Summary
It is proposed that a well organized vortex street behind the device is a necessary requirement for turbulent boundary layer drag reduction with outer-layer manipulators.
The communication from the outer region to the wall is inhibited through the existence of the vortex layer. The necessary wake structure is obtained with an open laminar separation on the front device. At low Reynolds numbers most flat plates and airfoils can provide this. At high Reynolds numbers different shapes are required, since conventional airfoils with sharp trailing edges produce a turbulent wake when the Reynolds numbers are increased. For tandem devices, the two airfoils shed vortices at slightly different frequencies, causing development of a strong, low-frequency component in the transitional wake.
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Bertelrud, A. (1990). The Importance of Lebu Device Shape for Turbulent Drag Reduction. In: Coustols, E. (eds) Turbulence Control by Passive Means. Fluid Mechanics and its Applications, vol 4. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-2159-7_4
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DOI: https://doi.org/10.1007/978-94-009-2159-7_4
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