CFD Online Discussion Forums - "a variable = C

CFD Online Discussion Forums (https://www.cfd-online.com/Forums/)

- Fluent UDF and Scheme Programming (https://www.cfd-online.com/Forums/fluent-udf/)

- - "a variable = C_UDMI(c,t,0)" does not work. (https://www.cfd-online.com/Forums/fluent-udf/207170-variable-c_udmi-c-t-0-does-not-work.html)

"a variable = C_UDMI(c,t,0)" does not work.

Lift force on bubble is Zero in this macro. Other variables to compute "FL" have their own numbers. Not zero.

I have no idea why "FL" goes to zero on each cell. C_UDMI problem?

Would you give me any advices to fix this issue? Thanks in advance.
--------------------------------------------------------------------------------------

DEFINE_SOURCE(Lift,c,tc,dS,eqn)

{

real source;

real sum_vv;

real sum_fa_top;

real sum_fa_bot;

real Re;

real CL;

real sum_fa;

real Gs;

real FL;

real db;

real M;

real xc[ND_ND];

real mu = 3.994e-4;

real Uz = 1.4;

real rho = 1615;

sum_vv = C_UDMI(c,tc,0);

sum_fa_top = C_UDMI(c,tc,1);

sum_fa_bot = C_UDMI(c,tc,2);

sum_fa = sum_fa_top + sum_fa_bot;

Thread **pt = THREAD_SUB_THREADS(tc);

Thread *ts = pt[1];

if(sum_vv > 0. && sum_fa > 0.)

{

if(C_VOF(c,ts) >= 0.5)

{

db = sum_vv/sum_fa;
printf("db: %e\n", db);

Re = rho*C_W(c,tc)*db/mu;
printf("Re: %e\n", Re);

Gs = fabs(C_DWDY(c,tc))*db/2/C_W(c,tc);
printf("Gs: %e\n", Gs);

M = 1/(pow(Re,2))+0.014*pow(Gs,2);
printf("M: %e\n", M);

CL = 3.877*pow(Gs,1/2)*pow(M,1/4);
printf("CL: %e\n", CL);

FL = 3/4*CL*rho*C_W(c,tc)*C_W(c,tc)/db; -----> !!! ISSUE
printf("FL: %e\n", FL);

C_CENTROID(xc,c,tc);

if(xc[1] < 0.0025)

{

source = FL;

dS[eqn] = 0;

printf("positive: %e\n", source);

}

else if(xc[1] > 0.0025)

{

source = -FL;

dS[eqn] = 0;

printf("negative: %e\n", source);

}

else

{
printf("nothing");

source = 0;

dS[eqn] = 0;

}

}

}

return source;

}

did you define your UDMIs before calculation?

best regards

Hi jm86groove,

If you think that FL should not be zero, but you get answers that suggest it is, then you can start debugging by printing details:

Code:

FL = 3/4*CL*rho*C_W(c,tc)*C_W(c,tc)/db;

if(fabs(FL)<1e-6) {

  Message("Bah! FL is near zero again: %g\n",FL);

  Message("   CL=%g, rho=%g, C_W=%g, db=%g\n",CL,rho,C_W,db);

  /* and eventually you become paranoid: */

  Message("   3/4 = %g\n",3/4);  /* This might well give zero!!!!! */

  Message("   3./4. = %g\n",3./4.);

  Message("   3./4.*CL = %g\n",3./4.*CL);

  Message("   3./4.*CL*rho = %g\n",3./4.*CL*rho);

  Message("   3./4.*CL*rho*C_W(c,tc) = %g\n",3./4.*CL*rho*C_W(c,tc));

  Message("   3./4.*CL*rho*C_W(c,tc)*C_W(c,tc) = %g\n",3./4.*CL*rho*C_W(c,tc)*C_W(c,tc));

  Message("   3./4.*CL*rho*C_W(c,tc)*C_W(c,tc)/db = %g\n",3./4.*CL*rho*C_W(c,tc)*C_W(c,tc)/db);

}

If the model is large and that gives an overwhelming amount of text to the screen, you can select some cells arbitrarily: replace

Code:

if(fabs(FL)<1e-6) {

with

Code:

if(c%10000==1234 && fabs(FL)<1e-6) {

A couple of points:
-- I would advise always using Message rather than printf.
-- Integer division rounds to the nearest integer, so there is always a risk that "3/4" is calculated to be "0" before it starts being floating-point. The same applies where you have typed "1/2" and "1/4" -- you should never type this! "1./2." is better, and "0.5" is better still. And "sqrt(Gs)" is probably quicker than "pow(Gs,0.5)", but speed is less important than correctness.

-- I picked 1e-6 as a small number, but it might not be small enough for your example -- in that case, try 1e-20 or whatever. But "if(FL==0.0)" is much more specific. Just like alarm bells should ring if you ever type "1/3", you should ask yourself whether a tiny value is just as bad as "(FL==0.0)". Maybe, in the circumstances, you should also ask whether infinity or NaN (not a number) are also possibilities.