[TRNSYS-users] "Average value over time step" in TRNSYS Types

[knut.erik.enerstvedt at niva.no]

Jeff,

Thanks again for your answer. However, since my solution is known only for 
Ti and Tf and nowhere between, any attempt at integrating numerically over 
the time step and dividing by the time step length will result in a linear 
approximation for Tavg as before. I can of course solve the equation to 
find T at the midpoint of the time step, and then use a three-point 
numerical integration rule. I can't  think of anything else at the moment.

Kind regards,
Knut Erik



From:
"Jeff Thornton" <thornton at tess-inc.com>
To:
knut.erik.enerstvedt at niva.no
Cc:
trnsys-users at engr.wisc.edu
Date:
08/31/2009 07:04 PM
Subject:
Re: [TRNSYS-users] "Average value over time step" in TRNSYS Types



<If I solve the model equation for the time  t(i-1/2), would this be a
correct approach to find Tavg? I can then solve for both Tavg and Tf, and
use Tf as Ti in the next time step. Is this how it's done in the
standard/TESS TRNSYS components? >

No - in the TRNSYS components that have mass we solve for the average
temperature over the timestep and don't approximate it with a linear
solution or a solution at the midpoint of the timestep.

Take your solution for the temperature and integrate it over the timestep
and then divide by the timestep to get the average value.  Refer to the
Programmers guide in the section for differential equations for a more
detailed explanation when you have a simple linear DiffEq.

Jeff


Jeff Thornton
President - TESS, LLC

22 North Carroll Street - Suite 370
Madison WI 53703 USA

Phone: 608-274-2577
Fax: 608-278-1475
E-mail: thornton at tess-inc.com
Web: www.tess-inc.com




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