Effects of Weak Free Stream Nonuniformity on Boundary Layer Transition

Experiments are described in which well-defined weak Free Stream Nonuniformity (FSN) is introduced by placing fine wires upstream of the leading edge of a flat plate. Large amplitude spanwise thickness variations form in the boundary layer as a result of the interaction between the steady laminar wakes from the wires and the leading edge. The centerline of a region of elevated layer thickness is aligned with the centerline of the wake in the freestream and the response is shown to be remarkably sensitive to the spanwise length-scale of the wakes. The region of elevated thickness is equivalent to a long narrow low speed streak in the layer. Elevated Free Stream Turbulence (FST) levels are known to produce randomly forming arrays of long narrow low speed streaks in laminar boundary layers. Therefore the characteristics of the streaks resulting from the FSN are studied in detail in an effort to gain some insight into bypass transition that occurs at elevated FST levels. The shape factors of the profiles in the vicinity of the streak appear to be unaltered from the Blasius value, even though the magnitude of the local thickness variations are as large as 60% of that of the undisturbed layer. Regions of elevated background unsteadiness appear on either side of the streak and it is shown that they are most likely the result of small amplitude spanwise modulation of the layer thickness. The background unsteadiness shares many of the characteristics of Klebanoff modes observed at elevated FST levels. However, the layer remains laminar to the end of the test section $(Rx≈1.4×106)$ and there is no evidence of bursting or other phenomena associated with breakdown to turbulence. A vibrating ribbon apparatus is used to examine interactions between the streak and Tollmien-Schlichting (TS) waves. The deformation of the mean flow introduced by the streak is responsible for substantial phase and amplitude distortion of the waves and the breakdown of the distorted waves is more complex and it occurs at a lower Reynolds number than the breakdown of the $K$-type secondary instability that is observed when the FSN is not present.