It is a small focused collaborative project funded under the European Transport Item of the FP7 Cooperation Work Program. Project Context and Objectives: BUTERFLI is the acronym for "BUffet and Transition delay control investigated within European-Russian cooperation for improved FLIght performanceā€¯. The main recommendation is to continue research efforts, both experimentally and numerically, on buffet control techniques, particularly in laminar regime, on swept wing configurations. Several realizations of flow control technologies using DBD and plasma actuators have been evaluated in laboratories and wind tunnels. Incontestable scientific advances have been made on the control of turbulent buffet and on the characterization and control of laminar buffet on two-dimensional profiles, and on the control of transverse flow. Numerical simulations have contributed both to the design of flow control technologies and to the re-building and analysis of results. This low TRL project permitted the development of several new flow control technologies then tested on sub-scale wing configurations in wind tunnels. About the VR-type actuators, numerical studies indicated that an order of magnitude stronger forcing would have been needed. The spatial distribution of the induced forcing achieved by the actuators designed for transition control by crossflow velocity reduction may have been too inhomogeneous. A key issue of all present designs was the unwanted but significant promotion of unsteady boundary-layer instabilities. The second consists in the formation of successive micro-jets normal to the wall creating a series of virtual roughnesses (VR) whose spacing corresponds to a wavelength "killer" of that present in the transverse flow. The first principle consists in the creation of a counter-current stopping the transverse flow. The objective of WP3 was the study of two flow control principles using DBD actuators to delay the laminar-turbulent transition generated by the presence of transverse flow on a swept profile. The passive control by bump 3D has shown an attenuation of the phenomenon without completely removing it. Active control by blowing upstream of the shock was very convincing to delay the occurrence of the buffet. A numerical simulation of the LES type has well reproduced the dynamics of laminar buffet. Tests carried out in the ONERA S3Ch transonic wind tunnel have successfully characterized the occurrence of laminar buffet in the Mach-Incidence envelope. The objective of WP2 was to characterize and control the laminar buffet on a two-dimensional profile. The other two WPs, WP2 and WP3, concerned wing profiles operating in laminar regime. The other plasma actuators by spark discharge have shown some ability to reduce the amplitude of the buffet phenomenon, but not to suppress it. The DBD actuator check was ineffective for the tested configuration. Only the process using a tangential jet flow control was effective in T-112 TsAGI wind tunnel tests. The aim of WP1 was to control the buffet on a two-dimensional supercritical wing profile operating in turbulent regime. The study of these flow control principles has been segmented into three WPs. Executive Summary: The overall objective of the project was to study the principles of flow control aimed at improving the aerodynamic performance of the wings of transport aircraft and thereby reducing their fuel consumption.
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