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Active Control and Reduced-Order Modeling of Transition in Shear Flows
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis direct numerical simulation is used to investigate the possibilityto delay the transition from laminar to turbulent in boundary layer flows.Furthermore, modal analysis is used to reveal the coherent structures in highdimensional dynamical systems arising in the flow problems.Among different transition scenarios, the classical transition scenario isanalysed. In this scenario, the laminar-turbulent transition occurs when Tollmien-Schlichting waves are triggered inside the boundary layer and grow exponentiallyas they move downstream in the domain. The aim is to attenuate the amplitudeof these waves using active control strategy based on a row of spatiallylocalised sensors and actuators distributed near the wall inside the boundarylayer. To avoid the high dimensional system arises from discretisation of theNavier Stokes equation, a reduced order model (ROM) based on EigensystemRealisation Algorithm (ERA) is obtained and a linear controller is designed.A plasma actuator is modelled and implemented as an external forcing on theflow. To account for the limitation of the plasma actuators and to further reducethe complexity of the controller several control strategies are examinedand compared. The outcomes reveal successful performance in mitigating theenergy of the disturbances inside the boundary layer.To extract coherent features of the wind turbine wakes, modal decompositiontechnique is employed where a large scale dynamical system is reduced toa fewer number of degrees of freedom. Two decomposition techniques are employed:proper orthogonal decomposition and dynamic mode decomposition.In the former procedure, the flow is decomposed into a set of uncorrelated structureswhich are rank according to their energy. In the latter, the eigenvaluesand eigenvectors of the underlying approximate linear operator is computedwhere each mode is associated with a specific frequency and growth rate. Theresults revealed the structures which are dynamically significant to the onsetof instability in the wind turbine wakes.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , v, 31 p.
Trita-MEK, ISSN 0348-467X ; 2013:12
National Category
Fluid Mechanics and Acoustics Aerospace Engineering Applied Mechanics
Research subject
SRA - E-Science (SeRC)
URN: urn:nbn:se:kth:diva-122594ISBN: 978-91-7501-801-0OAI: diva2:622905
2013-06-13, E3, Osquars Backe 14, KTH, Stockholm, 10:15 (English)
Swedish Research Council

QC 20130531

Available from: 2013-05-31 Created: 2013-05-23 Last updated: 2013-06-25Bibliographically approved
List of papers
1. Output feedback control of flow on a flat plate past a leading edge using plasma actuators
Open this publication in new window or tab >>Output feedback control of flow on a flat plate past a leading edge using plasma actuators
(English)In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385XArticle in journal (Refereed) Accepted
Abstract [en]

The evolution and control of a two dimensional (2D) wavepacket developing on a flat plate with a leading edge is investigated by means of direct numerical simulation (DNS).

The aim is to identify and suppress the wavepackets generated by freestream perturbations. A sensor is placed close to the wall in order to detect the upcoming wavepacket, while an actuator is placed further downstream to control it. A plasma actuator is modelled as an external forcing on the flow using a model based and validated on experimental investigations. A Linear Quadratic Gaussian (LQG) controller is designed and an output projection is used to build the objective function. Moreover, by appropriate selection of the Proper Orthogonal Decomposition (POD) modes, we identify the disturbances to be damped. A reduced-order model of the input-output system is constructed by using system identification via the Eigensystem Realization Algorithm (ERA) algorithm.

A limitation of the plasma actuators is the uni-directional forcing of the generated wall jet, which is predetermined by the electrodes location. In this paper, we address this limitation by proposing and comparing two different solutions: i) by introducing an offset in the control signal such that the resulting total forcing is oriented along one direction; ii) by using two plasma actuators acting in opposite directions. The results are compared with the ideal case where constraints are not accounted for the control design. We show that the resulting controllers based on plasma actuators can successfully attenuate the amplitude of the wavepacket developing inside the boundary layer.

National Category
Other Engineering and Technologies not elsewhere specified
urn:nbn:se:kth:diva-117914 (URN)

QP 2013

Available from: 2013-02-07 Created: 2013-02-07 Last updated: 2013-05-31Bibliographically approved
2. Instability of the tip vortices behind a wind turbine: a wind turbine
Open this publication in new window or tab >>Instability of the tip vortices behind a wind turbine: a wind turbine
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Mechanical Engineering
urn:nbn:se:kth:diva-120585 (URN)

QS 2013

Available from: 2013-04-12 Created: 2013-04-12 Last updated: 2013-05-31Bibliographically approved

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