Control of a swept-wing boundary layer perturbed by free-stre am turbulence

Wall suction has been used extensively to reduce the growth of laminar boundary layer disturbances, such as Tollmien-Schlichting waves, cross-flow vortices and, more recently, Klebanoff modes, namely unsteady streamwise-elongated low-frequency disturbances generated by free-stream vortical perturbations. Wall suction has resulted in a delay of transition to turbulence with major benefits, such as reduction of viscous drag, noise, and fuel consumption.The central aim of the proposed work is to apply wall suction to attenuate the intensity of Klebanoff modes arising in swept-wing laminar boundary layers due to free-stream turbulence. These flow structures have never been studied over large scale aerodynamic bodies, despite their vast importance in numerous industrial and technological applications, such as the design of aircraft wings and turbine blades. The effects of wall suction have not been investigated for these technologically relevant flows. A reason for the dearth of works is the extreme difficulty in predicting and controlling such flows.The problem will be tackled by a combination of mathematical and numerical techniques. The proposed approach will offer a fast and numerically robust toolkit in line with the long-standing effort of the fluid mechanics community to control pre-transitional flows in an efficient manner.

The proposed research will have a major impact on different academic disciplines and on industry, global economic performance, and on the society in general. Both applied mathematicians interested in perturbation and asymptotic methods and fluid mechanics engineers carrying out wind-tunnel experiments or high-level computations will largely benefit once the results are available. The findings will give rise to huge incentive for validation and guidance on future research. Future interactions with academics in the fluid mechanics community will highlight the limitations of the theory, and this will lead to improvements of the theoretical framework. More advanced models for transition prediction will be developed based on our results. At a more fundamental level, the impact on the academic community will be large because this will be the first study worldwide on Klebanoff modes on large-scale aerodynamic bodies with and without suction. The asymptotic framework is likely to elucidate further the physical mechanism for the formation of the Klebanoff modes. The impact on technological advancement, economic performances and well-being of society is also vast because the results will work toward the objective of maintaining laminar flows over aiplanes for improved aerodynamic performances. This will improve the quality of life through energy saving. Wall suction has been shown in numerous cases to suppress laminar boundary layer disturbances and to be an efficient control method to guarantee a delay of transition to turbulence. Such extension of the laminar region leads to an aerodynamic drag reduction. The benefits are immense. A better system efficiency is achieved, smaller power units can be used, and a significant reduction of fuel consumption is obtained. This is particularly important for the aeronautical industry, where airplanes consume a large amount of energy during flight. The impact in economic terms will also be of vast proportions. The reduction in size of power units and limited fuel consumptions will be beneficial because billions of pounds per year could be saved. As the oil price is increasing steadily, the reduction of fuel consumption has become a priority for airlines and manufacturing industry. It has been estimated that a fully-efficient wall-suction laminar flow control system will be able to reduce the fuel burned by about 30\%. As a consequence, the operating costs of commercial airplanes and ticket prices will reduce. The benefits on the quality of life, health and well-being of the global community are also relevant. A reduction of fuel consumption will imply a radical decrease of emission of pollutants in the atmosphere. Aircrafts will also be less noisy, with benefits on densely populated areas in the proximity of airports.