Custom Field Functions: Data from different locations

pisykl · May 23, 2016, 00:21

Hello,

I'm a greenhorn in CFD and so my question regarding Custom Field Functions may be really basic:

I simulate a flow around a blade of a turbomachine. Now I would like to calculate the loss coefficient zeta at a measurement line 50 mm behind the blade with zeta = delta(ptot)/((rho/2)/c1^2). I tried to do this with a custom field function, but for the calculation I need data from two different locations: The total pressure and the velocity from the inlet and the total pressure at the measurement line. So far I entered the data from the inlet manually. But is there a way so integrate these data automatically? Thanks for your help!

Greetings
Chris

nvarma · May 23, 2016, 16:04

As far as I know, CFFs are field functions, ie. they are general functions for the entire domain and has a value at every point.

You cannot take input values from a particular boundary for a CFF. CFF is a "function".

You might want to look into macros.

Or better, if you have CFX you can use the expressions there to calculate this accurately.

LuckyTran · May 24, 2016, 21:12

Custom field functions are field functions.

What you are trying to do is a simple expression. It's awkward to do this in Fluent, much easier to do in CFD post, excel, matlab, by hand, etc.

Although, you can do the integration in Fluent using surface monitors. But to evaluate the complete expression is a little weird. You can do it with Fluent scheme commands, but what is the point? What would you do with the resulting value? Is automation in Fluent really needed?

pisykl · May 25, 2016, 07:28

Thanks! I'll use matlab for this – this is the most comfortable way for me. I can export the data files I need (total pressure, velocity etc.) and write a script in Matlab.

May 23, 2016, 00:21	Custom Field Functions: Data from different locations	#1
pisykl New Member Chris Join Date: May 2016 Posts: 10 Rep Power: 10	Hello, I'm a greenhorn in CFD and so my question regarding Custom Field Functions may be really basic: I simulate a flow around a blade of a turbomachine. Now I would like to calculate the loss coefficient zeta at a measurement line 50 mm behind the blade with zeta = delta(ptot)/((rho/2)/c1^2). I tried to do this with a custom field function, but for the calculation I need data from two different locations: The total pressure and the velocity from the inlet and the total pressure at the measurement line. So far I entered the data from the inlet manually. But is there a way so integrate these data automatically? Thanks for your help! Greetings Chris

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May 23, 2016, 16:04		#2
nvarma Senior Member nm Join Date: Mar 2013 Posts: 100 Rep Power: 13	As far as I know, CFFs are field functions, ie. they are general functions for the entire domain and has a value at every point. You cannot take input values from a particular boundary for a CFF. CFF is a "function". You might want to look into macros. Or better, if you have CFX you can use the expressions there to calculate this accurately.

May 24, 2016, 21:12		#3
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,751 Rep Power: 66	Custom field functions are field functions. What you are trying to do is a simple expression. It's awkward to do this in Fluent, much easier to do in CFD post, excel, matlab, by hand, etc. Although, you can do the integration in Fluent using surface monitors. But to evaluate the complete expression is a little weird. You can do it with Fluent scheme commands, but what is the point? What would you do with the resulting value? Is automation in Fluent really needed?

May 25, 2016, 07:28		#4
pisykl New Member Chris Join Date: May 2016 Posts: 10 Rep Power: 10	Thanks! I'll use matlab for this – this is the most comfortable way for me. I can export the data files I need (total pressure, velocity etc.) and write a script in Matlab.