On the convectively unstable nature of optimal streaks in boundary layers

L. Brandt 1 Carlo Cossu 2, * Jean-Marc Chomaz 2, * Patrick Huerre 2, * D.S. Henningson 1
* Auteur correspondant
1 Department of Mechanics, Royal Institute of Technology (KTH), Stockholm S-10044, Sweden
2 LadHyX - Laboratoire d'hydrodynamique
Abstract : The objective of the study is to determine the absolute/convective nature of the secondary instability experienced by finite-amplitude streaks in the flat-plate boundary layer. A family of parallel streaky base flows is defined by extracting velocity profiles from direct numerical simulations of nonlinearly saturated optimal streaks. The computed impulse response of the streaky base flows is then determined as a function of streak amplitude and streamwise station. Both the temporal and spatio-temporal instability properties are directly retrieved from the impulse response wave packet, without solving the dispersion relation or applying the pinching point criterion in the complex wavenumber plane. The instability of optimal streaks is found to be unambiguously convective for all streak amplitudes and streamwise stations. It is more convective than the Blasius boundary layer in the absence of streaks; the trailing edge-velocity of a Tollmien-Schlichting wave packet in the Blasius boundary layer is around 35% of the free-stream velocity, while that of the wave packet riding on the streaky base flow is around 70%. This is because the streak instability is primarily induced by the spanwise shear and the associated Reynolds stress production term is located further away from the wall, in a larger velocity region, than for the Tollmien-Schlichting instability. The streak impulse response consists of the sinuous mode of instability triggered by the spanwise wake-like profile, as confirmed by comparing the numerical results with the absolute/convective instability properties of the family of two-dimensional wakes introduced by Monkewitz (1988). The convective nature of the secondary streak instability implies that the type of bypass transition studied here involves streaks that behave as amplifiers of external noise.
Type de document :
Article dans une revue
Journal of Fluid Mechanics, Cambridge University Press (CUP), 2003, may (485), pp.221-242. 〈10.1017/s0022112003004427〉
Liste complète des métadonnées

Littérature citée [47 références]  Voir  Masquer  Télécharger
https://hal-polytechnique.archives-ouvertes.fr/hal-01024920
Contributeur : Denis Roura <>
Soumis le : mercredi 27 août 2014 - 12:53:59
Dernière modification le : jeudi 7 février 2019 - 16:59:53
Document(s) archivé(s) le : vendredi 28 novembre 2014 - 10:11:22

Fichier

S0022112003004427a.pdf
Fichiers éditeurs autorisés sur une archive ouverte

Identifiants

Collections

X-LADHYX | X | PARISTECH | X-DEP-MECA

Citation

L. Brandt, Carlo Cossu, Jean-Marc Chomaz, Patrick Huerre, D.S. Henningson. On the convectively unstable nature of optimal streaks in boundary layers. Journal of Fluid Mechanics, Cambridge University Press (CUP), 2003, may (485), pp.221-242. 〈10.1017/s0022112003004427〉. 〈hal-01024920〉

Exporter

BibTeX TEI DC DCterms EndNote

Partager

Métriques

Consultations de la notice

218

Téléchargements de fichiers

185

Contact

support.ccsd.cnrs.fr
hal.support@ccsd.cnrs.fr
Logo CCSD