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Receptivity of crossflow instabilities to free-stream turbulence and surface roughness
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0002-3251-8328
(English)Article in journal (Other academic) Submitted
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-13916OAI: oai:DiVA.org:kth-13916DiVA: diva2:328228
Note
QS 20120328Available from: 2010-07-02 Created: 2010-07-02 Last updated: 2012-03-28Bibliographically approved
In thesis
1. An experimental investigation of disturbance growth in boundary layer flows
Open this publication in new window or tab >>An experimental investigation of disturbance growth in boundary layer flows
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis deals with the early stages of transition to turbulence in two different baseflows, namely the Falkner-Skan-Cooke boundary layer (FSC) and the asymptotic suction boundary layer (ASBL). Grid-generated turbulence is studied in order to characterise the isotropy levels, free-stream turbulence levels and characteristic length scales that will be present in the receptivity study. By varying the grids and their location it is possible to control the turbulence intensity level, Tu, and the integral length scale independently. Comparisons with other studies show that for increasing Re_M the isotropy levels and the rate of kinetic energy decay asymptotically approach the theoretical values. The FSC describes a a 3D boundary layer subjected to a pressure gradient. The FSC is stable to TS-waves, but becomes susceptible to both travelling and stationary crossflow disturbances. In the experiments the travelling modes were triggered using free-stream turbulence (FST) and the stationary modes were triggered using an array of cylindrical roughness elements. The receptivity phase to FST was linear as well as the initial growth. For high enough $Tu$ inside the boundary layer, nonlinear behaviour was observed further downstream. The stationary mode could only be triggered using tall roughness elements, with low heights resulting in no noticeable disturbances. The receptivity is found to be nonlinear for the roughness heights tested and the growth of the disturbances is exponential. For low levels of FST, Tu < 0.25%, the travelling mode as well as the stationary mode grew. The ASBL is formed when uniform suction is applied to the surface of a porous plate with a flow over it. This baseflow is very stable to TS-waves, and was used to study the transient growth. For the ASBL, stationary disturbances were triggered using a spanwise array or cylindrical roughness elements. The velocity signals were decomposed using a spatial Fourier transform to study the growth of individual modes. The fundamental mode as well as some harmonics were seen to undergo transient growth, before finally decaying exponentially. Comparisons were made to the experimental data using optimal perturbation theory. The global optimals did not describe the transient growth effects well. The calculations were redone for suboptimal times and showed agreement with the experimental data, showing that optimal perturbation theory can describe transient growth if the initial disturbance state is known.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. viii, 41 p.
Series
Trita-MEK, ISSN 0348-467X ; 2010:04
National Category
Fluid Mechanics and Acoustics Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-12949 (URN)978-91-7415-654-6 (ISBN)
Public defence
2010-06-11, F3, Linstedsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC20100702Available from: 2010-05-24 Created: 2010-05-19 Last updated: 2010-07-02Bibliographically approved

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Fransson, Jens H. M.

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CiteExportLink to record
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Citation style
  • apa
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