Global optimisation-based control algorithms applied to boundary layer transition problems

Turbulent flow has a significantly higher drag than the corresponding laminar flow at the same flow conditions. The presence of turbulent flow over a large part of an aircraft therefore incurs a significant penalty of increased fuel consumption due to the extra thrust required. One possible way of decreasing the drag is to apply surface suction to delay the transition from laminar to turbulent flow. However, in order for the gain from the reduction in drag to outweigh the extra costs associated with the suction system, the suction must be distributed in an optimum, or near optimum, manner. In this paper two practical cases are considered. In the first of these a flat plate with panels whose positions are adjustable but do not overlap is treated. Since the cost function in this case is multimodal, non-smooth and non-convex, methods for solving the optimisation problems necessary to design multi-panel suction systems based on direct search techniques are developed. In the second case considered the problem is that of linear distributed suction over the front part of an aerofoil. For this case, the computational load increases so significantly that in some cases it is not really feasible to continue the investigation using a single processor code. To overcome this, three parallel global optimisation algorithms are developed for the design of multi-panel suction systems on an aerofoil and it is shown that good solutions can be found efficiently. r 2003 Elsevier Ltd. All rights reserved.