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-   -   Which method is better to post-process for mass and energy flows? (http://www.cfd-online.com/Forums/cfx/95291-method-better-post-process-mass-energy-flows.html)

 Chander December 12, 2011 15:21

Which method is better to post-process for mass and energy flows?

If one wants to calculate mass and energy flows at various planes in a domain, which one of the following methods is more accurate?

1. Calculate these inside CFX-Post using functions like massFlowInt, massflowave etc. on variables like velocity u, velocity v, velocity w, area, static enthalpy etc.

2. Export these values on the required planes to text files and then use these text files to calculate the relevant quantities through say MATLAB.

 ghorrocks December 12, 2011 17:13

Option 1 is most accurate. This will use the integration points used by the solver so will maintain the solver accuracy. Option 2 only uses the nodal values and so will introduce a difference to approach used int he solver.

 Chander December 13, 2011 05:21

I agree completely.
However, in one of my simulations, I had to calculate the heat transfer coefficient (HTC) for heat transfer from wall to fluid in channel flow at various axial locations along the channel. I defined HTC at any axial location x as per the traditional definition as
Average heat flux at x = HTC (x)*(Average Twall(x) - Tbulk(x))

Since Average heat flux at x, Average Twall(x) at any axial location x and Tbulk(x) at the corresponding axial location were not available from CFX-Post, I had to export the nodal heat flux, Twall and Tfluid values along several axial planes and then use this info available in exported text files to calculate HTC(x) in MATLAB.
I understand that using nodal values would not have given me accurate values of HTC(x), but could I have done anything better?

 ghorrocks December 13, 2011 16:08

Quote:
 I understand that using nodal values would not have given me accurate values of HTC(x)
No, I never said it was not accurate. I said using the full integration in CFX was more accurate. The approach you describe might well have been accurate enough for what you are doing.

This could also be done with a script in CFD-Post which takes strips of your geometry and does integrations on the strip. It would have required a little programming of session files to work I suspect.

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