The Experts below are selected from a list of 22242 Experts worldwide ranked by ideXlab platform
Steven L Ceccio - One of the best experts on this subject based on the ideXlab platform.
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flow induced degradation of drag reducing polymer solutions within a high reynolds number turbulent boundary layer
Journal of Fluid Mechanics, 2011Co-Authors: Brian R Elbing, Marc Perlin, David R. Dowling, Michael J Solomon, Steven L CeccioAbstract:Polymer drag reduction, diffusion and degradation in a high-Reynolds-number turbulent boundary layer (TBL) flow were investigated. The TBL developed on a flat plate at free-stream speeds up to 20ms ―1 Measurements were acquired up to 10.7 m Downstream of the leading edge, yielding Downstream-Distance-based Reynolds numbers up to 220 million. The test model surface was hydraulically smooth or fully rough. Flow diagnostics included local skin friction, near-wall polymer concentration, boundary layer sampling and rheological analysis of polymer solution samples. Skin-friction data revealed that the presence of surface roughness can produce a local increase in drag reduction near the injection location (compared with the flow over a smooth surface) because of enhanced mixing. However, the roughness ultimately led to a significant decrease in drag reduction with increasing speed and Downstream Distance. At the highest speed tested (20 m s ―1 ) no drag reduction was discernible at the first measurement location (0.56 m Downstream of injection), even at the highest polymer injection flux (10 times the flux of fluid in the near-wall region). Increased polymer degradation rates and polymer mixing were shown to be the contributing factors to the loss of drag reduction. Rheological analysis of liquid drawn from the TBL revealed that flow-induced polymer degradation by chain scission was often substantial. The inferred polymer molecular weight was successfully scaled with the local wall shear rate and residence time in the TBL. This scaling revealed an exponential decay that asymptotes to a finite (steady-state) molecular weight. The importance of the residence time to the scaling indicates that while individual polymer chains are stretched and ruptured on a relatively short time scale (∼10 ―3 s), because of the low percentage of individual chains stretched at any instant in time, a relatively long time period (∼0.1 s) is required to observe changes in the mean molecular weight. This scaling also indicates that most previous TBL studies would have observed minimal influence from degradation due to insufficient residence times.
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high reynolds number turbulent boundary layer friction drag reduction from wall injected polymer solutions
Journal of Fluid Mechanics, 2009Co-Authors: Eric S Winkel, Marc Perlin, David R. Dowling, Ghanem F Oweis, Siva A Vanapalli, Michael J Solomon, Steven L CeccioAbstract:A set of controlled high-Reynolds-number experiments has been conducted at the William B. Morgan Large Cavitation Channel (LCC) in Memphis, Tennessee to investigate the friction drag reduction achieved by injecting aqueous poly(ethylene oxide) (PEO) solutions at three different mean molecular weights into the near-zero-pressure-gradient turbulent boundary layer that forms on a smooth flat test surface having a length of nearly 11m. The test model spanned the 3.05m width of the LCC test section and had an overall length of 12.9m. Skin-friction drag was measured with six floating-plate force balances at Downstream-Distance-based Reynolds numbers as high as 220 million and free stream speeds up to 20ms −1 . For a given polymer type, the level of drag reduction was measured for a range of free stream speeds, polymer injection rates and concentrations of the injected solution. Polymer concentration fields in the near-wall region (0 y + 3 ) were examined at three locations Downstream of the injector using near-wall planar laser-induced-fluorescence imaging. The development and extent of drag reduction and polymer mixing are compared to previously reported results using the traditional K -factor scaling. Unlike smaller scale and lower speed experiments, speed dependence is observed in the K -scaled results for the higher molecular weight polymers and it is postulated that this dependence is caused by molecular aggregation and/or flow-induced polymer degradation (chain scission). The evolution of near-wall polymer concentration is divided into three regimes: (i) the development region near the injector where drag reduction increases with Downstream Distance and the polymer is highly inhomogeneous forming filaments near the wall, (ii) the transitional mixing region where drag reduction starts to decrease as the polymer mixes across the boundary layer and where filaments are less pronounced and (iii) the final region where the polymer mixing and dilution is set by the rate of boundary layer growth. Unlike pipe-flow friction-drag reduction, the asymptotic maximum drag reduction (MDR) either was not reached or did not persist in these experiments. Instead, the nearest approach to MDR was transitory and occurred between the development and transitional regions. The length of the development region was observed to increase monotonically with increasing polymer molecular weight, injection rate, concentration and decreasing free stream speed. And finally, the near-wall polymer concentration is correlated to the measured drag reduction for the three polymer molecular weights in the form of a proposed empirical drag-reduction curve.
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bubble friction drag reduction in a high reynolds number flat plate turbulent boundary layer
Journal of Fluid Mechanics, 2006Co-Authors: Wendy Sanders, Marc Perlin, Eric S Winkel, David R. Dowling, Steven L CeccioAbstract:Turbulent boundary layer skin friction in liquid flows may be reduced when bubbles are present near the surface on which the boundary layer forms. Prior experimental studies of this phenomenon reached Downstream-Distance-based Reynolds numbers (m. At the lowest test speed and highest air injection rate, buoyancy pushed the air bubbles to the plate surface where they coalesced to form a nearly continuous gas film that persisted to the end of the plate with near-100% skin-friction drag reduction. At the higher two flow speeds, the bubbles generally remained distinct and skin-friction drag reduction was observed when the bubbly mixture was closer to the plate surface than 300 wall units of the boundary-layer flow without air injection, even when the bubble diameter was more than 100 of these wall units. Skin-friction drag reduction was lost when the near-wall shear induced the bubbles to migrate from the plate surface. This bubble-migration phenomenon limited the persistence of bubble-induced skin-friction drag reduction to the first few metres Downstream of the air injector in the current experiments.
H Y Peng - One of the best experts on this subject based on the ideXlab platform.
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study of wake characteristics of a vertical axis wind turbine by two and three dimensional computational fluid dynamics simulations
Renewable Energy, 2016Co-Authors: H Y PengAbstract:In this work, the near and far wakes of a low-solidity two-straight-bladed vertical axis wind turbine (VAWT) were, for the first time, investigated with two- and three-dimensional computational fluid dynamics (CFD) simulations. The wake velocity field and turbulence field from 1 to 10 turbine diameters (1D to 10D) Downstream were examined. Structured meshes were generated throughout the computational domain for calculation accuracy and efficiency. Both the transition shear stress transport (SST) and the detached eddy simulation (DES) models were used to close the unsteady Reynolds-averaged Navier–Stokes (URANS) equations. The CFD models were validated by particle image velocimetry (PIV) test results from the literature. The regions of the near and far wakes were defined based on the occurrence of the maximum velocity deficit. In the near wake (within 3D), the velocity suffered a drastic deficit of about 85%. In the far wake (beyond 3D), major velocity recovery occurred with the average stream-wise velocity reaching approximately 75% at 10D. The wake asymmetry grew as the Downstream Distance increased, and the causes behind it were examined. Further, investigation into the dimensional effects of the CFD models, and the blade tip and span vortices was conducted.
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Investigation into the wake aerodynamics of a five-straight-bladed vertical axis wind turbine by wind tunnel tests
Journal of Wind Engineering and Industrial Aerodynamics, 2016Co-Authors: H Y Peng, H. F. Lam, C. F. LeeAbstract:Wake characteristics have significant effects on the performance design of standalone turbines and the optimal placement of multiple turbines. In the literature to date, little experimentation has been done on the wake of vertical axis wind turbines (VAWTs), and understanding of such wake is far from adequate. In this work, systematic measurements are presented of both the near and mid-range wake of a five-straight-bladed VAWT in a wind tunnel. The blockage ratio of the VAWT was 1.8%, and no correction of the measured data was required. The wake flow fields were measured up to 10 turbine diameters (10D) to the Downstream. The wake exhibited high asymmetry in the horizontal direction. In addition, the wake expanded more in the horizontal direction than in the vertical direction. The causes of the asymmetry were analyzed and discussed through the experimental results. An engineering wake model was proposed to characterize the wake edges and the average velocities. The existence of a pair of counter-rotating vortical structures in the wake was detected. Moreover, the integral length scale was found to steadily grow with the Downstream Distance. This work contributes to the knowledge of the VAWTs' wake and the application of VAWTs in wind farm layout design.
David R. Dowling - One of the best experts on this subject based on the ideXlab platform.
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flow induced degradation of drag reducing polymer solutions within a high reynolds number turbulent boundary layer
Journal of Fluid Mechanics, 2011Co-Authors: Brian R Elbing, Marc Perlin, David R. Dowling, Michael J Solomon, Steven L CeccioAbstract:Polymer drag reduction, diffusion and degradation in a high-Reynolds-number turbulent boundary layer (TBL) flow were investigated. The TBL developed on a flat plate at free-stream speeds up to 20ms ―1 Measurements were acquired up to 10.7 m Downstream of the leading edge, yielding Downstream-Distance-based Reynolds numbers up to 220 million. The test model surface was hydraulically smooth or fully rough. Flow diagnostics included local skin friction, near-wall polymer concentration, boundary layer sampling and rheological analysis of polymer solution samples. Skin-friction data revealed that the presence of surface roughness can produce a local increase in drag reduction near the injection location (compared with the flow over a smooth surface) because of enhanced mixing. However, the roughness ultimately led to a significant decrease in drag reduction with increasing speed and Downstream Distance. At the highest speed tested (20 m s ―1 ) no drag reduction was discernible at the first measurement location (0.56 m Downstream of injection), even at the highest polymer injection flux (10 times the flux of fluid in the near-wall region). Increased polymer degradation rates and polymer mixing were shown to be the contributing factors to the loss of drag reduction. Rheological analysis of liquid drawn from the TBL revealed that flow-induced polymer degradation by chain scission was often substantial. The inferred polymer molecular weight was successfully scaled with the local wall shear rate and residence time in the TBL. This scaling revealed an exponential decay that asymptotes to a finite (steady-state) molecular weight. The importance of the residence time to the scaling indicates that while individual polymer chains are stretched and ruptured on a relatively short time scale (∼10 ―3 s), because of the low percentage of individual chains stretched at any instant in time, a relatively long time period (∼0.1 s) is required to observe changes in the mean molecular weight. This scaling also indicates that most previous TBL studies would have observed minimal influence from degradation due to insufficient residence times.
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high reynolds number turbulent boundary layer friction drag reduction from wall injected polymer solutions
Journal of Fluid Mechanics, 2009Co-Authors: Eric S Winkel, Marc Perlin, David R. Dowling, Ghanem F Oweis, Siva A Vanapalli, Michael J Solomon, Steven L CeccioAbstract:A set of controlled high-Reynolds-number experiments has been conducted at the William B. Morgan Large Cavitation Channel (LCC) in Memphis, Tennessee to investigate the friction drag reduction achieved by injecting aqueous poly(ethylene oxide) (PEO) solutions at three different mean molecular weights into the near-zero-pressure-gradient turbulent boundary layer that forms on a smooth flat test surface having a length of nearly 11m. The test model spanned the 3.05m width of the LCC test section and had an overall length of 12.9m. Skin-friction drag was measured with six floating-plate force balances at Downstream-Distance-based Reynolds numbers as high as 220 million and free stream speeds up to 20ms −1 . For a given polymer type, the level of drag reduction was measured for a range of free stream speeds, polymer injection rates and concentrations of the injected solution. Polymer concentration fields in the near-wall region (0 y + 3 ) were examined at three locations Downstream of the injector using near-wall planar laser-induced-fluorescence imaging. The development and extent of drag reduction and polymer mixing are compared to previously reported results using the traditional K -factor scaling. Unlike smaller scale and lower speed experiments, speed dependence is observed in the K -scaled results for the higher molecular weight polymers and it is postulated that this dependence is caused by molecular aggregation and/or flow-induced polymer degradation (chain scission). The evolution of near-wall polymer concentration is divided into three regimes: (i) the development region near the injector where drag reduction increases with Downstream Distance and the polymer is highly inhomogeneous forming filaments near the wall, (ii) the transitional mixing region where drag reduction starts to decrease as the polymer mixes across the boundary layer and where filaments are less pronounced and (iii) the final region where the polymer mixing and dilution is set by the rate of boundary layer growth. Unlike pipe-flow friction-drag reduction, the asymptotic maximum drag reduction (MDR) either was not reached or did not persist in these experiments. Instead, the nearest approach to MDR was transitory and occurred between the development and transitional regions. The length of the development region was observed to increase monotonically with increasing polymer molecular weight, injection rate, concentration and decreasing free stream speed. And finally, the near-wall polymer concentration is correlated to the measured drag reduction for the three polymer molecular weights in the form of a proposed empirical drag-reduction curve.
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bubble friction drag reduction in a high reynolds number flat plate turbulent boundary layer
Journal of Fluid Mechanics, 2006Co-Authors: Wendy Sanders, Marc Perlin, Eric S Winkel, David R. Dowling, Steven L CeccioAbstract:Turbulent boundary layer skin friction in liquid flows may be reduced when bubbles are present near the surface on which the boundary layer forms. Prior experimental studies of this phenomenon reached Downstream-Distance-based Reynolds numbers (m. At the lowest test speed and highest air injection rate, buoyancy pushed the air bubbles to the plate surface where they coalesced to form a nearly continuous gas film that persisted to the end of the plate with near-100% skin-friction drag reduction. At the higher two flow speeds, the bubbles generally remained distinct and skin-friction drag reduction was observed when the bubbly mixture was closer to the plate surface than 300 wall units of the boundary-layer flow without air injection, even when the bubble diameter was more than 100 of these wall units. Skin-friction drag reduction was lost when the near-wall shear induced the bubbles to migrate from the plate surface. This bubble-migration phenomenon limited the persistence of bubble-induced skin-friction drag reduction to the first few metres Downstream of the air injector in the current experiments.
Marc Perlin - One of the best experts on this subject based on the ideXlab platform.
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flow induced degradation of drag reducing polymer solutions within a high reynolds number turbulent boundary layer
Journal of Fluid Mechanics, 2011Co-Authors: Brian R Elbing, Marc Perlin, David R. Dowling, Michael J Solomon, Steven L CeccioAbstract:Polymer drag reduction, diffusion and degradation in a high-Reynolds-number turbulent boundary layer (TBL) flow were investigated. The TBL developed on a flat plate at free-stream speeds up to 20ms ―1 Measurements were acquired up to 10.7 m Downstream of the leading edge, yielding Downstream-Distance-based Reynolds numbers up to 220 million. The test model surface was hydraulically smooth or fully rough. Flow diagnostics included local skin friction, near-wall polymer concentration, boundary layer sampling and rheological analysis of polymer solution samples. Skin-friction data revealed that the presence of surface roughness can produce a local increase in drag reduction near the injection location (compared with the flow over a smooth surface) because of enhanced mixing. However, the roughness ultimately led to a significant decrease in drag reduction with increasing speed and Downstream Distance. At the highest speed tested (20 m s ―1 ) no drag reduction was discernible at the first measurement location (0.56 m Downstream of injection), even at the highest polymer injection flux (10 times the flux of fluid in the near-wall region). Increased polymer degradation rates and polymer mixing were shown to be the contributing factors to the loss of drag reduction. Rheological analysis of liquid drawn from the TBL revealed that flow-induced polymer degradation by chain scission was often substantial. The inferred polymer molecular weight was successfully scaled with the local wall shear rate and residence time in the TBL. This scaling revealed an exponential decay that asymptotes to a finite (steady-state) molecular weight. The importance of the residence time to the scaling indicates that while individual polymer chains are stretched and ruptured on a relatively short time scale (∼10 ―3 s), because of the low percentage of individual chains stretched at any instant in time, a relatively long time period (∼0.1 s) is required to observe changes in the mean molecular weight. This scaling also indicates that most previous TBL studies would have observed minimal influence from degradation due to insufficient residence times.
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high reynolds number turbulent boundary layer friction drag reduction from wall injected polymer solutions
Journal of Fluid Mechanics, 2009Co-Authors: Eric S Winkel, Marc Perlin, David R. Dowling, Ghanem F Oweis, Siva A Vanapalli, Michael J Solomon, Steven L CeccioAbstract:A set of controlled high-Reynolds-number experiments has been conducted at the William B. Morgan Large Cavitation Channel (LCC) in Memphis, Tennessee to investigate the friction drag reduction achieved by injecting aqueous poly(ethylene oxide) (PEO) solutions at three different mean molecular weights into the near-zero-pressure-gradient turbulent boundary layer that forms on a smooth flat test surface having a length of nearly 11m. The test model spanned the 3.05m width of the LCC test section and had an overall length of 12.9m. Skin-friction drag was measured with six floating-plate force balances at Downstream-Distance-based Reynolds numbers as high as 220 million and free stream speeds up to 20ms −1 . For a given polymer type, the level of drag reduction was measured for a range of free stream speeds, polymer injection rates and concentrations of the injected solution. Polymer concentration fields in the near-wall region (0 y + 3 ) were examined at three locations Downstream of the injector using near-wall planar laser-induced-fluorescence imaging. The development and extent of drag reduction and polymer mixing are compared to previously reported results using the traditional K -factor scaling. Unlike smaller scale and lower speed experiments, speed dependence is observed in the K -scaled results for the higher molecular weight polymers and it is postulated that this dependence is caused by molecular aggregation and/or flow-induced polymer degradation (chain scission). The evolution of near-wall polymer concentration is divided into three regimes: (i) the development region near the injector where drag reduction increases with Downstream Distance and the polymer is highly inhomogeneous forming filaments near the wall, (ii) the transitional mixing region where drag reduction starts to decrease as the polymer mixes across the boundary layer and where filaments are less pronounced and (iii) the final region where the polymer mixing and dilution is set by the rate of boundary layer growth. Unlike pipe-flow friction-drag reduction, the asymptotic maximum drag reduction (MDR) either was not reached or did not persist in these experiments. Instead, the nearest approach to MDR was transitory and occurred between the development and transitional regions. The length of the development region was observed to increase monotonically with increasing polymer molecular weight, injection rate, concentration and decreasing free stream speed. And finally, the near-wall polymer concentration is correlated to the measured drag reduction for the three polymer molecular weights in the form of a proposed empirical drag-reduction curve.
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bubble friction drag reduction in a high reynolds number flat plate turbulent boundary layer
Journal of Fluid Mechanics, 2006Co-Authors: Wendy Sanders, Marc Perlin, Eric S Winkel, David R. Dowling, Steven L CeccioAbstract:Turbulent boundary layer skin friction in liquid flows may be reduced when bubbles are present near the surface on which the boundary layer forms. Prior experimental studies of this phenomenon reached Downstream-Distance-based Reynolds numbers (m. At the lowest test speed and highest air injection rate, buoyancy pushed the air bubbles to the plate surface where they coalesced to form a nearly continuous gas film that persisted to the end of the plate with near-100% skin-friction drag reduction. At the higher two flow speeds, the bubbles generally remained distinct and skin-friction drag reduction was observed when the bubbly mixture was closer to the plate surface than 300 wall units of the boundary-layer flow without air injection, even when the bubble diameter was more than 100 of these wall units. Skin-friction drag reduction was lost when the near-wall shear induced the bubbles to migrate from the plate surface. This bubble-migration phenomenon limited the persistence of bubble-induced skin-friction drag reduction to the first few metres Downstream of the air injector in the current experiments.
Eric S Winkel - One of the best experts on this subject based on the ideXlab platform.
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high reynolds number turbulent boundary layer friction drag reduction from wall injected polymer solutions
Journal of Fluid Mechanics, 2009Co-Authors: Eric S Winkel, Marc Perlin, David R. Dowling, Ghanem F Oweis, Siva A Vanapalli, Michael J Solomon, Steven L CeccioAbstract:A set of controlled high-Reynolds-number experiments has been conducted at the William B. Morgan Large Cavitation Channel (LCC) in Memphis, Tennessee to investigate the friction drag reduction achieved by injecting aqueous poly(ethylene oxide) (PEO) solutions at three different mean molecular weights into the near-zero-pressure-gradient turbulent boundary layer that forms on a smooth flat test surface having a length of nearly 11m. The test model spanned the 3.05m width of the LCC test section and had an overall length of 12.9m. Skin-friction drag was measured with six floating-plate force balances at Downstream-Distance-based Reynolds numbers as high as 220 million and free stream speeds up to 20ms −1 . For a given polymer type, the level of drag reduction was measured for a range of free stream speeds, polymer injection rates and concentrations of the injected solution. Polymer concentration fields in the near-wall region (0 y + 3 ) were examined at three locations Downstream of the injector using near-wall planar laser-induced-fluorescence imaging. The development and extent of drag reduction and polymer mixing are compared to previously reported results using the traditional K -factor scaling. Unlike smaller scale and lower speed experiments, speed dependence is observed in the K -scaled results for the higher molecular weight polymers and it is postulated that this dependence is caused by molecular aggregation and/or flow-induced polymer degradation (chain scission). The evolution of near-wall polymer concentration is divided into three regimes: (i) the development region near the injector where drag reduction increases with Downstream Distance and the polymer is highly inhomogeneous forming filaments near the wall, (ii) the transitional mixing region where drag reduction starts to decrease as the polymer mixes across the boundary layer and where filaments are less pronounced and (iii) the final region where the polymer mixing and dilution is set by the rate of boundary layer growth. Unlike pipe-flow friction-drag reduction, the asymptotic maximum drag reduction (MDR) either was not reached or did not persist in these experiments. Instead, the nearest approach to MDR was transitory and occurred between the development and transitional regions. The length of the development region was observed to increase monotonically with increasing polymer molecular weight, injection rate, concentration and decreasing free stream speed. And finally, the near-wall polymer concentration is correlated to the measured drag reduction for the three polymer molecular weights in the form of a proposed empirical drag-reduction curve.
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bubble friction drag reduction in a high reynolds number flat plate turbulent boundary layer
Journal of Fluid Mechanics, 2006Co-Authors: Wendy Sanders, Marc Perlin, Eric S Winkel, David R. Dowling, Steven L CeccioAbstract:Turbulent boundary layer skin friction in liquid flows may be reduced when bubbles are present near the surface on which the boundary layer forms. Prior experimental studies of this phenomenon reached Downstream-Distance-based Reynolds numbers (m. At the lowest test speed and highest air injection rate, buoyancy pushed the air bubbles to the plate surface where they coalesced to form a nearly continuous gas film that persisted to the end of the plate with near-100% skin-friction drag reduction. At the higher two flow speeds, the bubbles generally remained distinct and skin-friction drag reduction was observed when the bubbly mixture was closer to the plate surface than 300 wall units of the boundary-layer flow without air injection, even when the bubble diameter was more than 100 of these wall units. Skin-friction drag reduction was lost when the near-wall shear induced the bubbles to migrate from the plate surface. This bubble-migration phenomenon limited the persistence of bubble-induced skin-friction drag reduction to the first few metres Downstream of the air injector in the current experiments.