[mamalili]

hi every 1
I want to write below function to take b values of a values in CFD-post to define new expression and so new variable, but I dont know how to do it,

b: when a=0 --> b=0 & when a is not zero --> b=1/a
tank U my friends

[ghorrocks]

b = if(a=0,0,1/a)

[mamalili]

tank U dear ghorrocks
I try it in details of expressions in the expressions tab, but I see this error:

"""""""""""""""""""""""""""""""""""""""""""""""""" """"""""""""""""""""""

WARNING
Syntax error detected in the expression assigned to 'h2VF'.
Successfully read 26 characters:
if(H2 at STP.Mass Fraction
then error detected at:
=0,0,1/(1+(0.09/1.185)*(1/H2 at STP.Mass Fraction-1)))
Details - Unexpected character.


ERROR
Syntax error detected in the expression assigned to 'h2VF'.
Successfully read 26 characters:
if(H2 at STP.Mass Fraction
then error detected at:
=0,0,1/(1+(0.09/1.185)*(1/H2 at STP.Mass Fraction-1)))
Details - Unexpected character.


"""""""""""""""""""""""""""""""""""""""" """"""""""""""""""""""""""""""""""""


my variable is HVF ( hydrogen volume fraction) and define so:
HVF=1/(1+(0.09/1.185)*(1/H2 at STP.Mass Fraction-1))

but where "H2 at STP.Mass Fraction" is zero HVF undefined

[ghorrocks]

Your variable name is hydrogen VOLUME FRACTION but the CEL expression is using the H2 MASS FRACTION. Are you talking about mass fractions or volume fractions?

[mamalili]

Deer ghorrocks
Thank you for all of your efforts.
I explain it so:
My solution contains H2 Mass Fraction(HMF) and I need H2 Volume Fraction(HVF), and we know that:


HVF=1/(1+(0.09/1.185)*(1/HMF-1))

I created a new expression (VHF) in expression tab,
Now please exactly tell me: what I write in the expression details?
I really appreciate it if you can guide me, on this.
Thank you for your time

[ghorrocks]

I suspect CFX is looking for the C equality test (==), not the variable assignment operator (=). So try if(a==0,0,1/a)

You might need to put a bit of a tolerance around this to handle numerical noise. But as you just seem to be handling the special case for when a=0 maybe not.

[mamalili]

thank U very much deer ghorrocks
your answer was true.

I faced with new problem:

I decide to define "Hazard region" so:

where: 0.04<HVF<0.07

with this relation, when or where hazard region existe, CFD post show me '1' and when it not existe, CFD post referd '0' to me.

I try to define (0.04<HVF) and it was successful , but the try to define it (0.04<HVF<0.07) was unsuccessful and CFD post dont refered any thing to me.

[ghorrocks]

CFX uses C logic operations so look at a C reference manual for the correct syntax. There is also a breif description in the CFX reference manual.

try something like if(HVF>0.04 && HVF<0.07,do this,or do that)

[mamalili]

Yes it is working very well
Thank U dignified ghorrocks!