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Flow control and reduced-order modelling of transition in shear flows


Respondent	Huvudhandledare	Bihandledare	Datum
Reza Dadfar	Dan Henningson	Ardeshir Hanifi	2014-06-10

Opponent
Vassilis Theofilis, Universidad Politécnica de Madrid

Betygsnämd
Markus Kloker, Institut für Aerodynamik und Gasdynamik, Universität Stuttgart, Germany Lennart Löfdahl, CTH Jesper Oppelstrup, KTH, NADA

Abstract

In this thesis direct numerical simulation is used to investigate the possibility to delay the transition from a laminar to a turbulent flow in boundary layer flows. Furthermore, modal analysis techniques are used to identify the coherent structures in wind turbine wake. Among different transition scenarios, the classical transition scenario is considered where the laminar-turbulent transition occurs due to Tollmien- Schlichting (TS) waves. These waves are convectively unstable and when trig- gered inside the boundary layer, they grow exponentially in amplitude as they are advected downstream of the domain. The aim is to suppress these waves using flow control strategies based on a row of spatially localised sensors and actuators distributed near the wall inside the boundary layer. To avoid the high dimensionality arising from discretisation of the Navier–Stokes equations, a reduced order model (ROM) based on the Eigensystem Realisation Algorithm (ERA) is obtained and based on that a linear controller is designed. To manip- ulate the flow, a plasma actuator is modelled and implemented as an external forcing. To account for the restrictions of the plasma actuators, several strate- gies are proposed and tested within the LQG framework. We studied also the design of a faster controller based on decentralised approach and compared the performance to a more expensive centralised controller. The outcomes reveal successful performance in mitigating the energy of the disturbances inside the boundary layer and suppressing the TS waves. To extract coherent features of the wind turbine wakes, modal decompo- sition techniques are employed. In this method a large dynamical system is reduced to a fewer number of degrees of freedom. Two decomposition tech- niques are employed, namely proper orthogonal decomposition and dynamic mode decomposition. In the former, the flow is decomposed into a set of or- thogonal structures which are ranked according to their energy contents in a hierarchical manner. In the latter, the eigenvalues and eigenvectors of the un- derlying approximate linear operator of the system is evaluated. In particular each mode is associated with a specific frequency and growth rate. The results revealed the coherent structures which are dynamically significant for the onset of instability in the wind turbine wakes.
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