Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Revival of Classical Vortex Generators Now for Transition Delay
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0003-3310-0633
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0002-3251-8328
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
2012 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, no 7, 074501- p.Article in journal (Refereed) Published
Abstract [en]

Classical vortex generators, known for their efficiency in delaying or even inhibiting boundary layer separation, are here shown to be coveted devices for transition to turbulence delay. The present devices are miniature with respect to classical vortex generators but are tremendously powerful in modulating the laminar boundary layer in the direction orthogonal to the base flow and parallel to the surface. The modulation generates an additional term in the perturbation energy equation, which counteracts the wall-normal production term and, hence, stabilizes the flow. Our experimental results show that these devices are really effective in delaying transition, but we also reveal their Achilles' heel.

Place, publisher, year, edition, pages
2012. Vol. 109, no 7, 074501- p.
Keyword [en]
Number-Independent Instability, Boundary-Layer, Flat Surface, Streaks, Waves
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-102913DOI: 10.1103/PhysRevLett.109.074501ISI: 000307709400022Scopus ID: 2-s2.0-84865209849OAI: oai:DiVA.org:kth-102913DiVA: diva2:557625
Funder
EU, European Research Council
Note

QC 20120928

Available from: 2012-09-28 Created: 2012-09-28 Last updated: 2017-12-07Bibliographically approved
In thesis
1. An experimental study on streamwise streaks in transitional boundary layers
Open this publication in new window or tab >>An experimental study on streamwise streaks in transitional boundary layers
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The present experimental study focuses on two topics, a passive transition-delay method for different types of wave disturbances and the effect of free-stream turbulence characteristics on boundary layer transition over a at plate. In the investigations, hot-wire anemometry was employed and the experiments were performed in a well-controlled wind tunnel facility.

In the past streamwise vortices, which generate high and low velocity streaks in the spanwise direction, has been employed successfully in order to damp boundary layer perturbations and eventually postpone transition to the turbulence. In the previous experiments the perturbations have been Tollmien- Schilichting waves (TSW) and the vortices, generating the streaky boundary layer, were produced by bluff obstacles, i:e: cylindrical surface roughnesses on the plate. In the first investigation of this thesis, it is shown that vortex generators originally used to control boundary layer separation, have a strong damping effect on boundary layer perturbations and are able to postpone the transition to turbulence. The present vortex generators are however miniature with respect to classical ones and are in the following denoted MVGs, for miniature vortex generators. The benefit of using MVGs is that the streaks are more stable compared to cylindrical surface roughnesses due to the fact that vortex shedding does not appear behind the MVGs. In addition, for the first time the perturbations are generated upstream of the MVGs, a configuration, which is closer to real applications. The effect of the streaks, which are generated by MVGs, can be characterized by a new integral-based amplitude definition. This amplitude definition, which scales on boundary layer parameters and geometrical parameters of the vortex generators, takes spanwise variations into account, which are neglected in the classical amplitude definition. Besides TSW, the effect of the vortex generators are investigated on other types of wave like disturbances, such as a single oblique wave (SOW) and a pair of oblique waves (POW). In the linear regime, in which the perturbations are of the order of and up to 1% of the free-stream velocity, it is observed for all the wave types that after an initial increase of the disturbance amplitude, they decay quickly in the streamwise direction in presence of the MVGs. Moreover, in the non-linear regime, a length of 1-2 meter transition delay is achieved in the presence of the MVGs. At the same time, the energy of the fluctuations are typically four orders of magnitude smaller in the modulated boundary layers compared to the uncontrolled case. It should be pointed out that there is a limitation for the streak amplitude in order to damp the disturbances. When the streak amplitude amplifies more than 30% using the new definition, the streaks become unstable and transition may occur even further upstream compared to the unmodulated boundary layer.

As a second study, another transition scenario was investigated, namely when the free-stream turbulence (FST) level is high enough to cause by-pass transition. FST can be generated by mounting a grid inside the wind tunnel upstream of the leading edge. By manufacturing grids by pipes (instead of solid bars), it is possible to pressurize them and then have a secondary ow injection through orifices in the pipes into the main stream pointing in the upstream direction. With this feature using several different FST grids the turbulence intensity can be varied, and along with a relative position of the grid to the leading edge, 42 different cases are provided in terms of different turbulent intensities, Tu = urms=U∝, and integral length scales, Λx, in the free-stream. In the first step, a universal streamwise distribution of turbulence intermittency, γ is introduced and then the effect of FST characteristics, such as turbulent intensity and integral length scale, are studied on the onset and length of the transition region. It is shown that the transition location is advanced by increasing turbulence intensity and goes with the exponent of -2, and the integral length scale has opposite effect on the transition onset at low and high turbulence intensities. While at low Tu, transition is postponed by decreasing the integral length scale, it is advanced at higher turbulence intensities. Moreover, it is argued that, even at high turbulence intensities, there exists a minimum distance for the turbulent boundary layer to be selfsustained.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. viii, 30 p.
Series
Trita-MEK, ISSN 0348-467X ; 2013:09
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-121184 (URN)978-91-7501-721-1 (ISBN)
Public defence
2013-05-03, Sal F3, Lindstedsvägen 26, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20130424

Available from: 2013-04-24 Created: 2013-04-22 Last updated: 2013-04-24Bibliographically approved
2. 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
3. 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

Open Access in DiVA

No full text

Other links

Publisher's full textScopus

Authority records BETA

Sattarzadeh, Sohrab ShirvanFransson, Jens H. M.

Search in DiVA

By author/editor
Shahinfar, ShahabSattarzadeh, Sohrab ShirvanFransson, Jens H. M.Talamelli, Alessandro
By organisation
Fluid PhysicsLinné Flow Center, FLOWMechanics
In the same journal
Physical Review Letters
Physical Sciences

Search outside of DiVA

GoogleGoogle Scholar

doi
urn-nbn

Altmetric score

doi
urn-nbn
Total: 280 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf