Change search
ReferencesLink to record
Permanent link

Direct link
On the convectively unstable nature of optimal streaks in boundary layers
KTH, Superseded Departments, Mechanics.ORCID iD: 0000-0002-4346-4732
Show others and affiliations
2003 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 485, 221-242 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2003. Vol. 485, 221-242 p.
Keyword [en]
spatially developing flows, self-sustaining process, near-wall turbulence, shear flows, secondary instabilities, localized disturbances, optimal perturbations, absolute instability, bypass transition, batchelor vortex
URN: urn:nbn:se:kth:diva-22659DOI: 10.1017/s0022112003004427ISI: 000184058800011OAI: diva2:341357
QC 20100525Available from: 2010-08-10 Created: 2010-08-10Bibliographically approved

Open Access in DiVA

No full text

Other links

Publisher's full text

Search in DiVA

By author/editor
Brandt, LucaHenningson, Dan S.
By organisation
In the same journal
Journal of Fluid Mechanics

Search outside of DiVA

GoogleGoogle Scholar
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

Altmetric score

Total: 20 hits
ReferencesLink to record
Permanent link

Direct link