Numerical problem

CrisCFD · February 10, 2012, 04:40

Hello! I have inserted a constant in my model (dielectric constant), which is very low (2,21 E-10) and the simulation does not converge.
What can I do?

ghorrocks · February 10, 2012, 05:23

You question is meaningless. What does the constant do? How is it coupled to the simulation? What are you modelling?

CrisCFD · February 10, 2012, 06:27

Quote:

Originally Posted by ghorrocks

You question is meaningless. What does the constant do? How is it coupled to the simulation? What are you modelling?

I previously described my case, in another thread...
I'm modelling a reverse electrodialysis system...so I have Na+ (cations) and Cl- (anions).
So I have two transport equation (cations and anions)
I have to consider the electrical interactions in the transport equation as an additional term, as a source term.
I just found an expression like this: 1) S+ = - σ / ε * (C-A) (cations)
and
2) S- = σ / ε * (C-A) (anions)
Where A and C are the concentrations of anions and cations, respectively
But the very low value of the costant "ε " (dielectric constant = 2,21 E-10) is a problem for the solver, a numerical problem, I think.
Also theese source terns which are dependent on each other could be a problem...I know it is a particular issue...
What do you think about it ?

ghorrocks · February 11, 2012, 05:24

Just because a constant term is small does not tell you it has a big effect or small. It is how it interacts with the flow which is important. There will be some form of non dimensional number to describe these effects, pesumably the ratio of electrical forces to fluid flow inertial forces or something like that. That sort of number starts telling you how significant your constant term is.

CrisCFD · February 15, 2012, 12:20

Quote:

Originally Posted by ghorrocks

Just because a constant term is small does not tell you it has a big effect or small. It is how it interacts with the flow which is important. There will be some form of non dimensional number to describe these effects, pesumably the ratio of electrical forces to fluid flow inertial forces or something like that. That sort of number starts telling you how significant your constant term is.

I have no information about this sort of number...this is the reason I'm modelling this process

ghorrocks · February 15, 2012, 17:13

Starting simulation without knowing which effects are important is a reciepe for disaster. You need to do some back of the envelope calculations to work out what is significant and what is not before starting modelling.

CrisCFD · February 18, 2012, 09:06

Quote:

Originally Posted by ghorrocks

Starting simulation without knowing which effects are important is a reciepe for disaster. You need to do some back of the envelope calculations to work out what is significant and what is not before starting modelling.

My supervisor ordered me (

) to consider these electrical interactions

ghorrocks · February 18, 2012, 20:07

Do you know how many times it has come back to "my supervisor told me to"...

Regardless of what your supervisor says, do some back of the envelope calculations to check it is significant. And the degree of significance will tell you what sort of approach you need to take.

CrisCFD · March 6, 2012, 11:06

Quote:

Originally Posted by ghorrocks

Do you know how many times it has come back to "my supervisor told me to"...

Regardless of what your supervisor says, do some back of the envelope calculations to check it is significant. And the degree of significance will tell you what sort of approach you need to take.

Experimental measurements showed that these terms are relevant. What can I do?

ghorrocks · March 6, 2012, 16:06

My previous post explains what you must do. If the electrical terms are relevant, then what about the viscous terms? The inertial terms? If any of these are irrelevant then you can select the correct options to proceed. Otherwise you are just guessing.

But what is the physics of what you are modelling anyway? I seem to remember you asking about imposing a constant drift velocity on top the fluid velocity - is this the same issue?

CrisCFD · March 8, 2012, 13:26

Quote:

Originally Posted by ghorrocks

My previous post explains what you must do. If the electrical terms are relevant, then what about the viscous terms? The inertial terms? If any of these are irrelevant then you can select the correct options to proceed. Otherwise you are just guessing.

But what is the physics of what you are modelling anyway? I seem to remember you asking about imposing a constant drift velocity on top the fluid velocity - is this the same issue?

I'm modelling the flux of cations and anions through electrodialysis membranes.
So I have to consider the electric interations as source terms in the transport equations (cations and anions). We have a laminar flow in the channel

ghorrocks · March 8, 2012, 17:26

My question is what is the physics of what you are modelling. To model it you need to know exactly how each important force works because you need to describe an equation which accurately describes it.

Questions include:
* How is charge balance maintained? Does it have a conservation equation?
* How is motion of the charges coupled to the fluid flow?
* Is the charge motion independant of the fluid flow?

But my point about working out the significant forces needs to be considered first. It will tell you what approach you need to take and what things can be ignored. It is quite likely that CFX is not appropriate for what you are modelling at all - you will only know once you know what forces are significant.

February 10, 2012, 04:40	Numerical problem	#1
CrisCFD New Member Join Date: Feb 2012 Posts: 10 Rep Power: 14	Hello! I have inserted a constant in my model (dielectric constant), which is very low (2,21 E-10) and the simulation does not converge. What can I do?

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February 10, 2012, 05:23		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	You question is meaningless. What does the constant do? How is it coupled to the simulation? What are you modelling?

February 11, 2012, 05:24		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Just because a constant term is small does not tell you it has a big effect or small. It is how it interacts with the flow which is important. There will be some form of non dimensional number to describe these effects, pesumably the ratio of electrical forces to fluid flow inertial forces or something like that. That sort of number starts telling you how significant your constant term is.

February 15, 2012, 17:13		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Starting simulation without knowing which effects are important is a reciepe for disaster. You need to do some back of the envelope calculations to work out what is significant and what is not before starting modelling.

February 18, 2012, 20:07		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Do you know how many times it has come back to "my supervisor told me to"... Regardless of what your supervisor says, do some back of the envelope calculations to check it is significant. And the degree of significance will tell you what sort of approach you need to take.

March 6, 2012, 16:06		#10
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	My previous post explains what you must do. If the electrical terms are relevant, then what about the viscous terms? The inertial terms? If any of these are irrelevant then you can select the correct options to proceed. Otherwise you are just guessing. But what is the physics of what you are modelling anyway? I seem to remember you asking about imposing a constant drift velocity on top the fluid velocity - is this the same issue?

March 8, 2012, 17:26		#12
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	My question is what is the physics of what you are modelling. To model it you need to know exactly how each important force works because you need to describe an equation which accurately describes it. Questions include: * How is charge balance maintained? Does it have a conservation equation? * How is motion of the charges coupled to the fluid flow? * Is the charge motion independant of the fluid flow? But my point about working out the significant forces needs to be considered first. It will tell you what approach you need to take and what things can be ignored. It is quite likely that CFX is not appropriate for what you are modelling at all - you will only know once you know what forces are significant.