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Shedding light on the bluff body wake instability
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The flow around a vehicle, for example, results in a large resistance and can generate a complex swirl pattern behind the body of the vehicle, which can lead to strong alternating lateral forces. There is therefore a clear need for more fundamental understanding of the instability of this so-called wake region, which arises behind bluff bodies. In this thesis, the focus lies on the relation between the wake inlet conditions and the wake characteristics of a bluff body. This relation is studied experimentally, in the Reynolds number range 2.9E3 - 5.5E4.  The experiments are carried out in a specially designed test-section, which comprises a rectangular forebody with a smooth leading edge and a blunt trailing edge. The perforated top and bottom surface of this body allow for boundary layer modification by means of wall suction. 

Hot-wire anemometry and pressure measurements show that wall suction, in the order of 1% of the free-stream velocity, leads to a significant change of the boundary-layer properties; the boundary-layer thickness decreases and the wall-shear stress increases.  Laminar boundary layers take the form of the asymptotic suction boundary layer and turbulent boundary layers are shown to relaminarize, when subject to wall suction above a critical value. A modification of the boundary layer leads to an increase of the vortex shedding frequency and a decrease of the base pressure. Empirical relations for the Strouhal number and the base pressure coefficient are derived. The boundary layer conditions are included in these relations and as result, they hold for both laminar and turbulent boundary layers.

Two-dimensional velocity fields, obtained by means of particle image velocimetry, reveal the effect of boundary-layer modification on the near-wake topology. By dividing the velocity component into (i) mean, (ii) periodic and (iii) random contributions, it can be shown that the confluence point moves upstream, the curvature of streamlines enclosing the trailing edge increases, and the Reynolds normal and shear stresses increase. 

The experimental study is continued by introducing interference elements in the wake. The qualitative effect of a splitter plate on the wake characteristics is not altered by boundary-layer modification, but the critical splitter plate length and gap width for which the vortex shedding frequency is suppressed do change. A sensitivity map is obtained experimentally by placing a control rod at various locations in the wake. Considering all limitations of such an experiment, it was found to be in good agreement with the result of a linear stability analysis carried out on the measured mean velocity field. 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , viii, 60 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2016:04
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-183545ISBN: 978-91-7595-893-4 (print)OAI: oai:DiVA.org:kth-183545DiVA: diva2:912433
Public defence
2016-04-08, sal D2, Lindstedtsvägen 5, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20160316

Available from: 2016-03-16 Created: 2016-03-16 Last updated: 2016-03-16Bibliographically approved
List of papers
1. A new test-section for wind tunnel studies on wake instability and its control
Open this publication in new window or tab >>A new test-section for wind tunnel studies on wake instability and its control
2009 (English)Report (Other academic)
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-47325 (URN)
Note
QC 20111108Available from: 2011-11-08 Created: 2011-11-08 Last updated: 2016-03-16Bibliographically approved
2. An experimental study on the wake of a rectangular forebody with suction applied over the surfaces
Open this publication in new window or tab >>An experimental study on the wake of a rectangular forebody with suction applied over the surfaces
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-142855 (URN)
Note

QS 2014

Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2016-03-16Bibliographically approved
3. Boundary layer modification by means of wall suction and the effect on the wake behind a rectangular forebody
Open this publication in new window or tab >>Boundary layer modification by means of wall suction and the effect on the wake behind a rectangular forebody
2014 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 26, no 12, 125105- p.Article in journal (Refereed) Published
Abstract [en]

The wake characteristics of a two-dimensional rectangular forebody with a smooth leading edge and a blunt trailing edge are investigated. Wall suction is applied along the forebody in order to modify the developing boundary layer. An initially laminar boundary layer subject to suction yields an asymptotic suction boundary layer at the trailing edge of the body, whereas a high enough suction coefficient relaminarizes an initially turbulent boundary layer. The critical suction velocity required to achieve this significant modification of the boundary layer properties is typically in the order of 1% of the free-stream velocity, where the critical suction coefficient depends on the Reynolds number. We show that a thinner boundary layer induces a higher vortex shedding frequency and a lower base pressure. Furthermore, the boundary layer state, laminar or turbulent, has a significant influence on the wake. For example, the Strouhal number based on the effective body thickness is being reduced by 25% from laminar to turbulent inlet conditions.

Keyword
Circular-Cylinder, Pressure-Gradients, Porous Cylinder, Base-Pressure, Aspect Ratio, Bluff-Body, Flow, Turbulence, Injection, Subject
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-159389 (URN)10.1063/1.4904376 (DOI)000347161600046 ()2-s2.0-84919935697 (Scopus ID)
Funder
Swedish Research CouncilEU, European Research Council
Note

QC 20150130

Available from: 2015-01-30 Created: 2015-01-29 Last updated: 2017-12-05Bibliographically approved
4. An experimental study on the combined effect of boundary layer modification and a splitter plate on the wake characteristics
Open this publication in new window or tab >>An experimental study on the combined effect of boundary layer modification and a splitter plate on the wake characteristics
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-183543 (URN)
Note

QS 2016

Available from: 2016-03-16 Created: 2016-03-16 Last updated: 2016-03-16Bibliographically approved
5. On the effect of boundary layer modification on the near wake topology of a rectangular forebody
Open this publication in new window or tab >>On the effect of boundary layer modification on the near wake topology of a rectangular forebody
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-183542 (URN)
Note

QS 2016

Available from: 2016-03-16 Created: 2016-03-16 Last updated: 2016-03-16Bibliographically approved
6. An experimental study on the relation between the wake inlet conditions and the near wake topology
Open this publication in new window or tab >>An experimental study on the relation between the wake inlet conditions and the near wake topology
2016 (English)In: Springer Proceedings in Physics, Springer Science+Business Media B.V., 2016, 133-138 p.Conference paper, Published paper (Refereed)
Abstract [en]

A study on the relation between the wake inlet conditions and the wake characteristics of a bluff body by means of Particle Image Velocimetry is presented. The wake inlet condition, being a laminar boundary layer at the trailing edge of the body, are varied by means of wall-suction. Measurements are carried out at Reh = 6.7 × 103 based on the body thickness h. The induced radius of curvature of the streamlines is shown to be a promising parameter in explaining the increase in base drag and decrease in vortex shedding frequency associated with a thinner boundary layer.

Place, publisher, year, edition, pages
Springer Science+Business Media B.V., 2016
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-183541 (URN)10.1007/978-3-319-30602-5_17 (DOI)000387431400017 ()2-s2.0-84979031674 (Scopus ID)9783319306001 (ISBN)
Conference
5th International Conference on Jets, Wakes and Separated Flows, ICJWSF2015, 15 June 2015 through 18 June 2015
Note

QC 20160316

Available from: 2016-03-16 Created: 2016-03-16 Last updated: 2017-02-23Bibliographically approved
7. Investigation of passive control of the wake past a thick plate by stability and sensitivity analysis of experimental data
Open this publication in new window or tab >>Investigation of passive control of the wake past a thick plate by stability and sensitivity analysis of experimental data
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-183544 (URN)
Note

QS 2016

Available from: 2016-03-16 Created: 2016-03-16 Last updated: 2016-03-16Bibliographically approved

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