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Title: Spatial evolution of optimal disturbances in two fluid boundary layers
Author: Burini, Luca
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2013
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The transition to turbulence of a laminar boundary layer can be affected by free-stream turbulence (FST). High levels of FST can cause the exponentially growing Tollmien-Schlichting wave to be replaced by algebraically growing streaks. Transient growth theory successfully identified counter-rotating, streamwise-oriented vortices as optimal flow configuration for maximum streaks’ amplification. The introduction of a wall film can significantly alter the amplification and evolution of streaks, influencing the penetration of FST inside the boundary layers, and its effects on modal and non-modal stability are yet to be addressed within the appropriate spatially-developing framework. In this work, the outcome of the introduction of a wall film on spatial transient amplification is analysed. Under base flow parallelism, optimal disturbances are computed by solving an optimal evolution problem. This study demonstrates that decreasing the viscosity of the film has a direct impact on the efficacy of the vortex tilting mechanism, leading to transient growth reduction. The different spreading rates of the boundary layer edge and the interface height imply that the effects of stratification vary in the streamwise direction. The assumption of base flow parallelism is therefore relaxed and a framework for the determination of optimal disturbances and spatial transient amplification in two-phase spreading boundary layers is presented, using adjoint optimization techniques. It is shown that spreading effects can alter the prediction of linear stability on streaks amplification. Beyond the early linear stage, the amplitude of the instability waves and interface deformation become appreciable and nonlinear effects cannot be neglected. As a result, an accurate description of the evolution of disturbances in two-fluid boundary layers must account for nonlinear interactions and mean flow distortion. Direct Numerical Simulation (DNS) with interface tracking is used to study the nonlinear evolution of the optimal disturbances in the two-fluid flow.
Supervisor: Zaki, Tamer Sponsor: Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available