During the last decades, riblets have shown a potential for viscous drag reduction in turbulent boundary layers. 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 question of where riblets must be applied on the surface of a compressor blade is still not sufficiently answered. In a first step, the profile loss reduction by ideal triangular riblets with a trapezoidal groove and a constant geometry along the surface on the suction and pressure sides of a compressor blade is investigated. The results show a higher potential on the profile loss reduction by riblets on the suction side. In a second step, the effect of laser-structured ribs on the laminar separation bubble and the influence of these structures on the laminar boundary layer near the leading edge are investigated. After clarifying the best choices where riblets should be applied on the blade surface, a strategy for locally adapted riblets is presented. The suction side of a compressor blade is laser-structured with segmented riblets with a constant geometry in each segment. The measured profile loss reduction shows the increasing effect on the profile loss reduction of this locally adapted structure compared to a constant riblet-geometry along the surface. Furthermore, the particle deposition on a riblet-structured compressor blade is investigated and compared to the particle deposition on a smooth surface. Results show a primary particle deposition on the riblet tips followed by an agglomeration. The particle deposition on the smooth surface is stochastic.
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.
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.
During the last decades, riblets have shown a potential for viscous drag reduction in turbulent boundary layers. 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 question where riblets must be applied on the surface of a compressor blade is still not sufficiently answered. In a first step, the profile loss reduction by ideal triangular riblets with a trapezoidal groove and a constant geometry along the surface on the suction and pressure side of a compressor blade is investigated. The results show a higher potential on the profile loss reduction by riblets on the suction side. In a second step, the effect of laser-structured ribs on the laminar separation bubble and the influence of these structures on the laminar boundary layer near the leading edge are investigated. After clarifying the best choices where riblets should be applied on the blade surface, a strategy for locally adapted riblets is presented. The suction side of a compressor blade is laser-structured with a segmented riblet-like structure with a constant geometry in each segment. The measured profile loss reduction shows the increasing effect on the profile loss reduction of this locally adapted structure compared to a constant riblet-geometry along the surface. Furthermore, the particle deposition on a riblet-structured compressor blade is investigated and compared to the particle deposition on a smooth surface. Results show a primary particle deposition on the riblet tips followed by an agglomeration. The particle deposition on the smooth surface is stochastic.
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