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An experimental study on the wake of a rectangular forebody with suction applied over the surfaces
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: 0000-0002-3251-8328
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-142855OAI: oai:DiVA.org:kth-142855DiVA: diva2:704738
Note

QS 2014

Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2016-03-16Bibliographically approved
In thesis
1. An experimental study on the wake behind a rectangular forebody with variable inlet conditions
Open this publication in new window or tab >>An experimental study on the wake behind a rectangular forebody with variable inlet conditions
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The wake behind a rectangular forebody with variable inlet conditions is investigated. The perforated surface of the two-dimensional rectangular forebody, with a smooth leading edge and a blunt trailing edge, allows for boundary layer modification by means of wall suction. The test section, of which the rectangular forebody is the main part, is experimentally evaluated with a series of hot-wire and Prandtl tube measurements in the boundary layer and the wake.

For a suction coefficient of Γ>9, corresponding to 0.9% suction of the free stream velocity, the asymptotic suction boundary layer (ASBL) is obtained at the trailing edge of the forebody for laminar boundary layers (Rex=1.6×105−3.8×105). The key feature of the ASBL, a spatially invariant boundary thickness which can be modified independent of the Reynolds number, is used to perform a unique parametrical study.

Turbulent boundary layers (Rex=4.5×105−3.0×106) subject to wall suction are also investigated. For a critical suction coefficient Γcrit, which depends on Rex, the boundary layer relaminarizes. Strong evidence is found to support the hypothesis that turbulent boundary layers will ultimately attain the ASBL as well, provided that the wall suction is strong enough.

The effect of the modulated laminar and turbulent boundary layers on the wake characteristics is studied. The shape of the mean wake velocity profile, scaled with the velocity deficit U0and the wake half width ∆y1/2, is found tobe independent of x/h, for x/h> 6 and Reh >6.7×103. The wake width is shown to scale with the effective thickness of the body h+2δ1, where the ratio is expected to vary with the downstream location.

A decrease of the displacement thickness leads to a decrease of the base pressure, with Cp,b = −0.36 in the ASBL limit. The Strouhal number based on the effective thickness becomes Sth+2δ1 ≈ 0.29 in the ASBL limit and independent of the plate thickness (h) Reynolds number, in the range Reh = 2.9×103 − 6.7×103. For the turbulent boundary Sth+2δ1 is found to be 25% lower, which shows that the wake characteristics depend on the state of the boundary layer at the trailing edge.

The total drag is found to be reduced by as much as 30% for Reh = 2.7×104 when a wall normal velocity of only 3.5% of the free stream velocity is applied. Wall suction successively reduces the total drag with increasing wall suction, at least in the Reynolds number rangeReh = 8.0×103−5.5×104.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. vi, 31 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2014:06
Keyword
Bluff bodies, wake flow, asymptotic suction boundary layer, flow control
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-142859 (URN)978-91-7595-044-0 (ISBN)
Presentation
2014-03-19, E3, Osquarsbacke 14, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20140312

Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2014-03-13Bibliographically approved
2. Shedding light on the bluff body wake instability
Open this publication in new window or tab >>Shedding light on the bluff body wake instability
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:nbn:se:kth:diva-183545 (URN)978-91-7595-893-4 (ISBN)
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

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