Experiments in a boundary layer subjected to free stream turbulence. Part 2. The role of TS-waves in the transition process

Бойко, А. В.; Westin, K. J. A.; Klingmann, Barbro G. B.; Козлов, В. В.; Alfredsson, P. Henrik

doi:10.1017/s0022112094003095

Cited by 126 publications

(65 citation statements)

References 23 publications

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“…1 Others show that streaks suppress the amplification of primary TS waves, 2-4 yet transition is promoted. 2,4 Our recent direct numerical simulation ͑DNS͒ studies 5,6 provide some degree of empirical resolution: streaks do, indeed, reduce the growth rate of primary TS waves and therefore can delay transition. Nevertheless, streaks can also enhance transition by promoting the formation of ⌳ shaped velocity contours, which precede the appearance of turbulent spots and breakdown of the boundary layer.…”

Section: Floquet Analysis Of Secondary Instability Of Boundary Layersmentioning

confidence: 99%

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Floquet analysis of secondary instability of boundary layers distorted by Klebanoff streaks and Tollmien–Schlichting waves

2008

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Previous studies of the interaction between boundary layer streaks and Tollmien-Schlichting (TS) waves have shown puzzling effects. Streaks were shown to reduce the growth rate of primary TS waves and, thereby, to delay transition; however, they can also promote transition by inducing a secondary instability. The outcome of the interaction depends on the spanwise wavelength and intensity of the streaks as well as on the amplitude of the TS waves. A Floquet analysis of secondary instability is able to explain many of these features. The base state is periodic in two directions: it is an Ansatz composed of a saturated TS wave (periodic in x) and steady streaks (periodic in z). Secondary instability analysis is extended to account for the doubly periodic base flow. Growth rate computations show that, indeed, the streak can either enhance or diminish the overall stability of the boundary layer. The stabilizing effect is a reduction in the growth rate of the primary two-dimensional TS wave; the destabilizing effect is a secondary instability. Secondary instability falls into two categories, depending on the spanwise spacing of the streaks. The response of one category to perturbations is dominated by fundamental and subharmonic instability; the response of the other is a detuned instability. Keywordsboundary layers, flow interactions, groundwater flow, growth (materials), hydrodynamics, base flows, boundary layer streaks, destabilizing effects, doubly periodic, floquet analysis, puzzling effects, secondary instabilities, secondary instability analysis, stabilizing effects, subharmonic instabilities, Ts waves, aerodynamics Disciplines Aerospace Engineering | Mechanical Engineering CommentsThe following article appeared in Physics of Fluids 20, 124102 (2008) Previous studies of the interaction between boundary layer streaks and Tollmien-Schlichting ͑TS͒ waves have shown puzzling effects. Streaks were shown to reduce the growth rate of primary TS waves and, thereby, to delay transition; however, they can also promote transition by inducing a secondary instability. The outcome of the interaction depends on the spanwise wavelength and intensity of the streaks as well as on the amplitude of the TS waves. A Floquet analysis of secondary instability is able to explain many of these features. The base state is periodic in two directions: it is an Ansatz composed of a saturated TS wave ͑periodic in x͒ and steady streaks ͑periodic in z͒. Secondary instability analysis is extended to account for the doubly periodic base flow. Growth rate computations show that, indeed, the streak can either enhance or diminish the overall stability of the boundary layer. The stabilizing effect is a reduction in the growth rate of the primary two-dimensional TS wave; the destabilizing effect is a secondary instability. Secondary instability falls into two categories, depending on the spanwise spacing of the streaks. The response of one category to perturbations is dominated by fundamental and subharmonic instability; the response of the o...

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Section: Floquet Analysis Of Secondary Instability Of Boundary Layersmentioning

confidence: 99%

“…In the laboratory, streaks can not only be artificially excited by rods 8 but also naturally develop in response to forcing by free-stream turbulence. 2,9 In the last case they are called Klebanoff streaks and are quite commonly seen. The term streak has its origin in flow visualization.…”

Section: Floquet Analysis Of Secondary Instability Of Boundary Layersmentioning

confidence: 99%

Floquet analysis of secondary instability of boundary layers distorted by Klebanoff streaks and Tollmien–Schlichting waves

2008

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“…When the level of FST is >1 %, the disturbances in the boundary layer develop rapidly and the breakdown occurs much earlier than that predicted by the traditional transition theory based on the TS instability. This scenario of transition is referred to as bypass transition, which typically occurs at a Reynolds number of the order of 10 5 or lower (see, for example, Klebanoff 1971;Boiko et al 1994).…”

Section: Bypass Transitionmentioning

confidence: 99%

Transition of transient channel flow after a change in Reynolds number

Seddighi

2015

J. Fluid Mech.

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It has previously been shown that the transient flow in a channel following a step increase of Reynolds number from 2800 to 7400 (based on channel half-height and bulk velocity) is effectively a laminar-turbulent bypass transition even though the initial flow is turbulent (He & Seddighi, J. Fluid Mech., vol. 715, 2013, pp. 60-102). In this paper, it is shown that the transient flow structures exhibit strong contrasting characteristics in large and small flow perturbation scenarios. When the increase of Reynolds number is large, the flow is characterized by strong elongated streaks during the initial period, followed by the occurrence and spreading of isolated turbulent spots, as shown before. By contrast, the flow appears to evolve progressively and the turbulence regeneration process remains largely unchanged during the flow transient when the Reynolds number ratio is low, and streaks do not appear to play a significant role. Despite the major apparent differences in flow structures, the transient flow under all conditions considered is unambiguously characterized by laminar-turbulent transition, which exhibits itself clearly in various flow statistics. During the pre-transition period, the time-developing boundary layers in all the cases show a strong similarity to each other and follow closely the Stokes solution for a transient laminar boundary layer. The streamwise fluctuating velocity also shows good similarity in the various cases, irrespective of the appearance of elongated streaks or not, and the maximum energy growth exhibits a linear rate similar to that in a spatially developing boundary layer. The onset of transition is clearly definable in all cases using the minimum friction factor, and the critical time thus defined is strongly correlated with the free-stream turbulence in a power-law form.

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“…However, experimental, numerical and theoretical efforts have not yielded a unified view as to whether this modification is stabilizing or destabilizing. Boiko et al (1994) experimentally observed that boundary layers subject to freestream turbulence (FST) levels of 1.5% can support growing T-S waves. Their growth rate, however, was lower than in an undisturbed Blasius flow.…”

Section: The Influence Of Streaks On Tollmien-schlichting-type Instabmentioning

confidence: 99%

Stability of zero-pressure-gradient boundary layer distorted by unsteady Klebanoff streaks

2011

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The secondary instability of a zero-pressure-gradient boundary layer, distorted by unsteady Klebanoff streaks, is investigated. The base profiles for the analysis are computed using direct numerical simulation (DNS) of the boundary-layer response to forcing by individual free-stream modes, which are low frequency and dominated by streamwise vorticity. Therefore, the base profiles take into account the nonlinear development of the streaks and mean flow distortion, upstream of the location chosen for the stability analyses. The two most unstable modes were classified as an inner and an outer instability, with reference to the position of their respective critical layers inside the boundary layer. Their growth rates were reported for a range of frequencies and amplitudes of the base streaks. The inner mode has a connection to the TollmienSchlichting (T-S) wave in the limit of vanishing streak amplitude. It is stabilized by the mean flow distortion, but its growth rate is enhanced with increasing amplitude and frequency of the base streaks. The outer mode only exists in the presence of finite amplitude streaks. The analysis of the outer instability extends the results of Andersson et al. (J. Fluid Mech. vol. 428, 2001, p. 29) to unsteady base streaks. It is shown that base-flow unsteadiness promotes instability and, as a result, leads to a lower critical streak amplitude. The results of linear theory are complemented by DNS of the evolution of the inner and outer instabilities in a zero-pressure-gradient boundary layer. Both instabilities lead to breakdown to turbulence and, in the case of the inner mode, transition proceeds via the formation of wave packets with similar structure and wave speeds to those reported by Nagarajan, Lele & Ferziger (J. Fluid Mech., vol. 572, 2007, p. 471).

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Experiments in a boundary layer subjected to free stream turbulence. Part 2. The role of TS-waves in the transition process

Cited by 126 publications

References 23 publications

Floquet analysis of secondary instability of boundary layers distorted by Klebanoff streaks and Tollmien–Schlichting waves

Floquet analysis of secondary instability of boundary layers distorted by Klebanoff streaks and Tollmien–Schlichting waves

Transition of transient channel flow after a change in Reynolds number

Stability of zero-pressure-gradient boundary layer distorted by unsteady Klebanoff streaks

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