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Unstable flow structures in the Blasius boundary layer
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0002-5913-5431
2014 (English)In: The European Physical Journal E Soft matter, ISSN 1292-8941, E-ISSN 1292-895X, Vol. 37, no 4, 34- p.Article in journal (Refereed) Published
Abstract [en]

Finite amplitude coherent structures with a reflection symmetry in the spanwise direction of a parallel boundary layer flow are reported together with a preliminary analysis of their stability. The search for the solutions is based on the self-sustaining process originally described by Waleffe (Phys. Fluids 9, 883 (1997)). This requires adding a body force to the Navier-Stokes equations; to locate a relevant nonlinear solution it is necessary to perform a continuation in the nonlinear regime and parameter space in order to render the body force of vanishing amplitude. Some states computed display a spanwise spacing between streaks of the same length scale as turbulence flow structures observed in experiments (S.K. Robinson, Ann. Rev. Fluid Mech. 23, 601 (1991)), and are found to be situated within the buffer layer. The exact coherent structures are unstable to small amplitude perturbations and thus may be part of a set of unstable nonlinear states of possible use to describe the turbulent transition. The nonlinear solutions survive down to a displacement thickness Reynolds number Re-* = 496, displaying a 4-vortex structure and an amplitude of the streamwise root-mean-square velocity of 6% scaled with the free-stream velocity. At this Re-* the exact coherent structure bifurcates supercritically and this is the point where the laminar Blasius flow starts to cohabit the phase space with alternative simple exact solutions of the Navier-Stokes equations.

Place, publisher, year, edition, pages
2014. Vol. 37, no 4, 34- p.
Keyword [en]
Exact Coherent Structures, Parameterized Continuation Process, Self-Sustaining Process, Amplitude Steady Waves, Pipe-Flow, Traveling-Waves, Secondary Instability, Shear Flows, Optimal Disturbances, Reynolds-Number
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URN: urn:nbn:se:kth:diva-145823DOI: 10.1140/epje/i2014-14034-1ISI: 000335160300004Scopus ID: 2-s2.0-84901945636OAI: oai:DiVA.org:kth-145823DiVA: diva2:721517
Note

QC 20140604

Available from: 2014-06-04 Created: 2014-06-02 Last updated: 2017-12-05Bibliographically approved

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Hanifi, Ardeshir

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