Active Control of Boundary Layer and Separation
Active control means in the context of these lectures the use of additional artificial influences beside the natural boundary and initial conditions on stability or excitation of boundary layers and on separation in flows. These additional influences are
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G.E.A. Meier DLR Institute for Fluid Mechanics, Gottingen, Germany
Abstract Active control means in the context of these lectures the use of additional artificial influences beside the natural boundary and initial conditions on stability or excitation of boundary layers and on separation in flows. These additional influences are active influences because they imply an unsteady or at least adaptive change of boundary or flow conditions with the help of external energy. Examples given in the lectures are the influence of periodic surface waves or periodic blowing and suction on the amplification of Tollmien/Schlichting waves generated by a periodic excitation of the boundary layer. For certain phase shifts of the artificial distortions a suppression of the Tollmien/Schlichting waves and a delay of transition to turbulent boundary layer flow can be achieved. - By acoustic excitation of the boundary layer in the leading edge area of airfoils the flow separation at high angles of incidence can be influenced. For this purpose normal sound waves from a loudspeaker can be used. Another more efficient technique uses sound waves focused by a mirror on a small region of a certain wing area. This results in a stronger localised excitation which reduces laminar flow separation occurring at high angles of attack. - By means of suction also the unsteady behaviour of transonic boundary layers in the case of shock boundary layer interaction (buffet) can be influenced strongly. The result is not only a reduction of the buffet oscillation but also a reduced tendency for flow separation and a change in shock structure and drag. - Another application of suction is suppression of the stagnation line and cross flow instability which lead to early transition to turbulence in case of airfoils with large sweep angles. - The unsteady motion of the surface of a body in a certain flow field is another method to suppress or influence unsteady behaviour of wakes. - Finally heat addition or cooling are also able to act as tools for suppression of flow instabilities. G. E. A. Meier et al. (eds.), Control of Flow Instabilities and Unsteady Flows © Springer-Verlag Wien 1996
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G.E.A. Meier
1. Introduction Because of friction effects, energy and mass transfer flows tend to have an unpredictable behaviour with respect to the designers ideas about flow in ducts, the flow around bodies of given shape and also flows in natural environment. This often unpredictable behaviour or even the difficulties with wanted or designed flow configurations which is not correctable by simple measures leads to the general problem of flow control. In this respect flow control is a measure to influence a given flow configuration in such a way that it is changed or improved into a wanted flow performance. The reasons for flow control are mainly due to the fact that the above mentioned unpredictable flow behaviour leads to severe problems in the application of flow machinery or in the environment of natural flows. The main effects are the friction in boundary layers, t
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