# divergence + define_uds_flux + axisymmetric flow

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September 5, 2014, 02:29
divergence + define_uds_flux + axisymmetric flow
#1
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Jacky
Join Date: Jul 2011
Posts: 24
Rep Power: 7
Hi All,

I am defining one UDS eqn by having udf for its mass flux using DEFINE_UDS_FLUX(). Actual object of interest for uds application is a annular porous cylindrical region. So I have taken a section of object along its axis and have define made it as a rectangular domain...(pic of mesh is attached).
My UDF is given below....

DEFINE_UDS_FLUX(my_uds_flux,f,t,i)
{
cell_t c0, c1 = -1;
real gamma, lambda_l, lambda_v, krl, krv, s;
real NV_VEC(psi_vec), NV_VEC(A), flux;

flux = 0.0;

c0 = F_C0(f,t);
F_AREA(A, f, t);

/* If face lies at domain boundary, use face values; */
/* If face lies IN the domain, use average of adjacent cells. */

if (BOUNDARY_FACE_THREAD_P(t)) /*Most face values will be available*/
{
s = C_UDMI(c0,t0,1);
lambda_l = C_UDMI(c0,t0,4);
gamma = lambda_l*( 1.0 + (ROG*(1.0-s))/(ROL*s)) ;

NV_DS(psi_vec, =, F_U(f,t), F_V(f,t), 0.0, *, gamma);

flux = NV_DOT(psi_vec, A); /* flux through Face */
// Message("s, lambda_l, gamma, F_U(f,t), F_V(f,t), psi_vec[0], psi_vec[1], flux = %f %f %f %g %g %g %g %g \n", s, lambda_l, gamma, F_U(f,t), F_V(f,t), psi_vec[0], psi_vec[1], flux );
// Message("A[0], A[1] = %g %g \n", A[0], A[1] );

}
else
{
c1 = F_C1(f,t); /* Get cell on other side of face */

s = 0.5*( C_UDMI(c0,t0,1) + C_UDMI(c1,t1,1) );
lambda_l = 0.5*( C_UDMI(c0,t0,4) + C_UDMI(c1,t1,4) );

gamma = lambda_l*( 1.0 + (ROG*(1.0-s))/(ROL*s)) ;

NV_DS(psi_vec, =, C_U(c0,t0),C_V(c0,t0),C_W(c0,t0),*,gamma);
NV_DS(psi_vec, +=, C_U(c1,t1),C_V(c1,t1),C_W(c1,t1),*,gamma);

flux = NV_DOT(psi_vec, A)/2.0; /* Average flux through face */
C_UDMI(c0,t0,11) = flux;
}

// Message(" FLUX, F_FLUX(f,t) = %g %g \n", flux, F_FLUX(f,t) );

return flux;
}

I have checked the value of flux going into fluent solver to be within a range of -3.63E-10 to 3.09E-14.

Now when ever I am using this in UDS-0 I am getting following error:

Error: Divergence detected in AMG solver: uds-0
Error Object: #f

To make sure what could be the reason I made verbosity in AMG panel to be 1 and got following reply:

iter continuity x-velocity y-velocity uds-0 time/iter

x-momentum equation:
tol. 1.0000e-20
0 3.1019e-25
1 1.3428e-30

y-momentum equation:
tol. 8.8308e-19
0 1.7662e-06
1 1.5347e-11

pressure correction equation:
tol. 6.6436e-17
0 1.3287e-04
1 1.9282e-03
2 2.2744e-03
3 1.3370e-03
4 1.0036e-03
5 7.2081e-04
6 3.1648e-04
7 2.1811e-04

uds-0 equation:
tol. 3.1881e-08
0 4.2003e+00
1 1.#QNBe+00

That means uds-0 residuals are becoming NAN but I dont know how to correct it.

I am using flexible cycle for uds-0 with no stabilization.
Pressure discretization = standard
Momentum " " = second order upwind
UDS-0 " " = power law.

Under Relaxation Factor for uds-0 is 0.1.

SO can anybody point out what could be the reason for this error and how to correct it ?????

Bharat
Attached Images
 cfd-online-mesh-axisymmetric.jpg (17.5 KB, 7 views)

Last edited by bharat.cmeri; September 5, 2014 at 02:44. Reason: attachment added

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