# compute turbulent diffusion coefficient for UDS

 Analagous method needed for computing diffusivity for user-defined scalars (UDS) in turbulent flow as for species eqns.Solution methodology is as followsif turbulence is not on, thenrho D = rho (D_lam)otherwiserho D = rho (D_lam + mu_turbulent / (rho *Schmidt_turb))the Schmidt_turb is set in the cortex variable species/sct; Instructions for modifying this variable are in the source code below....A UDF has been written and follows immediately:/*************************************************** UDF to compute turbulent UDS diffusivity by William Wangard, Ph.D. ww@fluent.com Fluent, Inc. All Rights reserved. Fluent, Inc. does not guarantee the rigor of this UDF****************************************************************/#include "udf.h"DEFINE_DIFFUSIVITY(uds_diff_coeff,c,t,i){ /* Note: the diffusivity in GUI requires not D, but rho*D */ real schmidt, d_turb, d_eff, mu_turb; /* Laminar diff coeff */ d_eff = 1.0E-3; /* Multiply this by the density */ d_eff *= C_R(c,t); /* If turbulence is off, then mu_turb = 0.0, and d_eff = d_lam */ if (rp_turb) { /* Turbulent viscosity */ mu_turb = C_MU_T(c,t); /* Schmidt number */ /* This default is 0.7.... to change the value to 0.9, type the following in the FLUENT GUI: (rpsetvar 'species/sct 0.9) to check the value, type the following in the FLUENT GUI: (rpgetvar 'species/sct) */ schmidt = RP_Get_Real("species/sct"); /* Turbulent diffusivity (* rho) */ d_turb = mu_turb / schmidt; /* Effective diffusivity (* rho)*/ d_eff += d_turb; } return d_eff;}

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