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Unsuccessfully trying to use Given/find within iterations

Unsuccessfully trying to use Given/find within iterations

Unsuccessfully trying to use Given/find within iterations

(OP)
I am unsuccessfully trying to use Given/find within  iterations.  I am trying to calculate friction factor (f) as a function of Reynolds number Re(dP) within iterations of pressure loss (dP) for a channel using the Colebrook correlation which has the form,
1/f = C1 +C2 -2*log(C3 + C4/(C5*Re(dP)*f^1/2)
If I iterate on Reynolds number then I can calculate f for each Re using Given/Find.
But when I iterate on dP, then calculate Re(dP) and then try to calculate f, then the expression is declared to be illegal. Any ideas about what I am doing wrong? I have tried writing f as f(dP) but that does not work either. I need to iterate on dP rather than Re since this is for parallel channel flow with the same dP across each channel but different Re in each channel.

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