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Pr Viswanath - One of the best experts on this subject based on the ideXlab platform.

  • Aircraft viscous drag reduction using Riblets
    Progress in Aerospace Sciences, 2002
    Co-Authors: Pr Viswanath
    Abstract:

    The last two decades have seen considerable research activity on the use of Riblets for viscous drag reduction. Experimental results concerning the performance of 3 M Riblets on airfoils, wings and wing-body or aircraft configurations at different speed regimes are reviewed; these applications bring in additional effects like pressure gradients and three dimensionality.In addition to drag reduction, aspects of altered flow features due to Riblets are discussed based on detailed wind tunnel measurements at low speeds.The available results obtained from wind tunnels as well as flight tests firmly establish the effectiveness of Riblets from low speed to moderate supersonic Mach numbers. With optimized Riblets, skin friction drag reduction in the range of 5–8% have been measured on 2D airfoils at low incidence and in mild adverse pressure gradients; strong evidence exist at low speeds to indicate that Riblets are more effective in adverse pressure gradients.On wings of moderate sweep relevant to transport aircraft, Riblets remain effective providing drag reduction comparable to 2D airfoils, as long as the local angle between the surface streamlines and riblet orientation is relatively small (o101).Limited data available on wing-body configurations show that total drag reduction of about 2–3% is likely.Certain suggestions for future research are outlined. r 2002 Elsevier Science Ltd.All rights reserved.

  • Riblets on airfoils and wings - A review AIAA Paper 99-3402
    1999
    Co-Authors: Pr Viswanath
    Abstract:

    This review paper summarizes the current status of research on the use of Riblets for viscous drag reduction on 2D airfoils and swept wings. These applications have additional effects, such as streamwise pressure gradients and three-dimensionality. The effectiveness of Riblets under such conditions are discussed along with associated flow features. Two major observations emerge from the experimental results: first, Riblets provide higher drag reduction under adverse pressure gradients; second, the effectiveness of Riblets on a moderately swept wing is retained as long as the angle between surface streamlines and riblet orientation is generally small (less than 10 deg). Available results on transonic airfoils show that compressibility effects do not have any adverse effect on riblet action. Finally, the effects of Riblets on other engineering parameters, such as lift characteristics and stall angle, are discussed on the basis of the available results. Certain suggestions for future research are outlined.13; 13;

  • EFFECT OF Riblets ON AXISYMMETRIC BASE PRESSURE
    Journal of Spacecraft and Rockets, 1997
    Co-Authors: Krishnan, Pr Viswanath, S Rudrakumar
    Abstract:

    Experiments have been performed assessing riblet effects on axisymemtric base pressure at low speeds. The tests were conducted in the 0.91-m-diam. low-speed wind tunnel at a freestream velocity of 20 m/s. It is shown that, with a large-scale or massive separation, Riblets do not decrease the base drag at low speeds; on the other hand, there is a progressive increase in base drag with h+ and the drag penalty is about 8 percent for the optimized drag reducing riblet. Riblets may provide a small pressure or base drag reduction on streamlined bodies with a sharp trailing edge like an airfoil or turbine blade, because the effects of viscous-induced displacement thickness will be lower on the riblet surface.

  • viscous drag reduction using Riblets on naca 0012 airfoil to moderate incidence
    AIAA Journal, 1996
    Co-Authors: S Sundaram, Pr Viswanath, S Rudrakumar
    Abstract:

    Results of viscous drag reduction using Riblets from 3M on a NACA 0012 airfoil model up to moderate angles of attack are presented. Measurements made consisted of model surface pressure distributions, mean velocity and streamwise turbulence intensity profiles in the boundary layer (just ahead of the trailing edge), and total airfoil drag for two riblet heights of 0.152 and 0.076 mm. Results show significantly higher skin friction drag reduction with incidence compared to plat plate flows; the reduction was as high as 16% at ? = 6 deg. Results of mean velocity profiles show that a larger contribution to drag reduction results from the suction side of the airfoil, indicating increased effectiveness of Riblets in adverse pressure gradients. Examination of turbulence intensity profiles in the wall region indicates an appreciable reduction in the presence of Riblets; correspondingly, the spectra show reduced energy levels at low frequencies.

  • Study on turbulent drag reduction using Riblets on a flat plate
    1992
    Co-Authors: S Sundaram, Pr Viswanath
    Abstract:

    Results of viscous drag reduction on a flat plate using Riblets at low speeds are presented. Measurements of mean velocity and streamwise turbulence intensity profiles were made on the riblet surface as well as on the smooth flat plate for one riblet geometry. Drag reduction estimated by applying boundary layer momentum integral technique on the riblet surface as well as from the far wake measurements showed good agreement. Comparisons of spectra of streamwise velocity fluctuations with and without the riblet surface, indicated noticeable reduction in the energy levels in the near wall consistent with earlier observations reported in literature.

Fotis Sotiropoulos - One of the best experts on this subject based on the ideXlab platform.

  • Riblet drag reduction in mild adverse pressure gradients: A numerical investigation
    International Journal of Heat and Fluid Flow, 2015
    Co-Authors: Aaron Boomsma, Fotis Sotiropoulos
    Abstract:

    Abstract Riblet films are a passive method of turbulent boundary layer control that can reduce viscous drag. They have been studied with great detail for over 30 years. Although common riblet applications include flows with Adverse Pressure Gradients (APG), nearly all research thus far has been performed in channel flows. Recent research has provided motivation to study Riblets in more complicated turbulent flows with claims that riblet drag reduction can double in mild APG common to airfoils at moderate angles of attack. Therefore, in this study, we compare drag reduction by scalloped riblet films between Riblets in a zero pressure gradient and those in a mild APG using high-resolution large eddy simulations. In order to gain a fundamental understanding of the relationship between drag reduction and pressure gradient, we simulated several different riblet sizes that encompassed a broad range of s + (riblet width in wall units), similarly to many previously published experimental studies. We found that there was only a slight improvement in drag reduction for Riblets in the mild APG. We also observed that peak values of streamwise turbulence intensity, turbulent kinetic energy, and streamwise vorticity scale with riblet width. Primary Reynolds shear stresses and turbulence kinetic energy production however scale with the ability of the riblet to reduce skin-friction.

  • Drag reduction of large wind turbine blades through Riblets: Evaluation of riblet geometry and application strategies
    Renewable Energy, 2013
    Co-Authors: Leonardo P Chamorro, Roger E. A. Arndt, Fotis Sotiropoulos
    Abstract:

    Abstract Wind tunnel experiments were performed to quantify the drag reduction on a wind turbine airfoil partially or fully covered with Riblets. A full-scale 2.5 MW wind turbine airfoil section, typical for the near tip, was placed in the free stream flow of the wind tunnel at the Saint Anthony Falls Laboratory, University of Minnesota. Various sizes and geometries of experimental Riblets were provided by 3M Company and tested at angles of attack ranging from 0° ≤ α ≤ 10° (0.25 ≤ CL ≤ 1.14) and at a Reynolds number of Re = 2.2 × 106. Mean drag was measured via wake survey (momentum deficit) and with a sensitive force balance. Lift was measured directly from the force balance. Tests included the cases of complete and partial riblet coverage on the wing. Results indicated that Riblets could provide an overall reduction of skin friction drag, and that the amount of the decrease varied with riblet height and geometry. Partial riblet coverage appears in some cases more efficient than its complete coverage counterpart. The percentage of drag the Riblets reduced varied greatly and in some cases the Riblets were even detrimental to the airfoil. The most efficient riblet for a completely covered airfoil was found to be the V-groove shape of 100 μm height. It produced a reduction of roughly 6% in the operational range expected in a turbine airfoil. On the other hand, the most efficient riblet size for partial coverage was also a V-groove shape and seemed to shift slightly to a smaller peak height of 80 μm. This configuration produced a reduction of roughly 4% in the range of angle of attack that is typical for operation in the field. The average non-dimensional square root of the groove cross-section, l+, defined in terms of the drag coefficient at design angle of attack for the optimum riblet configuration in the fully coverage case was found to be l + ≈ 10 , which is very close to the optimum value found for planar surfaces. Based on our results we propose a formulation for the optimum riblet size in airfoil considering the mean drag coefficient and chord length Reynolds number. Even though the optimum full coverage case showed better performance that the partial case, the additional drag reduction benefit may be offset by the additional application cost.

  • drag reduction in large wind turbines through Riblets evaluation of different geometries
    50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, 2012
    Co-Authors: Roger E. A. Arndt, Leonardo P Chamorro, Fotis Sotiropoulos
    Abstract:

    Wind tunnel experiments were performed with a wind turbine airfoil that was fitted with different riblet geometries. The foil represents the cross section of a full-scale 2.5MW wind turbine blade close to the tip. The test foil was placed in the free stream flow of the wind tunnel at the Saint Anthony Falls Laboratory, University of Minnesota. Various sizes and geometries of Riblets were tested at angles of attack ranging from 0 ≤α≤10 , corresponding to a range of lift coefficient of 0.25≤ CL ≤1.14, and at a Reynolds number of Re=2.2 X 10. Mean drag was measured via a wake survey (momentum deficit) and with a sensitive force balance. Lift was measured directly from the force balance. Tests included the cases of complete and partial riblet coverage on the wing. The results indicate that Riblets could provide an overall reduction of skin friction drag, and that the amount of the decrease varied with riblet height and geometry. Partial riblet coverage appears, in some cases, to be more efficient than its complete coverage counterpart. The percentage of drag the Riblets reduced varied greatly and in some cases the Riblets were even detrimental to the airfoil. The most efficient riblet for a completely covered airfoil was found to be the V-groove shape of 100 μm height. It produced a reduction of roughly 6% in the operational range expected for a turbine airfoil. On the other hand, the most efficient riblet size for partial coverage was also a V-groove shape but had a slightly smaller peak height of 80 μm. This configuration produced a reduction of roughly 4% in the range of angle of attack that is typical for operation in the field. Our testing was limited to one size airfoil. However, comparison of our test data with recent numerical simulations by A-Mayoral and Jimenez suggest that an optimal size riblet configuration can be expressed in normalized form, involving only Reynolds number and the drag coefficient at design angle of attack.

  • on the skin friction drag reduction in large wind turbines using sharp v grooved Riblets
    49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2011
    Co-Authors: Leonardo P Chamorro, Roger E. A. Arndt, Fotis Sotiropoulos
    Abstract:

    Skin friction drag reduction through the use of Riblets has been a topic of intensive research during the last decades. Main efforts have been placed on both numerical (mainly DNS) and experimental (wind tunnel and fluid channel) approaches. In spite of the valuable efforts to date, the fundamental mechanisms that induce drag reduction are not well established and, in particular, the potential benefits associated with the use of Riblets in wind turbines are unclear. In this research, wind tunnel experiments were performed to quantify the drag reduction in a wind turbine airfoil using V-groove riblet structures. A full-scale 2.5MW wind turbine airfoil section (typical of the 75-80% blade span), was placed in the freestream flow of the wind tunnel at the Saint Anthony Falls Laboratory (SAFL), University of Minnesota. Different sizes of V-groove Riblets were tested at different angles of attack at full scale Reynolds number. Force sensors were used to measure lift and drag. Momentum deficit was also measured in the wake of the airfoil to determine the net drag. Flat plate experiments were also performed to determine the distribution of the skin friction coefficient in a developing boundary layer. The experimental measurements will be used to develop and test the performance of near-wall boundary conditions for the velocity field that can account for the effects of riblet roughness in the context of RANS and hybrid RANS/LES models. The numerical simulations along with the laboratory experiments will be used in the future to determine the best riblet configuration and guide the application and testing of Riblets in a 2.5MW wind turbine under real-life wind conditions.

Shan Zhong - One of the best experts on this subject based on the ideXlab platform.

  • control of laminar flow separation over a backward facing rounded ramp with c d Riblets the effects of riblet height spacing and yaw angle
    International Journal of Heat and Fluid Flow, 2020
    Co-Authors: Tongbiao Guo, Shan Zhong, Tim Craft
    Abstract:

    Abstract In the work presented in this paper, a section of convergent-divergent (C-D) Riblets is applied upstream of a backward-facing rounded ramp in a fully developed laminar channel flow at a Reynolds number based on the channel height of 400. Numerical simulations are undertaken to examine the effects of riblet geometry and yaw angle on the strength of secondary flow produced by the Riblets and the extent of flow separation zone. It is found that, in comparison with the baseline case with no Riblets, flow separation is delayed and the reattachment occurs earlier leading to a smaller separation zone around the diverging line. The opposite phenomenon occurs around the converging line. Our results also show that for Riblets with a given height a maximum strength of the secondary flow motion upstream of the ramp and a maximum reduction in the spanwise-averaged length of separation are obtained at s / h = 4 . At a given s / h = 4 , as the riblet height increases, the strength of the secondary flow motion generated by C-D Riblets increases and a minimum riblet height of 3.75% of the channel height is required to produce a net reduction in the spanwise-averaged length of separation. For Riblets with a given height and spacing, as yaw angle increases both the strength of secondary flow and net reduction of separation zone exhibit a parabolic trend and both of them peak at γ = 45 ° . Overall, with the riblet setting tested here the benefit of suppressing the laminar separation bubble has not resulted in a net reduction in the total pressure losses compared to that in the baseline case. Nevertheless, an examination of the pressure losses from three different sections along the channel separately has produced some insights about the loss mechanisms and pointed to the possible ways by which the pressure losses could be reduced.

  • Investigation of Riblet Geometry and Start Locations of Herringbone Riblets on Pressure Losses in a Linear Cascade at Low Reynolds Numbers
    Journal of Turbomachinery, 2020
    Co-Authors: Qiang Liu, Shan Zhong
    Abstract:

    Abstract In this paper, the effects of an array of herringbone Riblets with different riblet geometry (height and spacing) and start locations on the pressure losses in a cascade of diffuser blades are investigated over a range of low Reynolds numbers (0.50 × 105–1.00 × 105). The herringbone Riblets with a given geometry are found to produce a profound modification to the wake structure above certain critical Reynolds numbers. It is also found that within the range of parameters tested an increase in riblet height and riblet spacing results in an onset of significant control effect at a lower Reynolds number, which is accompanied by a slight reduction in zone-averaged loss coefficient and flow turning angle. An upstream shift of the start position of the riblet array along the blades enables the Riblets to become effective at a lower Reynolds number at the expense of a reduced loss reduction and flow turning angle. A semi-empirical relationship between the ratio of riblet height to local baseline boundary layer displacement thickness and the critical Reynolds number is established using the present experimental data. A preliminary methodology for designing the herringbone Riblets to ensure an effective control of 2D flow separations around the mid-span of diffuser blades over a specified range of Reynolds numbers is also proposed.

  • Control of laminar flow separation over a backward-facing rounded ramp with C-D Riblets – The effects of riblet height, spacing and yaw angle
    International Journal of Heat and Fluid Flow, 2020
    Co-Authors: Tongbiao Guo, Shan Zhong, Tim Craft
    Abstract:

    Abstract In the work presented in this paper, a section of convergent-divergent (C-D) Riblets is applied upstream of a backward-facing rounded ramp in a fully developed laminar channel flow at a Reynolds number based on the channel height of 400. Numerical simulations are undertaken to examine the effects of riblet geometry and yaw angle on the strength of secondary flow produced by the Riblets and the extent of flow separation zone. It is found that, in comparison with the baseline case with no Riblets, flow separation is delayed and the reattachment occurs earlier leading to a smaller separation zone around the diverging line. The opposite phenomenon occurs around the converging line. Our results also show that for Riblets with a given height a maximum strength of the secondary flow motion upstream of the ramp and a maximum reduction in the spanwise-averaged length of separation are obtained at s / h = 4 . At a given s / h = 4 , as the riblet height increases, the strength of the secondary flow motion generated by C-D Riblets increases and a minimum riblet height of 3.75% of the channel height is required to produce a net reduction in the spanwise-averaged length of separation. For Riblets with a given height and spacing, as yaw angle increases both the strength of secondary flow and net reduction of separation zone exhibit a parabolic trend and both of them peak at γ = 45 ° . Overall, with the riblet setting tested here the benefit of suppressing the laminar separation bubble has not resulted in a net reduction in the total pressure losses compared to that in the baseline case. Nevertheless, an examination of the pressure losses from three different sections along the channel separately has produced some insights about the loss mechanisms and pointed to the possible ways by which the pressure losses could be reduced.

  • vortical structures and development of laminar flow over convergent divergent Riblets
    Physics of Fluids, 2018
    Co-Authors: Shan Zhong, Shanying Zhang
    Abstract:

    In this work, the development of a laminar boundary layer over a rectangular convergent-divergent riblet section with a finite streamwise length is studied experimentally using dye visualization and particle image velocimetry in a water flume. The flow topology over this highly directional spanwise roughness is established from this study. It is shown that convergent-divergent Riblets generate a spanwise flow above the Riblets from the diverging line toward the adjacent converging line. This consequently leads to the formation of a weak recirculating secondary flow in cross-stream planes across the boundary layer that creates a downwash motion over the diverging line and an upwash motion over the converging line. It is found that the fluid inside the riblet valley follows a helicoidal path and it also interacts with the crossflow boundary layer hence playing a key role in determining the structure of the secondary flow across the boundary layer. The impact of riblet wavelength on vortical structures is also revealed for the first time. A larger riblet wavelength is seen to produce a stronger upwash/downwash and hence a more intense secondary flow as well as a stronger deceleration effect on the crossflow. Furthermore, the streamwise development of the flow over the riblet section can be divided into a developing stage followed by a developed stage. In the developing stage, the magnitude of induced streamwise velocity and vorticity over the converging line continues to increase, whereas in the developed stage the values of these parameters remain essentially unchanged.In this work, the development of a laminar boundary layer over a rectangular convergent-divergent riblet section with a finite streamwise length is studied experimentally using dye visualization and particle image velocimetry in a water flume. The flow topology over this highly directional spanwise roughness is established from this study. It is shown that convergent-divergent Riblets generate a spanwise flow above the Riblets from the diverging line toward the adjacent converging line. This consequently leads to the formation of a weak recirculating secondary flow in cross-stream planes across the boundary layer that creates a downwash motion over the diverging line and an upwash motion over the converging line. It is found that the fluid inside the riblet valley follows a helicoidal path and it also interacts with the crossflow boundary layer hence playing a key role in determining the structure of the secondary flow across the boundary layer. The impact of riblet wavelength on vortical structures is al...

Leonardo P Chamorro - One of the best experts on this subject based on the ideXlab platform.

  • Drag reduction of large wind turbine blades through Riblets: Evaluation of riblet geometry and application strategies
    Renewable Energy, 2013
    Co-Authors: Leonardo P Chamorro, Roger E. A. Arndt, Fotis Sotiropoulos
    Abstract:

    Abstract Wind tunnel experiments were performed to quantify the drag reduction on a wind turbine airfoil partially or fully covered with Riblets. A full-scale 2.5 MW wind turbine airfoil section, typical for the near tip, was placed in the free stream flow of the wind tunnel at the Saint Anthony Falls Laboratory, University of Minnesota. Various sizes and geometries of experimental Riblets were provided by 3M Company and tested at angles of attack ranging from 0° ≤ α ≤ 10° (0.25 ≤ CL ≤ 1.14) and at a Reynolds number of Re = 2.2 × 106. Mean drag was measured via wake survey (momentum deficit) and with a sensitive force balance. Lift was measured directly from the force balance. Tests included the cases of complete and partial riblet coverage on the wing. Results indicated that Riblets could provide an overall reduction of skin friction drag, and that the amount of the decrease varied with riblet height and geometry. Partial riblet coverage appears in some cases more efficient than its complete coverage counterpart. The percentage of drag the Riblets reduced varied greatly and in some cases the Riblets were even detrimental to the airfoil. The most efficient riblet for a completely covered airfoil was found to be the V-groove shape of 100 μm height. It produced a reduction of roughly 6% in the operational range expected in a turbine airfoil. On the other hand, the most efficient riblet size for partial coverage was also a V-groove shape and seemed to shift slightly to a smaller peak height of 80 μm. This configuration produced a reduction of roughly 4% in the range of angle of attack that is typical for operation in the field. The average non-dimensional square root of the groove cross-section, l+, defined in terms of the drag coefficient at design angle of attack for the optimum riblet configuration in the fully coverage case was found to be l + ≈ 10 , which is very close to the optimum value found for planar surfaces. Based on our results we propose a formulation for the optimum riblet size in airfoil considering the mean drag coefficient and chord length Reynolds number. Even though the optimum full coverage case showed better performance that the partial case, the additional drag reduction benefit may be offset by the additional application cost.

  • drag reduction in large wind turbines through Riblets evaluation of different geometries
    50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, 2012
    Co-Authors: Roger E. A. Arndt, Leonardo P Chamorro, Fotis Sotiropoulos
    Abstract:

    Wind tunnel experiments were performed with a wind turbine airfoil that was fitted with different riblet geometries. The foil represents the cross section of a full-scale 2.5MW wind turbine blade close to the tip. The test foil was placed in the free stream flow of the wind tunnel at the Saint Anthony Falls Laboratory, University of Minnesota. Various sizes and geometries of Riblets were tested at angles of attack ranging from 0 ≤α≤10 , corresponding to a range of lift coefficient of 0.25≤ CL ≤1.14, and at a Reynolds number of Re=2.2 X 10. Mean drag was measured via a wake survey (momentum deficit) and with a sensitive force balance. Lift was measured directly from the force balance. Tests included the cases of complete and partial riblet coverage on the wing. The results indicate that Riblets could provide an overall reduction of skin friction drag, and that the amount of the decrease varied with riblet height and geometry. Partial riblet coverage appears, in some cases, to be more efficient than its complete coverage counterpart. The percentage of drag the Riblets reduced varied greatly and in some cases the Riblets were even detrimental to the airfoil. The most efficient riblet for a completely covered airfoil was found to be the V-groove shape of 100 μm height. It produced a reduction of roughly 6% in the operational range expected for a turbine airfoil. On the other hand, the most efficient riblet size for partial coverage was also a V-groove shape but had a slightly smaller peak height of 80 μm. This configuration produced a reduction of roughly 4% in the range of angle of attack that is typical for operation in the field. Our testing was limited to one size airfoil. However, comparison of our test data with recent numerical simulations by A-Mayoral and Jimenez suggest that an optimal size riblet configuration can be expressed in normalized form, involving only Reynolds number and the drag coefficient at design angle of attack.

  • on the skin friction drag reduction in large wind turbines using sharp v grooved Riblets
    49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2011
    Co-Authors: Leonardo P Chamorro, Roger E. A. Arndt, Fotis Sotiropoulos
    Abstract:

    Skin friction drag reduction through the use of Riblets has been a topic of intensive research during the last decades. Main efforts have been placed on both numerical (mainly DNS) and experimental (wind tunnel and fluid channel) approaches. In spite of the valuable efforts to date, the fundamental mechanisms that induce drag reduction are not well established and, in particular, the potential benefits associated with the use of Riblets in wind turbines are unclear. In this research, wind tunnel experiments were performed to quantify the drag reduction in a wind turbine airfoil using V-groove riblet structures. A full-scale 2.5MW wind turbine airfoil section (typical of the 75-80% blade span), was placed in the freestream flow of the wind tunnel at the Saint Anthony Falls Laboratory (SAFL), University of Minnesota. Different sizes of V-groove Riblets were tested at different angles of attack at full scale Reynolds number. Force sensors were used to measure lift and drag. Momentum deficit was also measured in the wake of the airfoil to determine the net drag. Flat plate experiments were also performed to determine the distribution of the skin friction coefficient in a developing boundary layer. The experimental measurements will be used to develop and test the performance of near-wall boundary conditions for the velocity field that can account for the effects of riblet roughness in the context of RANS and hybrid RANS/LES models. The numerical simulations along with the laboratory experiments will be used in the future to determine the best riblet configuration and guide the application and testing of Riblets in a 2.5MW wind turbine under real-life wind conditions.

Joerg R. Seume - One of the best experts on this subject based on the ideXlab platform.

  • Correlation-Based Riblet Model for Turbomachinery Applications
    Journal of Turbomachinery, 2017
    Co-Authors: Viktor Koepplin, Florian Herbst, Joerg R. Seume
    Abstract:

    An empirical riblet model for manufactured V-shaped and trapezoidal Riblets which is suitable for turbomachinery application is presented. The implementation of the riblet effect employs a correlation-based correction for the damping of the specific dissipation rate ω in the vicinity of the wall which has been previously presented by other researchers. In the current paper, the correlations are extended into the drag-increasing regime and are extended to account for the effect of misalignment of the Riblets relative to the flow and for the effect of adverse pressure gradients. In order to account for the latter in modern, massive parallel Reynolds-averaged Navier–Stokes (RANS) codes, a local Clauser parameter has been newly derived. The model is implemented in a three-dimensional (3D) turbomachinery design code and validated with flat plate measurement data and a NACA6510 compressor cascade. The predictions of the experimental values are in very good agreement with the experimental data, showing the capability of the model for designing riblet structured turbomachinery blading.

  • Correlation-Based Riblet Model for Turbomachinery Applications
    Volume 2C: Turbomachinery, 2016
    Co-Authors: Viktor Koepplin, Florian Herbst, Joerg R. Seume
    Abstract:

    An empirical riblet model for manufactured V-shaped and trapezoidal Riblets which is suitable for turbomachinery application is presented. The implementation of the riblet effect employs a correlation-based correction for the damping of the specific dissipation rate omega in the vicinity of the wall which has been previously presented by other researchers. In the current paper the correlations are extended into the drag-increasing regime and are extended to account for the effect of misalignment of the Riblets relative to the flow and for the effect of adverse pressure gradients. In order to account for the latter in modern, massive parallel CFD-codes, a local Clauser-Parameter has been newly derived. The model is implemented in the 3D turbomachinery design code TRACE and validated with flat plate measurement data and a NACA6510 compressor cascade. The predictions of the experimental values are in very good agreement with the experimental data, showing the capability of the model for designing riblet structured turbomachinery blading.Copyright © 2016 by ASME

  • Optimal Application of Riblets on Compressor Blades and Their Contamination Behavior
    Journal of Turbomachinery-transactions of The Asme, 2012
    Co-Authors: Christoph Lietmeyer, Karsten Oehlert, Joerg R. Seume
    Abstract:

    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.

  • Optimal Application of Riblets on Compressor Blades and Their Contamination Behavior
    Volume 7: Turbomachinery Parts A B and C, 2011
    Co-Authors: Christoph Lietmeyer, Karsten Oehlert, Joerg R. Seume
    Abstract:

    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.Copyright © 2011 by ASME

  • Exploratory Experiments on Machined Riblets for 2-D Compressor Blades
    Volume 1: Advances in Aerospace Technology, 2007
    Co-Authors: Karsten Oehlert, Joerg R. Seume, Frank Siegel, Andreas Ostendorf, Bo Wang, Berend Denkena, Taras Vynnyk, Eduard Reithmeier, Wolfram Hage, Karsten Knobloch
    Abstract:

    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%.Copyright © 2007 by ASME