CFD-Post: How to get the correct values

¥yy · September 18, 2014, 19:50

Hi everybody,
I hope you forgive my stupid questions

I have to compare some data of a CFD-simulation with the values I measured on a test-stand. Having never worked before with CFX I am ... at the end of my abilities. All I need are some numbers, so I used the table-view and tried to get the following values: pressure, total-pressure, temperature, total-temperature as well as absolute and relative velocity.
I read on this forum that it's better to use mass-averaging values for total-pressure and area-averaging for static-pressure source... I assume the same goes for temp and total temp, but I'm very unsure about the velocities:
The model in CFX is built in 4 parts: nozzle, volute, turbine, diffuser. As far as I understand I have 3 stationary parts and one rotating. To get the absolute velocity of the fluid I used "Velocity in Stn Frame" - is that correct? Do I have to use area or mass-averaged values? What do I have to use the relative speed?
Apart from the absolute Velocity I need c_m as well as c_u - the axial and circumference amount of the absolute speed. I used a "dirty" trick for both: c_m as massflow/(density*Area) and c_u as sqrt(c^2 - c_m^2)... are there better ways?

Maybe you don't believe me, but I searched for my questions, but as my English is limited I might have missed the right keyword, so be gentle

Greetings and thanks in advance to everybody who at least read this far!

ghorrocks · September 19, 2014, 07:09

When you apply some form of averaging over a flow to get a global quantity it is best to think of what parameter you are actually measuring to get the right type of averaging.

For instance: for pressures, the pressure integrated over the area is a force. This is often of physical significance, so area averaging for pressures makes sense. For temperatures, massflowAve gives a representation of the energy content of the gas so massflowAve makes sense there.

For your velocity, if you take the area average you are looking at the volume flow rate. If you that the massflowAve you are looking at the mass flow rate. So it is up to you which is more important.

To get your velocity components you can define a local coordinate frame and get them easily from there, or use the rotating machinery macro to generate it all.

September 18, 2014, 19:50	CFD-Post: How to get the correct values	#1
¥yy New Member Join Date: Sep 2014 Posts: 1 Rep Power: 0	Hi everybody, I hope you forgive my stupid questions I have to compare some data of a CFD-simulation with the values I measured on a test-stand. Having never worked before with CFX I am ... at the end of my abilities. All I need are some numbers, so I used the table-view and tried to get the following values: pressure, total-pressure, temperature, total-temperature as well as absolute and relative velocity. I read on this forum that it's better to use mass-averaging values for total-pressure and area-averaging for static-pressure source... I assume the same goes for temp and total temp, but I'm very unsure about the velocities: The model in CFX is built in 4 parts: nozzle, volute, turbine, diffuser. As far as I understand I have 3 stationary parts and one rotating. To get the absolute velocity of the fluid I used "Velocity in Stn Frame" - is that correct? Do I have to use area or mass-averaged values? What do I have to use the relative speed? Apart from the absolute Velocity I need c_m as well as c_u - the axial and circumference amount of the absolute speed. I used a "dirty" trick for both: c_m as massflow/(density*Area) and c_u as sqrt(c^2 - c_m^2)... are there better ways? Maybe you don't believe me, but I searched for my questions, but as my English is limited I might have missed the right keyword, so be gentle Greetings and thanks in advance to everybody who at least read this far!

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September 19, 2014, 07:09		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,852 Rep Power: 144	When you apply some form of averaging over a flow to get a global quantity it is best to think of what parameter you are actually measuring to get the right type of averaging. For instance: for pressures, the pressure integrated over the area is a force. This is often of physical significance, so area averaging for pressures makes sense. For temperatures, massflowAve gives a representation of the energy content of the gas so massflowAve makes sense there. For your velocity, if you take the area average you are looking at the volume flow rate. If you that the massflowAve you are looking at the mass flow rate. So it is up to you which is more important. To get your velocity components you can define a local coordinate frame and get them easily from there, or use the rotating machinery macro to generate it all.