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Turbulent spots in the asymptotic suction boundary layer
KTH, School of Engineering Sciences (SCI), Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0001-7864-3071
2007 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 584, 397-413 p.Article in journal (Refereed) Published
Abstract [en]

Amplitude thresholds for transition of localized disturbances, their breakdown to turbulence and the development of turbulent spots in the asymptotic suction boundary layer are studied using direct numerical simulations. A parametric study of the horizontal scales of the initial disturbance is performed and the disturbances that lead to the highest growth under the conditions investigated are used in the simulations. The Reynolds-number dependence of the threshold amplitude of a localized disturbance is investigated for 500 <= Re <= 1200, based on the free-stream velocity and the displacement thickness. It is found that the threshold amplitude scales as Re-1.5 for the considered Reynolds numbers. For Re <= 367, the localized disturbance does not lead to a turbulent spot and this provides an estimate of the critical Reynolds number for the onset of turbulence. When the localized disturbance breaks down to a turbulent spot, it happens through the development of hairpin and spiral vortices. The shape and spreading rate of the turbulent spot are determined for Re = 500, 800 and 1200. Flow visualizations reveal that the turbulent spot takes a bullet-shaped form that becomes more distinct for higher Reynolds numbers. Long streaks extend in front of the spot and in its wake a calm region exists. The spreading rate of the turbulent spot is found to increase with increasing Reynolds number.

Place, publisher, year, edition, pages
2007. Vol. 584, 397-413 p.
Keyword [en]
plane poiseuille flow, adverse pressure-gradient, free-stream turbulence, couette-flow, localized disturbances, subcritical transition, numerical simulations, bypass transition, channel flows, shear flows
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-8903DOI: 10.1017/S0022112007006544ISI: 000249137500018Scopus ID: 2-s2.0-34547787137OAI: oai:DiVA.org:kth-8903DiVA: diva2:14384
Note
Uppdaterad från manuskript till artikel: 20101025. QC 20101025Available from: 2005-12-08 Created: 2005-12-08 Last updated: 2010-10-25Bibliographically approved
In thesis
1. Numerical studies of transtion in wall-bounded flows
Open this publication in new window or tab >>Numerical studies of transtion in wall-bounded flows
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Disturbances introduced in wall-bounded flows can grow and lead to transition from laminar to turbulent flow. In order to reduce losses or enhance mixing in energy systems, a fundamental understanding of the flow stability and transition mechanism is important. In the present thesis, the stability, transition mechanism and early turbulent evolution of wall-bounded flows are studied. The stability is investigated by means of linear stability equations and the transition mechanism and turbulence are studied using direct numerical simulations. Three base flows are considered, the Falkner-Skan boundary layer, boundary layers subjected to wall suction and the Blasius wall jet. The stability with respect to the exponential growth of waves and the algebraic growth of optimal streaks is studied for the Falkner-Skan boundary layer. For the algebraic growth, the optimal initial location, where the optimal disturbance is introduced in the boundary layer, is found to move downstream with decreased pressure gradient. A unified transition prediction method incorporating the influences of pressure gradient and free-stream turbulence is suggested. The algebraic growth of streaks in boundary layers subjected to wall suction is calculated. It is found that the spatial analysis gives larger optimal growth than temporal theory. Furthermore, it is found that the optimal growth is larger if the suction begins a distance downstream of the leading edge. Thresholds for transition of periodic and localized disturbances as well as the spreading of turbulent spots in the asymptotic suction boundary layer are investigated for Reynolds number Re=500, 800 and 1200 based on the displacement thickness and the free-stream velocity. It is found that the threshold amplitude scales like Re^-1.05 for transition initiated by streamwise vortices and random noise, like Re^-1.3 for oblique transition and like Re^-1.5 for the localized disturbance. The turbulent spot is found to take a bullet-shaped form that becomes more distinct and increases its spreading rate for higher Reynolds number. The Blasius wall jet is matched to the measured flow in an experimental wall-jet facility. Both the linear and nonlinear regime of introduced waves and streaks are investigated and compared to measurements. It is demonstrated that the streaks play an important role in the breakdown process where they suppress pairing and enhance breakdown to turbulence. Furthermore, statistics from the early turbulent regime are analyzed and reveal a reasonable self-similar behavior, which is most pronounced with inner scaling in the near-wall region.

Place, publisher, year, edition, pages
Stockholm: KTH, 2005. viii, 48 p.
Series
Trita-MEK, ISSN 0348-467X ; 2005:17
Keyword
boundary layer, suction, wall jet, streaks, waves, periodic disturbance, localized disturbance, turbulent spot, algebraic growth, exponential growth, stability, transition thresholds, transition prediction, PSE, DNS
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-546 (URN)
Public defence
2005-12-16, F3, F-huset, Lindstedsvägen 26, Stockholm, 10:15
Opponent
Supervisors
Note
QC 20101025Available from: 2005-12-08 Created: 2005-12-08 Last updated: 2010-10-25Bibliographically approved

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