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Carlo Brignoli, Lorenzo Cacciatori
The application of riblets on a commercial Unmanned Aerial Vehicle (UAV) is studied by means of RANS simulations. Riblets are micro-grooves capable of altering skin-friction drag, but their small scale precludes a direct representation in the geometric model of the UAV, and we resort to a model to introduce their effect via a partial-slip boundary condition implemented in the OpenFOAM CFD code.
The approach is validated first on simpler test cases, and then applied to the UAV in cruise condition, a fixed-wing aircraft flying at low speed for which riblets data are unavailable. Several configurations of riblets coverage are considered to extract the potential for drag reduction of different body parts. Installing riblets with optimal size and orientation over the full surface of the UAV leads to a relatively drag reduction of 3%. Besides friction reduction, analysis shows a significant additional form drag reduction localized on the wing. Because of this effect, the configuration where riblets are installed only on the upper surface of the UAV wing, still provides an interesting drag reduction of 1.7%. Considering that area coverage is only 28%, this is the configuration with the best cost-benefit ratio.