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Experimental study of the rotating-disk boundary-layer flow with surface roughness
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.ORCID iD: 0000-0002-1146-3241
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. Institute of Continuing Education, University of Cambridge.
2014 (English)Report (Other academic)
Place, publisher, year, edition, pages
2014. , 34 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2014:27
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:kth:diva-158970OAI: oai:DiVA.org:kth-158970DiVA: diva2:781432
Note

QC 20150116

Available from: 2015-01-16 Created: 2015-01-16 Last updated: 2015-01-19Bibliographically approved
In thesis
1. Studies of the rotating-disk boundary-layer flow
Open this publication in new window or tab >>Studies of the rotating-disk boundary-layer flow
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The rotating-disk boundary layer is not only a simpler model for the study of cross-flow instability than swept-wing boundary layers but also a useful simplification of many industrial-flow applications where rotating configurations are present. For the rotating disk, it has been suggested that a local absolute instability, leading to a global instability, is responsible for the small variation in the observed laminar-turbulent transition Reynolds number however the exact nature of the transition is still not fully understood. This thesis aims to clarify certain aspects of the transition process. Furthermore, the thesis considers the turbulent rotating-disk boundary layer, as an example of a class of three-dimensional turbulent boundary-layer flows.

The rotating-disk boundary layer has been investigated in an experimental apparatus designed for low vibration levels and with a polished glass disk that gave a smooth surface. The apparatus provided a low-disturbance environment and velocity measurements of the azimuthal component were made with a single hot-wire probe. A new way to present data in the form of a probability density function (PDF) map of the azimuthal fluctuation velocity, which gives clear insights into the laminar-turbulent transition region, has been proposed. Measurements performed with various disk-edge conditions and edge Reynolds numbers showed that neither of these conditions a↵ect the transition process significantly, and the Reynolds number for the onset of transition was observed to be highly reproducible.

Laminar-turbulent transition for a ‘clean’ disk was compared with that for a disk with roughness elements located upstream of the critical Reynolds number for absolute instability. This showed that, even with minute surface roughness elements, strong convectively unstable stationary disturbances were excited. In this case, breakdown of the flow occurred before reaching the absolutely unstable region, i.e. through a convectively unstable route. For the rough disk, the breakdown location was shown to depend on the amplitude of individual stationary vortices. In contrast, for the smooth (clean-disk) condition, the amplitude of the stationary vortices did not fix the breakdown location, which instead was fixed by a well-defined Reynolds number. Furthermore, for the clean-disk case, travelling disturbances have been observed at the onset of nonlinearity, and the associated disturbance profile is in good agreement with the eigenfunction of the critical absolute instability.

Finally, the turbulent boundary layer on the rotating disk has been investigated. The azimuthal friction velocity was directly measured from the azimuthal velocity profile in the viscous sublayer and the velocity statistics, normalized by the inner scale, are presented. The characteristics of this three-dimensional turbulent boundary-layer flow have been compared with those for the two-dimensional flow over a flat plate and close to the wall they are found to be quite similar but with rather large differences in the outer region. 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. viii, 58 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2014:27
Keyword
Fluid mechanics, laminar-turbulent transition, convective instability, absolute instability, secondary instability, hot-wire anemometry
National Category
Other Physics Topics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-158973 (URN)978-91-7595-416-5 (ISBN)
Public defence
2015-01-30, F3, Lindstedsv. 26, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 76324
Note

QC 20150119

Available from: 2015-01-19 Created: 2015-01-16 Last updated: 2015-01-19Bibliographically approved

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Alfredsson, Henrik

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