Laminar-turbulent transition in CFX 5.5

Louwrens

How does CFX 5.5 handle flow that is initially laminar but goes turbulent? Case in point is turbulent flow over a smooth flat plate, which of course has an initial laminar region at the leading edge. I've simulated this with k-e and SST turbulence models and with 1st order, full second order (blend factor=1) and high resolution advection on both structured and unstructured meshes.

I'm comparing the numerically determined values for Cf with the following equation, valid for turbulent flow:

Cf=0.455/(ln^2(0.06 Re_x)) (Eq.1)

as given in White, Viscous Fluid Flow, Chapter 6.

On the structured mesh, Cf compares well with theory for Re_x>4.7e6 (which is the "highest" value for transition according to White) and not good at all for Re_x<4.7e6. They do not follow a typical transition curve, or a laminar solution at these low Re_x though.

On the unstrutured mesh, the numerical values follow Eq. 1 closely, even to Re_x<1e6, which is supposed to be strictly laminar. The question therefore is what happens during a turbulent solution when the flow is supposed to be laminar? Does the eddy viscosity just become very small, in which case turbulent effects become negligible (which does not seem to be the case) or does it "enforce" turbulence, almost like one would do experimentally using a trip wire at the leading edge?