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

[Miguel Casas <miguelcasasarredondo@xxxxxxxxx>]

Hi Saleh, once again.
I have two questions for you, I'd really appreciate it if you'd be so kind to answer.
1.
In a previous e-mail you told me the convergence criteria was calculated as such:
C_c = abs(sum (T node_i^m- T node_i^m-1) / sum(T node_i^m) ),
instead, couldn't you have meant:
C_c = sum(abs (T node_i^m- T node_i^m-1) / sum(T node_i^m) ) ?

2.
I already have the E+ code, but I can't seem to find the part where the convergence criteria is given. Could you tell me where to look for it?


Yours sincerely,
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...