BC Transition Model Term2 update

Updated the Term2 of the BC Transition model, which eliminates some deficiencies in the previous version. The update is based on:

1- Cakmakcioglu, S. C., Bas, O., Mura, R., and Kaynak, U., "A Revised One-Equation Transitional Model for External Aerodynamics," AIAA Paper 2020-2706, June 2020, https://doi.org/10.2514/6.2020-2706.
2- Mura, R. and Cakmakcioglu, S. C., "A Revised One-Equation Transitional Model for External Aerodynamics - Part I: Theory, Validation and Base Cases," AIAA Paper 2020-2714, June 2020, https://doi.org/10.2514/6.2020-2714.

which is also presented here: https://turbmodels.larc.nasa.gov/sa-bc_1eqn.html

Author: sametcaka

SU2_CFD/src/numerics/turbulent/turb_sources.cpp

@@ -77,7 +77,7 @@ CNumerics::ResidualType<> CSourcePieceWise_TurbSA::ComputeResidual(const CConfig
 
 //  BC Transition Model variables
   su2double vmag, rey, re_theta, re_theta_t, re_v;
-  su2double tu , nu_cr, nu_t, nu_BC, chi_1, chi_2, term1, term2, term_exponential;
+  su2double tu, nu_t, chi_1, chi_2, term1, term2, term_exponential;
 
   if (incompressible) {
     Density_i = V_i[nDim+2];
@@ -140,19 +140,19 @@ CNumerics::ResidualType<> CSourcePieceWise_TurbSA::ComputeResidual(const CConfig
 
 //    BC model constants
       chi_1 = 0.002;
-      chi_2 = 5.0;
+      chi_2 = 50.0;
 
       nu_t = (TurbVar_i[0]*fv1); //S-A variable
-      nu_cr = chi_2/rey;
-      nu_BC = (nu_t)/(vmag*dist_i);
+      //nu_cr = chi_2/rey;
+      //nu_BC = (nu_t)/(vmag*dist_i);
 
       re_v   = ((Density_i*pow(dist_i,2.))/(Laminar_Viscosity_i))*Omega;
       re_theta = re_v/2.193;
       re_theta_t = (803.73 * pow((tu + 0.6067),-1.027)); //MENTER correlation
       //re_theta_t = 163.0 + exp(6.91-tu); //ABU-GHANNAM & SHAW correlation
 
       term1 = sqrt(max(re_theta-re_theta_t,0.)/(chi_1*re_theta_t));
-      term2 = sqrt(max(nu_BC-nu_cr,0.)/(nu_cr));
+      term2 = sqrt(max((nu_t*chi_2)/nu,0.));
       term_exponential = (term1 + term2);
       gamma_BC = 1.0 - exp(-term_exponential);