Control of Turbulent Flows for Skin Friction Drag Reduction
The purpose of these lectures is to provide an uptodate overview of some of the most recent concepts for turbulent skin friction drag reduction. After reviewing what is known about the structure of a turbulent boundary layer, active and passive techniques
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E. Coustols CERT-ONERA, Toulouse, France
Summary The purpose of these lectures is to provide an uptodate overview of some of the most recent concepts for turbulent skin friction drag reduction. After reviewing what is known about the structure of a turbulent boundary layer, active and passive techniques offering nett drag benefits will be discussed. Emphasis will be put on the conc~pts which appear to have potential for possible transonic aircraft applications via manipulation of either the inner or the outer region of a turbulent boundary layer. That means that approaches, which are applicable only to hydrodynamic flows or require additives or equipment which are obviously out of question for aircraft will not be discussed. Thus, the goal of these lectures should be to put forward recent experimental and numerical results concerning the following techniques: wall suction, injection, active and selective control of turbulent boundary layers, passive control of turbulent boundary layers using LEBUs and Riblets. Some turbulent separation flow control will be briefly evoked.
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Introduction
The classical theory of aircraft range performance clearly shows up how propulsion, structural and aerodynamical efficiencies interact to affect aircraft performance. For instance, aerodynamic efficiency of aircraft configuration can be improved by ensuring that drag forces generated during any motion are as small as possible. Since this G. E. A. Meier et al. (eds.), Control of Flow Instabilities and Unsteady Flows © Springer-Verlag Wien 1996
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E. Coustols
may also be a prime consideration for hydrodynamic or ground vehicles, much of the discussion presented here is equally relevant to these. The search for practical means of achieving significant drag reductions has stimulated aerodynamicists imagination for almost half a century, since ideas which are at the origin of actual developments could be found in a book published in 1961 [1]. Since that time, a growing research effort has been developed in a large number of countries, to investigate both novel ideas and ideas which have been looked at in the past but not pursued to the point of application. Moreover, interest in reducing drag is strictly related to the impact on the "Direct Operational Cost" when dealing with civil transport aircraft, since such reductions translate directly into either increased range, speed and manoeuvrability or decreased fuel consumption [2]. The major sources of drag for climb and cruise conditions, which account for almost 90% of the fuel consumption for subsonic civil transport aircraft, are: skin friction drag, induced drag or drag due to lift, afterbody or separation drag, wave drag, interference drag, roughness or excrescence drag. Though, it is not always worthwile to address only the more significant drag contributions [3], it is reasonable to assume that drag reductions may be achieved by focusing research on the two major sources: friction drag due to development of boundary layers and lift-induced-drag associated with wing-tip vor
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