Implicit expression

Luigi · February 3, 2014, 08:26

I have to calculate a variable by an implicit expression and I would use at the Right Hand Side of the expression the value of the variable at the previous iteration (steady-state simulation).
Is it possible? Is there a Predefined Expression in CEL to recall the previous value of the variable?

ghorrocks · February 3, 2014, 17:15

The simple answer is no, you cannot do this in CFX.

If you have an implicit expression you will probably need user fortran to solve it.

Your comment about using the previous time step value is puzzling - this suggests your expression is explicit, not implicit.

But if you insist on doing this with CEL anyway, there are a few posts on the forum about how to get previous values. They are not pretty but they work.

Luigi · February 4, 2014, 06:56

Thanks for your reply
I will try to explain better my problem. I have an expression where the variable to be calculated can not be analytically made explicit. I thought that the code could be able to use such expression, since it solve numerically the equations; but it found cyclic the variable!!
So I need to solve somehow this implicit expression. Which could be the way??
I thought to recall the previous value of the variable on the right hand side of the expression, so that it would became explicit. How is it possible to do?

ghorrocks · February 4, 2014, 18:14

Your questions are answered in my previous post.

stumpy · February 11, 2014, 18:13

Try making an algebraic Additional Variable equal to the right hand side, then make your expression equal to the Additional Variable. You might need to provide initial values for the AV by using a previous solution. Still not sure if it would work.
There's info elsewhere on this forum for how to do this in a transient simulation (search for TRANS_LOOP), but since you are running steady state it should be simpler.

February 3, 2014, 08:26	Implicit expression	#1
Luigi New Member Luigi Gurreri Join Date: Jul 2012 Posts: 18 Rep Power: 14	I have to calculate a variable by an implicit expression and I would use at the Right Hand Side of the expression the value of the variable at the previous iteration (steady-state simulation). Is it possible? Is there a Predefined Expression in CEL to recall the previous value of the variable?

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February 3, 2014, 17:15		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	The simple answer is no, you cannot do this in CFX. If you have an implicit expression you will probably need user fortran to solve it. Your comment about using the previous time step value is puzzling - this suggests your expression is explicit, not implicit. But if you insist on doing this with CEL anyway, there are a few posts on the forum about how to get previous values. They are not pretty but they work.

February 4, 2014, 06:56		#3
Luigi New Member Luigi Gurreri Join Date: Jul 2012 Posts: 18 Rep Power: 14	Thanks for your reply I will try to explain better my problem. I have an expression where the variable to be calculated can not be analytically made explicit. I thought that the code could be able to use such expression, since it solve numerically the equations; but it found cyclic the variable!! So I need to solve somehow this implicit expression. Which could be the way?? I thought to recall the previous value of the variable on the right hand side of the expression, so that it would became explicit. How is it possible to do?

February 4, 2014, 18:14		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	Your questions are answered in my previous post.

February 11, 2014, 18:13		#5
stumpy Senior Member Join Date: Apr 2009 Posts: 531 Rep Power: 21	Try making an algebraic Additional Variable equal to the right hand side, then make your expression equal to the Additional Variable. You might need to provide initial values for the AV by using a previous solution. Still not sure if it would work. There's info elsewhere on this forum for how to do this in a transient simulation (search for TRANS_LOOP), but since you are running steady state it should be simpler.