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Transition induced by free-stream turbulence
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0002-3251-8328
KTH, School of Engineering Sciences (SCI), Mechanics.
2005 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 527, 1-25 p.Article in journal (Refereed) Published
Abstract [en]

Free-stream turbulence (FST) is perhaps the most important source inducing by-pass transition in boundary layer flows. The present study describes the initial energy growth of streamwise-oriented disturbances in the boundary layer originating from the presence of FST with intensities between 1.4% and 6.7%, but the study is mainly focused on the modelling of the transition zone. For this study three passive and one active turbulence-gene rating grids were used. The active grid was used in order to vary the turbulence intensity (Tu(2)) without changing the setup in the test section. It is shown that the initial disturbance energy in the boundary layer is proportional to Tu(2). The experiments also show that the energy grows in linear proportion to the Reynolds number based on the downstream distance. Furthermore the transitional Reynolds number is shown to be inversely proportional to Tu(2) for the whole range of Tu studied. The intermittency in the transitional zone was determined and it was shown that the intermittency function has a universal shape if the downstream distance is scaled with the length of the transition zone. The Reynolds number based on this transition zone length was found to increase linearly with the transition Reynolds number; however it was also noted that this non-dimensional length has a minimum value. With these results we were able to formulate an expression for the spot production rate which has a better physical base than previous models.

Place, publisher, year, edition, pages
2005. Vol. 527, 1-25 p.
Keyword [en]
National Category
Mechanical Engineering
URN: urn:nbn:se:kth:diva-13240DOI: 10.1017/S0022112004002770ISI: 000228618000001ScopusID: 2-s2.0-17044423373OAI: diva2:322564
QC 20100607Available from: 2010-06-07 Created: 2010-06-07 Last updated: 2010-07-16Bibliographically approved
In thesis
1. Flow control of boundary lagers and wakes
Open this publication in new window or tab >>Flow control of boundary lagers and wakes
2003 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Both experimental and theoretical studies have beenconsidered on flat plate boundary layers as well as on wakesbehind porous cylinders. The main thread in this work iscontrol, which is applied passively and actively on boundarylayers in order to inhibit or postpone transition toturbulence; and actively through the cylinder surface in orderto effect the wakecharacteristics.

An experimental set-up for the generation of the asymptoticsuction boundary layer (ASBL) has been constructed. This studyis the first, ever, that report a boundary layer flow ofconstant boundary layer thickness over a distance of 2 metres.Experimental measurements in the evolution region, from theBlasius boundary layer (BBL) to the ASBL, as well as in theASBL are in excellent agreement with boundary layer analysis.The stability of the ASBL has experimentally been tested, bothto Tollmien-Schlichting waves as well as to free streamturbulence (FST), for relatively low Reynolds numbers (Re). For the former disturbances good agreement is foundfor the streamwise amplitude profiles and the phase velocitywhen compared with linear spatial stability theory. However,the energy decay factor predicted by theory is slightlyoverestimated compared to the experimental findings. The latterdisturbances are known to engender streamwise elongated regionsof high and low speeds of fluid, denoted streaks, in a BBL.This type of spanwise structures have been shown to appear inthe ASBL as well, with the same spanwise wavelength as in theBBL, despite the fact that the boundary layer thickness issubstantially reduced in the ASBL case. The spanwise wavenumberof the optimal perturbation in the ASBL has been calculated andis β = 0.53, when normalized with the displacementthickness. The spanwise scale of the streaks decreases withincreasing turbulence intensity (Tu) and approaches the scale given by optimalperturbation theory. This has been shown for the BBL case aswell.

The initial energy growth of FST induced disturbances hasexperimentally been found to grow linearly as Tu2Rexin the BBL, the transitional Reynolds numberto vary as Tu-2, and the intermittency function to have a relativelywell-defined distribution, valid for all Tu.

The wake behind a porous cylinder subject to continuoussuction or blowing has been studied, where amongst other thingsthe Strouhal number (St) has been shown to increase strongly with suction,namely, up to 50% for a suction rate of 2.5% of the free streamvelocity. In contrast, blowing shows a decrease ofStof around 25% for a blowing rate of 5% of the freestream velocity in the considered Reynolds number range.

Keywords:Laminar-turbulent transition, asymptoticsuction boundary layer, free stream turbulence,Tollmien-Schlichting wave, stability, flow control, cylinderwake.

Place, publisher, year, edition, pages
Stockholm: KTH, 2003. viii, 60 p.
Trita-MEK, ISSN 0348-467X ; 2003:18
Laminar-turbulent transition, asymptotic suction boundary layer, free stream turbulence, Tollmien-Schlichting wave, stability, flow control, cylinder wake
National Category
Mechanical Engineering
urn:nbn:se:kth:diva-3664 (URN)
Public defence
2003-12-12, 00:00
QC 20100607Available from: 2003-12-05 Created: 2003-12-05 Last updated: 2010-06-07Bibliographically approved

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