LARGE-AREA PRODUCTION OF DYNAMICALLY SCALED MICROSTRUCTURES USING DIFFRACTIVE OPTICAL ELEMENTS

摘要

Drag reduction of turbine compressor blades can bernachieved by structuring small riblets, which runrnparallel to the airflow direction, into the blade’srnsurface. Machining of these riblets has been alreadyrnsuccessfully demonstrated by using single-spotrnpicosecond laser ablation. This approach is successfulrnbut lacks an industry-compatible process speed andrnuses only fractions of the available power of a state-ofthe-rnart picosecond laser system. Therefore beamsplittingrnDiffractive Optical Elements (DOEs) are usedrnto parallelize the machining and to exploit the full laserrnpower which is provided without side effects onrnquality.rnUntil now a DOE can only help in machiningrnstructures bearing a fixed riblet pitch into the blade’srnsurface, but to further optimize the drag reductionrndynamic changes of these pitches along the directionrnof the airflow are necessary. Since dynamicallyrnchanging the DOE is not yet an acceptable solution,rnthis limitation is overcome by rotating the DOE to tiltrnthe spot array and thus change the resulting spotrndistances.rnIn this paper an algorithmic approach is presentedrnwhich optimizes the structure fill factor during thernmachining process, as rotation of the DOE causes arnchange of the overall width of the ablation zone.rnExperimental results presented show the feasibility ofrnthis approach using an integrated combination of arnseven-spot DOE, translational and rotational axes, andrna laser scanner to machine drag-reducing riblets inrnturbine-grade stainless steel.rnFurthermore, the experiments show that using thisrnapproach dead-end riblets can be suppressed and thusrnthe number of turbulence seeds can be minimized.