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Experimental studies of bypass transition and its control
KTH, Superseded Departments, Mechanics.
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 [en]
fluid mechanics, active control, Laminar-turbulent transition, free-stream turbulence
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-3480OAI: oai:DiVA.org:kth-3480DiVA: diva2:9284
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
List of papers
1. Velocity statistics and flow structures observed in bypass transition using stereo PTV
Open this publication in new window or tab >>Velocity statistics and flow structures observed in bypass transition using stereo PTV
Show others...
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.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13097 (URN)10.1007/S00348-002-0551-3 (DOI)000181540600008 ()
Note
QC 20100527Available from: 2010-05-27 Created: 2010-05-27 Last updated: 2017-12-12Bibliographically approved
2. Streamwise scaling of streaks in laminar boundary layers subjected to free-stream turbulence
Open this publication in new window or tab >>Streamwise scaling of streaks in laminar boundary layers subjected to free-stream turbulence
2004 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 16, no 5, 1814-1817 p.Article in journal (Refereed) 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.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13098 (URN)10.1063/1.1683170 (DOI)000220832600054 ()2-s2.0-2442706346 (Scopus ID)
Note
QC 20100527Available from: 2010-05-27 Created: 2010-05-27 Last updated: 2017-12-12Bibliographically approved
3. Streak oscillations of finite length: Disturbance evolution and growth
Open this publication in new window or tab >>Streak oscillations of finite length: Disturbance evolution and growth
2004 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 16, no 8, 3227-3230 p.Article in journal (Refereed) Published
Abstract [en]

Secondary instabilities of low-velocity streaks are studied in a plane channel flow. The secondary instability is forced by a sinusoidal signal consisting of N periods, where N ranges from 0.5 to 30. The disturbance distribution of the secondary instability is found to be similar to previous measurements on streaks forced by infinitely long wave trains. The amplitude growth of the localized secondary instabilities is close to exponential for N>4. However, for N<4 it is found that the exponential growth factor varies considerably both in the streamwise direction and with N with high values at upstream positions. It is proposed that these effects are due to transient growth of the localized secondary instabilities.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13100 (URN)10.1063/1.1758152 (DOI)000222528300044 ()2-s2.0-4344625564 (Scopus ID)
Note
QC 20100527Available from: 2010-05-27 Created: 2010-05-27 Last updated: 2017-12-12Bibliographically approved
4. Experiments on control of streamwise streaks
Open this publication in new window or tab >>Experiments on control of streamwise streaks
2003 (English)In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 22, no 3, 279-290 p.Article in journal (Refereed) Published
Abstract [en]

Control of streamwise velocity streaks are studied experimentally in a plane channel flow. High and low-velocity streaks are created by suction through streamwise slots and, further downstream, the secondary instability of the streaks is forced by speakers. The streaks are controlled by localized suction downstream of the disturbance generation. In a modified setup, reactive control is used in order to delay transition of low-velocity regions appearing at known spanwise positions randomly in time. As expected, the growth rate of the secondary instability decreases when localized suction is applied below a low-velocity streak. With control applied, transition is substantially delayed. The suction position and, in the case of reactive control of randomly appearing disturbances, the time instants at which control suction was turned on/off, were varied. The parameter study shows that the control suction has to be applied within a narrow region (10% of a streak width) around the centre of a low-velocity streak. The timing of the control suction is seen to be less critical.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13102 (URN)10.1016/S0997-7546(03)00032-3 (DOI)000184383400006 ()
Note
QC 20100527Available from: 2010-05-27 Created: 2010-05-27 Last updated: 2017-12-12Bibliographically approved
5. Reactive control of transition induced by free-stream turbulence: an experimental demonstration
Open this publication in new window or tab >>Reactive control of transition induced by free-stream turbulence: an experimental demonstration
2007 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 585, 41-71 p.Article in journal (Refereed) Published
Abstract [en]

The present wind-tunnel experiment demonstrates that a reactive control system is able to decrease the amplitude of random disturbances in a flat-plate boundary layer. The disturbances were induced in a laminar boundary layer by a turbulent free stream. The control system consisted of upstream wall-shear-stress sensors (wall wires) and downstream actuators (suction through holes). An ad hoe threshold-and-delay control algorithm is evaluated and parameter variations were performed in order to find a suitable working point of the control system. Detailed measurements of the flow field show how the control influences the disturbances in the boundary layer, whereas the effect on the mean flow owing to the control is minute. The control system manages to inhibit the growth of the fluctuations of the streamwise velocity component for a considerable distance downstream of the two actuator positions. Further downstream, however, the amplitudes of the fluctuations grow again. The flow rate used to obtain the control effect is one sixth of that necessary if continuous distributed suction is used to reach the same control objective. Finally, correlations and spectra show that the elongation of the structures in the streamwise direction is eliminated in the regions where the control has the largest effect. The spanwise scale of the disturbances is not affected by the control.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13099 (URN)10.1017/S0022112007006490 (DOI)000249080200002 ()2-s2.0-34547756306 (Scopus ID)
Note
QC 20100527Available from: 2010-05-27 Created: 2010-05-27 Last updated: 2017-12-12Bibliographically approved
6. Streak control by a surface-mounted piezo-ceramic flap
Open this publication in new window or tab >>Streak control by a surface-mounted piezo-ceramic flap
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-13096 (URN)
Note
QC 20100527Available from: 2010-05-27 Created: 2010-05-27 Last updated: 2010-05-27Bibliographically approved
7. Pulse-width modulated blowing/suction as a flow control actuator
Open this publication in new window or tab >>Pulse-width modulated blowing/suction as a flow control actuator
2003 (English)In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 35, no 5, 502-504 p.Article in journal (Refereed) Published
Abstract [en]

The effect of pulse-width modulated (PWM) blowing/suction through a hole in a flat plate, above which a Blasius boundary layer develops, is studied. A hot wire is used to measure the streamwise velocity downstream of the hole. The PWM blowing/suction is found to create amplitude-modulated variations in the streamwise velocity, which are well correlated with the signal generating the disturbance.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13101 (URN)10.1007/s00348-003-0671-4 (DOI)000186777900013 ()
Note
QC 20100527Available from: 2010-05-27 Created: 2010-05-27 Last updated: 2017-12-12Bibliographically approved

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