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An experimental study on streamwise streaks in transitional boundary layers
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
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: urn:nbn:se:kth:diva-121184ISBN: 978-91-7501-721-1 (print)OAI: oai:DiVA.org:kth-121184DiVA: diva2:617605
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
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. On transitional boundary layers caused by free-stream turbulence
Open this publication in new window or tab >>On transitional boundary layers caused by free-stream turbulence
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The present measurement campaign on the free-stream turbulence (FST) induced boundary layer transition scenario has provided a unique set of experimental data, with potential to enhance the understanding of the effect of the free-stream turbulence characteristic length scales on the transition location and not only the turbulence intensity, which has been the focus in most previous studies. Recent investigations where the turbulence intensity has been kept essentially constant, while the integral length scale has been changed, show that the transition location is advanced for increasing length scale. The present data confirm this result for low Tu-levels, but show the opposite effect for higher Tu-level, i:e. a delay in the transition location for longer integral length scales

National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-121271 (URN)
Note

QS 2013

Available from: 2013-04-24 Created: 2013-04-24 Last updated: 2013-04-24Bibliographically approved

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