Improving the performance of aerofoil sections using momentum transfer via a secondary flow
Aerodynamic flow control can improve aerofoil performance by influencing the natural growth of boundary layers, which develop on the surface of vehicles moving in viscous fluids. Many active and passive techniques have been developed to reduce drag and/or increase the lift of aerofoil sections. The work presented in this thesis is concerned with the active excitation of the boundary layer on the suction side of aerofoil sections through momentum transfer via a secondary flow. The secondary flow was achieved by air passing through an air breathing device (ABD) which was implemented in the aerofoil surface. This resulted in an almost tangential and uni-directional fluid interaction. Numerical and experimental work showed a beneficial influence of the secondary flow on the aerodynamic characteristics of the studied aerofoil sections. A Taguchi analysis was initially used to confirm findings from previous work on the use of an ABD on a NACA0012 aerofoil section. The resulting parameter ranking showed general agreement with previous data in that the most important parameters are the gap-size i.e. the length over which the two fluids are in contact and the velocity gradient between the two fluids. However, it also raised questions that required an additional in-depth analysis of the parameters governing the flow control process. Due to the greater importance to the modern aviation industry of the NACA65-415 aerofoil section this particular cambered aerofoil section was used for further investigations. This study highlighted the importance of the velocity gradient between the main and secondary flows as well as the location of interaction of the ABD. In addition the gap-size is also important. Consideration of the power requirements for the ABD indicated that this may limit exploitation of the device. An evolutionary search strategy based on genetic algorithms, was employed to optimize the air breathing geometry. This optimisation produced non-intuitive geometries which revealed the importance of promoting an inner fluid recirculation in the device. Finally experimental data in a closed loop wind-tunnel showed trends which were in general agreement with the numerical predictions. However, the measurements indicated significantly greater enhancements of lift forces than those predicted by the numerical investigation.
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