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Licentiate seminar

Active Control and Reduced-Order Modeling of Transition in Shear Flows


Defendant Main Advisor Extra Advisor Date
Reza Dadfar Dan Henningson Ardeshir Hanifi 2013-06-10

Opponent
Vassilis Theofilis, Universidad Politécnica de Madrid

Evaluation committee

Abstract

In this thesis direct numerical simulation is used to investigate the possibility to delay the transition from laminar to turbulent in boundary layer flows.Furthermore, modal analysis is used to reveal the coherent structures in high dimensional dynamical systems arising in the flow problems.Among different transition scenarios, the classical transition scenario is analysed. In this scenario, the laminar-turbulent transition occurs when Tollmien-Schlichting waves are triggered inside the boundary layer and grow exponentially as they move downstream in the domain. The aim is to attenuate the amplitude of these waves using active control strategy based on a row of spatially localised sensors and actuators distributed near the wall inside the boundary layer. To avoid the high dimensional system arises from discretisation of the Navier Stokes equation, a reduced order model (ROM) based on Eigensystem Realisation Algorithm (ERA) is obtained and a linear controller is designed.A plasma actuator is modelled and implemented as an external forcing on the flow. To account for the limitation of the plasma actuators and to further reduce the complexity of the controller several control strategies are examined and compared. The outcomes reveal successful performance in mitigating the energy of the disturbances inside the boundary layer.To extract coherent features of the wind turbine wakes, modal decomposition technique is employed where a large scale dynamical system is reduced to a 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 structures which are rank according to their energy. In the latter, the eigenvalues and eigenvectors of the underlying approximate linear operator is computed where each mode is associated with a specific frequency and growth rate. The results revealed the structures which are dynamically significant to the onset of instability in the wind turbine wakes.
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