Large Scale Periodic Motions in Turbulent Shear Layers

Turbulent boundary layers and turbulent wakes are studied by means of flow visualization methods. It is shown that a streamwise large scale periodic structure generally exists in turbulent shear layers, and that the wavelength of the periodic structure is directly proportional to the displacement thickness of the shear layer. The present experiments strongly suggest that the large scale periodic motion originates in the instability of the mean flow.