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Transition thresholds 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
2005 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 17, no 12Article in journal (Refereed) Published
Abstract [en]

Energy thresholds for transition to turbulence in an asymptotic suction boundary layer is calculated by means of temporal direct numerical simulations. The temporal assumption limits the analysis to periodic disturbances with horizontal wave numbers determined by the computational box size. Three well known transition scenarios are investigated: oblique transition, the growth and breakdown of streaks triggered by streamwise vortices, and the development of random noise. Linear disturbance simulations and stability diagnostics are also performed for a base flow consisting of the suction boundary layer and a streak. The scenarios are found to trigger transition by similar mechanisms as obtained for other flows. Transition at the lowest initial energy is provided by the oblique wave scenario for the considered Reynolds numbers 500, 800, and 1200. The Reynolds number dependence on the energy thresholds are determined for each scenario. The threshold scales like Re-2.6 for oblique transition and like Re-2.1 for transition initiated by streamwise vortices and random noise, indicating that oblique transition has the lowest energy threshold also for larger Reynolds numbers.

Place, publisher, year, edition, pages
2005. Vol. 17, no 12
Keyword [en]
free-stream turbulence, plane poiseuille flow, localized disturbances, subcritical transition, oblique transition, bypass transition, pressure-gradient, algebraic growth, channel flows, shear flows
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-8902DOI: 10.1063/1.2136900ISI: 000233603000019Scopus ID: 2-s2.0-30044443822OAI: oai:DiVA.org:kth-8902DiVA: diva2:14383
Note
Uppdaterad från "In press" till published: 20101021. QC 20101021Available from: 2005-12-08 Created: 2005-12-08 Last updated: 2017-12-14Bibliographically 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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