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Transient growth on boundary layer streaks
KTH, School of Engineering Sciences (SCI), Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0002-4346-4732
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0001-7864-3071
2005 (English)In: Journal of fluid mechanics, ISSN 0022-1120, Vol. 537, 91-100 p.Article in journal (Refereed) Published
Abstract [en]

The linear perturbations evolving on streamwise boundary layer streaks which yield maximum energy growth are computed. The steady and spanwise-periodic streaks arising from the nonlinear saturation of optimally growing streamwise vortices are considered as base flow. It is shown that significant transient growth may occur for both sinuous antisymmetric perturbations and for varicose symmetric modes. The energy growth is observed at amplitudes significantly below the threshold beyond which the streaks become linearly unstable and is largest for sinuous perturbations, to which the base flow considered first become unstable. The optimal initial condition consists of velocity perturbations localized in the regions of highest shear of the streak base flow, tilted upstream from the wall. The optimal response is still localized in the areas of largest shear but it is tilted in the flow direction. The most amplified perturbations closely resemble the unstable eigenfunctions obtained for streaks of higher amplitudes. The results suggest the possibility of a transition scenario characterized by the non-modal growth of primary perturbations, the streaks, followed by the secondary transient growth of higher frequency perturbations. The implication for turbulent flow is also discussed.

Place, publisher, year, edition, pages
2005. Vol. 537, 91-100 p.
Keyword [en]
viscous shear-flow, optimal perturbations, optimal excitation, pressure-gradient, turbulence, transition, breakdown, wall; instability, subject
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-5630DOI: 10.1017/S0022112005005203ISI: 000231935700004Scopus ID: 2-s2.0-23944441154OAI: oai:DiVA.org:kth-5630DiVA: diva2:10059
Note
QC 20100830Available from: 2006-04-27 Created: 2006-04-27 Last updated: 2010-12-06Bibliographically approved
In thesis
1. Stability and control of shear flows subject to stochastic excitations
Open this publication in new window or tab >>Stability and control of shear flows subject to stochastic excitations
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

In this thesis, we adapt and apply methods from linear control theory to shear flows. The challenge of this task is to build a linear dynamic system that models the evolution of the flow, using the Navier--Stokes equations, then to define sensors and actuators, that can sense the flow state and affect its evolution. We consider flows exposed to stochastic excitations. This framework allows to account for complex sources of excitations, often present in engineering applications. Once the system is built, including dynamic model, sensors, actuators, and sources of excitations, we can use standard optimization techniques to derive a feedback law. We have used feedback control to stabilize unstable flows, and to reduce the energy level of sensitive flows subject to external excitations.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. 86 p.
Series
Trita-MEK, ISSN 0348-467X ; 2006:08
Keyword
control, estimation, stochastic excitations, feedback, model reduction
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-3929 (URN)
Public defence
2006-05-05, Sal F3, Lindstedtsvägen 26, Stockholm, 10:15
Opponent
Supervisors
Note
QC 20100830Available from: 2006-04-27 Created: 2006-04-27 Last updated: 2011-12-16Bibliographically approved

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Brandt, L.ucaHenningson, Dan S.

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