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Transition in boundary layers subject to free-stream turbulence
KTH, Tidigare Institutioner, Mekanik.ORCID-id: 0000-0002-4346-4732
KTH, Tidigare Institutioner, Mekanik.ORCID-id: 0000-0001-9627-5903
KTH, Tidigare Institutioner, Mekanik.ORCID-id: 0000-0001-7864-3071
2004 (Engelska)Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 517, s. 167-198Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

The effect of high levels of free-stream turbulence on the transition in a Blasius boundary layer is studied by means of direct numerical simulations, where a synthetic turbulent inflow is obtained as superposition of modes of the continuous spectrum of the Orr-Sommerfeld and Squire operators. In the present bypass scenario the flow in the boundary layer develops streamwise elongated regions of high and low streamwise velocity and it is suggested that the breakdown into turbulent spots is related to local instabilities of the strong shear layers associated with these streaks. Flow structures typical of the spot precursors are presented and these show important similarities with the flow structures observed in previous studies on the secondary instability and breakdown of steady symmetric streaks. Numerical experiments are performed by varying the energy spectrum of the incoming perturbation. It is shown that the transition location moves to lower Reynolds numbers by increasing the integral length scale of the free-stream turbulence. The receptivity to free-stream turbulence is also analysed and it is found that two distinct physical mechanisms are active depending on the energy content of the external disturbance. If low-frequency modes diffuse into the boundary layer, presumably at the leading edge, the streaks Lire induced by streamwise vorticity through the linear lift-up effect. If, conversely, the free-stream perturbations are mainly located above the boundary layer a nonlinear process is needed to create streamwise vortices inside the shear layer. The relevance of the two mechanisms is discussed.

Ort, förlag, år, upplaga, sidor
2004. Vol. 517, s. 167-198
Nyckelord [en]
orr-sommerfeld equation, near-wall turbulence, self-sustaining process, bypass transition, continuous-spectrum, shear flows, localized disturbances, optimal perturbations, channel flows, receptivity
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URN: urn:nbn:se:kth:diva-23881DOI: 10.1017/s0022112004000941ISI: 000225173200008Scopus ID: 2-s2.0-8144222576OAI: oai:DiVA.org:kth-23881DiVA, id: diva2:342580
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QC 20100525 QC 20110914Tillgänglig från: 2010-08-10 Skapad: 2010-08-10 Senast uppdaterad: 2017-12-12Bibliografiskt granskad

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Brandt, LucaSchlatter, PhilippHenningson, Dan S.

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