The Experts below are selected from a list of 309 Experts worldwide ranked by ideXlab platform
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, Luc Stephane Leblanc, Ambarish Jayant Kulkarni, Vaibhav Bahadur, 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, Luc Stephane Leblanc, Ambarish Jayant Kulkarni, Vaibhav Bahadur, 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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bubble induced Skin Friction Drag reduction and the abrupt transition to air layer Drag reduction
Journal of Fluid Mechanics, 2008Co-Authors: Brian R Elbing, Steven L. Ceccio, David R. Dowling, Eric S Winkel, 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.
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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, David R. Dowling, Marc Perlin, Eric S Winkel, 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 (Re x ) of several million, but potential applications may occur at Re x orders of magnitude higher. This paper presents results for Re x as high as 210 million from Skin-Friction Drag-reduction experiments conducted in the USA Navy's William B. Morgan Large Cavitation Channel (LCC). Here, a near-zero-pressure-gradient flat-plate turbulent boundary layer was generated on a 12.9m long hydraulically smooth flat plate that spanned the 3m wide test section. The test surface faced downward and air was injected at volumetric rates as high as 0.38 m 3 s -1 through one of two flush-mounted 40 μm sintered-metal strips that nearly spanned the test model at upstream and downstream locations. Spatially and temporally averaged shear stress and bubble-image-based measurements are reported here for nominal test speeds of 6, 12 and 18 m s -1 , The mean bubble diameter was ∼300μm. 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.
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, Luc Stephane Leblanc, Ambarish Jayant Kulkarni, Vaibhav Bahadur, 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, Luc Stephane Leblanc, Ambarish Jayant Kulkarni, Vaibhav Bahadur, 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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bubble induced Skin Friction Drag reduction and the abrupt transition to air layer Drag reduction
Journal of Fluid Mechanics, 2008Co-Authors: Brian R Elbing, Steven L. Ceccio, David R. Dowling, Eric S Winkel, 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.
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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, David R. Dowling, Marc Perlin, Eric S Winkel, 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 (Re x ) of several million, but potential applications may occur at Re x orders of magnitude higher. This paper presents results for Re x as high as 210 million from Skin-Friction Drag-reduction experiments conducted in the USA Navy's William B. Morgan Large Cavitation Channel (LCC). Here, a near-zero-pressure-gradient flat-plate turbulent boundary layer was generated on a 12.9m long hydraulically smooth flat plate that spanned the 3m wide test section. The test surface faced downward and air was injected at volumetric rates as high as 0.38 m 3 s -1 through one of two flush-mounted 40 μm sintered-metal strips that nearly spanned the test model at upstream and downstream locations. Spatially and temporally averaged shear stress and bubble-image-based measurements are reported here for nominal test speeds of 6, 12 and 18 m s -1 , The mean bubble diameter was ∼300μm. 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.
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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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, Bettina Frohnapfel, Yutaka Hasegawa, Elisabetta De Angelis, 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 $Re$.
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Skin Friction Drag reduction via steady streamwise oscillations of spanwise velocity
11th EUROMECH European Turbulence Conference (ETC11), 2007Co-Authors: Maurizio Quadrio, Claudio Viotti, Paolo LuchiniAbstract:Reducing the Skin-Friction Drag in turbulent wall flows has seen a growing interest in recent years, owing to potential energetic and environmental advantages. Passive techniques (like riblets) are not yet in widespread use, notwithstanding their applicative appeal; most of the strategies currently under investigation are active techniques. One of the simplest and most interesting amongst active approaches is the oscillating-wall technique [1], where the wall moves according to:
Changhwan Choi - One of the best experts on this subject based on the ideXlab platform.
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experimental study of Skin Friction Drag reduction on superhydrophobic flat plates in high reynolds number boundary layer flow
Physics of Fluids, 2013Co-Authors: Elias Aljallis, Mohammad Amin Sarshar, Raju Datla, Vinod K Sikka, Andrew K Jones, Changhwan ChoiAbstract:In this paper, we report the measurement of Skin Friction Drag on superhydrophobic-coated flat plates in high Reynolds (Re) number boundary layer flows, using a high-speed towing tank system. Aluminum flat plates with a large area (4 feet × 2 feet, 3/8 in. thick) and sharpened leading/trailing edges (1 in. long) were prepared as a boundary layer flow model. Spray coating of hydrophobic nanoparticles was applied to make two different types of superhydrophobic coatings: one with low contact angle and high contact angle hysteresis, and the other with high contact angle and low contact angle hysteresis. Skin Friction Drag of the superhydrophobic plates was measured in the flow speed up to 30 ft/s to cover transition and turbulent flow regimes (105 < ReL < 107), and was compared to that of an uncoated bare aluminum plate. A significant Drag reduction was observed on the superhydrophobic plate with high contact angle and low contact angle hysteresis up to ∼30% in transition regime (105 < ReL < 106), which is at...
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experimental study of Skin Friction Drag reduction on superhydrophobic flat plates in high reynolds number boundary layer flow
Physics of Fluids, 2013Co-Authors: Elias Aljallis, Mohammad Amin Sarshar, Raju Datla, Vinod K Sikka, Andrew K Jones, Changhwan ChoiAbstract:In this paper, we report the measurement of Skin Friction Drag on superhydrophobic-coated flat plates in high Reynolds (Re) number boundary layer flows, using a high-speed towing tank system. Aluminum flat plates with a large area (4 feet × 2 feet, 3/8 in. thick) and sharpened leading/trailing edges (1 in. long) were prepared as a boundary layer flow model. Spray coating of hydrophobic nanoparticles was applied to make two different types of superhydrophobic coatings: one with low contact angle and high contact angle hysteresis, and the other with high contact angle and low contact angle hysteresis. Skin Friction Drag of the superhydrophobic plates was measured in the flow speed up to 30 ft/s to cover transition and turbulent flow regimes (105 < ReL < 107), and was compared to that of an uncoated bare aluminum plate. A significant Drag reduction was observed on the superhydrophobic plate with high contact angle and low contact angle hysteresis up to ∼30% in transition regime (105 < ReL < 106), which is attributed to the shear-reducing air layer entrapped on the superhydrophobic surface. However, in fully turbulence regime (106 < ReL < 107), an increase of Drag was observed, which is ascribed to the morphology of the surface air layer and its depletion by high shear flow. The texture of superhydrophobic coatings led to form a rugged morphology of the entrapped air layer, which would behave like microscale roughness to the liquid flow and offset the Drag-reducing effects in the turbulent flow. Moreover, when the superhydrophobic coating became wet due to the removal of air by high shear at the boundary, it would amplify the surface roughness of solid wall and increase the Drag in the turbulent flow. The results illustrate that Drag reduction is not solely dependent on the superhydrophobicity of a surface (e.g., contact angle and air fraction), but the morphology and stability of the surface air layer are also critical for the effective Drag reduction using superhydrophobic surfaces, especially in high Re number turbulent flow regimes.
Davide Gatti - One of the best experts on this subject based on the ideXlab platform.
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Riblets in fully developed turbulent channel flow
2020Co-Authors: Lars Hendrik Von Deyn, Davide Gatti, Bettina FrohnapfelAbstract:This database features high-precision pressure measurements in a wind tunnel facility used to determine the Skin Friction Drag reduction of ribelts. Please refer to the README below for further information.
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Turbulent Duct Flow Controlled with Spanwise Wall Oscillations
Flow Turbulence and Combustion, 2017Co-Authors: Steffen Straub, Philipp Schlatter, Bettina Frohnapfel, Ricardo Vinuesa, Davide GattiAbstract:The spanwise oscillation of channel walls is known to substantially reduce the Skin-Friction Drag in turbulent channel flows. In order to understand the limitations of this flow control approach wh ...
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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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turbulent Skin Friction Drag reduction at high reynolds numbers
IEEE International Conference on High Performance Computing Data and Analytics, 2016Co-Authors: Davide GattiAbstract:Direct Numerical Simulation (DNS) of turbulent channel flows at moderately high values of the Reynolds number (Re) are performed to examine how Re affects the capabilities of wall-based spanwise-forcing techniques to achieve turbulent Skin-Friction Drag reduction. With the present new data, a relationship could be derived and validated, which predicts the amount of Drag reduction at several values of Re. The present study shows that a Drag reduction of nearly 30 % would still be possible for an airplane at flight Reynolds numbers thanks to the spanwise forcing.
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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 $Re$.