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Licentiate seminar

Optimal disturbances in boundary layer flows


Defendant Main Advisor Extra Advisor Date
Martin Byström Dan Henningson Ardeshir Hanifi 2007-06-12

Opponent
Stefan Hein, DLR - Institute of Aerodynamics and Flow Technology

Evaluation committee

Abstract

This thesis deals with algebraic growth in boundary layer ows, within the spatial framework. Adjoint-based optimization procedures are employed to identify the initial disturbance which experiences the maximum ampli cation. Such optimal disturbances are herein calculated in various boundary layer ows. Firstly, two-dimensional boundary layers a ected by suction is considered, and comparisons are made between a developing boundary layer, where the leading edge region is una ected by suction, and a non-developing boundary layer where suction is applied over the entire streamwise interval. Based on the numerical results, a hypothesis is presented to explain previous experimental ndings which indicates that the spanwise scale of the disturbances is set in the leading-edge region. Secondly, swept boundary layers subjected to adverse and favourable pressure gradients are considered. It is shown that the optimal disturbances take the form of tilted vortices in the cross- ow plane. As these algebraically growing disturbances evolve downstream, they are fed into exponentially growing cross- ow modes when the critical Reynolds number is exceeded. It is also shown that the basic shape of the disturbance remains the same as it evolves from the region of algebraic growth to the super-critical region of the boundary layer.
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