Drag reduction of turbine compressor blades can be achieved by structuring small riblets, which run parallel to the airflow direction, into the blade’s surface. Machining of these riblets has been already successfully demonstrated by using single-spot picosecond laser ablation. This approach is successful but lacks an industry-compatible process speed and uses only fractions of the available power of a state-of-the-art picosecond laser system. Therefore beam-splitting Diffractive Optical Elements (DOEs) are used to parallelize the machining and to exploit the full laser power which is provided without side effects on quality.
Until now a DOE can only help in machining structures bearing a fixed riblet pitch into the blade’s surface, but to further optimize the drag reduction dynamic changes of these pitches along the direction of the airflow are necessary. Since dynamically changing the DOE is not yet an acceptable solution, this limitation is overcome by rotating the DOE to tilt the spot array and thus change the resulting spot distances.
In this paper an algorithmic approach is presented which optimizes the structure fill factor during the machining process, as rotation of the DOE causes a change of the overall width of the ablation zone.
Experimental results presented show the feasibility of this approach using an integrated combination of a seven-spot DOE, translational and rotational axes, and a laser scanner to machine drag-reducing riblets in turbine-grade stainless steel.
Furthermore, the experiments show that using this approach dead-end riblets can be suppressed and thus the number of turbulence seeds can be minimized.