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-   -   imbalance in H-Energy (http://www.cfd-online.com/Forums/cfx/23230-imbalance-h-energy.html)

cmete October 28, 2006 15:23

imbalance in H-Energy
 
Hi, in my results except the H-Energy all other imbalances are nearly zero. but imbalance of H-Energy is very high and converge of H-Energy is slow than others. do these reasons show wrong the heat tranfer setup?

regards

cmete

Robin October 30, 2006 09:08

Re: imbalance in H-Energy
 
Try increasing the timestep on the energy equation (CFX-Pre, Solver Control > Advanced). If you have a very slow energy transport process such as conduction through your fluid (as opposed to convection), the energy timescale can be quite large. Setting the timestep of your energy equation to 10x or 100x larger than the fluid timescale can speed things up significantly.

Also, if your fluid properties are not temperature dependant and the hydrodynamic equations have already converged, consider turning off the solution of fluids by setting the following expert parameters:

solve fluids = f solve turbulence = f

You will find these in the expert parameters panel in Pre.

Regards, Robin

cmete November 2, 2006 16:37

Re: imbalance in H-Energy
 
I thank you, your advice about setting the timestep of y energy equation to 10x or 100x larger than the fluid timescale was come good for H-energy converge speed.

I did not select inc. viscous work term in Heat ransfer menu. could been that option selected for good converge?

again i thank you. best regards. cmete

TobiasZ November 3, 2006 10:03

Re: imbalance in H-Energy
 
Hi Robin,

I saw your comment on h-energy imbalances in solids when simulating an adjacent air domain. How about transient simulations? It is normally not possible to set a different time step for solid and fluid. In my case, a free convection flow in a room enclosed by solids, high T-energy imbalances (up to value 2-10)occured in some solids (particular in the horizontal top and bottom located solids) and low h-energy imbalances in the fluid. I believe the imbalance occurs because of the different thermal time constants of the solids and the fluid on one hand and on the GGI interface on the other hand (surface mesh at interfaces are not 1:1). Is it OK to neglect the energy imbalance, if the other convergence criteria e.g. RMS=1e-5 are met? Thanks in advance for your help. Regards Tobias

Robin November 6, 2006 09:27

Re: imbalance in H-Energy
 
Hi Tobiax,

The requirements for convergence are different in a transient simulation. It becomes importatant to converge the residuals within a timestep (this time including the transient term in the residual calcualtion), but it is no longer important to reduce global imbalances. This is not because global imbalances are unimportant, but rather because what you are trying to do is resolve the transient behavior accurately.

The difficulty with a transient CHT calculation is that the solid timescale is so much larger than the fluid timescale. To resolve the fluid behavior accurately, you must take a small timestep, but you still have to run for a long period of time to heat up your solid. The imbalance in your solid is simply indicating that it is still heating up (since the difference between what goes in and what goes out is what is accumulating in the domain).

If your fluid flow is nearly steady state, you can probably get away with a large timestep for your whole simulation. Better still, if the fluid temperature only changes slightly, you can assume constant properties relative to the temperature. In such a case it would be reasonable to solve a steady state flow and freeze this for the duration of your transient simulation. You would do this by initializing your transient simulation with the steady state solution and set the expert parameter 'solve fluids = f' and 'solve turbulence = f', leaving energy on. Your flow field would remain unchanged and you would only need to solve the energy equation, which is much cheaper, and you could use a large timestep.

Regards, Robin

TobiasZ November 6, 2006 10:38

Re: imbalance in H-Energy
 
Robin,

> The imbalance in your solid is simply indicating that it is
: still heating up (since the difference between what goes in
: and what goes out is what is accumulating in the domain).

this was exactly what I hoped to hear :)

If I understand it correctly, this means that it is OK having some imbalance in a solid domain (with high thermal mass) when going to the next time step since the imbalance does not show numerical error but only the process of heating up due to the capacity. Correct?

Thank you for your kind help, Robin.

Kind regards Tobias

Robin November 6, 2006 15:02

Re: imbalance in H-Energy
 
Hi Tobias,

This is exactly right. The control volume equations include flux terms for quantities flowing into and out of a control volume and a transient term, which accounts for the accumulation within a control volume. If the equations are solved correctly, these should balance out, i.e. what goes in should be equal to what goes out plus what accumulates. The residual is obtained by taking the sum of all three of these. If the equations are not fully solve, the residual is not zero.

At steady state the transient term should go to zero and therefore the fluxes should balance out themselves. So in a steady state calculation, CFX does not include the transient term in the residual.

In a transient simulation, it is important to ensure the equations solved within a timestep accurately represent the transient evolution of the control volume. In this case the residual does include the transient term, because how the control volume changes is important. This is why you can converge within a timestep in a transient simulation and why the residuals are different than in a steady state.

Regards, Robin

TobiasZ November 7, 2006 02:33

Re: imbalance in H-Energy
 
Robin, Excellent. Thank you again! Kind regards Tobias

happy September 5, 2012 05:01

question
 
Quote:

Originally Posted by Robin
;79037
Hi Tobiax,

The requirements for convergence are different in a transient simulation. It becomes importatant to converge the residuals within a timestep (this time including the transient term in the residual calcualtion), but it is no longer important to reduce global imbalances. This is not because global imbalances are unimportant, but rather because what you are trying to do is resolve the transient behavior accurately.

The difficulty with a transient CHT calculation is that the solid timescale is so much larger than the fluid timescale. To resolve the fluid behavior accurately, you must take a small timestep, but you still have to run for a long period of time to heat up your solid. The imbalance in your solid is simply indicating that it is still heating up (since the difference between what goes in and what goes out is what is accumulating in the domain).

If your fluid flow is nearly steady state, you can probably get away with a large timestep for your whole simulation. Better still, if the fluid temperature only changes slightly, you can assume constant properties relative to the temperature. In such a case it would be reasonable to solve a steady state flow and freeze this for the duration of your transient simulation. You would do this by initializing your transient simulation with the steady state solution and set the expert parameter 'solve fluids = f' and 'solve turbulence = f', leaving energy on. Your flow field would remain unchanged and you would only need to solve the energy equation, which is much cheaper, and you could use a large timestep.

Regards, Robin

Hi Robin,,
After the energy equation is convergenace how I can get my overall solution. I mean the flow field solution and enrgy solution.
please help me ASAP.
Best Regards
CFD user

ghorrocks September 5, 2012 06:07

Robin has not been sighted on this forum for years.

My preferred method of getting CHT simulations to convergence is by using a solid time scale factor. For typical air/steel systems (for example) a time scale factor of 1000 would be a good starting point.

Your comment suggests you have miunderstood robin's comment. You only freeze the fluids equation after it is converged.

happy September 7, 2012 04:43

Quote:

Originally Posted by ghorrocks (Post 380312)
Robin has not been sighted on this forum for years.

My preferred method of getting CHT simulations to convergence is by using a solid time scale factor. For typical air/steel systems (for example) a time scale factor of 1000 would be a good starting point.

Your comment suggests you have miunderstood robin's comment. You only freeze the fluids equation after it is converged.

Hi again
yes, I did it after I post my question:D.
Actually, the fluid field solution is remained and has been used to complate the convergance for energy equation within solid domain. After, it get convergance, the complete solution will be together within res file.
great hint:cool:
Bye


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