The initial increase of turbulence intensity a laminar boundary layer at the leading edge

Abstract

Experimental measurements of the degree of turbulence taken inside a laminar boundary layer near the leading edge and farther downstreamreveal a strong increase of this turbulence beyound that one of the freestream. These increases were observed at subcritical Reynolds numbers in some and at supercritical Reynolds numbers in other cases.Increases beyond the level of the original turbulence were observed inside a laminar boundary layer for the downstream and crossstream direction. In the normal direction to the wall the turbulence was almost unaffected. For both increases, the subcritical and the supercritical, different explanations are given. The supercritical increase which was observed at a free stream turbulence of around 0.2 % is explained by using results from receptivity theory. The subcritical increase which was observed at a free-stream turbulence of around 1% is explained as a result of the interaction of an oncoming vortex with its image vortex due to the boundary condition at the wall. In the latter case a bypass transition was observed. This suggests the existence of a threshold degree of free stream turbulence beyond which a bypass transition occurs.