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August 21, 2009, 06:09 
udfoutlet BChow to get flow rate from former iteration?

#1 
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christophe
Join Date: Aug 2009
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I am a newbie in udf and I need 2 udfs for my model: outlets and inlet's BC.
For the outlets BC I would like to provide a pressure value from the volume flow rate obtained at the former iteration. So my question is: it is possible to get the value of the volume flow rate through a thread, or a face. otherwise, it is possible to get the velocity through a face (with the face area that will give me the volume flow rate). I haven't found anything about this in the udf manual (but it's more than 600 pages so it's possible that I missed it). I am using fluent 6.3. Thanks for helping. ps by reading through again the udf manual, I think I can use the F_U_M1(f,t) macro. But I am not sure of the meanning for U,V,W. I just want the legth of the velocity vector. Does U,V,W stand for the cartesian coordinates of the velocity? Last edited by jjchristophe; August 21, 2009 at 09:33. 

August 21, 2009, 11:47 

#2 
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I don't know if there's a volume flow rate macro, but there are mass flow rate and density macros. You might try getting volume flow rate from those (volume flow rate = mass flow rate / density)...


August 24, 2009, 09:15 

#3 
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christophe
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Thanks for the reply, but as I said, with the F_U_M1(f,t) macro I can get the velocity, and the velocity can easily provide me the volume flow rate.
So, as there is three velocity macros (U,V, W) my question is does those macros provide the components of the velocity vector for a given face? 

August 24, 2009, 14:06 

#4 
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I can't find anything on that macro in the UDF manual or on the web. I would guess that U, V, and W are the cartesian axes, but in my experience macros such as this use subscripts [0], [1], and [2] for x, y, and z components, rather than separate macros.
Try posting your question at http://university.fluent.com/forum/  an ANSYS admin might be able to help you more. 

August 25, 2009, 04:15 

#5 
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Max
Join Date: Mar 2009
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Yes, F_U = xcomponent of velocity vector, F_V = ycomponent, F_W = zcomponent.
If you put these components into a vector "vel" you can use NV_MAG(vel) to get its magnitude. However, I am not sure if F_U_M1 is really available or if it will return what you want. I know C_U_M1 which returns the cell value of the last timestep in transient simulations but never tried F_U_M1. Maybe you should use a UDM to store the required values of the last iteration. cheers 

August 25, 2009, 04:49 

#6 
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christophe
Join Date: Aug 2009
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Thank you for your help.
As I can't have access to Fluent before mid september, I have to wait until then to check my codes, but I will look for more informations about UDM and the availability of F_U_M1 macro until then. 

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