# Expression for two domains

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 May 29, 2008, 09:35 Expression for two domains #1 Blee Guest   Posts: n/a Hi there, Using Expression, how can I define a volume averaged mass fraction at two domains? For one domain, the Expression I used is like this: "volumeAve(Water Vapour at 25 C.Mass fraction)@One domain" I want to measure the same quantity at both One domain and Two domain. Thanks.

 May 29, 2008, 13:33 Re: Expression for two domains #2 andy2O Guest   Posts: n/a You want a single number giving the average over 2 domains, right? You just need to recall that if WholeDomain is formed from the combination of Domain1 and Domain2 (and assuming that Domain1 and Domain2 do not overlap) then mathematics says that: (Integral over WholeDomain) = (Integral over Domain1) + (Integral over Domain2) That's generally true, so now just apply this rule to computing a volume average in CFX over the combined domain. Then you find: (Volume Average over WholeDomain) = (Integral of (Variable) over WholeDomain) / (Volume of WholeDomain)= (volumeInt(Variable)@Domain1 + volumeInt(Variable)@Domain2) / (volume()Domain1 + volume()Domain2) So the answer is, in CFX terms: (volumeInt(Variable)@Domain1 + volumeInt(Variable)@Domain2) / (volume()Domain1 + volume()Domain2) I hope that makes sense! Obviously my "Variable" needs to be replaced with your mass fraction variable, etc and my domain names "Domain1" and "Domain2" need to be changed to be your real domain names. Andy

 May 30, 2008, 11:21 Re: Expression for two domains #3 Blee Guest   Posts: n/a Good tip, Andy. Thanks for it.

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