Minor modifications

   SASPH/Turbulence. Wall-function depth changed from 0.75dx to 0.5dx.
      Different numerical initialization for the turbulent viscosity.

src/SA_SPH/Turbulence/wall_function_for_SASPH.f90

@@ -62,11 +62,13 @@ subroutine wall_function_for_SASPH(u_t_0,d_50,rho_0,r_0w,slip_coefficient_0w,  &
 ! Initializations
 !------------------------
 slip_coefficient_0w = 0.d0
-mu_T_0w = 0.d0
+! Turbulent viscosity is initialized to a non-null value just to avoid the 
+! product "0*0" when assessing the shear stress out of the wall-function depth.
+mu_T_0w = 1.d-12
 !------------------------
 ! Statements
 !------------------------
-if (r_0w<=(0.75d0*Domain%dx)) then
+if (r_0w<=(0.5d0*Domain%dx)) then
 ! Only the particle layer close to the boundary is selected
    z_0 = d_50 / 30.d0
    if (r_0w<(z_0*Nepero_number)) then