[ahvz]

Hi,

I am trying to write a UDF for the Specific heat (Cp) as a function of Temperature.
I used "curve fitting" technique to obtain the formula.

my First question, am I not sure if I put the below formula correctly (the original one is attached) ?

Cp=4.3568+27.57838*exp(-0.5*((K-19.08514)/2.00369)^2)

Second question, why the code is not interrupting ("float" error)? please guide me if through the code is right or not.

As its for a transient analysis so, I expected to call the temperature from current temperature.


#include "udf.h"
DEFINE_SPECIFIC_HEAT(my_user_cp,c,K,thread, position)
{
real cp; /*specific heat as a function of temperature*/
real K; /* Temperature*/


face_t f;
begin_f_loop(f,thread)
{
F_CENTROID(K,f,thread);
Cp = CURRENT_TEMPERATURE;

Cp=4.3568+27.57838*exp(-0.5*((K-19.08514)/2.00369)^2)

F_PROFILE(C, K, position) =Cp;
}
end_f_loop(C,K)
}


regards,

[blackmask]

As far as I can recall, the binary operator "^" in C means BIT-AND and it expects both operand to be integer, so you should replace
(...)^2
to
pow( (...), 2.0 )
when you need to calculate the power of a float number.

Correct me if I am wrong

[ahvz]

Thank you for reply,

Actually, I recived this error for the formula :

pow: too few arguments supplied (argument 2)

which its :

cp=4.3568+27.57838*exp(-0.5*(pow((T-19.08514)/2.00369),2.0)

[blackmask]

Your ")" does not match "("

cp=4.3568+27.57838*exp(-0.5*( pow( (T-19.08514)/2.00369,2.0) ) );

[ahvz]

thank you! the formula's error now solved.

but still have error "invalid type for binary expression: int + pointer to float" it belongs to the line after formula I guess.


#include "udf.h"
DEFINE_SPECIFIC_HEAT(my_user_cp,c,T,thread, position)
{
real cp; /*specific heat as a function of temperature*/
real T; /* Temperature*/


face_t f;
begin_f_loop(f,thread)
{


cp=4.3568+27.57838*exp(-0.5*( pow( (T-19.08514)/2.00369,2.0) ) );


F_CENTROID(T,f,thread);
y = CURRENT_TEMPERATURE;
F_PROFILE(f, thread, position) =cp;
}
end_f_loop(C,T)
}


regards,

[blackmask]

Try this one, adapted from the fluent udf manual.

#include "udf.h"
DEFINE_SPECIFIC_HEAT(my_user_cp, T, Tref, h, yi)
{
real cp=4.3568+27.57838*exp(-0.5*( pow( (T-19.08514)/2.00369,2.0) ) );
*h = cp*(T-Tref);
return cp;
}

[ahvz]

thank you very much,

It was hooked to the Fluent.

As I have two UDF file, Is it possible to hook two UDF for analysis at same time ?

it seems impossible as I tryied to do!

what I should to do ?


regards,

[blackmask]

I think at least you can concatenate the two files into a single file.

[ahvz]

I did and its work very well. I just paste the second code at the end of first code. let see the results...

many thanks,

[ahvz]

would you please tell me how to change inital temperature value in Fluent ? and in C program code as UDF file ?

as I can see the initial temperature (at step 0) for the model, its 26 C by default by Fluent. how can I change it ? where is the option menu in Fluent ? or in this such a case that in transient analysis I must define at UDF file which its already interrupted to the Fluent ?

Note: when I am using hybrid initialization!
regards,

[blackmask]

You can use DEFINE_INIT to specify the initial values.

[ahvz]

Done!

it works very well, thank you very much,

best regards,

[ghost82]

Originally Posted by blackmask

Try this one, adapted from the fluent udf manual.

#include "udf.h"
DEFINE_SPECIFIC_HEAT(my_user_cp, T, Tref, h, yi)
{
real cp=4.3568+27.57838*exp(-0.5*( pow( (T-19.08514)/2.00369,2.0) ) );
*h = cp*(T-Tref);
return cp;
}

It's an old post, but take care that the equation for enthalpy is wrong, since it's valid for constant cp(f(T)).
Correct equation is integral of cp.

[thanhho]

Originally Posted by ghost82

It's an old post, but take care that the equation for enthalpy is wrong, since it's valid for constant cp(f(T)).
Correct equation is integral of cp.

In which case, how would one write the integral of cp to correctly represent the enthalpy in UDF?

[rampal]

Dear friends, I have to write udf for specific heat of water as function of temperature. I have written according to DEFINE_PROPERTY macro, but it is not working. Specific heat of water as a function of temperature (in deg Celsius) is as follows:
cp = 4.2174356 - 0.0056181625*temp + 0.0012992528*pow(temp,1.5) - 0.00011535353*pow(temp,2.0) + 4.14964*pow(10.0,-6.0)*pow(temp,2.5)

I have seen in UDF manual DEFINE_SPECIFIC_HEAT, but don't understand how can I write for above defined function. Please help me.

Thanks in advance