| 1. |
- Chernoray, Valery, 1975, et al.
(författare)
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Secondary instability of a swept-wing boundary layer disturbed by controlled roughness elements
- 2010
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Ingår i: Journal of Visualization. - : Springer Science and Business Media LLC. - 1343-8875 .- 1386-6478 .- 1875-8975. ; 13:3, s. 251-256
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Tidskriftsartikel (refereegranskat)abstract
- Wind-tunnel data on velocity perturbations evolving in a laminar swept-wing flow under low subsonic conditions are reported. The focus of the present experiments are secondary disturbances of the boundary layer which is modulated by stationary streamwise vortices. Both the stationary vortices and the secondary oscillations of interest are generated in a controlled manner. The experimental data are obtained through hot-wire measurements. Thus, evolution of the vortices, either isolated or interacting with each other, is reconstructed in detail. As is found, the secondary disturbances, initiating the laminar-flow breakdown, are strongly affected by configuration of the stationary boundary-layer perturbation that may have an implication to laminar-turbulent transition control.
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| 2. |
- Chernoray, Valery, 1975, et al.
(författare)
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Spatial hot-wire visualization of the Lambda-structure transformation into the turbulent spot on the smooth flat plate surface and riblet effect on this process
- 2010
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Ingår i: Journal of Visualization. - : Springer Science and Business Media LLC. - 1343-8875 .- 1386-6478 .- 1875-8975. ; 13:2, s. 151-158
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Tidskriftsartikel (refereegranskat)abstract
- Evolution of the Lambda-vortex and its transformation into the turbulent spot in a flat plate boundary layer are investigated experimentally. Extensive measurements visualizing the Lambda-structure transformation into the turbulent spot on the smooth and ribbed surfaces of the flat plate are presented. The flow behavior in the course of spatial evolution of the Lambda-structure and turbulent spot is discussed. Specific features of the downstream evolution of Lambda-structure and turbulent spot on the smooth and ribbed surfaces are demonstrated, such as suppression by riblets of the Lambda-vortex transformation into the turbulent spot, appearance of the coherent structures of Lambda-vortex within ensemble-averaged turbulent spot, and oblique waves generation both by the Lambda-vortex and turbulent spot.
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| 3. |
- Kalpakli, Athanasia, et al.
(författare)
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Dean vortices in turbulent flows : rocking or rolling?
- 2012
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Ingår i: Journal of Visualization. - : Springer. - 1343-8875 .- 1875-8975. ; 15:1, s. 37-38
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Tidskriftsartikel (refereegranskat)abstract
- Flows in pipe bends have been studied extensively over the last decades due to their occurrence both in the human respiratory and blood systems as well as in many technical applications. When a fluid flows through a pipe bend an adverse pressure gradient is generated forcing high velocity fluid towards the outer wall which is then replaced by low velocity fluid moving along the wall towards the inner side of the bend. The physical effect is that the high velocity fluid is experiencing a large centrifugal force, resulting in an unstable ‘‘stratification’’ making the high velocity fluid in the centre deflect outwards along the pipe bend, thereby forming two counter-rotating roll cells, so-called Dean vortices. While their behavior in laminar flows has been nicely visualized, the picture of their unsteady behavior in turbulent flows still remains rather blurry, and in fact ‘‘the questions, for example, whether the Dean vortices stay symmetric with respect to the geometric plane of symmetry or whether the strength of the Dean vortices varies with time are hardly addressed’’ (Rütten et al 2005). In the present study, stereoscopic particle image velocimetry has been employed to seize the unsteady behavior of the Dean vortices at the exit of a 90 degree pipe bend at a Reynolds number and Dean number of 34,000 and 19,000, respectively. While the time-averaged flow field shows two symmetrical roll cells, that can be observed both in the streamwise and cross stream velocities, as well as in the streamwise vorticity, the instantaneous snapshots reveal an unsteady behavior where the roll cells are pushing one another alternatively towards the lower or upper half of the pipe, in what could be described as a ‘‘rocking’’ motion of the high speed ‘‘stem’’ in between the roll cells. Hence, the real question is not whether ‘‘to be, or not to be’’ in regards to the instantaneous existence of the Dean vortices in turbulent flows, but rather why, when and how they roll (as their time-averaged counterpart) or rock between the states caught in the presented snapshots.
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| 4. |
- Talamelli, Alessandro, et al.
(författare)
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"Lock-in" phenomenon in coaxial jets
- 2011
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Ingår i: Journal of Visualization. - : Springer Science and Business Media LLC. - 1343-8875 .- 1875-8975. ; 14:4, s. 305-306
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Tidskriftsartikel (refereegranskat)abstract
- Flow visualizations have been performed in order to study the vortex interactions in the near field of a coaxial jet configuration. Smoke has been injected in the outer stream and a continuous Argon Laser sheet has been used as a light source. Instantaneous pictures have been acquired at a rate of 1 kHz by means of DANTEC Nanosense MK1 camera. In order to remove the background noise the average of all the available images has been subtracted from each snapshot. Different tests have been performed at moderately large Reynolds number, for different velocity ratios, r=Uo/Ui, [1], whereUiandUoare the inner and outer jet velocity, respectively. Three main regimes have emerged depending on the velocity ratio. Although for ru≪1 the coaxial jets dynamics may be related to the independent instability of the inner and outer shear layer (see figure 1a), in this type of configuration (visible for ru≫1 in figure 1c) it is typically the stronger outer shear layer that dominates the near field vortex dynamics, [2]. However, for runearly unitary, and for a thick enough separating wall, a clear vortex shedding behind the separating wall is produced as a result of a global instability mechanism. Figure 1b shows that the generated vortices impose their own dynamics on the entire jet near field synchronizing the instability of the outer shear layer. This ”lock-in” phenomenon can be used as a ”passive” mean, i.e. requiring no energy input, to control the mixing between the two streams and between the jet and the ambient fluid.
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| 5. |
- Örlü, Ramis, 1977-, et al.
(författare)
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The life of a vortex in an axisymmetric jet
- 2011
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Ingår i: Journal of Visualization. - : Springer. - 1343-8875 .- 1875-8975. ; 14:1, s. 5-6
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Tidskriftsartikel (refereegranskat)abstract
- An axisymmetric jet is produced whenever a fluid is ejected from a round nozzle. The internal and external disturbances present in and surrounding the jet will trigger the formation of Kelvin–Helmoltz waves that are amplified and grow until they saturate and roll up in discrete vortices undergoing different processes such as (vortex) pairing and tearing until they collapse generating a complete turbulent flow. Concerning the evolution of the vortices within the axisymmetric shear layer, a typical scenario, describing the stages in a life of a vortex, is well represented by the flow visualization sequences shown in Fig. 1 as well as the movie in Online Resource 1.
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