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Streamwise scaling of streaks in laminar boundary layers subjected to free-stream turbulence
KTH, Tidigare Institutioner                               , Mekanik.
KTH, Tidigare Institutioner                               , Mekanik.ORCID-id: 0000-0002-1146-3241
2004 (Engelska)Ingår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 16, nr 5, s. 1814-1817Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

A flat plate laminar boundary layer subjected to free-stream turbulence, generated by grids upstream of the leading edge, has been studied. Correlations between a wall wire and a hot wire in the boundary layer have been obtained. The hot wire has been traversed to 328 positions for each of the five wall-wire positions studied. The length, height, and width of the correlation distributions is seen to increase in the downstream direction being self-similar in coordinates scaled with the boundary-layer length scale. The propagation speeds of the structures are obtained from the data and its variation in the streamwise and wall-normal directions is found to agree with what is obtained from a simple kinematic model.

Ort, förlag, år, upplaga, sidor
2004. Vol. 16, nr 5, s. 1814-1817
Nationell ämneskategori
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Identifikatorer
URN: urn:nbn:se:kth:diva-13098DOI: 10.1063/1.1683170ISI: 000220832600054Scopus ID: 2-s2.0-2442706346OAI: oai:DiVA.org:kth-13098DiVA, id: diva2:320848
Anmärkning
QC 20100527Tillgänglig från: 2010-05-27 Skapad: 2010-05-27 Senast uppdaterad: 2017-12-12Bibliografiskt granskad
Ingår i avhandling
1. Experimental studies of bypass transition and its control
Öppna denna publikation i ny flik eller fönster >>Experimental studies of bypass transition and its control
2003 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Stockholm: KTH, 2003. s. x, 68
Serie
Trita-MEK, ISSN 0348-467X ; 03:03
Nyckelord
fluid mechanics, active control, Laminar-turbulent transition, free-stream turbulence
Nationell ämneskategori
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Identifikatorer
urn:nbn:se:kth:diva-3480 (URN)
Disputation
2003-03-14, 00:00 (Engelska)
Anmärkning
QC 20100527Tillgänglig från: 2003-03-06 Skapad: 2003-03-06 Senast uppdaterad: 2010-05-27Bibliografiskt granskad

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