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Experimental investigation on the steady and unsteady disturbances in a flat plate boundary layer
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0003-3310-0633
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0002-3251-8328
2014 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 26, no 12, 124103- p.Article in journal (Refereed) Published
Abstract [en]

Recent experiments have shown that miniature vortex generators (MVGs) are coveted devices to stabilize unsteady disturbances in flat plate boundary layers and to delay the onset of turbulence by modulating the base flow in the spanwise direction. The spanwise modulation is a result from the non-modal transient growth of steady and spanwise periodic streamwise vortices being generated by the MVGs. The present experimental investigation aims at studying the transient growth of non-modal disturbances induced by a spanwise periodic array of MVGs and its stabilizing effect on non-linear unsteady disturbances in the boundary layer originating from planar Tollmien-Schlichting (TS) waves. Measurements consist of cross-stream planes at different downstream locations in the boundary layer and a spatio-temporal analysis of different modes of the disturbances is carried out. In the streaky boundary layer generated by the MVGs the fundamental spanwise mode, with the same wavelength as the MVG pairs in the array, and its first harmonic, both undergo transient growth whereas the higher harmonics decay immediately downstream of the array. In the unstable region formed in the wake of the MVG blades, i.e., just downstream of the array, a wide range of spanwise modes contributes to an initial growth in the energy of unsteady disturbances. Similar behavior is observed upstream of branch II position of the neutral stability curve where the unsteady disturbances undergo a second energy growth in the unstable region. It is shown that the spatial gradients of the base flow in the wall-normal and spanwise directions are contributing to the amplification and attenuation of the TS wave disturbances, respectively, in the streaky boundary layer.

Place, publisher, year, edition, pages
2014. Vol. 26, no 12, 124103- p.
Keyword [en]
Tollmien-Schlichting Waves, Number-Independent Instability, Streamwise Vortices, Bypass Transition, Transient Growth, Shear Flows, Streaks, Stability, Surface, Stabilization
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-139047DOI: 10.1063/1.4902395ISI: 000347161600037Scopus ID: 2-s2.0-84918789723OAI: oai:DiVA.org:kth-139047DiVA: diva2:682767
Funder
EU, European Research Council
Note

QC 20150203. Updated from manuscript to article in journal.

Previous title: "Experimental investigation on the steady and traveling disturbances in a flat plate boundary layer".

Available from: 2013-12-30 Created: 2013-12-30 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Attenuation of boundary layer disturbances by means of streamwise vortices
Open this publication in new window or tab >>Attenuation of boundary layer disturbances by means of streamwise vortices
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The stability of the boundary layer developed on a flat plate is studied in the presence of streamwise streaks where disturbances of different types are introduced in the wall-bounded shear layer. The experimental investigations are aimed at flow control by means of passively damping the energy growth of the disturbances in the streaky boundary layer and furthermore delay the onset of the transition in the streamwise direction.

The streamwise streaks are introduced to the two-dimensional Falkner-Skan boundary layer by means of a spanwise-periodic array of miniature vortex generators (MVGs), mounted on the flat plate, that generate pairs of counterrotating vortices elongated in the streamwise direction. The spanwise modulation of the boundary layer into regions of high and low speed streaks is hence obtained passively since the MVG blades utilize the existing momentum in the shear flow to set up the streamwise vortices with no energy input to the boundary layer. The disturbances are introduced as planar Tollmien-Schlichting (TS) waves as well as three dimensional oblique and pair of oblique waves. In an attempt to obtain a more realistic configuration, the disturbance slot is moved upstream of the MVG array as opposed to the earlier studies. It is shown that the passive control method is capable of stabilizing the disturbance waves in the linear regime for a wide range of frequencies albeit an initial receptivity of the disturbance amplitudes is observed immediately downstream of the MVG array. The control method is proven to be extended in the non-linear regime of the disturbances where transition to turbulence delay is obtained. This results in significant drag reduction when comparing the skin friction drag of a laminar boundary layer to a turbulent one. In addition, a parameter study on a wide range of MVG configurations is performed in order to investigate the transient growth of the streaks as well as the damping effects. A universal scaling of the streak amplitudes is found based on empiricism where an integral amplitude definition is proposed for the streaks. The damping effect 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. Moreover, it is shown that the passive control method can be extended in the streamwise direction by placing a second array of the MVGs in an already streaky boundary layer which results in further delay of the transition to turbulence location.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013
Series
TRITA-MEK, ISSN 0348-467X ; 2013:22
Keyword
boundary layer stability, Tollmien-Schlichting waves, oblique waves, transition to turbulence, streamwise vortices, miniature vortex generators, instability control, drag reduction
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-139050 (URN)978-91-7501-984-0 (ISBN)
Presentation
2014-01-20, Q2, Osquldasväg 10, KTH, Stockholm, 13:15 (English)
Opponent
Supervisors
Funder
EU, European Research Council
Note

QC 20140117

Available from: 2014-01-17 Created: 2013-12-30 Last updated: 2014-01-17Bibliographically approved
2. 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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Sattarzadeh, Sohrab S.Fransson, Jens H. M.

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