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ST October 25, 2005 01:35

Problem for Mass flow average in Post...
I recently run a transient simulation for my problem and wish to take mass flow averaged total pressure at each time period. However, I can't actually get the mass flow average value from the caculator when I use the minimal output option for the transient results. I did choose pressure,total pressure,velocity,absolute pressure in the output variable list. It worked okay when I click everything in the output control but it takes lots of space.

Just wonder if anyone know what variables I should add in the output list to get the mass flow averaged values of a plane. Thanks....

ST October 25, 2005 01:36

Re: Problem for Mass flow average in Post...
I'm using CFX5.7.1 with SST model at the moment.

Robin October 27, 2005 13:35

Re: Problem for Mass flow average in Post...
Hi ST,

If you have pressure, then absolute pressure is redundant since it is simply 'pressure + reference pressure'. You will need Density to calculate the mass flow rate.

Normally when you calculate mass flow, Post will use the integration point (ip) flows written to the RES file by the solver. This is the most accurate, since the details on how the solver computes the flows at each subface are complicated. When you write a partial results file, the ip flows are not included. The closest estimate is to calculate the area integral of density times normal velocity, thus:

Vnorm = Normal X * Velocity u + Normal Y * Velocity w + Normal Z * Velocity w
rho = xxx [kg/s] #or write density to .trn if it is variable
mass flow = areaInt(rho*Vnorm)@Plane 1

Note that this is only an approximation to the actual mass flow, since it assume contant mass flow across the face, which is not what the solver is calculating.


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