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Optimal Design of Natural and Hybrid Laminar Flow Control on Wings


Respondent	Huvudhandledare	Bihandledare	Datum
Jan Pralits	Dan Henningson	Ardeshir Hanifi	2003-10-07

Opponent
Patrick Huerre, LadHyX - Ecole Polytechnique, France

Betygsnämd
Per Lötstedt, TDB, Uppsala University Jesper Oppelstrup, KTH, NADA Yngve Sedin, Saab Aerospace, Saab AB, Linköping

Abstract

Methods for optimal design of different means of control are developed in this thesis. The main purpose is to maintain the laminar flow on wings at a chord Reynolds number beyond what is usually transitional or turbulent. Linear stability analysis is used to compute the exponential amplification of infinitesimal disturbances, which can be used to predict the location of laminar-turbulent transition. The controls are computed using gradient-based optimization techniques where the aim is to minimize an objective function based upon, or related to, the disturbance growth. The gradients of the objective functions with respect to the controls are evaluated from the solutions of adjoint equations. Sensitivity analysis using the gradients of the disturbance kinetic energy with respect to different periodic forcing show where and by what means control is most efficiently made. The results are presented for flat plate boundary layer flows with different free stream Mach numbers. A method to compute optimal steady suction distributions to minimize the disturbance kinetic energy is presented for both incompressible and compressible boundary layer flows. It is shown how to formulate an objective function in order to minimize simultaneously different types of disturbances which might exist in two, and three-dimensional boundary layer flows. The problem formulation also includes control by means of realistic pressure chambers, and results are presented where the method is applied on a swept wing designed for commercial aircraft. Optimal temperature distributions for disturbance control are presented for flat plate boundary layer flows. It is shown that the efficiency of the control depends both on the free stream Mach number, and whether the wall downstream of the control domain is insulated, or heat transfer occurs. Shape optimization is presented with the aim of reducing the aerodynamic drag, while maintaining operational properties. Results of optimized airfoils are presented for cases where both the disturbance kinetic energy, and wave drag are reduced simultaneously while lift, and pitch-moment coefficients as well as the volume are kept at desired values.
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