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Hui Meng - One of the best experts on this subject based on the ideXlab platform.
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Particle radial distribution function and relative velocity measurement in turbulence at small Particle Pair separations
Journal of Fluid Mechanics, 2021Co-Authors: Adam Hammond, Hui MengAbstract:Particle collisions in turbulent flow are critical to Particle agglomeration and droplet coalescence. The collision kernel can be evaluated by radial distribution function (RDF) and radial relative velocity (RV) between Particles at small separations can be attributed to Particle–Particle interactions including hydrodynamic interactions, which are not well-understood. Uncertainty analysis substantiates the observed trends. This first-ever simultaneous RDF and RV measurement at small separations provides a clear glimpse into the clustering and relative velocities of Particles in turbulence near-contact.
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Particle radial distribution function and relative velocity measurement in turbulence at small Particle Pair separations
arXiv: Fluid Dynamics, 2021Co-Authors: Adam Hammond, Hui MengAbstract:The collision rate of Particles suspended in turbulent flow is critical to Particle agglomeration and droplet coalescence. The collision kernel can be evaluated by the radial distribution function (RDF) and radial relative velocity (RV) between Particles at small separations $r$. Previously, the smallest $r$ was limited to roughly the Kolmogorov length $\eta$ due to Particle position uncertainty and image overlap. We report a new approach to measure RDF and RV near contact ($r/a\: \approx$ 2.07, $a$ Particle radius) overcoming these limitations. Three-dimensional Particle tracking velocimetry using four-pulse Shake-the-Box algorithm recorded short Particle tracks with the interpolated midpoints registered as Particle positions to avoid image overlap. This strategy further allows removal of mismatched tracks using their characteristic false RV. We measured RDF and RV in a one-meter-diameter isotropic turbulence chamber with Taylor Reynolds number $Re_\lambda=324$ with Particles of 12-16 $\mu$m radius and Stokes number $\approx$ 0.7. While at large $r$ the measured RV agrees with the literature, when $r \eta$, RDF scales as $r^{-0.39}$ reflecting RDF scaling for polydisperse Particles in the literature , but when $r\lessapprox\eta$ RDF scales as $r^{-6}$, yielding 1000 times higher near-contact RDF than simulations. Such extreme clustering and relative velocity enhancement can be attributed to Particle-Particle interactions. Uncertainty analysis substantiates the observed trends. This first-ever simultaneous RDF and RV measurement at small separations provides a clear glimpse into the clustering and relative velocities of Particles in turbulence near-contact.
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effects of reynolds number and stokes number on Particle Pair relative velocity in isotropic turbulence a systematic experimental study
Journal of Fluid Mechanics, 2018Co-Authors: Zhongwang Dou, Lance R. Collins, Andrew D. Bragg, Adam Hammond, Zach Liang, Hui MengAbstract:The effects of Reynolds number ( ) and Stokes number ( ) on Particle-Pair relative velocity (RV) are investigated systematically using a recently developed planar four-frame Particle tracking technique in a novel homogeneous and isotropic turbulence chamber. We compare the measured results with direct numerical simulation (DNS), verifying whether the conclusions of the DNS for simplified conditions and limited are still valid in reality. Two experiments are performed: varying between 246 and 357 at six values, and varying between 0.02 and 4.63 at five values. The measured mean inward Particle-Pair RV as a function of separation distance is compared with the DNS under closely matched conditions. At all experimental conditions, an excellent agreement is achieved, except when the Particle separation distance ( is the Kolmogorov length scale), where the experimental is consistently higher, possibly due to Particle polydispersity and finite laser thickness in the experiments (Dou et al., arXiv:1712.07506, 2017). At any fixed is essentially independent of , echoing the DNS finding of Ireland et al. (J. Fluid Mech., vol. 796, 2016, pp. 617–658). At any fixed , increases with at small , showing dominance of the path-history effect in the dissipation range when , but decreases with at large , indicating dominance of inertial filtering. We further compare the and RV variance from experiments with DNS and theoretical predictions by Pan & Padoan (J. Fluid Mech., vol. 661, 2010, pp. 73–107). For , experimental and match these values well at , but they are higher than both DNS and theory at . For , from all three match well, except for , for which experimental values are higher, while from experiment and DNS are much higher than theoretical predictions. We discuss potential causes of these discrepancies. What this study shows is the first experimental validation of and effect on inertial Particle-Pair in homogeneous and isotropic turbulence.
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Effects of Reynolds number and Stokes number on Particle-Pair relative velocity in isotropic turbulence: a systematic experimental study
Journal of Fluid Mechanics, 2018Co-Authors: Zhongwang Dou, Lance R. Collins, Andrew D. Bragg, Adam Hammond, Zach Liang, Hui MengAbstract:The effects of Reynolds number and Stokes number on Particle-Pair relative velocity (RV) were investigated systematically using a recently developed planar four-frame Particle tracking technique in a novel homogeneous and isotropic turbulence chamber.
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Particle Pair relative velocity measurement in high reynolds number homogeneous and isotropic turbulence using 4 frame Particle tracking velocimetry
Experiments in Fluids, 2018Co-Authors: Zhongwang Dou, Lance R. Collins, Andrew D. Bragg, Zach Liang, Peter J Ireland, Hui MengAbstract:The radial relative velocity (RV) between Particles suspended in turbulent flow plays a critical role in droplet collision and growth. We present a simple and accurate approach to RV measurement in isotropic turbulence—planar 4-frame Particle tracking velocimetry—using routine PIV hardware. It improves Particle positioning and Pairing accuracy over the 2-frame holographic approach by de Jong et al. (Int J Multiphas Flow 36:324–332; de Jong et al., Int J Multiphas Flow 36:324–332, 2010) without using high-speed cameras and lasers as in Saw et al. (Phys Fluids 26:111702, 2014). Homogeneous and isotropic turbulent flow ( $${R_\lambda }=357$$ ) in a new, fan-driven, truncated iscosahedron chamber was laden with either low-Stokes (mean $$St=0.09$$ , standard deviation 0.05) or high-Stokes aerosols (mean $$St=3.46$$ , standard deviation 0.57). For comparison, DNS was conducted under similar conditions ( $${R_\lambda }=398$$ ; $$St=0.10$$ and 3.00, respectively). Experimental RV probability density functions (PDF) and mean inward RV agree well with DNS. Mean inward RV increases with $$St$$ at small Particle separations, $$r$$ , and decreases with $$St$$ at large $$r$$ , indicating the dominance of “path-history” and “inertial filtering” effects, respectively. However, at small $$r$$ , the experimental mean inward RV trends higher than DNS, possibly due to the slight polydispersity of Particles and finite light sheet thickness in experiments. To confirm this interpretation, we performed numerical experiments and found that Particle polydispersity increases mean inward RV at small $$r$$ , while finite laser thickness also overestimates mean inward RV at small $$r$$ , This study demonstrates the feasibility of accurately measuring RV using routine hardware, and verifies, for the first time, the path-history and inertial filtering effects on Particle-Pair RV at large Particle separations experimentally.
Lance R. Collins - One of the best experts on this subject based on the ideXlab platform.
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effects of reynolds number and stokes number on Particle Pair relative velocity in isotropic turbulence a systematic experimental study
Journal of Fluid Mechanics, 2018Co-Authors: Zhongwang Dou, Lance R. Collins, Andrew D. Bragg, Adam Hammond, Zach Liang, Hui MengAbstract:The effects of Reynolds number ( ) and Stokes number ( ) on Particle-Pair relative velocity (RV) are investigated systematically using a recently developed planar four-frame Particle tracking technique in a novel homogeneous and isotropic turbulence chamber. We compare the measured results with direct numerical simulation (DNS), verifying whether the conclusions of the DNS for simplified conditions and limited are still valid in reality. Two experiments are performed: varying between 246 and 357 at six values, and varying between 0.02 and 4.63 at five values. The measured mean inward Particle-Pair RV as a function of separation distance is compared with the DNS under closely matched conditions. At all experimental conditions, an excellent agreement is achieved, except when the Particle separation distance ( is the Kolmogorov length scale), where the experimental is consistently higher, possibly due to Particle polydispersity and finite laser thickness in the experiments (Dou et al., arXiv:1712.07506, 2017). At any fixed is essentially independent of , echoing the DNS finding of Ireland et al. (J. Fluid Mech., vol. 796, 2016, pp. 617–658). At any fixed , increases with at small , showing dominance of the path-history effect in the dissipation range when , but decreases with at large , indicating dominance of inertial filtering. We further compare the and RV variance from experiments with DNS and theoretical predictions by Pan & Padoan (J. Fluid Mech., vol. 661, 2010, pp. 73–107). For , experimental and match these values well at , but they are higher than both DNS and theory at . For , from all three match well, except for , for which experimental values are higher, while from experiment and DNS are much higher than theoretical predictions. We discuss potential causes of these discrepancies. What this study shows is the first experimental validation of and effect on inertial Particle-Pair in homogeneous and isotropic turbulence.
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Effects of Reynolds number and Stokes number on Particle-Pair relative velocity in isotropic turbulence: a systematic experimental study
Journal of Fluid Mechanics, 2018Co-Authors: Zhongwang Dou, Lance R. Collins, Andrew D. Bragg, Adam Hammond, Zach Liang, Hui MengAbstract:The effects of Reynolds number and Stokes number on Particle-Pair relative velocity (RV) were investigated systematically using a recently developed planar four-frame Particle tracking technique in a novel homogeneous and isotropic turbulence chamber.
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Particle Pair relative velocity measurement in high reynolds number homogeneous and isotropic turbulence using 4 frame Particle tracking velocimetry
Experiments in Fluids, 2018Co-Authors: Zhongwang Dou, Lance R. Collins, Andrew D. Bragg, Zach Liang, Peter J Ireland, Hui MengAbstract:The radial relative velocity (RV) between Particles suspended in turbulent flow plays a critical role in droplet collision and growth. We present a simple and accurate approach to RV measurement in isotropic turbulence—planar 4-frame Particle tracking velocimetry—using routine PIV hardware. It improves Particle positioning and Pairing accuracy over the 2-frame holographic approach by de Jong et al. (Int J Multiphas Flow 36:324–332; de Jong et al., Int J Multiphas Flow 36:324–332, 2010) without using high-speed cameras and lasers as in Saw et al. (Phys Fluids 26:111702, 2014). Homogeneous and isotropic turbulent flow ( $${R_\lambda }=357$$ ) in a new, fan-driven, truncated iscosahedron chamber was laden with either low-Stokes (mean $$St=0.09$$ , standard deviation 0.05) or high-Stokes aerosols (mean $$St=3.46$$ , standard deviation 0.57). For comparison, DNS was conducted under similar conditions ( $${R_\lambda }=398$$ ; $$St=0.10$$ and 3.00, respectively). Experimental RV probability density functions (PDF) and mean inward RV agree well with DNS. Mean inward RV increases with $$St$$ at small Particle separations, $$r$$ , and decreases with $$St$$ at large $$r$$ , indicating the dominance of “path-history” and “inertial filtering” effects, respectively. However, at small $$r$$ , the experimental mean inward RV trends higher than DNS, possibly due to the slight polydispersity of Particles and finite light sheet thickness in experiments. To confirm this interpretation, we performed numerical experiments and found that Particle polydispersity increases mean inward RV at small $$r$$ , while finite laser thickness also overestimates mean inward RV at small $$r$$ , This study demonstrates the feasibility of accurately measuring RV using routine hardware, and verifies, for the first time, the path-history and inertial filtering effects on Particle-Pair RV at large Particle separations experimentally.
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Particle Pair relative velocity measurement in high reynolds number homogeneous and isotropic turbulence using 4 frame Particle tracking velocimetry
arXiv: Fluid Dynamics, 2017Co-Authors: Zhongwang Dou, Lance R. Collins, Andrew D. Bragg, Zach Liang, Peter J Ireland, Hui MengAbstract:The radial relative velocity between Particles suspended in turbulent flow plays a critical role in droplet collision and growth. We present a simple and accurate approach to RV measurement in isotropic turbulence - planar 4-frame Particle tracking velocimetry - using routine PIV hardware. This study demonstrates the feasibility of accurately measuring RV using routine hardware and verifies, for the first time, the path-history and inertial filtering effects on Particle-Pair RV at large Particle separations experimentally.
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the effect of reynolds number on inertial Particle dynamics in isotropic turbulence part 1 simulations without gravitational effects
Journal of Fluid Mechanics, 2016Co-Authors: Peter J Ireland, Andrew D. Bragg, Lance R. CollinsAbstract:In this study, we analyse the statistics of both individual inertial Particles and inertial Particle Pairs in direct numerical simulations of homogeneous isotropic turbulence in the absence of gravity. The effect of the Taylor microscale Reynolds number, , on the Particle statistics is examined over the largest range to date (from to 597), at small, intermediate and large Kolmogorov-scale Stokes numbers . We first explore the effect of preferential sampling on the single-Particle statistics and find that low- inertial Particles are ejected from both vortex tubes and vortex sheets (the latter becoming increasingly prevalent at higher Reynolds numbers) and preferentially accumulate in regions of irrotational dissipation. We use this understanding of preferential sampling to provide a physical explanation for many of the trends in the Particle velocity gradients, kinetic energies and accelerations at low , which are well represented by the model of Chun et al. (J. Fluid Mech., vol. 536, 2005, pp. 219–251). As increases, inertial filtering effects become more important, causing the Particle kinetic energies and accelerations to decrease. The effect of inertial filtering on the Particle kinetic energies and accelerations diminishes with increasing Reynolds number and is well captured by the models of Abrahamson (Chem. Engng Sci., vol. 30, 1975, pp. 1371–1379) and Zaichik & Alipchenkov (Intl J. Multiphase Flow, vol. 34 (9), 2008, pp. 865–868), respectively. We then consider Particle-Pair statistics, and focus our attention on the relative velocities and radial distribution functions (RDFs) of the Particles, with the aim of understanding the underlying physical mechanisms contributing to Particle collisions. The relative velocity statistics indicate that preferential sampling effects are important for and that path-history/non-local effects become increasingly important for . While higher-order relative velocity statistics are influenced by the increased intermittency of the turbulence at high Reynolds numbers, the lower-order relative velocity statistics are only weakly sensitive to changes in Reynolds number at low . The Reynolds-number trends in these quantities at intermediate and large are explained based on the influence of the available flow scales on the path-history and inertial filtering effects. We find that the RDFs peak near of order unity, that they exhibit power-law scaling for low and intermediate and that they are largely independent of Reynolds number for low and intermediate . We use the model of Zaichik & Alipchenkov (New J. Phys., vol. 11, 2009, 103018) to explain the physical mechanisms responsible for these trends, and find that this model is able to capture the quantitative behaviour of the RDFs extremely well when direct numerical simulation data for the structure functions are specified, in agreement with Bragg & Collins (New J. Phys., vol. 16, 2014a, 055013). We also observe that at large , changes in the RDF are related to changes in the scaling exponents of the relative velocity variances. The Particle collision kernel closely matches that computed by Rosa et al. (New J. Phys., vol. 15, 2013, 045032) and is found to be largely insensitive to the flow Reynolds number. This suggests that relatively low-Reynolds-number simulations may be able to capture much of the relevant physics of droplet collisions and growth in the adiabatic cores of atmospheric clouds.
Andrew D. Bragg - One of the best experts on this subject based on the ideXlab platform.
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effects of reynolds number and stokes number on Particle Pair relative velocity in isotropic turbulence a systematic experimental study
Journal of Fluid Mechanics, 2018Co-Authors: Zhongwang Dou, Lance R. Collins, Andrew D. Bragg, Adam Hammond, Zach Liang, Hui MengAbstract:The effects of Reynolds number ( ) and Stokes number ( ) on Particle-Pair relative velocity (RV) are investigated systematically using a recently developed planar four-frame Particle tracking technique in a novel homogeneous and isotropic turbulence chamber. We compare the measured results with direct numerical simulation (DNS), verifying whether the conclusions of the DNS for simplified conditions and limited are still valid in reality. Two experiments are performed: varying between 246 and 357 at six values, and varying between 0.02 and 4.63 at five values. The measured mean inward Particle-Pair RV as a function of separation distance is compared with the DNS under closely matched conditions. At all experimental conditions, an excellent agreement is achieved, except when the Particle separation distance ( is the Kolmogorov length scale), where the experimental is consistently higher, possibly due to Particle polydispersity and finite laser thickness in the experiments (Dou et al., arXiv:1712.07506, 2017). At any fixed is essentially independent of , echoing the DNS finding of Ireland et al. (J. Fluid Mech., vol. 796, 2016, pp. 617–658). At any fixed , increases with at small , showing dominance of the path-history effect in the dissipation range when , but decreases with at large , indicating dominance of inertial filtering. We further compare the and RV variance from experiments with DNS and theoretical predictions by Pan & Padoan (J. Fluid Mech., vol. 661, 2010, pp. 73–107). For , experimental and match these values well at , but they are higher than both DNS and theory at . For , from all three match well, except for , for which experimental values are higher, while from experiment and DNS are much higher than theoretical predictions. We discuss potential causes of these discrepancies. What this study shows is the first experimental validation of and effect on inertial Particle-Pair in homogeneous and isotropic turbulence.
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Effects of Reynolds number and Stokes number on Particle-Pair relative velocity in isotropic turbulence: a systematic experimental study
Journal of Fluid Mechanics, 2018Co-Authors: Zhongwang Dou, Lance R. Collins, Andrew D. Bragg, Adam Hammond, Zach Liang, Hui MengAbstract:The effects of Reynolds number and Stokes number on Particle-Pair relative velocity (RV) were investigated systematically using a recently developed planar four-frame Particle tracking technique in a novel homogeneous and isotropic turbulence chamber.
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Particle Pair relative velocity measurement in high reynolds number homogeneous and isotropic turbulence using 4 frame Particle tracking velocimetry
Experiments in Fluids, 2018Co-Authors: Zhongwang Dou, Lance R. Collins, Andrew D. Bragg, Zach Liang, Peter J Ireland, Hui MengAbstract:The radial relative velocity (RV) between Particles suspended in turbulent flow plays a critical role in droplet collision and growth. We present a simple and accurate approach to RV measurement in isotropic turbulence—planar 4-frame Particle tracking velocimetry—using routine PIV hardware. It improves Particle positioning and Pairing accuracy over the 2-frame holographic approach by de Jong et al. (Int J Multiphas Flow 36:324–332; de Jong et al., Int J Multiphas Flow 36:324–332, 2010) without using high-speed cameras and lasers as in Saw et al. (Phys Fluids 26:111702, 2014). Homogeneous and isotropic turbulent flow ( $${R_\lambda }=357$$ ) in a new, fan-driven, truncated iscosahedron chamber was laden with either low-Stokes (mean $$St=0.09$$ , standard deviation 0.05) or high-Stokes aerosols (mean $$St=3.46$$ , standard deviation 0.57). For comparison, DNS was conducted under similar conditions ( $${R_\lambda }=398$$ ; $$St=0.10$$ and 3.00, respectively). Experimental RV probability density functions (PDF) and mean inward RV agree well with DNS. Mean inward RV increases with $$St$$ at small Particle separations, $$r$$ , and decreases with $$St$$ at large $$r$$ , indicating the dominance of “path-history” and “inertial filtering” effects, respectively. However, at small $$r$$ , the experimental mean inward RV trends higher than DNS, possibly due to the slight polydispersity of Particles and finite light sheet thickness in experiments. To confirm this interpretation, we performed numerical experiments and found that Particle polydispersity increases mean inward RV at small $$r$$ , while finite laser thickness also overestimates mean inward RV at small $$r$$ , This study demonstrates the feasibility of accurately measuring RV using routine hardware, and verifies, for the first time, the path-history and inertial filtering effects on Particle-Pair RV at large Particle separations experimentally.
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Particle Pair relative velocity measurement in high reynolds number homogeneous and isotropic turbulence using 4 frame Particle tracking velocimetry
arXiv: Fluid Dynamics, 2017Co-Authors: Zhongwang Dou, Lance R. Collins, Andrew D. Bragg, Zach Liang, Peter J Ireland, Hui MengAbstract:The radial relative velocity between Particles suspended in turbulent flow plays a critical role in droplet collision and growth. We present a simple and accurate approach to RV measurement in isotropic turbulence - planar 4-frame Particle tracking velocimetry - using routine PIV hardware. This study demonstrates the feasibility of accurately measuring RV using routine hardware and verifies, for the first time, the path-history and inertial filtering effects on Particle-Pair RV at large Particle separations experimentally.
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the effect of reynolds number on inertial Particle dynamics in isotropic turbulence part 1 simulations without gravitational effects
Journal of Fluid Mechanics, 2016Co-Authors: Peter J Ireland, Andrew D. Bragg, Lance R. CollinsAbstract:In this study, we analyse the statistics of both individual inertial Particles and inertial Particle Pairs in direct numerical simulations of homogeneous isotropic turbulence in the absence of gravity. The effect of the Taylor microscale Reynolds number, , on the Particle statistics is examined over the largest range to date (from to 597), at small, intermediate and large Kolmogorov-scale Stokes numbers . We first explore the effect of preferential sampling on the single-Particle statistics and find that low- inertial Particles are ejected from both vortex tubes and vortex sheets (the latter becoming increasingly prevalent at higher Reynolds numbers) and preferentially accumulate in regions of irrotational dissipation. We use this understanding of preferential sampling to provide a physical explanation for many of the trends in the Particle velocity gradients, kinetic energies and accelerations at low , which are well represented by the model of Chun et al. (J. Fluid Mech., vol. 536, 2005, pp. 219–251). As increases, inertial filtering effects become more important, causing the Particle kinetic energies and accelerations to decrease. The effect of inertial filtering on the Particle kinetic energies and accelerations diminishes with increasing Reynolds number and is well captured by the models of Abrahamson (Chem. Engng Sci., vol. 30, 1975, pp. 1371–1379) and Zaichik & Alipchenkov (Intl J. Multiphase Flow, vol. 34 (9), 2008, pp. 865–868), respectively. We then consider Particle-Pair statistics, and focus our attention on the relative velocities and radial distribution functions (RDFs) of the Particles, with the aim of understanding the underlying physical mechanisms contributing to Particle collisions. The relative velocity statistics indicate that preferential sampling effects are important for and that path-history/non-local effects become increasingly important for . While higher-order relative velocity statistics are influenced by the increased intermittency of the turbulence at high Reynolds numbers, the lower-order relative velocity statistics are only weakly sensitive to changes in Reynolds number at low . The Reynolds-number trends in these quantities at intermediate and large are explained based on the influence of the available flow scales on the path-history and inertial filtering effects. We find that the RDFs peak near of order unity, that they exhibit power-law scaling for low and intermediate and that they are largely independent of Reynolds number for low and intermediate . We use the model of Zaichik & Alipchenkov (New J. Phys., vol. 11, 2009, 103018) to explain the physical mechanisms responsible for these trends, and find that this model is able to capture the quantitative behaviour of the RDFs extremely well when direct numerical simulation data for the structure functions are specified, in agreement with Bragg & Collins (New J. Phys., vol. 16, 2014a, 055013). We also observe that at large , changes in the RDF are related to changes in the scaling exponents of the relative velocity variances. The Particle collision kernel closely matches that computed by Rosa et al. (New J. Phys., vol. 15, 2013, 045032) and is found to be largely insensitive to the flow Reynolds number. This suggests that relatively low-Reynolds-number simulations may be able to capture much of the relevant physics of droplet collisions and growth in the adiabatic cores of atmospheric clouds.
Adam Hammond - One of the best experts on this subject based on the ideXlab platform.
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Particle radial distribution function and relative velocity measurement in turbulence at small Particle Pair separations
Journal of Fluid Mechanics, 2021Co-Authors: Adam Hammond, Hui MengAbstract:Particle collisions in turbulent flow are critical to Particle agglomeration and droplet coalescence. The collision kernel can be evaluated by radial distribution function (RDF) and radial relative velocity (RV) between Particles at small separations can be attributed to Particle–Particle interactions including hydrodynamic interactions, which are not well-understood. Uncertainty analysis substantiates the observed trends. This first-ever simultaneous RDF and RV measurement at small separations provides a clear glimpse into the clustering and relative velocities of Particles in turbulence near-contact.
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Particle radial distribution function and relative velocity measurement in turbulence at small Particle Pair separations
arXiv: Fluid Dynamics, 2021Co-Authors: Adam Hammond, Hui MengAbstract:The collision rate of Particles suspended in turbulent flow is critical to Particle agglomeration and droplet coalescence. The collision kernel can be evaluated by the radial distribution function (RDF) and radial relative velocity (RV) between Particles at small separations $r$. Previously, the smallest $r$ was limited to roughly the Kolmogorov length $\eta$ due to Particle position uncertainty and image overlap. We report a new approach to measure RDF and RV near contact ($r/a\: \approx$ 2.07, $a$ Particle radius) overcoming these limitations. Three-dimensional Particle tracking velocimetry using four-pulse Shake-the-Box algorithm recorded short Particle tracks with the interpolated midpoints registered as Particle positions to avoid image overlap. This strategy further allows removal of mismatched tracks using their characteristic false RV. We measured RDF and RV in a one-meter-diameter isotropic turbulence chamber with Taylor Reynolds number $Re_\lambda=324$ with Particles of 12-16 $\mu$m radius and Stokes number $\approx$ 0.7. While at large $r$ the measured RV agrees with the literature, when $r \eta$, RDF scales as $r^{-0.39}$ reflecting RDF scaling for polydisperse Particles in the literature , but when $r\lessapprox\eta$ RDF scales as $r^{-6}$, yielding 1000 times higher near-contact RDF than simulations. Such extreme clustering and relative velocity enhancement can be attributed to Particle-Particle interactions. Uncertainty analysis substantiates the observed trends. This first-ever simultaneous RDF and RV measurement at small separations provides a clear glimpse into the clustering and relative velocities of Particles in turbulence near-contact.
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effects of reynolds number and stokes number on Particle Pair relative velocity in isotropic turbulence a systematic experimental study
Journal of Fluid Mechanics, 2018Co-Authors: Zhongwang Dou, Lance R. Collins, Andrew D. Bragg, Adam Hammond, Zach Liang, Hui MengAbstract:The effects of Reynolds number ( ) and Stokes number ( ) on Particle-Pair relative velocity (RV) are investigated systematically using a recently developed planar four-frame Particle tracking technique in a novel homogeneous and isotropic turbulence chamber. We compare the measured results with direct numerical simulation (DNS), verifying whether the conclusions of the DNS for simplified conditions and limited are still valid in reality. Two experiments are performed: varying between 246 and 357 at six values, and varying between 0.02 and 4.63 at five values. The measured mean inward Particle-Pair RV as a function of separation distance is compared with the DNS under closely matched conditions. At all experimental conditions, an excellent agreement is achieved, except when the Particle separation distance ( is the Kolmogorov length scale), where the experimental is consistently higher, possibly due to Particle polydispersity and finite laser thickness in the experiments (Dou et al., arXiv:1712.07506, 2017). At any fixed is essentially independent of , echoing the DNS finding of Ireland et al. (J. Fluid Mech., vol. 796, 2016, pp. 617–658). At any fixed , increases with at small , showing dominance of the path-history effect in the dissipation range when , but decreases with at large , indicating dominance of inertial filtering. We further compare the and RV variance from experiments with DNS and theoretical predictions by Pan & Padoan (J. Fluid Mech., vol. 661, 2010, pp. 73–107). For , experimental and match these values well at , but they are higher than both DNS and theory at . For , from all three match well, except for , for which experimental values are higher, while from experiment and DNS are much higher than theoretical predictions. We discuss potential causes of these discrepancies. What this study shows is the first experimental validation of and effect on inertial Particle-Pair in homogeneous and isotropic turbulence.
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Effects of Reynolds number and Stokes number on Particle-Pair relative velocity in isotropic turbulence: a systematic experimental study
Journal of Fluid Mechanics, 2018Co-Authors: Zhongwang Dou, Lance R. Collins, Andrew D. Bragg, Adam Hammond, Zach Liang, Hui MengAbstract:The effects of Reynolds number and Stokes number on Particle-Pair relative velocity (RV) were investigated systematically using a recently developed planar four-frame Particle tracking technique in a novel homogeneous and isotropic turbulence chamber.
Zhongwang Dou - One of the best experts on this subject based on the ideXlab platform.
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effects of reynolds number and stokes number on Particle Pair relative velocity in isotropic turbulence a systematic experimental study
Journal of Fluid Mechanics, 2018Co-Authors: Zhongwang Dou, Lance R. Collins, Andrew D. Bragg, Adam Hammond, Zach Liang, Hui MengAbstract:The effects of Reynolds number ( ) and Stokes number ( ) on Particle-Pair relative velocity (RV) are investigated systematically using a recently developed planar four-frame Particle tracking technique in a novel homogeneous and isotropic turbulence chamber. We compare the measured results with direct numerical simulation (DNS), verifying whether the conclusions of the DNS for simplified conditions and limited are still valid in reality. Two experiments are performed: varying between 246 and 357 at six values, and varying between 0.02 and 4.63 at five values. The measured mean inward Particle-Pair RV as a function of separation distance is compared with the DNS under closely matched conditions. At all experimental conditions, an excellent agreement is achieved, except when the Particle separation distance ( is the Kolmogorov length scale), where the experimental is consistently higher, possibly due to Particle polydispersity and finite laser thickness in the experiments (Dou et al., arXiv:1712.07506, 2017). At any fixed is essentially independent of , echoing the DNS finding of Ireland et al. (J. Fluid Mech., vol. 796, 2016, pp. 617–658). At any fixed , increases with at small , showing dominance of the path-history effect in the dissipation range when , but decreases with at large , indicating dominance of inertial filtering. We further compare the and RV variance from experiments with DNS and theoretical predictions by Pan & Padoan (J. Fluid Mech., vol. 661, 2010, pp. 73–107). For , experimental and match these values well at , but they are higher than both DNS and theory at . For , from all three match well, except for , for which experimental values are higher, while from experiment and DNS are much higher than theoretical predictions. We discuss potential causes of these discrepancies. What this study shows is the first experimental validation of and effect on inertial Particle-Pair in homogeneous and isotropic turbulence.
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Effects of Reynolds number and Stokes number on Particle-Pair relative velocity in isotropic turbulence: a systematic experimental study
Journal of Fluid Mechanics, 2018Co-Authors: Zhongwang Dou, Lance R. Collins, Andrew D. Bragg, Adam Hammond, Zach Liang, Hui MengAbstract:The effects of Reynolds number and Stokes number on Particle-Pair relative velocity (RV) were investigated systematically using a recently developed planar four-frame Particle tracking technique in a novel homogeneous and isotropic turbulence chamber.
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Particle Pair relative velocity measurement in high reynolds number homogeneous and isotropic turbulence using 4 frame Particle tracking velocimetry
Experiments in Fluids, 2018Co-Authors: Zhongwang Dou, Lance R. Collins, Andrew D. Bragg, Zach Liang, Peter J Ireland, Hui MengAbstract:The radial relative velocity (RV) between Particles suspended in turbulent flow plays a critical role in droplet collision and growth. We present a simple and accurate approach to RV measurement in isotropic turbulence—planar 4-frame Particle tracking velocimetry—using routine PIV hardware. It improves Particle positioning and Pairing accuracy over the 2-frame holographic approach by de Jong et al. (Int J Multiphas Flow 36:324–332; de Jong et al., Int J Multiphas Flow 36:324–332, 2010) without using high-speed cameras and lasers as in Saw et al. (Phys Fluids 26:111702, 2014). Homogeneous and isotropic turbulent flow ( $${R_\lambda }=357$$ ) in a new, fan-driven, truncated iscosahedron chamber was laden with either low-Stokes (mean $$St=0.09$$ , standard deviation 0.05) or high-Stokes aerosols (mean $$St=3.46$$ , standard deviation 0.57). For comparison, DNS was conducted under similar conditions ( $${R_\lambda }=398$$ ; $$St=0.10$$ and 3.00, respectively). Experimental RV probability density functions (PDF) and mean inward RV agree well with DNS. Mean inward RV increases with $$St$$ at small Particle separations, $$r$$ , and decreases with $$St$$ at large $$r$$ , indicating the dominance of “path-history” and “inertial filtering” effects, respectively. However, at small $$r$$ , the experimental mean inward RV trends higher than DNS, possibly due to the slight polydispersity of Particles and finite light sheet thickness in experiments. To confirm this interpretation, we performed numerical experiments and found that Particle polydispersity increases mean inward RV at small $$r$$ , while finite laser thickness also overestimates mean inward RV at small $$r$$ , This study demonstrates the feasibility of accurately measuring RV using routine hardware, and verifies, for the first time, the path-history and inertial filtering effects on Particle-Pair RV at large Particle separations experimentally.
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Particle Pair relative velocity measurement in high reynolds number homogeneous and isotropic turbulence using 4 frame Particle tracking velocimetry
arXiv: Fluid Dynamics, 2017Co-Authors: Zhongwang Dou, Lance R. Collins, Andrew D. Bragg, Zach Liang, Peter J Ireland, Hui MengAbstract:The radial relative velocity between Particles suspended in turbulent flow plays a critical role in droplet collision and growth. We present a simple and accurate approach to RV measurement in isotropic turbulence - planar 4-frame Particle tracking velocimetry - using routine PIV hardware. This study demonstrates the feasibility of accurately measuring RV using routine hardware and verifies, for the first time, the path-history and inertial filtering effects on Particle-Pair RV at large Particle separations experimentally.