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Laminar-Turbulent Transition in Aerodynamics Boundary Layers

During flight an aircraft drags the air along its surface by friction forces thus generating a thin boundary-layer flow. The friction drag, but also the proneness to boundary-layer separation, depends on the state of this boundary layer: The calm, laminar, stratified, steady flow state causes significantly less friction drag than the chaotic, turbulent unsteady state which, on the other hand, resists flow separation much better.

Turbulence is a consequence of the instability of the laminar state. Laminar FlowControl (LFC) comprises techniques to maintain a laminar flow for as long as possible and is nowadays considered a key technology for eco-efficient flight. A comprehensive understanding of the three-dimensional dynamic instability processes is a pre-requisite for successful LFC. One LFC technique is partial suction of the boundary layer by distributed discrete orifices in the surface, thus stabilizing the laminar flow state. The example shows successful control of the boundary layer flow on a swept airliner wing which is especially difficult to control. Grown steady crossflow vortices, caused by the primary instability of the laminar flow, induce a secondary instability with respect to unsteady disturbances invoking turbulence. By localised suction of air through a few holes only the crossflow vortices can be crucially weakened, and thus secondary instability and turbulence are suppressed.

Project FENFLOSSLaminar Flow Control through „Pinpoint Suction“: The three images show disturbing crossflow vortices originating at the wing’s nose (at right boarder of each image) which generate turbulence. Pinpoint suction leads to a significant reduction of vortices, which results in an almost completely suppressed turbulence (image on the left).
Source: IAG, University Stuttgart

Prof. Dr. Ewald Krämer
Institute of Aerodynamics and Gas Dynamics, University of Stuttgart
Pfaffenwaldring 21, D-70569 Stuttgart