FDS Source: rework TAU_U to be consistent with Tech Guide

Author: rmcdermo

Source/fire.f90

@@ -778,9 +778,10 @@ SUBROUTINE COMBUSTION_MODEL(T,DT,ZZ_GET,Q_OUT,MIX_TIME_OUT,CHI_R_OUT,CHEM_SUBIT_
             Q_REAC_SUB(1:N_REACTIONS),Q_REAC_1(1:N_REACTIONS),Q_REAC_2(1:N_REACTIONS),Q_REAC_4(1:N_REACTIONS),&
             Q_REAC_SUM(1:N_REACTIONS),Q_SUM_CHI_R,CHI_R_SUM,TIME_RAMP_FACTOR,&
             TOTAL_MIXED_MASS_1,TOTAL_MIXED_MASS_2,TOTAL_MIXED_MASS_4,TOTAL_MIXED_MASS,&
-            ZETA_1,ZETA_2,ZETA_4,D_F,TMP_IN,C_U,DT_SUB_OLD,ERR_EST(N_TRACKED_SPECIES),ERR_TOL(N_TRACKED_SPECIES),ERR_TINY,&
+            ZETA_1,ZETA_2,ZETA_4,D_F,TMP_IN,K_SGS,DT_SUB_OLD,ERR_EST(N_TRACKED_SPECIES),ERR_TOL(N_TRACKED_SPECIES),ERR_TINY,&
             ZZ_TEMP(1:N_TRACKED_SPECIES),ATOL(1:N_TRACKED_SPECIES)
 INTEGER :: NR,NS,ITER,TVI,RICH_ITER,TIME_ITER,RICH_ITER_MAX
+REAL(EB), PARAMETER :: C_U=0.4_EB,TAU_EPS=1.E-10_EB
 INTEGER, PARAMETER :: TV_ITER_MIN=5
 LOGICAL :: TV_FLUCT(1:N_TRACKED_SPECIES),EXTINCT,NO_REACTIONS,NO_REAC_2,NO_REAC_4
 TYPE(REACTION_TYPE), POINTER :: RN !,R1
@@ -801,9 +802,9 @@ SUBROUTINE COMBUSTION_MODEL(T,DT,ZZ_GET,Q_OUT,MIX_TIME_OUT,CHI_R_OUT,CHEM_SUBIT_
    TAU_D = DELTA**2/MAX(D_F,TWO_EPSILON_EB)                            ! FDS Tech Guide (5.14)
    SELECT CASE(SIM_MODE)
       CASE DEFAULT
-         C_U = 0.4_EB*C_DEARDORFF*SQRT(1.5_EB)
-         TAU_U = C_U*RHO_IN*DELTA**2/MAX(MU_IN,TWO_EPSILON_EB)         ! FDS Tech Guide (5.15)
-         TAU_G = SQRT(2._EB*DELTA/(GRAV+1.E-10_EB))                    ! FDS Tech Guide (5.16)
+         K_SGS = (MU_IN/(RHO_IN*C_DEARDORFF*DELTA))**2                 ! FDS Tech Guide (4.17)
+         TAU_U = C_U*DELTA/SQRT(TWTH*(K_SGS+TAU_EPS))                  ! FDS Tech Guide (5.15)
+         TAU_G = SQRT(2._EB*DELTA/(GRAV+TAU_EPS))                      ! FDS Tech Guide (5.16)
          MIX_TIME_OUT= MAX(TAU_CHEM,MIN(TAU_D,TAU_U,TAU_G,TAU_FLAME))  ! FDS Tech Guide (5.13)
       CASE (DNS_MODE)
          MIX_TIME_OUT= MAX(TAU_CHEM,TAU_D)