## Surface Conditions

The transition from laminar to turbulent flow is assumed to occur when the following condition has been reached:

ln(Reynolds_{d2}) >= 18.4 H32 - 21.74 - 0.3 R

where:

Reynolds_{d2} 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

and

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.