[lbj007]

I'm using incompressible ideal gas model,and when I use UDM to monitor the pressure,I find it is different from the value calculated by fluent's counter.I don't understand

#include"udf.h"                                                                                                     
#define H2 0
#define H2O 1
#define N2 2
#define O2 3
#define SO2 4
#define NUM_SPECS 5
#define R 8.31447
#define pi 3.14159
#define F 96485

DEFINE_DIFFUSIVITY(gas_diff,c,t,i)
{
	real v[4],y[4],Dkn[4],D[4][4];
	real M[4]={2e-3,18e-3,28e-3,32e-3};

	real tem=C_T(c,t);
    concen0=C_R(c,t)*C_YI(c,t,0)/M[0];     /*H2*/
    concen1=C_R(c,t)*C_YI(c,t,1)/M[1];     /*H2O*/
    concen2=C_R(c,t)*C_YI(c,t,2)/M[2];     /*N2*/
    concen3=C_R(c,t)*C_YI(c,t,3)/M[3];     /*O2*/
    concent1=concen0+concen1;               /*Anode*/                       
    concent2=concen2+concen3;               /*cathode*/                      
    p1=concent1*R*tem;                                             
    p2=concent2*R*tem;                                            
	C_UDMI(c,t,14)=p1;
	C_UDMI(c,t,15)=p2;

Here the mole fraction of SO2 is always zero,and there are H2 and H20(g）in the anode,N2 and O2 in the cathode, for additional explanation.

[lbj007]

does it due to that fluent uses Density = P_operate/(R*T),it is a constant,and in udf the density is not a constant?

[AlexanderZ]

not sure, but you may check it
macro to get operating pressure is

p_op = RP_Get_Real("operating-pressure");

[lbj007]

Originally Posted by AlexanderZ

not sure, but you may check it
macro to get operating pressure is

p_op = RP_Get_Real("operating-pressure");

but operating-pressure is the value you set in fluent,it is of course a constant..

[pakk]

It might help if you share the value that fluent shows, and the value that your UDF shows.

[lbj007]

Originally Posted by pakk

It might help if you share the value that fluent shows, and the value that your UDF shows.

I've already showed the picture.

[pakk]

Originally Posted by lbj007

I've already showed the picture.

I don't see any picture.

[lbj007]

Originally Posted by pakk

I don't see any picture.

can you open: http://cdn.img.headns.com/2010/3934b2dcef76f.png

[pakk]

Very nice picture.

Now back to the topic: which value does fluent give, and which bakkie does your UDF give? You say they are different, shoe me the difference!

[lbj007]

Originally Posted by pakk

Very nice picture.

Now back to the topic: which value does fluent give, and which bakkie does your UDF give? You say they are different, shoe me the difference!

what I've given you is fluent's value,and here is the udf's value:http://cdn.img.headns.com/2010/395056fbb492a.png

[pakk]

Looking at the minima: one says 101083 Pa, the other 101325 Pa. That is a difference of only 0.2%, very small.

So, what small inaccuracy do you have in your code? I think the values of the molecular weights.

[lbj007]

Originally Posted by pakk

Looking at the minima: one says 101083 Pa, the other 101325 Pa. That is a difference of only 0.2%, very small.

So, what small inaccuracy do you have in your code? I think the values of the molecular weights.

ok,I got it.Thanks