Large Eddy Breakup Devices (LEBU)
LEBU, similarly to the surface riblets, produce extensive downstream regions of reduced skin friction coefficients. However, the experimental data gathered over the last twenty years provided widely varying results, which are also depending on the Reynolds number.
The overall performance seems to be related to the drag of the devices themselves. The best drag reduction rates (in percentage) are not better than the surface riblets, roughly 7 to 8 percent. Among these, the tapered trailing edge devices have been found among the best.
Cases of interest include trailing edge flows, for examples thick flat plates and airfoils, where trailing edge separation is an issue at relatively large Reynolds numbers.
Some NACA 0009 LEBU devices were found to provide a local net skin friction reduction in the range of 30 percent. Similar experiments on cambered NACA 4409 gave no benefits, possibly because of boundary layer separation on the devices themselves.
LEBU performance at the higher Reynolds numbers and transonic conditions often required in aeronautics is strongly dependent on the drag of the devices. Their effectiveness is presumably much reduced at these conditions.
In some instances the LEBU have been coupled with the riblets, but the optimal
combination of these systems requires experimentation.