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Transition of streamwise streaks in zero-pressure-gradient boundary layers
KTH, Tidigare Institutioner                               , Mekanik.
KTH, Tidigare Institutioner                               , Mekanik.ORCID-id: 0000-0001-7864-3071
2002 (Engelska)Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 472, s. 229-261Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

A transition scenario initiated by streamwise low- and high-speed streaks in a flat-plate boundary layer is studied. In many shear flows, the perturbations that show the highest potential for transient energy amplification consist of streamwise-aligned vortices. Due to the lift-up mechanism these optimal disturbances lead to elongated streamwise streaks downstream, with significant spanwise modulation. In a previous investigation (Andersson et al. 2001), the stability of these streaks in a zero-pressure-gradient boundary layer was studied by means of Floquet theory and numerical simulations. The sinuous instability mode was found to be the most dangerous disturbance. We present here the first simulation of the breakdown to turbulence originating from the sinuous instability of streamwise streaks. The main structures observed during the transition process consist of elongated quasi-streamwise vortices located on the flanks of the low-speed streak. Vortices of alternating sign are overlapping in the streamwise direction in a staggered pattern. The present scenario is compared with transition initiated by Tollmien-Schlichting waves and their secondary instability and by-pass transition initiated by a pair of oblique waves. The relevance of this scenario to transition induced by free-stream turbulence is also discussed.

Ort, förlag, år, upplaga, sidor
2002. Vol. 472, s. 229-261
Nyckelord [en]
DIRECT NUMERICAL-SIMULATION, SELF-SUSTAINING PROCESS, PLANE POISEUILLE FLOW, CHANNEL FLOW, SECONDARY INSTABILITY, OPTIMAL PERTURBATIONS, BYPASS TRANSITION, REYNOLDS-NUMBER, TURBULENCE, STABILITY
Identifikatorer
URN: urn:nbn:se:kth:diva-12916DOI: 10.1017/S0022112002002331ISI: 000179810100010OAI: oai:DiVA.org:kth-12916DiVA, id: diva2:319588
Anmärkning
QC 20100518Tillgänglig från: 2010-05-18 Skapad: 2010-05-18 Senast uppdaterad: 2017-12-12Bibliografiskt granskad
Ingår i avhandling
1. Study of generation, growth and breakdown of streamwise streaks in a Blasius boundary layer.
Öppna denna publikation i ny flik eller fönster >>Study of generation, growth and breakdown of streamwise streaks in a Blasius boundary layer.
2001 (Engelska)Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

Transition from laminar to turbulent flow has beentraditionally studied in terms of exponentially growingeigensolutions to the linearized disturbance equations.However, experimental findings show that transition may occuralso for parameters combinations such that these eigensolutionsare damped. An alternative non-modal growth mechanism has beenrecently identified, also based on the linear approximation.This consists of the transient growth of streamwise elongateddisturbances, mainly in the streamwise velocity component,called streaks. If the streak amplitude reaches a thresholdvalue, secondary instabilities can take place and provoketransition. This scenario is most likely to occur in boundarylayer flows subject to high levels of free-stream turbulenceand is the object of this thesis. Different stages of theprocess are isolated and studied with different approaches,considering the boundary layer flow over a flat plate. Thereceptivity to free-stream disturbances has been studiedthrough a weakly non-linear model which allows to disentanglethe features involved in the generation of streaks. It is shownthat the non-linear interaction of oblique waves in thefree-stream is able to induce strong streamwise vortices insidethe boundary layer, which, in turn, generate streaks by thelift-up effect. The growth of steady streaks is followed bymeans of Direct Numerical Simulation. After the streaks havereached a finite amplitude, they saturate and a new laminarflow, characterized by a strong spanwise modulation isestablished. Using Floquet theory, the instability of thesestreaks is studied to determine the features of theirbreakdown. The streak critical amplitude, beyond which unstablewaves are excited, is 26% of the free-stream velocity. Theinstability appears as spanwise (sinuous-type) oscillations ofthe streak. The late stages of the transition, originating fromthis type of secondary instability, are also studied. We foundthat the main structures observed during the transition processconsist of elongated quasi-streamwise vortices located on theflanks of the low speed streak. Vortices of alternating signare overlapping in the streamwise direction in a staggeredpattern.

Descriptors:Fluid mechanics, laminar-turbulenttransition, boundary layer flow, transient growth, streamwisestreaks, lift-up effect, receptivity, free-stream turbulence,nonlinear mechanism, streak instability, secondary instability,Direct Numerical Simulation.

Ort, förlag, år, upplaga, sidor
Stockholm: Mekanik, 2001. s. vii, 17
Serie
Trita-MEK, ISSN 0348-467X ; 2001:10
Nyckelord
Fluid mechanics, laminar-turbulent transition, boundary layer flow, transient growth, streamwise streaks, lift-up effect, receptivity, free-stream turbulence, streak instability, secondary instability, Direct Numerical Simulation.
Identifikatorer
urn:nbn:se:kth:diva-1256 (URN)
Presentation
(Engelska)
Anmärkning
QC 20100518Tillgänglig från: 2001-09-24 Skapad: 2001-09-24 Senast uppdaterad: 2010-05-18Bibliografiskt granskad

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