Hello! I have a question about NACA (NASA) standart roughness on airfoils (in form of strip). I read, that this roughness is used to initiate boundary layer transition from laminar flow to turbuluent flow. In addition, the form and location of roughness is chosen in way, that minimize drag force (or drag coefficient). NACA published results, where both plots for smooth and with roughness surface are illustrated. Please explain me, for what do NASA researchers use roughness strip? Why do they try to initiate boundary layer transition? And how does roughness strip corresponds to turbulence models, which are used in finite-element software (such as star-cd and ansys). I have some results obtained from this software and don't know which plot (smooth of roughness) should be used to check these results.

You have several questions there. To try and answer one of them,

the roughness strip is used to 'trip' the boundary layer from laminar to turbulent because a turbulent boundary layer will remain attached to the surface for a longer distance, allowing more lift to be generated on a particular wing at a particular flight speed.

Generally a 'separated boundary layer' (contradiction but standard language!) results in a bubble of reverse flow above the separation point that destroys lift generation.

Thanks for answer. I have already answered my questions by myself