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Marc Perlin - One of the best experts on this subject based on the ideXlab platform.
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Freeman Scholar Review: Passive and Active Skin-Friction Drag Reduction in Turbulent Boundary Layers
Journal of Fluids Engineering-transactions of The Asme, 2016Co-Authors: Marc Perlin, David R. Dowling, Steven L CeccioAbstract:A variety of skin-Friction Drag reduction (FDR) methods for turbulent boundary layer (TBL) flows are reviewed. Both passive and active methods of Drag reduction are discussed, along with a review of the fundamental processes responsible for Friction Drag and FDR. Particular emphasis is given to methods that are applicable to external hydrodynamic flows where additives are diluted by boundary layer entrainment. The methods reviewed include those based on engineered surfaces (riblets, large eddy breakup devices (LEBUs), and superhydrophobic surfaces (SHS)), those based on additives (polymer injection and gas injection), and those based on morphological alterations in the boundary layer flow (air layers and partial cavity formation). A common theme for all methods is their disruption of one or more of the underlying physical processes responsible for the production of skin-Friction Drag in a TBL. Opportunities and challenges for practical implementation of FDR techniques are also discussed.
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Skin-Friction Drag reduction in the turbulent regime using random-textured hydrophobic surfaces
Physics of Fluids, 2014Co-Authors: Rahul Anil Bidkar, Ambarish Jayant Kulkarni, Vaibhav Bahadur, Luc Stephane Leblanc, Steven L Ceccio, Marc PerlinAbstract:Technologies for reducing hydrodynamic skin-Friction Drag have a huge potential for energy-savings in applications ranging from propulsion of marine vessels to transporting liquids through pipes. The majority of previous experimental studies using hydrophobic surfaces have successfully shown skin-Friction Drag reduction in the laminar and transitional flow regimes (typically Reynolds numbers less than ≃106 for external flows). However, this hydrophobicity induced Drag reduction is known to diminish with increasing Reynolds numbers in experiments involving wall bounded turbulent flows. Using random-textured hydrophobic surfaces (fabricated using large-length scalable thermal spray processes) on a flat plate geometry, we present water-tunnel test data with Reynolds numbers ranging from 106 to 9 × 106 that show sustained skin-Friction Drag reduction of 20%–30% in such turbulent flow regimes. Furthermore, we provide evidence that apart from the formation of a Cassie state and hydrophobicity, we also need a lo...
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skin Friction Drag reduction in the turbulent regime using random textured hydrophobic surfaces
Physics of Fluids, 2014Co-Authors: Rahul Anil Bidkar, Ambarish Jayant Kulkarni, Vaibhav Bahadur, Luc Stephane Leblanc, Steven L Ceccio, Marc PerlinAbstract:Technologies for reducing hydrodynamic skin-Friction Drag have a huge potential for energy-savings in applications ranging from propulsion of marine vessels to transporting liquids through pipes. The majority of previous experimental studies using hydrophobic surfaces have successfully shown skin-Friction Drag reduction in the laminar and transitional flow regimes (typically Reynolds numbers less than ≃106 for external flows). However, this hydrophobicity induced Drag reduction is known to diminish with increasing Reynolds numbers in experiments involving wall bounded turbulent flows. Using random-textured hydrophobic surfaces (fabricated using large-length scalable thermal spray processes) on a flat plate geometry, we present water-tunnel test data with Reynolds numbers ranging from 106 to 9 × 106 that show sustained skin-Friction Drag reduction of 20%–30% in such turbulent flow regimes. Furthermore, we provide evidence that apart from the formation of a Cassie state and hydrophobicity, we also need a low surface roughness and an enhanced ability of the textured surface to retain trapped air, for sustained Drag reduction in turbulent flow regimes. Specifically, for the hydrophobic test surfaces of the present and previous studies, we show that Drag reduction seen at lower Reynolds numbers diminishes with increasing Reynolds number when the surface roughness of the underlying texture becomes comparable to the viscous sublayer thickness. Conversely, test data show that textures with surface roughness significantly smaller than the viscous sublayer thickness and textures with high porosity show sustained Drag reduction in the turbulent flow regime. The present experiments represent a significant technological advancement and one of the very few demonstrations of skin-Friction reduction in the turbulent regime using random-textured hydrophobic surfaces in an external flow configuration. The scalability of the fabrication method, the passive nature of this surface technology, and the obtained results in the turbulent regime make such hydrophobic surfaces a potentially attractive option for hydrodynamic skin-Friction Drag reduction.
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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 induced skin Friction Drag reduction and the abrupt transition to air layer Drag reduction
Journal of Fluid Mechanics, 2008Co-Authors: Brian R Elbing, Eric S Winkel, Steven L Ceccio, David R. Dowling, Marc PerlinAbstract:To investigate the phenomena of skin-Friction Drag reduction in a turbulent boundary layer (TBL) at large scales and high Reynolds numbers, a set of experiments has been conducted at the US Navy's William B. Morgan Large Cavitation Channel (LCC). Drag reduction was achieved by injecting gas (air) from a line source through the wall of a nearly zero-pressure-gradient TBL that formed on a flat-plate test model that was either hydraulically smooth or fully rough. Two distinct Drag-reduction phenomena were investigated; bubble Drag reduction (BDR) and air-layer Drag reduction (ALDR). The streamwise distribution of skin-Friction Drag reduction was monitored with six skin-Friction balances at downstream-distance-based Reynolds numbers to 220 million and at test speeds to 20.0ms −1 . Near-wall bulk void fraction was measured at twelve streamwise locations with impedance probes, and near-wall (0 Y Results from the BDR experiments indicate that: significant Drag reduction (>25%) is limited to the first few metres downstream of injection; marginal improvement was possible with a porous-plate versus an open-slot injector design; BDR has negligible sensitivity to surface tension; bubble size is independent of surface tension downstream of injection; BDR is insensitive to boundary-layer thickness at the injection location; and no synergetic effect is observed with compound injection. Using these data, previous BDR scaling methods are investigated, but data collapse is observed only with the ‘initial zone’ scaling, which provides little information on downstream persistence of BDR. ALDR was investigated with a series of experiments that included a slow increase in the volumetric flux of air injected at free-stream speeds to 15.3ms −1 . These results indicated that there are three distinct regions associated with Drag reduction with air injection: Region I, BDR; Region II, transition between BDR and ALDR; and Region III, ALDR. In addition, once ALDR was established: Friction Drag reduction in excess of 80% was observed over the entire smooth model for speeds to 15.3ms −1 ; the critical volumetric flux of air required to achieve ALDR was observed to be approximately proportional to the square of the free-stream speed; slightly higher injection rates were required for ALDR if the surface tension was decreased; stable air layers were formed at free-stream speeds to 12.5ms −1 with the surface fully roughened (though approximately 50% greater volumetric air flux was required); and ALDR was sensitive to the inflow conditions. The sensitivity to the inflow conditions can be mitigated by employing a small faired step (10mm height in the experiment) that helps to create a fixed separation line.
Steven L Ceccio - One of the best experts on this subject based on the ideXlab platform.
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Freeman Scholar Review: Passive and Active Skin-Friction Drag Reduction in Turbulent Boundary Layers
Journal of Fluids Engineering-transactions of The Asme, 2016Co-Authors: Marc Perlin, David R. Dowling, Steven L CeccioAbstract:A variety of skin-Friction Drag reduction (FDR) methods for turbulent boundary layer (TBL) flows are reviewed. Both passive and active methods of Drag reduction are discussed, along with a review of the fundamental processes responsible for Friction Drag and FDR. Particular emphasis is given to methods that are applicable to external hydrodynamic flows where additives are diluted by boundary layer entrainment. The methods reviewed include those based on engineered surfaces (riblets, large eddy breakup devices (LEBUs), and superhydrophobic surfaces (SHS)), those based on additives (polymer injection and gas injection), and those based on morphological alterations in the boundary layer flow (air layers and partial cavity formation). A common theme for all methods is their disruption of one or more of the underlying physical processes responsible for the production of skin-Friction Drag in a TBL. Opportunities and challenges for practical implementation of FDR techniques are also discussed.
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Skin-Friction Drag reduction in the turbulent regime using random-textured hydrophobic surfaces
Physics of Fluids, 2014Co-Authors: Rahul Anil Bidkar, Ambarish Jayant Kulkarni, Vaibhav Bahadur, Luc Stephane Leblanc, Steven L Ceccio, Marc PerlinAbstract:Technologies for reducing hydrodynamic skin-Friction Drag have a huge potential for energy-savings in applications ranging from propulsion of marine vessels to transporting liquids through pipes. The majority of previous experimental studies using hydrophobic surfaces have successfully shown skin-Friction Drag reduction in the laminar and transitional flow regimes (typically Reynolds numbers less than ≃106 for external flows). However, this hydrophobicity induced Drag reduction is known to diminish with increasing Reynolds numbers in experiments involving wall bounded turbulent flows. Using random-textured hydrophobic surfaces (fabricated using large-length scalable thermal spray processes) on a flat plate geometry, we present water-tunnel test data with Reynolds numbers ranging from 106 to 9 × 106 that show sustained skin-Friction Drag reduction of 20%–30% in such turbulent flow regimes. Furthermore, we provide evidence that apart from the formation of a Cassie state and hydrophobicity, we also need a lo...
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skin Friction Drag reduction in the turbulent regime using random textured hydrophobic surfaces
Physics of Fluids, 2014Co-Authors: Rahul Anil Bidkar, Ambarish Jayant Kulkarni, Vaibhav Bahadur, Luc Stephane Leblanc, Steven L Ceccio, Marc PerlinAbstract:Technologies for reducing hydrodynamic skin-Friction Drag have a huge potential for energy-savings in applications ranging from propulsion of marine vessels to transporting liquids through pipes. The majority of previous experimental studies using hydrophobic surfaces have successfully shown skin-Friction Drag reduction in the laminar and transitional flow regimes (typically Reynolds numbers less than ≃106 for external flows). However, this hydrophobicity induced Drag reduction is known to diminish with increasing Reynolds numbers in experiments involving wall bounded turbulent flows. Using random-textured hydrophobic surfaces (fabricated using large-length scalable thermal spray processes) on a flat plate geometry, we present water-tunnel test data with Reynolds numbers ranging from 106 to 9 × 106 that show sustained skin-Friction Drag reduction of 20%–30% in such turbulent flow regimes. Furthermore, we provide evidence that apart from the formation of a Cassie state and hydrophobicity, we also need a low surface roughness and an enhanced ability of the textured surface to retain trapped air, for sustained Drag reduction in turbulent flow regimes. Specifically, for the hydrophobic test surfaces of the present and previous studies, we show that Drag reduction seen at lower Reynolds numbers diminishes with increasing Reynolds number when the surface roughness of the underlying texture becomes comparable to the viscous sublayer thickness. Conversely, test data show that textures with surface roughness significantly smaller than the viscous sublayer thickness and textures with high porosity show sustained Drag reduction in the turbulent flow regime. The present experiments represent a significant technological advancement and one of the very few demonstrations of skin-Friction reduction in the turbulent regime using random-textured hydrophobic surfaces in an external flow configuration. The scalability of the fabrication method, the passive nature of this surface technology, and the obtained results in the turbulent regime make such hydrophobic surfaces a potentially attractive option for hydrodynamic skin-Friction Drag reduction.
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Friction Drag reduction of external flows with bubble and gas injection
Annual Review of Fluid Mechanics, 2010Co-Authors: Steven L CeccioAbstract:The lubrication of external liquid flow with a bubbly mixture or gas layer has been the goal of engineers for many years, and this article presents the underlying principles and recent advances of this technology. It reviews the use of partial and supercavities for Drag reduction of axisymmetric objects moving within a liquid. Partial cavity flows can also be used to reduce the Friction Drag on the nominally two-dimensional portions of a horizontal surface, and the basic flow features of two-dimensional cavities are presented. Injection of gas can lead to the creation of a bubbly mixture near the flow surface that can significantly modify the flow within the turbulent boundary layer, and there have been significant advances in the understanding of the underlying physical process of Drag reduction. Moreover, with sufficient gas flux, the bubbles flowing beneath a solid surface can coalesce to form a thin Drag-reducing air layer. The current applications of these techniques to underwater vehicles and surface ships are discussed.
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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.
Hyunwook Park - One of the best experts on this subject based on the ideXlab platform.
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the effects of superhydrophobic surfaces on skin Friction Drag
2016Co-Authors: Hyunwook ParkAbstract:Direct numerical simulation of a turbulent boundary layer developing over a superhydrophobic surface (SHS) was performed in order to investigate the effects of SHS on skin Friction Drag. Significant modifications of near-wall turbulence structures were observed, which resulted in large skin Friction Drag reduction. For the considered Reynolds number ranges and SHS geometries, it was found that the effective slip length normalized by viscous wall units was a key parameter. It was shown that the effective slip length should be on the order of the buffer layer in order to have the maximum benefit of Drag reduction. It was also found that the width of SHS, relative to the spanwise width of near-wall turbulence structures, was also a key parameter to the total amount of Drag reduction. Similarities and differences between the present turbulent boundary layer over SHS and our earlier work of turbulent channel flows with SHS are also discussed.
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a numerical study of the effects of superhydrophobic surfaces on skin Friction Drag reduction in wall bounded shear flows
2015Co-Authors: Hyunwook ParkAbstract:Recent developments of superhydrophobic surfaces (SHSs) have attracted much attention because of the possibility of achieving substantial skin-Friction Drag reduction at high Reynolds number turbulent flows. An SHS, consisting of a hydrophobic surface combined with micro- or nano-scaled topological features, can yield an effective slip length on the order of several hundred microns. In this numerical study, direct numerical simulations of turbulent channel flows and turbulent boundary layers (TBLs) developing over SHSs were performed. An SHS was modeled through the shear-free boundary condition, assuming the sustainable gas-liquid interface remained as a flat surface. For the considered Reynolds number ranges and SHS geometries, it was found that the effective slip length normalized by viscous wall units was the key parameter and the effective slip length should be on the order of the buffer layer in order to have the maximum benefit of Drag reduction. The effective surface slip length can be interpreted as a depth of influence into which SHSs affect the flow in the wall-normal direction. This result demonstrates that an SHS achieves its Drag reduction by affecting the turbulence structures within the buffer layer of wall-bounded turbulent flow. It was also found that the width of an SHS, relative to the spanwise width of near-wall turbulence structures, was also a key parameter to the total amount of Drag reduction. Significant suppression of near-wall turbulence structures were observed, which resulted in large skin-Friction Drag reduction due to the lack of the shear over SHSs. A comparison between TBLs and turbulent channel flows over SHSs were also examined. In contrast to fully developed turbulent channel flows, the effective slip velocity and hence the effective slip length varied in the streamwise direction of TBL, implying that total Drag reduction of TBL would depend on the streamwise length of a given SHS. The present numerical study was compared with recent experimental results and showed good agreement. In addition to flow and SHS geometry conditions, the streamwise length of SHSs was also a key factor to understand the underlying physics of wall-bounded shear flows. Finally, it was found that the amount of Drag reduction was theoretically estimated as a function of the effective slip length normalized by viscous wall units.
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A numerical study of the effects of superhydrophobic surface on skin-Friction Drag in turbulent channel flow
Physics of Fluids, 2013Co-Authors: Hyunwook Park, Hyungmin ParkAbstract:Superhydrophobic surfaces have attracted much attention lately as they present the possibility of achieving a substantial skin-Friction Drag reduction in turbulent flows. In this paper, the effects of a superhydrophobic surface, consisting of microgrates aligned in the flow direction, on skin-Friction Drag in turbulent flows were investigated through direct numerical simulation of turbulent channel flows. The superhydrophobic surface was modeled through a shear-free boundary condition on the air-water interface. Dependence of the effective slip length and resulting skin-Friction Drag on Reynolds number and surface geometry was examined. In laminar flows, the effective slip length depended on surface geometry only, independent of Reynolds number, consistent with an existing analysis. In turbulent flows, the effective slip length was a function of Reynolds number, indicating its dependence on flow conditions near the surface. The resulting Drag reduction was much larger in turbulent flows than in laminar fl...
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a numerical study of the effects of superhydrophobic surface on skin Friction Drag in turbulent channel flow
Physics of Fluids, 2013Co-Authors: Hyunwook Park, Hyungmin ParkAbstract:Superhydrophobic surfaces have attracted much attention lately as they present the possibility of achieving a substantial skin-Friction Drag reduction in turbulent flows. In this paper, the effects of a superhydrophobic surface, consisting of microgrates aligned in the flow direction, on skin-Friction Drag in turbulent flows were investigated through direct numerical simulation of turbulent channel flows. The superhydrophobic surface was modeled through a shear-free boundary condition on the air-water interface. Dependence of the effective slip length and resulting skin-Friction Drag on Reynolds number and surface geometry was examined. In laminar flows, the effective slip length depended on surface geometry only, independent of Reynolds number, consistent with an existing analysis. In turbulent flows, the effective slip length was a function of Reynolds number, indicating its dependence on flow conditions near the surface. The resulting Drag reduction was much larger in turbulent flows than in laminar flows, and near-wall turbulence structures were significantly modified, suggesting that indirect effects resulting from modified turbulence structures played a more significant role in reducing Drag in turbulent flows than the direct effect of the slip, which led to a modest Drag reduction in laminar flows. It was found that the Drag reduction in turbulent flows was well correlated with the effective slip length normalized by viscous wall units.
Hyungmin Park - One of the best experts on this subject based on the ideXlab platform.
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A numerical study of the effects of superhydrophobic surface on skin-Friction Drag in turbulent channel flow
Physics of Fluids, 2013Co-Authors: Hyunwook Park, Hyungmin ParkAbstract:Superhydrophobic surfaces have attracted much attention lately as they present the possibility of achieving a substantial skin-Friction Drag reduction in turbulent flows. In this paper, the effects of a superhydrophobic surface, consisting of microgrates aligned in the flow direction, on skin-Friction Drag in turbulent flows were investigated through direct numerical simulation of turbulent channel flows. The superhydrophobic surface was modeled through a shear-free boundary condition on the air-water interface. Dependence of the effective slip length and resulting skin-Friction Drag on Reynolds number and surface geometry was examined. In laminar flows, the effective slip length depended on surface geometry only, independent of Reynolds number, consistent with an existing analysis. In turbulent flows, the effective slip length was a function of Reynolds number, indicating its dependence on flow conditions near the surface. The resulting Drag reduction was much larger in turbulent flows than in laminar fl...
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a numerical study of the effects of superhydrophobic surface on skin Friction Drag in turbulent channel flow
Physics of Fluids, 2013Co-Authors: Hyunwook Park, Hyungmin ParkAbstract:Superhydrophobic surfaces have attracted much attention lately as they present the possibility of achieving a substantial skin-Friction Drag reduction in turbulent flows. In this paper, the effects of a superhydrophobic surface, consisting of microgrates aligned in the flow direction, on skin-Friction Drag in turbulent flows were investigated through direct numerical simulation of turbulent channel flows. The superhydrophobic surface was modeled through a shear-free boundary condition on the air-water interface. Dependence of the effective slip length and resulting skin-Friction Drag on Reynolds number and surface geometry was examined. In laminar flows, the effective slip length depended on surface geometry only, independent of Reynolds number, consistent with an existing analysis. In turbulent flows, the effective slip length was a function of Reynolds number, indicating its dependence on flow conditions near the surface. The resulting Drag reduction was much larger in turbulent flows than in laminar flows, and near-wall turbulence structures were significantly modified, suggesting that indirect effects resulting from modified turbulence structures played a more significant role in reducing Drag in turbulent flows than the direct effect of the slip, which led to a modest Drag reduction in laminar flows. It was found that the Drag reduction in turbulent flows was well correlated with the effective slip length normalized by viscous wall units.
Maurizio Quadrio - One of the best experts on this subject based on the ideXlab platform.
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reynolds number dependence of turbulent skin Friction Drag reduction induced by spanwise forcing
Journal of Fluid Mechanics, 2016Co-Authors: Davide Gatti, Maurizio QuadrioAbstract:This paper examines how increasing the value of the Reynolds number $Re$ affects the ability of spanwise-forcing techniques to yield turbulent skin-Friction Drag reduction. The considered forcing is based on the streamwise-travelling waves of spanwise-wall velocity (Quadrio et al. , J. Fluid Mech. , vol. 627, 2009, pp. 161–178). The study builds upon an extensive Drag-reduction database created via direct numerical simulation of a turbulent channel flow for two fivefold separated values of $Re$ , namely $Re_{\unicode[STIX]{x1D70F}}=200$ and $Re_{\unicode[STIX]{x1D70F}}=1000$ . The sheer size of the database, which for the first time systematically addresses the amplitude of the forcing, allows a comprehensive view of the Drag-reducing characteristics of the travelling waves, and enables a detailed description of the changes occurring when $Re$ increases. The effect of using a viscous scaling based on the Friction velocity of either the non-controlled flow or the Drag-reduced flow is described. In analogy with other wall-based Drag-reduction techniques, like riblets for example, the performance of the travelling waves is well described by a vertical shift of the logarithmic portion of the mean streamwise velocity profile. Except when $Re$ is very low, this shift remains constant with $Re$ , at odds with the percentage reduction of the Friction coefficient, which is known to present a mild, logarithmic decline. Our new data agree with the available literature, which is however mostly based on low- $Re$ information and hence predicts a quick drop of maximum Drag reduction with $Re$ . The present study supports a more optimistic scenario, where for an airplane at flight Reynolds numbers a Drag reduction of nearly 30 % would still be possible thanks to the travelling waves.
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reynolds dependence of turbulent skin Friction Drag reduction induced by spanwise forcing
arXiv: Fluid Dynamics, 2015Co-Authors: Davide Gatti, Maurizio QuadrioAbstract:This paper examines how increasing the value of the Reynolds number $Re$ affects the ability of spanwise-forcing techniques to yield turbulent skin-Friction Drag reduction. The considered forcing is based on the streamwise-travelling waves of spanwise wall velocity (Quadrio {\em et al. J. Fluid Mech.}, vol. 627, 2009, pp. 161--178). The study builds upon an extensive Drag-reduction database created with Direct Numerical Simulation of a turbulent channel flow for two, 5-fold separated values of $Re$, namely $Re_\tau=200$ and $Re_\tau=1000$. The sheer size of the database, which for the first time systematically addresses the amplitude of the forcing, allows a comprehensive view of the Drag-reducing characteristics of the travelling waves, and enables a detailed description of the changes occurring when $Re$ increases. The effect of using a viscous scaling based on the Friction velocity of either the non-controlled flow or the Drag-reduced flow is described. In analogy with other wall-based Drag reduction techniques, like for example riblets, the performance of the travelling waves is well described by a vertical shift of the logarithmic portion of the mean streamwise velocity profile. Except when $Re$ is very low, this shift remains constant with $Re$, at odds with the percentage reduction of the Friction coefficient, which is known to present a mild, logarithmic decline. Our new data agree with the available literature, which is however mostly based on low-$Re$ information and hence predicts a quick drop of maximum Drag reduction with $Re$. The present study supports a more optimistic scenario, where for an airplane at flight Reynolds numbers a Drag reduction of nearly 30\% would still be possible thanks to the travelling waves.
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turbulent skin Friction Drag reduction by spanwise wall oscillation with generic temporal waveform
Eighth International Symposium on Turbulence and Shear Flow Phenomena, 2013Co-Authors: Andrea Cimarelli, Yutaka Hasegawa, Elisabetta De Angelis, Bettina Frohnapfel, Maurizio QuadrioAbstract:To generalize the well-known spanwise-oscillatingwall technique for Drag reduction, non-sinusoidal oscillations of a solid wall are considered as a means to alter the skin-Friction Drag in a turbulent channel flow. A series of Direct Numerical Simulations is conducted to evaluate the control performance of nine different waveforms, in addition to the usual sinusoid, systematically changing the maximum wave amplitude and the period for each waveform. The turbulent average spanwise motion is found to coincide with the laminar Stokes solution that can be constructed, for the generic waveform, through harmonic superposition. A newly defined penetration depth of the Stokes layer is then used to build a simple tool that allows predicting turbulent Drag reduction and net energy saving rate for any waveform. Among all the cases considered, the sinusoid at optimal amplitude and period is found to yield the maximum net energy saving rate. However, when the wave amplitude and period deviate from the optimal values, other waves are found to perform better than the sinusoid. This is potentially interesting in view of applications, where a particular actuator limitations might preclude reaching the optimal operating conditions for the sinusoidal wall oscillation. It is demonstrated that the present model can predict the locally optimal waveform for given wave amplitude and period, as well as the globally optimal sinusoidal wave. INTRODUCTION The efficient use of energy in systems where a relative motion between a solid wall and a fluid takes place is perhaps the most important driving factor that supports the current research effort into aerodynamic Drag reduction. We consider here skin-Friction turbulent Drag. Existing open-loop techniques provide higher Drag reduction than passive methods while being less complex than feedbackcontrol methods. In particular, open-loop techniques that rely on the spanwise forcing of the near-wall turbulent flow have been shown to yield large Drag reduction and interestingly positive energy budgets in numerical simulations (Quadrio, 2011), and first laboratory experiments have already been carried out (Auteri et al., 2010; Gouder, 2011; Choi et al., 2011). The present paper deals with the simplest and well-known spanwise oscillating-wall technique. Most existing open-loop control strategies assume a sinusoidal waveform as control input. On the other hand, when trying to verify these control strategies in experiments, various constraints are placed on the properties of a control input by the used actuators. Hence, it is of key importance to identify the optimal waveform to achieve best control performance: this is the aim of the present paper. As a starting point, we select a set of waveforms and comparatively study, via several numerical experiments, how the Drag-reduction and energetic performances of the oscillating wall depend on the waveform as well as on the oscillation amplitude and period. Guided by our numerical experiments, we then aim at obtaining results of more general validity, so that a predictive tool for the control performance of non-sinusoidal wall oscillations can eventually be developed. In this process, we take advantage of the laminar solution that exists for the spanwise flow alone (the Stokes oscillating boundary layer), by extending it to a generic (periodic) temporal waveform.
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what happens to turbulent skin Friction Drag reduction at high re
arXiv: Fluid Dynamics, 2012Co-Authors: Davide Gatti, Maurizio QuadrioAbstract:We address one of the capital problems in the field of turbulent skin-Friction Drag reduction, i.e. the performance of the known techniques at high values of the Reynolds number $Re$. We limit ourselves to considering open-loop techniques based on spanwise forcing (oscillating wall, streamwise-travelling waves), and explore via Direct Numerical Simulations (DNS) how quickly the Drag reduction and net energy savings decrease when the Friction Reynolds number is increased. We suggest an unexpected and interesting scenario where the performance of the Drag-reduction technique degrade with $Re$ with a markedly different rate depending on the parameters. In particular, the known optimal region turns out to be such at low-$Re$ only, since there Drag reduction degrades quite fast with $Re$, in line with available results. However, other regions are much less sensitive to $Re$, or insensitive at all. If one considers that the energy required to create the forcing presents a slightly favorable trend with $Re$, the possibility exists of net energy saving at very high Reynolds numbers. This interesting scenario remains speculative in nature, owing to the spatial truncation implied by the limited domain size. However, a few full-scale DNS for the traveling waves at $Re_\tau=400$ have been carried out, and their results fully support the suggested scenario, which, though appealing, would force us to rethink our current understanding of how these Drag reduction techniques work and behave at high $Re$.
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What happens to turbulent skin-Friction Drag reduction at high $Re$?
arXiv: Fluid Dynamics, 2012Co-Authors: Davide Gatti, Maurizio QuadrioAbstract:We address one of the capital problems in the field of turbulent skin-Friction Drag reduction, i.e. the performance of the known techniques at high values of the Reynolds number $Re$. We limit ourselves to considering open-loop techniques based on spanwise forcing (oscillating wall, streamwise-travelling waves), and explore via Direct Numerical Simulations (DNS) how quickly the Drag reduction and net energy savings decrease when the Friction Reynolds number is increased. We suggest an unexpected and interesting scenario where the performance of the Drag-reduction technique degrade with $Re$ with a markedly different rate depending on the parameters. In particular, the known optimal region turns out to be such at low-$Re$ only, since there Drag reduction degrades quite fast with $Re$, in line with available results. However, other regions are much less sensitive to $Re$, or insensitive at all. If one considers that the energy required to create the forcing presents a slightly favorable trend with $Re$, the possibility exists of net energy saving at very high Reynolds numbers. This interesting scenario remains speculative in nature, owing to the spatial truncation implied by the limited domain size. However, a few full-scale DNS for the traveling waves at $Re_\tau=400$ have been carried out, and their results fully support the suggested scenario, which, though appealing, would force us to rethink our current understanding of how these Drag reduction techniques work and behave at high $Re$.
