Laminar-turbulent transition is connected to disturbances like free-stream turbulence, surface roughness, noise, etc., which enter the laminar boundary layer. If the boundary layer is unstable, these disturbances excite eigenmodes which are at the beginning of a sequence of events, which finally lead to the turbulent state of the boundary layer. The kind of entry of the disturbances into the boundary layer is called boundary-layer receptivity.
Regarding the influence of surface and free-stream properties we give an overview of the main topics. We treat the two properties separately, although in reality they are involved in simultaneously active receptivity mechanisms.
Very important is the fact that surface properties, for instance roughness, influence not only the transition process, but also strongly—if the flow is turbulent—skin friction and the thermal state of the surface .