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Onset of global instability behind distributed surface roughness in a Falkner–Skan–Cooke boundary layer
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.ORCID iD: 0000-0001-9446-7477
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.ORCID iD: 0000-0001-9627-5903
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. Swedish Defence Research Agency, FOI.ORCID iD: 0000-0002-5913-5431
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2015 (English)Report (Other academic)
Abstract [en]

A three-dimensional linear global stability analysis of a Falkner–Skan–Cooke boundary layer with distributed three-dimensional surface roughness is performed. The Falkner–Skan–Cooke boundary layer models the flow over swept airplane wings, and investigation of the critical roughness size for which a global instability emerges is thus of great importance within aeronautical applications. The study considers high-order direct numerical simulations and shows that such a critical roughness height exists for the Falkner–Skan–Cooke boundary layer. The roughness Reynolds number and roughness element aspect ratio for which this happens is comparable to the transition data reported in the literature for two-dimensional boundary layers. This demonstrates the importance of the local flow conditions in the vicinity of the roughness for triggering a global instability, although the resulting breakdown scenario is completely different from that of two-dimensional boundary layers. This breakdown scenario is studied in detail, and a global energy analysis is used to reveal the structures and mechanisms responsible for production and dissipation of perturbation energy.

Place, publisher, year, edition, pages
2015. , 33 p.
National Category
Fluid Mechanics and Acoustics Aerospace Engineering
Research subject
Engineering Mechanics; Aerospace Engineering
Identifiers
URN: urn:nbn:se:kth:diva-175347OAI: oai:DiVA.org:kth-175347DiVA: diva2:860446
Note

QC 20151015

Available from: 2015-10-12 Created: 2015-10-12 Last updated: 2015-10-15Bibliographically approved
In thesis
1. Global stability analysis of three-dimensional boundary layer flows
Open this publication in new window or tab >>Global stability analysis of three-dimensional boundary layer flows
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis considers the stability and transition of incompressible boundary layers. In particular, the Falkner–Skan–Cooke boundary layer subject to a cylindrical surface roughness, and the Blasius boundary layer with applied localized suction are investigated. These flows are of great importance within the aviation industry, feature complex transition scenarios, and are strongly three-dimensional in nature. Consequently, no assumptions regarding homogeneity in any of the spatial directions are possible, and the stability of the flow is governed by an extensive three-dimensional eigenvalue problem.

The stability of these flows is addressed by high-order direct numerical simulations using the spectral element method, in combination with a Krylov subspace projection method. Such techniques target the long-term behavior of the flow and can provide lower limits beyond which transition is unavoidable. The origin of the instabilities, as well as the mechanisms leading to transition in the aforementioned cases are studied and the findings are reported.

Additionally, a novel method for computing the optimal forcing of a dynamical system is developed. This type of analysis provides valuable information about the frequencies and structures that cause the largest energy amplification in the system. The method is based on the inverse power method, and is discussed in the context of the one-dimensional Ginzburg–Landau equation and a two-dimensional flow case governed by the Navier–Stokes equations.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2015. x, 30 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2015:07
Keyword
Hydrodynamic stability, transition to turbulence, global analysis, boundary layers, roughness, laminar flow control, Stokes/Laplace preconditioner, optimal forcing, crossflow vortices, Ginzburg-Landau, Falkner-Skan-Cooke, Blasius, lid-driven cavity
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-175353 (URN)978-91-7595-725-8 (ISBN)
Presentation
2015-10-30, D3, Lindstedtsvägen 5, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20151015

Available from: 2015-10-15 Created: 2015-10-12 Last updated: 2015-10-15Bibliographically approved

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Brynjell-Rahkola, MattiasSchlatter, PhilippHanifi, ArdeshirHenningson, Dan S.

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