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Comparison between different passive vortex generators and their stabilizing effect on Tollmien-Schlichting waves
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.ORCID iD: 0000-0003-3310-0633
(English)Manuscript (preprint) (Other academic)
Abstract [en]

An experimental investigation is performed on the streaky boundary layers generated by three types of vortex generators; namely cylindrical roughness elements, and miniature vortex generators (MVG) with triangular and rectangular blades. The stability of the generated streaks and the amplitude of the spanwise modulations are compared together with their efficiency to stabilize Tollmien--Schlichting (TS) waves, where the TS waves are generated upstream of the vortex generators. It is shown that for constant geometrical and boundary layer parameters the amplitude of the streaks generated by rectangular MVGs is considerably higher compared to triangular MVGs and roughness elements, even though a higher critical roughness-height Reynolds numbers is obtained for MVGs with triangular blades. In the investigated streaky boundary layers similar TS wave amplitude attenuation rates are obtained for rectangular and triangular MVGs at branch~II of the neutral stability curve whereas a higher TS wave amplitude is obtained for roughness elements compared to the two-dimensional reference boundary layer.

National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-181898OAI: oai:DiVA.org:kth-181898DiVA: diva2:901350
Funder
EU, European Research Council
Note

QS 2016

Available from: 2016-02-07 Created: 2016-02-07 Last updated: 2016-02-08Bibliographically approved
In thesis
1. Boundary layer streaks as a novel laminar flow control method
Open this publication in new window or tab >>Boundary layer streaks as a novel laminar flow control method
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A novel laminar flow control based on generation of spanwise mean velocity gradients (SVG) in a flat plate boundary layer is investigated where disturbances of different types are introduced in the wall-bounded shear layer. The experimental investigations are aimed at; (i) generating stable and steady streamwise streaks in the boundary layer which set up spanwise gradients in the mean flow, and (ii) attenuating disturbance energy growth in the streaky boundary layers and hence delaying the onset of turbulence transition.

The streamwise streaks generated by four different methods are investigated, which are spanwise arrays of triangular/rectangular miniature vortex generators (MVGs) and roughness elements, non-linear pair of oblique waves, and spanwise-periodic finite discrete suction. For all the investigated methods the boundary layer is modulated into regions of high- and low speed streaks through formation of pairs of counter-rotating streamwise vortices. For the streaky boundary layers generated by the MVGs a parameter study on a wide range of MVG configurations is performed in order to investigate the transient growth of the streaks. A general scaling of the streak amplitudes is found based on empiricism where an integral amplitude definition is proposed for the streaks.

The disturbances are introduced as single- and broad band frequency twodimensional Tollmien–Schlichting (TS) waves, and three-dimensional single and a pair of oblique waves. In an attempt to obtain a more realistic configuration compared to previous investigations the disturbances are introduced upstream of the location were streaks are generated. It is shown that the SVG method is efficient in attenuating the growth of disturbance amplitudes in the linear regime for a wide range of frequencies although the disturbances have an initial amplitude response to the generation of the streaks. The attenuation rate of the disturbance amplitude is found to be optimized for an integral streak amplitude of 30% of the free-stream velocity which takes into account the periodic wavelength of the streaky base flow.

The stabilizing effect of the streamwise streaks can be extended to the nonlinear regime of disturbances which in turn results in transition to turbulence delay. This results in significant drag reduction when comparing the skin friction coefficient of a laminar- to a turbulent boundary layer. It is also shown that consecutive turbulence transition delay can be obtained by reinforcing the streaky boundary layer in the streamwise direction. For the streaky boundary layer generated by pair of oblique waves their forcing frequency sets the upper limit for the frequency of disturbances beyond which the control fails.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. xii, 54 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2016:01
Keyword
boundary layer stability, laminar to turbulent transition, laminar flow control, drag reduction, Tollmien-Schlichting waves, oblique waves, streaky boundary layers, miniature vortex generators, discrete suction
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-181899 (URN)978-91-7595-864-4 (ISBN)
Public defence
2016-02-26, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
EU, European Research Council
Note

QC 20160208

Available from: 2016-02-08 Created: 2016-02-07 Last updated: 2016-02-08Bibliographically approved

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S. Sattarzadeh, Sohrab
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