Recent advances in manufacturing of riblets on compressor blades and their aerodynamic impact

Since Oehlert et al. (2007), significant improvements in the manufacturing processes of riblets by laser-structuring and grinding have been achieved. In the present study, strategies for manufacturing small-scale grooves with a spacing smaller than 40  μm by metal bonded grinding wheels are presented. For the laser-structuring process, significant improvements of the production time by applying diffractive optical elements were achieved. Finally, strategies for evaluating the geometrical quality of the small-scale surface structures are shown and results obtained with two different measuring techniques (SEM and confocal microscope) are compared with each other. The aerodynamic impact of the different manufacturing processes is investigated based upon skin friction reduction data obtained on flat plates as well as the profile-loss reduction of riblet-structured compressor blades measured in a linear cascade wind tunnel. Numerical simulations with MISES embedded in a Monte Carlo Simulation (MCS) were performed in order to calculate the profile-loss reduction of a blade structured by grinding to define further improvements of the riblet-geometry. A numerical as well as experimental study quantifying the relevant geometrical parameters indicate how further improvements from the present 4 % reduction in skin friction can be achieved by an additional decrease of the riblet tip-diameter and a more trapezoidal shape of the groove in order to realize the 8 % potential reduction.