| 1. |
- Bakchinov, A.A., et al.
(författare)
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Experimental study of the K-regime of breakdown in straight and swept wing boundary layers
- 2000
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Ingår i: Abstracts of 53rd Ann. Meeting of the Div. of Fluid Dyn. APS, Washington, DC.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Experiments on the stability of three-dimensional boundary layers on the straight and swept airfoils have been conducted. Detailed measurements of the streamwise velocity field in (Y, Z) planes, as well as the 3D frequency-wavenumber spectra have revealed linear and non-linear evolutions of the disturbances generated by an external acoustic field in the airfoil boundary layers. The Tollmien-Schlichting waves, excited by the sound, were dominant in both configurations, and the disturbance flow field was found to remain highly deterministic and periodic both in time and space until the latest stages of the transition. To control a spanwise variation of the disturbance flow field roughness elements were placed on the surface and equidistantly aligned along the airfoil leading edge. The K-type transition was identified with the aligned order of so-called Lambda-patterns at non-linear stage of the transition. The experiment was conducted in a closed-circuit wind-tunnel at the department of Thermo and Fluid Dynamics, Calmers. This work was supported by the Royal Swedish Academy of Sciences (KVA).
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| 2. |
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| 3. |
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| 4. |
- Chernoray, Valery, 1975, et al.
(författare)
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Experimental study of the K-regime of breakdown in straight and swept wing boundary layers
- 2001
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Ingår i: Physics of Fluids. ; 13:7, s. 2129-2132
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Tidskriftsartikel (refereegranskat)abstract
- In this Brief Communication, the nonlinear evolution of periodical disturbances generated by an external sound field in a swept wing boundary layer is presented. All experimental results are compared with corresponding data for a straight wing configuration. The Tollmien-Schlichting instability has been studied, and it was found that the disturbance flow field remained highly deterministic and periodic in both time and space until the latest stages of the transition. A frequency-wave number Fourier analysis shows that the disturbance spectra comprise only the fundamental wave and its higher harmonics. The K-type breakdown scenario was observed, and the nonsymmetry of flow patterns in the swept wing boundary layer was found to be due to the presence of the cross flow.
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| 5. |
- Chernoray, Valery, 1975, et al.
(författare)
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Generation of the localized disturbances by the vibrating surface
- 2000
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Ingår i: Thermophysics and Aeromechanics. ; 7:3, s. 329-339
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
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| 7. |
- Chernoray, Valery, 1975, et al.
(författare)
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Secondary instability of stationary vortex packets in a swept wing boundary layer
- 2004
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Ingår i: In Proc. of XXI International Congress of Theoretical and Applied Mechanics, Warsaw, Poland.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The fact is that stationary vortex packets are most likely to be generated under natural flight conditions on the wing and these disturbances lead to breakdown the fastest. A detailed experimental study on the formation of crossflow vortex mode packets and their secondary instability in a swept wing boundary layer was carried out.
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| 8. |
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| 9. |
- Grek, G.R., et al.
(författare)
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Experimental study of a Λ-structure development and its transformation into the turbulent spot
- 2000
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Ingår i: Current Science. - 0011-3891. ; 79:6, s. 781-789
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
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| 11. |
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| 12. |
- Kozlov, V.V., et al.
(författare)
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Longitudinal structures in boundary layers and jets
- 2001
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Ingår i: In Proc. of the 8th Int. Conf. on Stability of Homogeneous and Heterogeneous Flows, Novosibirsk, Russia. ; , s. 84-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 13. |
- Kozlov, V.V., et al.
(författare)
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Role of localized streamwise structures in the process of transition to turbulence in boundary layers and jets (review)
- 2002
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Ingår i: Journal of Applied Mechanics and Technical Physics. ; 43:2, s. 224-236
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Tidskriftsartikel (refereegranskat)abstract
- Results of the analysis of specific features of the laminar-turbulent transition in various subsonic shear flows, which are caused by localizedstationary and nonstationary streamwise structures,are presented. One mechanism of flow turbulization is considered, which involves the origination and development of secondary high-frequency disturbances in regions of flow instability generated by its modulation by streamwise structures. It is shown that this process is identical in different types of shear flows (boundary layers and jets) and in flows of the type of localized streamwise structures (stationary or nonstationary).
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| 14. |
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| 15. |
- Litvinenko, M.V., et al.
(författare)
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A visualization study of the longitudinal structures of a plane wall-jet
- 2003
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Ingår i: Proc. of The 7th Asian symposium on visualization, Singapore.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In the present study the flow field near the orifice of a plane wall jet is in focus. Two main flow regions may be distinguished in the wall-jet, i.e. a free shear layer away from the wall and a boundary layer close to the surface. In both of these layers streamwise coherent structures are detected by means of smoke visualization and hot-wire measurements. The structures, which occur naturally, have different spanwise scales and emerge at different distance from the nozzle. Effects of the flow velocity, upstream perturbations and acoustic excitation on the generation and characteristics of the streamwise disturbances are investigated and especially the interaction between the two layers is studied. In order to resolve the complex 3D flow by means of hot-wires a system for accurate automated traversing and data acquisition has been developed. It was found that the value of outlet velocity and the frequency of Kelvin-Helmholts rolls have a clear influence on the size of the structures. Higher outlet velocities and higher frequencies of triggered two-dimensional roll-ups lead the size of longitudinal structures to decrease.
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| 16. |
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| 17. |
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| 18. |
- Litvinenko, Yu.A., et al.
(författare)
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Control of spanwise flow instability of a swept wing by suction
- 2003
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Ingår i: Thermophysics and Aeromechanics. ; 10:4, s. 541-548
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Tidskriftsartikel (refereegranskat)abstract
- The experimental results on control by disturbances development in a swept wing boundary layer by means of localized and distributed suction are presented. The control by spanwise flow stationary vortices and secondary periodical disturbances generated at late stages of these vortices development was studied. It is shown that the distributed suction is a simple way to suppress the secondary vortex instability of a spanwise flow as compared with a localized suction. It was shown that the localized suction through a small diameter hole may cause generation of additional stationary vortices in the boundary layer. In this case complex nonlinear interactions between both controlled and controlling disturbances take place. The distributed suction effect through a group of holes is similar to the slot suction and it introduces much less disturbances of vorticity into the boundary layer.
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| 19. |
- Litvinenko, Yu.A., et al.
(författare)
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Experimental investigation of a streaky structure varicose instability in a swept wing boundary layer
- 2004
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Ingår i: Thermophysics and Aeromechanics. ; 11:1, s. 13-22
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Tidskriftsartikel (refereegranskat)abstract
- The investigation results of the controlled experiment of a streaky structure varicose instability in a swept wing boundary layer are presented. The picture of a streaky structure spatial evolution and secondary high-frequency disturbances generated on it is obtained using hot-wire measurements. It is shown that exactly a varicose instability is realized. It has been found that the secondary disturbance increases owing to instability of mean velocity distribution on normal to the wall caused by the streaky structure development. The following features of a varicose breakdown of a longitudinal stationary streaky structure are presented: structure modulation in the longitudinal direction by a secondary disturbance frequency, downstream excitation of new streaky structures, formation and development of nonstationary structures localized in space. In contrast to the boundary layer flow of the unswept wing models, where these structures resemble classical horseshoe structures or lambda-vortices, only one vortex remains on a swept wing from the two making the vortex structure because of the crossflow.
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| 20. |
- Litvinenko, Yu.A., et al.
(författare)
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Nonlinear sinusoidal and varicose instability in the boundary layer (review)
- 2004
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Ingår i: Thermophysics and Aeromechanics. ; 11:3, s. 329-354
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Tidskriftsartikel (refereegranskat)abstract
- Results of studying sinusoidal and varicose instabilities of streaky structures at the nonlinear stage of the laminar-turbulent process in shear flows are presented. The flow behavior in the course of spatial evolution of streaky structures with a secondary high-frequency disturbance generated on them is discussed. Various scenarios of origination and development of coherent vortex structures examined in physical and numerical experiments are considered. Specific features of the development of sinusoidal and varicose cases of destruction of the steady streamwise streaky structure are demonstrated, such as transverse and streamwise modulation of the structure by the secondary-disturbance frequency, appearance of new streaky structures in the downstream direction, and emergence and evolution of unsteady Λ-type structures localized in space in both cases.
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| 21. |
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