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Title: Low order modelling of flow-control techniques for turbulent skin-friction reduction
Author: Duque Daza, Carlos Alberto
ISNI:       0000 0004 2732 7484
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2012
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In the present thesis a linearized formulation of the Navier-Stokes equations is used to study two main subjects: the generation of near-wall streaks in turbulent boundary layers and the response of turbulent wall-bounded flows to streamwise-travelling waves of spanwise oscillations of the bounding walls. For the purposes of the present work, these oscillations of the wall have been considered as a flow control mechanism. A mathematical model, based on a velocity-vorticity formulation linearized around a turbulent mean base flow, is adopted to simulate the fluid flow equations. A hybrid spectral-finite differences solver has been employed to numerically implement the linearized system. A review on turbulent streak generation is presented and the concepts of exponential growth, algebraic growth and viscous dissipation of small-scale perturbations are linked to the concept of transient growth. Mechanisms of generation of near-wall streaks are explored using a large set of sources of excitation for the linearized Navier- Stokes equations (LNSE). Two types of sources, here labelled as Excitation Mechanisms (EM), are employed: a body-force source and an initial condition. The selection of parameters for the excitation mechanisms is performed based on the definition of a multi-step optimization problem. The different EMs studied consist of a restricted number of parameters, and therefore can be considered as a Low Order Model (LOM) for the generation of streaks. It is shown that both types of EM produce satisfactory results for the streak generation process evaluated in terms of experimentally expected optimal spanwise scales. Finally, a large set of numerical experiments are conducted to evaluate the response of the LNSE with optimised EM, to the flow control by a spanwise oscillating wall. By comparing the results between the response of the LNSE considered here to this type of flow control against a drag-reduction map obtained by DNS in other studies, it is possible to assess the correlation between streak evolution and disruption of the skin-friction drag. A good agreement between these two responses is found for the parameter space of the streamwise-travelling spanwise oscillation waves.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QA Mathematics ; TA Engineering (General). Civil engineering (General)