[dinhanh]

Hi Fluent experts,

I am trying to make the UDF for the Realizable k-epsilon turbulent viscosity based on the paper of Shih et al. and that expressed in the Fluent theory guide.

The UDF code is for 2D simulation as follows:

#include "udf.h"
#include "math.h"

DEFINE_TURBULENT_VISCOSITY(tur_vis,c,t)
{
* real mu_tur;
* real S11, S12, S22, W12, W12r, S;
* real SMU, WAS, PHI, USTAR;
* real AS;
* real CMU, a1, a2, a3, a4, a5, a6;
* real PHI2, F2;

/***/
* real betas=0.09;
* real A0=4.04;
* real rho=C_R(c,t);
* real k=C_K(c,t);
* real w=C_D(c,t);
* real CMUL=C_MU_L(c,t);
* real y=C_WALL_DIST(c,t);

/***/
* real wr=-1.485446130; * * * /***rotation speed unit: 1/s***/

/***/
* S11=0.5*(C_DUDX(c,t)+C_DUDX(c,t));
* S12=0.5*(C_DUDY(c,t)+C_DVDX(c,t));
* S22=0.5*(C_DVDY(c,t)+C_DVDY(c,t));
*
* W12=0.5*(C_DVDX(c,t)-C_DUDY(c,t));

/*** Cal vorticity with angular velocity wr ***/
*
* W12r=W12-wr;

/*** Cal U* ****/

* USTAR=sqrt(S11*S11+2.0*S12*S12+S22*S22+2.0*W12r*W1 2r);

/*** Cal Phi ***/

* SMU=sqrt(S11*S11+2.0*S12*S12+S22*S22);
* WAS=(pow(S11,3)+pow(S22,3)+3.0*pow(S12,2)*(S11+S22 ))/pow(SMU,3);
* PHI=1.0/3.0*acos(max(-1.0,min(sqrt(6.0)*WAS,1.0)));

/*** Cal As ***/
*
* AS = sqrt(6.0)*cos(PHI);

/*** Cal CMU ***/
*
* CMU=1.0/(A0+AS*USTAR*k/w);

/*** Cal Viscosity ***/

* mu_tur=CMU*rho*k*k/w;
**
* return mu_tur;
}

The simulations were performed for both the UDF and the default viscous model. However, there is a large deviation between these simulations' averaged results. I do not know why and why this difference is because the UDF code was based on the expression in the Fluent theory guide. I also compared the simulation between the UDF code and the default turbulent viscosity of the SST k-omega model and got a good agreement between the results.

Could anyone please help?

Thank you very much.