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-   -   Resolving for one extra variable? (http://www.cfd-online.com/Forums/cfx/26771-resolving-one-extra-variable.html)

Matt December 7, 2008 08:49

Resolving for one extra variable?
 
Hi,

I've completed numerous runs and realised that I did not have density being output as a selected variable. This means that I cannot calculate the massflow for the affected timesteps.

Is there a simple or quick way, without re-solving each run, to make CFX calculate the density for each run?

Kind Regards, Matt.

Glenn Horrocks December 7, 2008 18:06

Re: Resolving for one extra variable?
 
Hi,

If the simulations are incompressible then density is constant so you can work out the volume flows and convert to massflows easily.

If the flow is compressible - well you need to have two independant gas properties to define the gas state. From any two properties you can calculate any other property. If you don't have two properties then your only choice is to re-run the simulation.

Glenn Horrocks

Matt December 7, 2008 21:05

Re: Resolving for one extra variable?
 
Thanks for the quick response Glenn.

I did export Pressure and Velocity, but without temperature I don't think I can calculate out the density.

Looks like I'll have to re-run some of the solutions.

Cheers, Matt.

Glenn Horrocks December 8, 2008 18:02

Re: Resolving for one extra variable?
 
Hi,

I think you have mis-understood me. If you are using an idal compressible gas you only need two parameters to define the state of the gas. State variables are temperature, pressure, density, enthalpy, entropy. If you have any two of these variables you can calculate any others. Velocity is not a state variable, but if you have total pressure, static pressure and velocity you will be able to get the density.

But if you only exported pressure and velocity and it is a compressible simulation then your only choice is to rerun to get density.

Glenn Horrocks


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