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Licentiate seminar

Stability analysis and transition prediction of wall-bounded flows


Defendant Main Advisor Extra Advisor Date
Ori Levin Dan Henningson 2003-12-17

Opponent
Ardeshir Hanifi, KTH, Mekanik

Evaluation committee

Abstract

Disturbances introduced in wall-bounded flows can grow and lead to transition from laminar to turbulent flow. In order to reduce losses or enhance mixing in energy systems, a fundamental understanding of the flow stability is important. In low disturbance environments, the typical path to transition is an exponential growth of modal waves. On the other hand, in large disturbance environments, such as in the presence of high levels of free-stream turbulence or surface roughness, algebraic growth of non-modal streaks can lead to transition. In the present work, the stability of wall-bounded flows is investigated by means of linear stability equations valid both for the exponential and algebraic growth scenario. An adjoint-based optimization technique is used to optimize the algebraic growth of streaks. The exponential growth of waves is maximized in the sense that the envelope of the most amplified eigenmode is calculated. Two wall-bounded flows are investigated, the Falkner-Skan boundary layer subject to favorable, adverse and zero pressure gradients and the Blasius wall jet. For the Falkner-Skan boundary layer, the optimization is carried out over the initial streamwise location as well as the spanwise wave number and the angular frequency. Furthermore, a unified transition-prediction method based on available experimental data is suggested. The Blasius wall jet is matched to the measured flow in an experimental wall-jet facility. Linear stability analysis with respect to the growth of two-dimensional waves and streamwise streaks are performed and compared to the experiments. The nonlinear interaction of introduced waves and streaks and the flow structures preceding the flow breakdown are investigated by means of direct numerical simulations.