A survey is given on fluid-dynamic effects caused by the structure and properties of biological surfaces. It is demonstrated that the results of investigations aiming at technological applications can also provide insights into biophysical phenomena. Techniques are described both for reducing wall shear stresses and for controlling boundary-layer separation. (a) Wall shear stress reduction was investigated experimentally for various riblet surfaces including a shark skin replica. The latter consists of 800 plastic model scales with compliant anchoring. Hairy surfaces are also considered, and surfaces in which the no-slip condition is modified. Self-cleaning surfaces such as that of lotus leaves represent an interesting option to avoid fluid-dynamic deterioration by the agglomeration of dirt. An example of technological implementation is discussed for riblets in long-range commercial aircraft. (b) Separation control is also an important issue in biology. After a few brief comments on vortex generators, the mechanism of separation control by bird feathers is described in detail. Self-activated movable flaps (= artificial bird feathers) represent a high-lift system enhancing the maximum lift of airfoils by about 20%. This is achieved without perceivable deleterious effects under cruise conditions. Finally, flight experiments on an aircraft with laminar wing and movable flaps are presented.

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Separation control is an important issue in the physiology of birdflight. Here, the adaption of the separation control mechanism by bird feathers to the requirements of engineering applications is described in detail. Self-activated movable flaps similar to artificial bird feathers represent a high-lift system for increasing the maximum lift of airfoils. Their effect on the unsteady flow around a two-dimensional airfoil configuration is investigated by a joint numerical and experimental study. First, attention is paid to the automatic opening and closing mechanism of the flap. Following this, its beneficial effect on lift is investigated for varying incidences and flap configurations. In-depth analysis of experimental and numerical results provides a detailed description of the important phenomena and the effect of self-adjusting flaps on the flow around the airfoil. In the second part of this paper, a contribution is made to verification of the applicability of unsteady Reynolds-averaged approaches using statistical turbulence models for unsteady flows with particular attention to turbulent time scales with comparison to the results of a hybrid simulation based on unsteady Reynolds-averaged Navier-Stokes equations and large-eddy simulation. Finally, flight experiments are described using an aircraft with movable flaps fitted on its laminar wing. Nomenclature A = amplitude of oscillation b = wing span c = chord length c F = flap-moment coefficient c G = gravity-moment coefficient c L ; c D = lift and drag coefficients c R = reverse flow parameter k = turbulent kinetic energy Lt = turbulent length scale l F = flap length M F = flap moment due to fluid force M G = flap moment due to gravity Re = Reynolds number based on chord length Sr = Strouhal number based on flap length u 0 = inflow velocity x d = detachment position = angle of attack , max = flap deflection angle, maximum angle = conventional flap angle = density ! = specific turbulent dissipation

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