The transition from laminar to turbulent flow is assumed to occur when the following condition has been reached:
ln(Reynoldsd2) >= 18.4 H32 - 21.74 - 0.3 R
Reynoldsd2 is the Reynolds number of the boundary layer momentum thickness
H32 is the ratio of boundary layer's energy thickness, d3, to its momentum thickness, d2
R is a roughness factor
A roughness factor of 0 is a natural transition on a smooth surface with little or no free-stream turbulence. A roughness factor of 4 corresponds approximately to a NACA standard roughness. (slider bar aligned over the NACA label)
It is also possible to specify a location on the surface where the laminar to turbulent transition will occur. If the flow is laminar at the specified chord location (in units of x/c), then the calculation switches to the turbulent flow equations.
A chord location of 1.0 will result in a natural transition.
Any of the values may be entered manually by left clicking over the numeric value and typing the desired value in the text box.