Abstract
The behavior of the two-dimensional periodic disturbances introduced into turbulent boundary layers is determined by means of the smoke wire method. It is found that (1)the disturbances whose wave number is between about 0.1 and 0.6 are amplified in the downstream direction (the wave number is 2πδ*/λ, where δ* is the displacement thickness and λ the wavelength), (2)the disturbances whose wave number is about 0.3 are amplified most strongly, and (3) the disturbances whose wave number is larger than about 0.6 are damped. The present experiments strongly suggest that the large scale coherent motion originates in the instability of the mean flow.
