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Re: [EnergyPlus_Support] Why does E+ lose accuracy for lower values of the space discretization constant?





Hi again Saleh.

As I offered you, I am sending you now a report in which we propose a modification for the convergence criteria used in E+ to solve the Gauss-Seidel iterative method.

We believe that this modification would yield higher accuracy when the number of nodes is increased (or the space discretization constant is decreased).

I hope this effort is on the right path and could make a contribution.


With regards,

Miguel Casas


De: saleh saadi <salehsnjsaadi404@xxxxxxxxx>
Para: "EnergyPlus_Support@xxxxxxxxxxxxxxx" <EnergyPlus_Support@xxxxxxxxxxxxxxx>
Enviado: Viernes, 24 de mayo, 2013 18:51:57
Asunto: Re: [EnergyPlus_Support] Why does E+ lose accuracy for lower values of the space discretization constant?

 

Based on my previous check of the E+ code (version 7), the convergence criteria is calculated as such:

C_c = abs(sum (T node_i^m- T node_i^m-1) / sum(T node_i^m) ), where m stands for iteration level.

As the number of nodes increases, it is good to use small convergence criteria as well as small time step. This seems to work well for me when
Gauss-Seidell solver is used with the equation of convergence criteria mentioned above

Are you modeling Phase Change Materials (PCM) or massive structure? If you are not modeling any of these structures, then go for CTF algorithm.

regards
Saleh


From: Miguel Casas <miguelcasasarredondo@xxxxxxxxx>
To: "EnergyPlus_Support@xxxxxxxxxxxxxxx" <EnergyPlus_Support@xxxxxxxxxxxxxxx>
Sent: Friday, May 24, 2013 5:37 PM
Subject: Re: [EnergyPlus_Support] Why does E+ lose accuracy for lower values of the space discretization constant?

 
Hi Saleh. I am glad you answered.

Yes, I am sure the number of nodes increase by decreasing the Space Discretization Constant (C). I quote from the Engineering Reference Documentation: "Lower values for the Space Discretization Constant yield more nodes, with higher values yield fewer nodes".

I already modified the Convergence Criteria, using the lowest value. My results got better, but still, the same tendecy came out: lower values for C would generate lower accuracy. Why could this be happening?

When I read the Engineering Reference Documentation I found kind of a definition for the Convergence Criteria. I quote: 

Because the solution is implicit... The Gauss-Seidell iteration loop is the inner-most solver and is called for each surface. It is limited to 30 iterations but will exit early when the sum of all the node temperatures changes between the last call and the current call, normalized by the sum of the temperature values, is below 0.000001C. This convergence criteria is typically met after 3 iterations...

From what I understood, the Convergence Criteria is given by

C_c = sum^i (T node_i^t - T node_i^t-1) / number of nodes.

Is this equation correct? Am I interpreting this parameter correctly?



From: Miguel Casas <miguelcasasarredondo@xxxxxxxxx>
To: "EnergyPlus_Support@xxxxxxxxxxxxxxx" <EnergyPlus_Support@xxxxxxxxxxxxxxx>
Sent: Wednesday, May 22, 2013 3:37 PM
Subject: [EnergyPlus_Support] Why does E+ lose accuracy for lower values of the space discretization constant?

 

¿ENERGY PLUS: Why does this software lose accuracy for lower values of the space discretization constant?

I am using Energy Plus to simulate a cubic test hut without air conditioning in a hot climate. When I modify the Space Discretization Constant (C), set as default as 3, awkard results come out. I was expecting that lower values for C would yield more accurate results. Nonetheless, when I use low values for C, the results tend to lose accuracy. The fully implicit finite difference scheme was used with the default convergence criteria.


Miguel Casas Arredondo
52 (777) 2245453
Insituto de Energías Renovables
UNAM










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