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Transition delay in a boundary layer flow using active control
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.ORCID iD: 0000-0002-8209-1449
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.ORCID iD: 0000-0002-4346-4732
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.ORCID iD: 0000-0001-7864-3071
2013 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 731, 288-311 p.Article in journal (Refereed) Published
Abstract [en]

Active linear control is applied to delay the onset of laminar-turbulent transition in the boundary layer over a flat plate. The analysis is carried out by numerical simulations of the nonlinear, transitional regime. A three-dimensional, localized initial condition triggering Tollmien-Schlichting waves of finite amplitude is used to numerically simulate the transition to turbulence. Linear quadratic Gaussian controllers based on reduced-order models of the linearized Navier-Stokes equations are designed, where the wall sensors and the actuators are localized in space. A parametric analysis is carried out in the nonlinear regime, for different disturbance amplitudes, by investigating the effects of the actuation on the flow due to different distributions of the localized actuators along the spanwise direction, different sizes of the actuators and the effort of the controllers. We identify the range of parameters where the controllers are effective and highlight the limits of the device for high amplitudes and strong control action. Despite the fully linear control approach, it is shown that the device is effective in delaying the onset of laminar-turbulent transition in the presence of packets characterized by amplitudes a approximate to 1% of the free stream velocity at the actuator location. Up to these amplitudes, it is found that a proper choice of the actuators positively affects the performance of the controller. For a transitional case, a approximate to 0.20 %, we show a transition delay of Delta Re-x = 3 .0 x 10(5).

Place, publisher, year, edition, pages
2013. Vol. 731, 288-311 p.
Keyword [en]
boundary layers, boundary layer control, flow control, instability
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-117917DOI: 10.1017/jfm.2013.299ISI: 000324425800016OAI: oai:DiVA.org:kth-117917DiVA: diva2:603872
Funder
Swedish Research CouncilSwedish e‐Science Research Center
Note

QC 20131017. Updated from submitted to published.

Available from: 2013-02-07 Created: 2013-02-07 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Active Control and Modal Structures in Transitional Shear Flows
Open this publication in new window or tab >>Active Control and Modal Structures in Transitional Shear Flows
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Flow control of transitional shear flows is investigated by means of numerical simulations. The attenuation of three-dimensional wavepackets of Tollmien-Schlichting (TS) and streaks in the boundary layer is obtained using active control in combination with localised sensors and actuators distributed near the rigid wall. Due to the dimensions of the discretized Navier-Stokes operator, reduced-order models are identified, preserving the dynamics between the inputs and the outputs of the system. Balanced realizations of the system are computed using balanced truncation and system identification.

We demonstrate that the energy growth of the perturbations is substantially and efficiently mitigated, using relatively few sensors and actuators. The robustness of the controller is analysed by varying the number of actuators and sensors, the Reynolds number, the pressure gradient and by investigating the nonlinear, transitional case. We show that delay of the transition from laminar to turbulent flow can be achieved despite the fully linear approach. This configuration can be reproduced in experiments, due to the localisation of sensing and actuation devices.

The closed-loop system has been investigated for the corresponding twodimensional case by using full-dimensional optimal controllers computed by solving an iterative optimisation based on the Lagrangian approach. This strategy allows to compare the results achieved using open-loop model reduction with model-free controllers. Finally, a parametric analysis of the actuators/ sensors placement is carried-out to deepen the understanding of the inherent dynamics of the closed-loop. The distinction among two different classes of controllers – feedforward and feedback controllers - is highlighted.

A second shear flow, a confined turbulent jet, is investigated using particle image velocimetry (PIV) measurements. Proper orthogonal decomposition (POD) modes and Koopman modes via dynamic mode decomposition (DMD) are computed and analysed for understanding the main features of the flow. The frequencies related to the dominating mechanisms are identified; the most energetic structures show temporal periodicity.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. vii, 72 p.
Series
Trita-MEK, ISSN 0348-467X ; 2013:03
Keyword
Flow control, flat-plate boundary layer, optimal controllers, model reduction, turbulent jet, POD, DMD, Koopman modes
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-117916 (URN)978-91-7501-640-5 (ISBN)
Public defence
2013-02-22, Sal E3, Osquars Backe 14, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20130207

Available from: 2013-02-07 Created: 2013-02-07 Last updated: 2013-02-07Bibliographically approved

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Bagheri, ShervinBrandt, LucaHenningson, Dan

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