Discussion Forum

What is the problem, result verification? Cant help without case describing, Grashof number, boundary conditions, flow model used.

Send me the model. I will check for you!

The problem that I am experiencing is setting up the model in 4.0a so that it accurately simulates the conditions and agrees (at least to a degree) with the results from 3.5a. I have attached the model of what I have so far. My main problem is simulating the air above the soil and concrete trough as well as the boundary conditions around the soil to simulate a continuation on the material properties. I greatly appreciate any help. Thank You

Hi all,
I'm also having problems with natural convection. I'm making a tutorial on free convection which is written for version 3.5a. I'm having great difficulties in reproducing this tutorial in version 4.0a. When I describe the air density (rho) through rho=rho0*(1-(T-Tinf)/Tinf). I had to change the boundary condition "convective flux" into an open boundary is the cf condition is no longer available in 4.0a. When I try to solve the problem I get the error that Comsol failed to evaluate the variable T.....
I have no idea what I'm doing wrong. anybody who has an idea? Thanks.

Convective flux has been renamed Outflow. The implementation is exactly the same. About the fact that T can not be evaluated: it could be that the temperature variable is called something else (T2?). Have a look in the Error node that should be available where more a more detailed error message should be available. If you still have problems, please send the model to COMSOL support.

Niklas Rom, COMSOL

Hi all,
I'm also having problems with natural convection. I'm making a tutorial on free convection which is written for version 3.5a. I'm having great difficulties in reproducing this tutorial in version 4.0a. When I describe the air density (rho) through rho=rho0*(1-(T-Tinf)/Tinf). I had to change the boundary condition "convective flux" into an open boundary is the cf condition is no longer available in 4.0a. When I try to solve the problem I get the error that Comsol failed to evaluate the variable T.....
I have no idea what I'm doing wrong. anybody who has an idea? Thanks.

--
Niklas

''Convective flux has been renamed Outflow. The implementation is exactly the same. ''

Thank you very much. I will check it using COMSOL 4.0a. Also, I have encountered another problem, that is, you can find 'insulation' condition in COMSOL 3.5 for the convection-diffusion module, however, you can not find it in COMSOL 4.0a. So, does anyone know the new name of it in COMSOL 4.0a.

Looking forward to any reply from any of you.

Thank you very much in advance.

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 aback 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 filed 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.

Dear Yoav,

Your reply is very interesting. I am modeling the flow of outdoor air with heated building's walls and heated street surface. However, I never obtained the converged solution. It always "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!" as you explained. So I am very delighted when I found yours but I cannot understand some of your explanation. My questions are

1. You suggested that after a few fluctuations we should stop the Solver and use that result as the solution, is that correct? My problem is that I want a solution after all simulation residuals are less than 1e-5 then I can have a confident that it is the converged solution.

2. I downloaded your attached file and openned it by COMSOL 4.3. It said that it cannot find some thing. Is that caused by using different versions of COMSOL?

3. I will try to use absolute pressure in my simulation as you suggested. However, I was wondering something. I think that we use relative pressure in solving because it can reduce a truncation error especially for the case of small pressure change like free convection. If we use absolute pressure in our calculation, will the solution suffer by the truncation error?

Thank you for your sharing.


Best regards,


Atit

Hi Dr. Atit Koonsrisuk

I am Ray S.Y. Hu, a postgraduate from Sun yat-sen Univ. in China. I am involoving in research on the solar updraft chimney technology which is just started in our group. I've spent months to read literatures and understand the theoritcal models of it. one of the literatures is from you, which evaluated the improvement of energy output with the divergent chimney. I am very interested on that. Personally, I am trying to take more attention to the innovation of the structure of the chimney part.

Now, I am looking for references of the natural convection model setup on the Internet for simulating the process in the chimney with COMSOL. Fortunately, I met you here. If it's convenient, could I mail you and discuss with you on some research issues?

Best wishes,

Ray

Dear Ray,

I am glad to hear that you are interested in the solar chimney. You are welcome to contact me. Next time please send e-mail directly to atit@sut.ac.th.


Best regards,


Atit

Dear Atit,

Thx for your reply. You're so nice.

Ray