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

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

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

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

Generation of the localized disturbances by the vibrating surface

Appearance and development of disturbances generated by 3-D vibrating surface in Blasius boundary layer are studied experimentally. The experiments were conducted under the controlled conditions where the preservation of phase information made it possible to obtain profound and accurate data, qualitative as well as quantitative, on the object under study. Surface vibration was provided by the loudspeaker working in the blowing-suction regime. Detailed hot-wire measurements showed that in case when the surface produces oscillations of low frequency and small amplitude, the packet of disturbances with the characteristics of Tollmien - Schlichting (TS) waves is observed downstream. When the effective amplitude is increased two times "blowing" leads to the appearance of the new type of disturbances: "puff"-structures, which have other characteristics of development in the boundary layer than Tollmien - Schlichting waves

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

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