Experimental study of mechanism of high-frequency breakdown of Λ-structure

Artikel i vetenskaplig tidskrift, 1999

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