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Free stream turbulence induced disturbances in boundary layers with wall suction
KTH, Superseded Departments, Mechanics.ORCID iD: 0000-0002-3251-8328
KTH, Superseded Departments, Mechanics.ORCID iD: 0000-0002-1146-3241
2004 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 16, no 10, 3530-3539 p.Article in journal (Refereed) Published
Abstract [en]

An experimental investigation of free stream turbulence (FST) induced disturbances in asymptotic suction boundary layers (ASBL) has been performed. In the present study four different suction rates are used and the highest is 0.40% of the free stream velocity, together with three different FST levels (Tu=1.6, 2.0, and 2.3%). A turbulence generating grid of the active type is used and offers the possibility to vary the Tu-level while the scales of the turbulence remain almost constant. It is known that FST induces elongated disturbances consisting of high and low velocity regions, usually denoted streaky structures, into the boundary layer. The experiments show that wall suction suppresses the disturbance growth and may significantly delay or inhibit the break-down to turbulence. Two-point correlation measurements in the spanwise direction show that the averaged streak spacing decreases with increasing FST-level, whereas the spanwise scale in the ASBL is more or less constant if scaled with the free stream velocity and viscosity. This is in contrast to what is observed in a Blasius boundary layer where streaks develop and adapt their spanwise scale close to the boundary layer thickness.

Place, publisher, year, edition, pages
2004. Vol. 16, no 10, 3530-3539 p.
Keyword [en]
National Category
Mechanical Engineering
URN: urn:nbn:se:kth:diva-13239DOI: 10.1063/1.1775222ISI: 000223822300003ScopusID: 2-s2.0-7244260423OAI: diva2:322560
QC 20100607 QC 20110922Available from: 2010-06-07 Created: 2010-06-07 Last updated: 2011-09-22Bibliographically 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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