During the last decades, riblets have shown a potential for viscous drag reduction. Several investigations and measurements of skin-friction in the boundary layer over flat plates and on turbomachinery type blades with ideal riblet geometry have been reported in the literature. The purpose of the present study is to investigate whether laser machined and ground riblet-like structures could be successfully employed on conventional 2-D (NACA) compressor blades in order to assess the potential of industrial machining processes for the creation of the riblet effect. Perfectly trapezoid riblets were designed specifically for the flow parameters in the wind tunnel. Parameters describing the geometry and the deviation from ideal riblets are developed. Riblet machining by high precision material ablation has the potential of achieving micro-machining quality. In comparison to ns-laser processing using either Q-switched solid-state lasers or excimer lasers, the results for high precision material ablation show the enormous potential of ps-laser radiation and achieve the required quality, free of thermally induced defects and, consequently, with high reproducibility. For grinding riblets, geometrically defined microprofiles must firstly be generated via a profile dressing process and then ground onto the work piece surface. A precise adjustment of the grinding wheel system (grit, bonding) and the dressing/grinding conditions is necessary, in order to satisfy the opposing requirements at both dressing and grinding. The blade specimens were geometrically measured with a confocal microscope as well as secondary electron microscope using a specially developed riblet-oriented analysis. For verifying the measurement results, an Atomic Force Microscope was used. The specimens, i.e. flat plates and compressor blades, are aerodynamically tested in a cascade wind tunnel and properly scaled model surfaces were tested in an oil channel in order to quantify skin-friction reduction. Wake measurements of a cascade with NACA-profiles which have the resulting riblet-like structured surface show that the laser shaped as well as ground riblets reduce skin-friction almost as well as the ideal ones, which means a skin friction reduction of up to 7%.
Since Oehlert et al. (2007, “Exploratory Experiments on Machined Riblets for 2-D Compressor Blades,” Proceedings of International Mechanical Engineering Conference and Exposition 2007, Seattle, WA, IMECE2007-43457), 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.
In this paper the photometric or the so called "shape from shading" method is presented. In comparison to existing methods the efficiency of the detector system was considered and the requirements of the cosine Lambert's law for the angle distribution of the emitted electrons are suppressed. This new method was experimentally verified by measuring a steel sphere, a holographic grating and a hologram.
Inverse fringe projection can be seen as an improvement to the classical fringe projection method to significantly speed up the measurement of geometry defects of optical cooperative workpieces requiring no hardware changes to the classical setup. The CAD model of an ideal specimen is used in a virtual fringe projection system to generate a single sophisticated inverse fringe projection pattern which is, then, projected onto the surface of the real workpiece. Subsequently, 3D-geometry defects can be extracted directly and very quickly from a single image captured by the real camera using elaborate 2D-algorithms. This allows for verification of allowed geometry tolerances with a significantly reduced latency time.
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