Experimental study of a Λ-structure development and its transformation into the turbulent spot

Artikel i vetenskaplig tidskrift, 2000

This paper presents the results of experimental research on the characteristics of Λ-structures, their development, and the mechanism of their transformation into turbulent spots. It has been shown that an isolated Λ-structure can damp as well as increase downstream and transform into a turbulent spot. The structure of the associated disturbances consists of two counter-rotating vortices (the "legs" of disturbance) closed by a "head" at the leading edge. The difference between the two types is that the Λ-structure that damps is a kind of a hairpin vortex which does not cross the upper boundary layer edge; the "head" of the increasing Λ-structure crosses the upper boundary layer edge and the disturbance attains the form of the Greek letter Λ. It has been proposed that the increasing Λ-structure is connected with the development of secondary high frequency disturbances on the "legs" of the structure. The reason for this is probably the local transverse velocity gradient ∂u / ∂z on the "legs" of the Λ-structure, which creates conditions for the development of secondary disturbances in it. It has also been shown that the frequency of the secondary disturbance decreases because of the continuous extension of a localized disturbance under its downstream propagation. Secondary high frequency breakdown of structures is also observed when there is periodical generation.