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Velocity statistics and flow structures observed in bypass transition using stereo PTV
KTH, Superseded Departments, Mechanics.
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2003 (English)In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 34, no 2, 242-252 p.Article in journal (Refereed) Published
Abstract [en]

It is known from smoke visualizations that in a transitional boundary layer subjected to free-stream turbulence, streaks appear and eventually break down to turbulence after wavy motions. In order to observe the streaky structures directly, a stereo particle-tracking velocimetry system using hydrogen bubbles in a water channel has been developed and validated against laser Doppler velocimetry. Mean flow statistics show good agreement with previous results. With the developed measurement system, the instantaneous spanwise distribution of the streamwise and wall-normal velocities can be measured fast enough to resolve the time development of the streaky structures. Measurements of instantaneous spanwise distributions of the streamwise and wall-normal velocity disturbances show strong negative correlation between the wall-normal and streamwise velocities in the streaks.

Place, publisher, year, edition, pages
2003. Vol. 34, no 2, 242-252 p.
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-13097DOI: 10.1007/S00348-002-0551-3ISI: 000181540600008OAI: oai:DiVA.org:kth-13097DiVA: diva2:320850
Note
QC 20100527Available from: 2010-05-27 Created: 2010-05-27 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Experimental studies of bypass transition and its control
Open this publication in new window or tab >>Experimental studies of bypass transition and its control
2003 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Bypass transition, i.e. transition of a boundary layer at subcritical Reynolds numbers, has been studied. Fundamental studies of the phenomenon as such have been performed side by side with experiments aimed at controlling, i.e. delaying, transition. The experiments have been performed in three different flow facilities, two with air as the working fluid (a plane channel flow and a wind-tunnel) and one with water (a water channel).

From the water channel data the well known low-speed streaks appearing in a boundary layer under a turbulent free stream are found to be correlated with upward motion in the boundary layer.

The streaks are found to scale in proportion to the boundary-layer thickness in both the streamwise and wall-normal directions. The streamwise length is around hundred boundary-layer thicknesses.

It is found that the secondary instability of the streaks grows slower for disturbances consisting of less than four wavelengths, as compared to continuous wavetrains.

Elongated low-speed structures are controlled, first in the plane channel flow and then by a reactive system in the wind-tunnel. In the channel, the breakdown of generated streaks is delayed by applying localized suction under the regions of low velocity. Measurements of the disturbance environment withand without control applied show that both the growth of the secondary instability and its spreading in the spanwise direction are reduced when applying the control. In order to be successful, the control has to be applied to a narrow region (about 1/10th of a streak width) around the position of minimum velocity.

The reactive system in the windtunnel, comprising four upstream sensors and four suction ports downstream, inhibits the growth of the amplitude of the streaks for a certain distance downstream of the suction ports. After the inhibited growth the disturbances start to grow again and far downstream the streak amplitude returns to close to the uncontrolled values.

Place, publisher, year, edition, pages
Stockholm: KTH, 2003. x, 68 p.
Series
Trita-MEK, ISSN 0348-467X ; 03:03
Keyword
fluid mechanics, active control, Laminar-turbulent transition, free-stream turbulence
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-3480 (URN)
Public defence
2003-03-14, 00:00 (English)
Note
QC 20100527Available from: 2003-03-06 Created: 2003-03-06 Last updated: 2010-05-27Bibliographically approved

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