The results of an experimental investigation of the effect of free-stream turbulence on the characteristics of the turbulent boundary layer on a flat plate are presented. With the exception of the final runs, the measurements available at the cessation of the work reported are of only moderately satisfactory accuracy. Despite this, the data presented are sufficient to demonstrate the following points.nThe normally employed correlations for the turbulent boundary layer on a plate are, in effect, a limiting law for low turbulence in the free stream. Since the self-generated turbulence intensity in a turbulent boundary layer with low free-stream turbulence is of the order of 5 to 10 percent near the wall, this limiting law holds up to relatively high values of free-stream turbulence. However, when the free-stream turbulence is equal to or greater than the self-generated turbulence of the shear layer, considerable alterations in the boundary-layer char¬acteristics occur. In particular, high values of free-stream turbulence increase the thickness of the boundary layer, create fuller velocity profiles, and raise the apparent value of wall shear. As would be expected, the distribution of intensity of velocity fluctuations through the boundary layer is also grossly altered. Such altered boundary layers do not conform to the "universal" velocity profiles in the form of the law of the wall or the law of the wake. A straight-line portion of the layer on a semilogarithmic plot still exists but its slope and location are altered and are shown to depend upon the imposed free-stream fluctuation intensity.
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