Hydrodynamic instability of boundary layers and separated flows (present state)

Review is given of recent experimental results on the mechanisms of turbulization of complex flows, such as gradient, spatial, and separated flows, and perspectives of studies in this area at subsonic flow speeds, taking into account the modern manufacturing technologies of wing planes (making surfaces reasonably smooth) for big transport aircrafts at large Reynolds numbers, and the design of wing planes for small-sized flying vehicles (avoiding flow separation) at small Reynolds numbers, are shown. Nonlinear stages of the secondary high-frequency instability of various flows modulated by streaks, as well as methods of the laminar-turbulent transition control for these cases using streak interaction, riblets, localized and distributed suction are considered. Various methods to control the separated flows using localized disturbance sources inside of separated regions, exposing acoustics, and shaping the surfaces by the corrugations are demonstrated. A method of control of the flow separation using the MEMS technique is described

Hydrodynamic instability of boundary layers and separated flows (present state)

Review is given of recent experimental results on the mechanisms of turbulization of complex flows, such as gradient, spatial, and separated flows, and perspectives of studies in this area at subsonic flow speeds, taking into account the modern manufacturing technologies of wing planes (making surfaces reasonably smooth) for big transport aircrafts at large Reynolds numbers, and the design of wing planes for small-sized flying vehicles (avoiding flow separation) at small Reynolds numbers, are shown. Nonlinear stages of the secondary high-frequency instability of various flows modulated by streaks, as well as methods of the laminar-turbulent transition control for these cases using streak interaction, riblets, localized and distributed suction are considered. Various methods to control the separated flows using localized disturbance sources inside of separated regions, exposing acoustics, and shaping the surfaces by the corrugations are demonstrated. A method of control of the flow separation using the MEMS technique is described

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

Study of the nonlinear instability of a longitudinal structure generated by a roughness element in the boundary layer of a straight wing

The results of experimental study of a nonlinear varicose instability of the streaky structure generated by roughness element in unswept-wing boundary layer are presented. Features of the varicose breakdown of longitudinal steady streaky structure such as modulation of structure in transverse and streamwise directions by secondary disturbance, occurrence of the new streaky structures and Lambda-structures downstream are shown. Spatio-temporal pictures of the hot-wire visualization of flow during spatial evolution of the streaky structures under influence of secondary high-frequency disturbance are discussed. Features of the adverse pressure gradient influence upon processes of the nonlinear varicose instability evolution and flow structure are revealed. Essential influence of the adverse pressure gradient on evolution of disturbances is shown. Comparison of varicose instability of the streaky structures generated in two different ways (the roughness element as in the given work, and continuous air blowing as in the earlier published work) is the carried out