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Attenuation of boundary layer disturbances by means of streamwise vortices
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. (Prof. Jens Fransson research group)ORCID iD: 0000-0003-3310-0633
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 [en]
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: urn:nbn:se:kth:diva-139050ISBN: 978-91-7501-984-0 (print)OAI: oai:DiVA.org:kth-139050DiVA: diva2:682783
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
List of papers
1. Revival of Classical Vortex Generators Now for Transition Delay
Open this publication in new window or tab >>Revival of Classical Vortex Generators Now for Transition Delay
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.

Keyword
Number-Independent Instability, Boundary-Layer, Flat Surface, Streaks, Waves
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-102913 (URN)10.1103/PhysRevLett.109.074501 (DOI)000307709400022 ()2-s2.0-84865209849 (Scopus ID)
Funder
EU, European Research Council
Note

QC 20120928

Available from: 2012-09-28 Created: 2012-09-28 Last updated: 2017-12-07Bibliographically approved
2. Scaling of streamwise boundary layer streaks and their ability to reduce skin-friction drag
Open this publication in new window or tab >>Scaling of streamwise boundary layer streaks and their ability to reduce skin-friction drag
2013 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 733, 1-32 p.Article in journal (Refereed) Published
Abstract [en]

Spanwise arrays of miniature vortex generators (MVGs)are used to generate energetic transient disturbance growth, which is able to modulate the boundary layer flow with steady and stable streak amplitudes up to 32% of the free-stream velocity. This type of modulation has previously been shown to act in a stabilizing manner on modal disturbance growth described by classical instability theory. In anattempt to reproduce a more realistic flow configuration, in the present experimental setup, Tollmien-Schlichting (TS) waves are generated upstream of the MVG array, allowingfor a complete interaction of the incoming wave with the array. Fifteen new MVG configurations are investigated and the stabilizing effect on the TS waves is quantified. We show that the streak amplitude definition is very importantwhen trying to relate it to the stabilization, since it may completely bypass information on the mean streamwise velocity gradient in the spanwise direction, which is an essential ingredient of the observed stabilization. Here, we use an integral-based streak amplitude definition along with a streak amplitude scaling relation based on empiricism,which takes the spanwise periodicity of the streaks into account. The results show that, applying the integral definition, the optimal streak amplitude for attenuating TS wave disturbance growth is around 30% of the free-stream velocity, which corresponds to ̃20% in the conventional definition when keeping the spanwise wavelength constant. The experiments also show that the disturbance energy level, based on the full velocity signal, is significantly reduced in the controlled case, and that the onset of transition may be inhibited altogether throughout the measured region in the presence of an MVG array.

Keyword
drag reduction, instability control, transition to turbulence
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-121220 (URN)10.1017/jfm.2013.431 (DOI)000325649100007 ()2-s2.0-84884567688 (Scopus ID)
Funder
EU, European Research Council
Note

QC 20131028. Updated from submitted to published.

Available from: 2013-04-24 Created: 2013-04-24 Last updated: 2017-12-06Bibliographically approved
3. Passive boundary layer control of oblique distrubances by finite-amplitude streaks
Open this publication in new window or tab >>Passive boundary layer control of oblique distrubances by finite-amplitude streaks
2014 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 749, 1-36 p.Article in journal (Refereed) Published
Abstract [en]

Recent experimental results on the attenuation of two-dimensional Tollmien-Schlichting wave (TSW) disturbances by means of passive miniature vortex generators (MVGs) have shed new light on the possibility of delaying transition to turbulence and hence accomplishing skin-friction drag reduction. A recurrent concern has been whether this passive flow control strategy would work for other types of disturbances than plane TSWs in an experimental configuration where the incoming disturbance is allowed to fully interact with the MVG array. In the present experimental investigation we show that not only TSW disturbances are attenuated, but also three-dimensional single oblique wave (SOW) and pair of oblique waves (POW) disturbances are quenched in the presence of MVGs, and that transition delay can be obtained successfully. For the SOW disturbance an unusual interaction between the wave and the MVGs occurs, leading to a split of the wave with one part travelling with a 'mirrored' phase angle with respect to the spanwise direction on one side of the MVG centreline. This gives rise to 3-vortices on the centreline, which force a low-speed streak on the centreline, strong enough to overcome the high-speed streak generated by the MVGs themselves. Both these streaky boundary layers seem to act stabilizing on unsteady perturbations. The challenge in a passive control method making use of a non-modal type of disturbances to attenuate modal disturbances lies in generating stable streamwise streaks which do not themselves break down to turbulence.

Keyword
boundary layer control, boundary layer stability, transition to turbulence
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-121270 (URN)10.1017/jfm.2014.211 (DOI)000337922700001 ()2-s2.0-84901017405 (Scopus ID)
Funder
EU, European Research Council
Note

QC 20140806. Updated from manuscript to article in journal.

Available from: 2013-04-24 Created: 2013-04-24 Last updated: 2017-12-06Bibliographically approved
4. Experimental investigation on the steady and unsteady disturbances in a flat plate boundary layer
Open this publication in new window or tab >>Experimental investigation on the steady and unsteady disturbances in a flat plate boundary layer
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.

Keyword
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:nbn:se:kth:diva-139047 (URN)10.1063/1.4902395 (DOI)000347161600037 ()2-s2.0-84918789723 (Scopus ID)
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
5. On the scaling of streamwise streaks and their efficiency to attenuate Tollmien-Schlichting waves
Open this publication in new window or tab >>On the scaling of streamwise streaks and their efficiency to attenuate Tollmien-Schlichting waves
2015 (English)In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 56, no 3, 58Article in journal (Refereed) Published
Abstract [en]

Streaky boundary layers generated by an array of miniature vortex generators (MVGs) mounted on a flat plate have recently shown to have a stabilizing effect on both two-and three-dimensional disturbances. An experimental study on the effect of the geometrical parameters of MVGs on the generated streamwise streaks in the flat plate boundary layer is carried out, and the corresponding stabilizing effect on Tollmien-Schlichting (TS) wave disturbances is quantified. The new experimental configurations have led to an improved empirical scaling law, which includes additional geometrical parameters of the MVGs compared to the previously reported relation. It is found that the MVG configuration can be optimized with respect to the attenuation of disturbances. In addition, the streamwise location of branch I of the neutral stability curve, with regard to the location of the MVG array, is found to be correlated with the initial receptivity of TS waves on the MVG array and the attenuation of the TS wave amplitude in the unstable region.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-139048 (URN)10.1007/s00348-015-1930-x (DOI)000350688000011 ()2-s2.0-84928666620 (Scopus ID)
Funder
EU, European Research Council
Note

QC 20150423. Updated from manuscript to article in journal.

Available from: 2013-12-30 Created: 2013-12-30 Last updated: 2017-12-06Bibliographically approved
6. Consecutive turbulence transition delay with reinforced passive control
Open this publication in new window or tab >>Consecutive turbulence transition delay with reinforced passive control
2014 (English)In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 89, no 6, 061001- p.Article in journal (Refereed) Published
Abstract [en]

Miniature vortex generators (MVGs) are able to delay the transition to turbulence in a flat plate boundary layer if properly designed. Unfortunately, the natural recovery of the modulated laminar base flow in the streamwise direction is of exponential space scale and hence the passive laminar control fades away fairly rapidly. Here we show that by placing a second array of MVGs downstream of the first one it is possible to nourish the counter-rotating streamwise vortices responsible for the modulation, which results in a prolonged streamwise extent of the control. With this control strategy it is possible to delay the transition to turbulence, consecutively, by reinforcing the control effect and with the ultimate implication of obtaining a net skin-friction drag reduction of at least 65%.

Keyword
Tollmien-Schlichting Waves, Free-Stream Turbulence, Boundary-Layer, Stabilization
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-139049 (URN)10.1103/PhysRevE.89.061001 (DOI)000339053500003 ()2-s2.0-84902602766 (Scopus ID)
Funder
EU, European Research Council
Note

QC 20140811. Updated from manuscript to article in journal.

Available from: 2013-12-30 Created: 2013-12-30 Last updated: 2017-12-06Bibliographically approved

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Sattarzadeh Shirvan, Sohrab

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  • ieee
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