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Creating a UDF for heat generation given data points 

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December 5, 2022, 07:09 
Creating a UDF for heat generation given data points

#1 
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Aman
Join Date: Dec 2022
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Hi, I am currently trying to create a UDF for a source term to input into ANSYS FLUENT. I am modeling heat generation within a Liion cell and have the current charging profile. The heat generated within the cell is modeled with the equation q = I^2*R where I is current, R is internal resistance and q is the heat generation.
I have this current charging profile which varies over time. The internal resistance value is constant throughout the charging. I would like to know how to write a UDF such that the current changes with time according to this curve (attached image) . 

December 5, 2022, 07:45 

#2 
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Anas Nur Fauzan
Join Date: Oct 2019
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To add a source term, I think it's pretty obvious that we should use DEFINE_SOURCE macro. And for the given data points you can convert these values to a polynomial equation that varies along the timestep. What about the constant values? I suggest you implement a boolean that checks the current timestep. If the current timestep is below 500 ms then use constant source value and then after the timestep above 500 ms then return the source value calculated from the polynomial.
I understand the polynomial itself won't give you the exact value as in your sample data. But I think it would be still pretty valid. 

December 5, 2022, 09:12 

#3  
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Aman
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Quote:
I'm not sure what you mean by "implement a boolean". How can this be done? 

December 5, 2022, 10:05 

#4 
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Anas Nur Fauzan
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According to your provided data points, you can't use a polynomial to define the whole curve because the inconsistency of the curve (the source value from 0 ms to 500 ms is almost constant). So, I was thinking to use polynomial only for the source value from 500 ms and so on. And for below 500 ms, you can use the constant amount of heat source. What would I do to achieve this is something like this:
Code:
if (t == 500) { source = 4.199; // Constant value from 0 ms to 500 ms } else { source == x; // The x value should be your polynomial function from >500 ms until 2400 ms. } 

December 5, 2022, 13:54 

#5 
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Aman
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Thank you luzikato!
I have attempted it and have got the following. Does this seem correct or am i making a mistake? The constant values were obtained from a curvefitting program. #include "udf.h" #define C1 5E08 //define constant term #define C2 0.0003 #define C3 0.5805 #define C4 428.16 #define C5 4.194E9 DEFINE_SOURCE(heat_gen_cell,cell,thread,dS,eqn) { real source; real time; time = CURRENT_TIME; //taking time value; if (time == 500) { source = 4.199; // Constant value from 0 ms to 500 ms } else { source == C1 * pow(time, 3) + C2 * pow(time, 2) + C3 * pow(time, 1) + C4 ; // The x value should be your polynomial function from >500 ms until 2400 ms. } dS[eqn] = 0; return source; } 

December 9, 2022, 08:32 

#6 
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Anas Nur Fauzan
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Hi! Sorry for the late reply. The codes seem alright to me. Have you tried inserting it to your case?


December 12, 2022, 01:43 

#7 
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Alexander
Join Date: Apr 2013
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UDF uses UNITS in SI
500 ms = 0.5 sec not 500
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ansys 2021 r2, fluent  udf, udf and programming 
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