[suiger]

i need to model species transport with additional varialble.Then the density of working fluid must be defines as a funtion of addition variable,such as:Av1*5 [ ] *1[ kg m^-3] +1.225[kg m^-3].(Av1 is the name of additional variable)
when i begin the solutin ,show the error:
Error processing expression 'Density'.
The expression is invalid because:
Av1 is not available for use in this term

Error processing expression: Density = Expression 1

the question is that how to define the density of the fuid as a function of additional variable when i model species transport with additional variable?
thank you very much！

[ghorrocks]

Why do this using an additional variable? Why not use a multicomponent mixture and all this functionality is built-in, including the numerics required to stabilise it.

[usamaperwez]

First of all, units of the expression should be stable.