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Optimal disturbances in suction boundary layers.
KTH, School of Engineering Sciences (SCI), Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0001-7864-3071
2007 (English)In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 26, no 3, 330-343 p.Article in journal (Refereed) Published
Abstract [en]

A well-known optimization procedure is used to find the optimal disturbances in two different suction boundary layers within the spatial framework. The maximum algebraic growth in the asymptotic suction boundary layer is presented and compared to previous temporal results. Furthermore, the spatial approach allows a study of a developing boundary layer in which a region at the leading edge is left free from suction. This new flow, which emulates the base flow of a recent wind-tunnel experiment, is herein denoted a semi-suction boundary layer. It is found that the optimal disturbances for these two suction boundary layers consist of streamwise vortices that develop into streamwise streaks, as previously found for a number of shear flows. It is shown that the maximum energy growth in the semi-suction boundary layer is obtained over the upstream region where no suction is applied. The result indicates that the spanwise scale of the streaks is set in this region, which is in agreement with previous experimental findings.

Place, publisher, year, edition, pages
2007. Vol. 26, no 3, 330-343 p.
Keyword [en]
Suction boundary layer, Optimal disturbance, Algebraic growth, Streamwise streaks, Free-stream turbulence
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-8900DOI: 10.1016/j.euromechflu.2006.07.003ISI: 000245207500002Scopus ID: 2-s2.0-33847247341OAI: oai:DiVA.org:kth-8900DiVA: diva2:14381
Note
Uppdaterad från "In press" till published: 20101021. QC 20101021Available from: 2005-12-08 Created: 2005-12-08 Last updated: 2010-10-21Bibliographically 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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