver at a different fluctuation the flow field will look slightly different 'Plum-wise' but the temperature will be identical) i have attached a working example, that has been lab tested to make sure the solver results are accurate. Best regards to all M.sc Yoav matia.

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

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Posted: 1 decade ago 29 mag 2012, 04:17 GMT-4
hI

Thanks for the detailed explanation, it's nice and instructif to exchange info like this, we all gain quite a lot of time too ;)

--
Good luck
Ivar
hI Thanks for the detailed explanation, it's nice and instructif to exchange info like this, we all gain quite a lot of time too ;) -- Good luck Ivar

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Posted: 9 years ago 22 giu 2015, 18:11 GMT-4

Hi all
i must say this is like the 5th thread i see dealing with natural convection and problems with the analysis.
this is also not the first time i have seen reference to "natural convection must be solved under transient solver".

well, the solution to the whole natural convection problem is complex but possible in both STATIONARY and TRANSIENT (i personally prefer for engineering application to use the stationary solution unless i am interested in the "Ramp-up" process) if only you define the physics properly , see below:

the system:
1. you need to create a large enough volume of fluid around the object to make sure that you don't get artifacts in the computation field due to the boundaries (usually 2-5 times the nominal dimension of the object in length in all directions)
2. define the fluid field: under 'absolute pressure' choose "pressure(nitf/fluid1)" , and as reference pressure enter the ambient pressure.

Mesh:
1. choose a "fine" mesh for most cases where the geometry is more elaborate then cubic presentations (you need a fine enough mesh to capture the fine temperature gradients in the near wall region, as well as the sharp directional acceleration gradients of the velocity field from the entrainment into the plum)

Boundary conditions:
1. the face where the plum (the heat plum raising from the object) is to be defined as "outflow" (under heat transfer BC) AND "outlet" (under laminar BC - set P0=P, ambient)
2. the faces where the fluid enters the system (to satisfy the entrainment of the plum) is to be defined as "open boundary" (with "no viscus stress" condition and T0=T, ambient)
3. Volume force, is to be defined as "-g_const*(nitf.rho-rho_ref)"
4. heat source, what ever is relevant to you system.

summation of BC:
1. volume force
2. open boundary
3. outflow
4. outlet
5. heat source

Solver:
1. choose the stationary solver and change
under study>solver configurations>solver1>fully coupled 1 , change the 'damping and termination' definition to being "automatic highly non-linear"

regarding convergence in Stationary studies:
as we all know and like you can find in many places in the literature, indeed natural convection has a transient element to it, as the plum and flow field fluctuates along the heated surface, HOWEVER, there is still the possibility to achieve a Stationary study that complies with the system.

the thing is, should you run a stationary solver, you will see that after some iterations as the convergence 'green bar' goes all the way to 90% it will then drop sharply to approximately 50%... this phenomena will fluctuate back and forth ENDLESSLY!
what you need to understand it that this is happening because the solver set on one solution then loss stability as it "moves" to another.

What you need to do: let the solution fluctuates back and forth 1-2 times, once you see the pattern - stop the solver, and there you have it!!! - your solution (you will see that should you stop the solver at a different fluctuation the flow field will look slightly different 'Plum-wise' but the temperature will be identical)

i have attached a working example, that has been lab tested to make sure the solver results are accurate.

Best regards to all
M.sc Yoav matia.


Hello Yoav Matia,

Can you please send the working example you mentioned above? I guess it was deleted over time.
Thank you.

Regards,
Akim
[QUOTE] Hi all i must say this is like the 5th thread i see dealing with natural convection and problems with the analysis. this is also not the first time i have seen reference to "natural convection must be solved under transient solver". well, the solution to the whole natural convection problem is complex but possible in both STATIONARY and TRANSIENT (i personally prefer for engineering application to use the stationary solution unless i am interested in the "Ramp-up" process) if only you define the physics properly , see below: the system: 1. you need to create a large enough volume of fluid around the object to make sure that you don't get artifacts in the computation field due to the boundaries (usually 2-5 times the nominal dimension of the object in length in all directions) 2. define the fluid field: under 'absolute pressure' choose "pressure(nitf/fluid1)" , and as reference pressure enter the ambient pressure. Mesh: 1. choose a "fine" mesh for most cases where the geometry is more elaborate then cubic presentations (you need a fine enough mesh to capture the fine temperature gradients in the near wall region, as well as the sharp directional acceleration gradients of the velocity field from the entrainment into the plum) Boundary conditions: 1. the face where the plum (the heat plum raising from the object) is to be defined as "outflow" (under heat transfer BC) AND "outlet" (under laminar BC - set P0=P, ambient) 2. the faces where the fluid enters the system (to satisfy the entrainment of the plum) is to be defined as "open boundary" (with "no viscus stress" condition and T0=T, ambient) 3. Volume force, is to be defined as "-g_const*(nitf.rho-rho_ref)" 4. heat source, what ever is relevant to you system. summation of BC: 1. volume force 2. open boundary 3. outflow 4. outlet 5. heat source Solver: 1. choose the stationary solver and change under study>solver configurations>solver1>fully coupled 1 , change the 'damping and termination' definition to being "automatic highly non-linear" regarding convergence in Stationary studies: as we all know and like you can find in many places in the literature, indeed natural convection has a transient element to it, as the plum and flow field fluctuates along the heated surface, HOWEVER, there is still the possibility to achieve a Stationary study that complies with the system. the thing is, should you run a stationary solver, you will see that after some iterations as the convergence 'green bar' goes all the way to 90% it will then drop sharply to approximately 50%... this phenomena will fluctuate back and forth ENDLESSLY! what you need to understand it that this is happening because the solver set on one solution then loss stability as it "moves" to another. What you need to do: let the solution fluctuates back and forth 1-2 times, once you see the pattern - stop the solver, and there you have it!!! - your solution (you will see that should you stop the solver at a different fluctuation the flow field will look slightly different 'Plum-wise' but the temperature will be identical) i have attached a working example, that has been lab tested to make sure the solver results are accurate. Best regards to all M.sc Yoav matia. [/QUOTE] Hello Yoav Matia, Can you please send the working example you mentioned above? I guess it was deleted over time. Thank you. Regards, Akim

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