Biological surfaces - laboratory and flight experiments on drag reduction and separation control

Dietrich Bechert, DLR, Berlin

A survey is given on the fluid-dynamical effects caused by structure and properties of biological surfaces. It is demonstrated that the end product of investigations whose aim is technological applications can also provide insights that help understand biophysical phenomena. The present paper describes techniques for both reducing wall shear stresses and for controlling boundary-layer separation.

i. Wall shear stress reduction has been 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 where the no-slip condition is modified. Self-cleaning surfaces like the one of lotus leaves represent an interesting option to avoid fluid-dynamical deterioration by the agglomeration of dirt. For riblets, two examples of technological implementation are discussed: on long-range commercial aircraft and in gas pipelines.

ii. Separation control is also and important issue in biology. After a few brief comments on vortex generators, two novel mechanisms of separation control by bird feathers are 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 with an aircraft with a laminar wing and movable flaps are shown.

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Last Modified: October 14, 1997