Hi everyone

I'm using Fluent for my masters thesis which is about external car aerodynamics. before starting my 3D cases I want to validate the Blassius's analytical solution for flow on a flat plate(2D). as everyone knows the flow becomes turbulent after a certain Re number (5e+5). so I don't know which turbulence model to use because the flow is laminar+turbulent. can anyone help me ?

Laminar to turbulent transition is a difficult thing to model. In Fluent, you either are laminar OR turbulent, not both.

In aerospace applications they usually use a turbulence model but use a very low freestream value for turbulent kinetic energy (quasi-laminar). They then patch a spot of kinetic energy to 'kick' the BL into turbulent transition. There is a lot of research going on for models to incorporate this transition.

The thing for you to do might be to apply a turbulent Blasius profile at the inlet of the domain and solve with a turbulence model, and verify that the non-dimensional BL properties far downstream still conform to the Blasius solution.