The Experts below are selected from a list of 306 Experts worldwide ranked by ideXlab platform
Christopher J. Hogan - One of the best experts on this subject based on the ideXlab platform.
-
Collision Rate coefficient for charged dust grains in the presence of linear shear.
Physical Review E, 2017Co-Authors: Huan Yang, Christopher J. HoganAbstract:Like and oppositely charged particles or dust grains in linear shear flows are often driven to collide with one another by fluid and/or electrostatic forces, which can strongly influence particle-size distribution evolution. In gaseous media, Collisions in shear are further complicated because particle inertia can influence differential motion. Expressions for the Collision Rate coefficient have not been developed previously which simultaneously account for the influences of linear shear, particle inertia, and electrostatic interactions. Here, we determine the Collision Rate coefficient accounting for the aforementioned effects by determining the Collision area, i.e., the area of the plane perpendicular to the shear flow defining the relative initial locations of particles which will collide with one another. Integration of the particle flux over this area yields the Collision Rate. Collision Rate calculations are parametrized as an enhancement factor, i.e., the ratio of the Collision Rate considering potential interactions and inertia to the traditional Collision Rate considering laminar shear only. For particles of constant surface charge density, the enhancement factor is found dependent only on the Stokes number (quantifying particle inertia), the electrostatic energy to shear energy ratio, and the ratio of colliding particle radii. Enhancement factors are determined for Stokes numbers in the 0-10 range and energy ratios up to 5. Calculations show that the influences of both electrostatic interactions and inertia are significant; for inertialess (St=0) equal-sized and oppositely charged particles, we find that even at energy ratios as low as 0.2, enhancement factors are in excess of 2. For the same situation but like-charged particles, enhancement factors fall below 0.5. Increasing the Stokes number acts to mitigate the influence of electrostatic potentials for both like and oppositely charged particles; i.e., inertia reduces the enhancement factor for oppositely charged particles and increases it for like-charged particles. Uniquely, at elevated Stokes numbers with attractive potentials we find Collisionless "pockets" within the Collision area, which are regions completely bounded by the Collision area but within which Collisions do not occur. Regression equations to results are provided, enabling calculation of the enhancement factor as a function of energy ratio and Stokes number. In total, this study both leads to insight into the Collision dynamics of finite-inertia, charged particles in shear flows, and provides a means to simply calculate the particle-particle Collision Rate coefficient.
-
The Single-Fiber Collision Rate and Filtration Efficiency for Nanoparticles II: Extension to Arbitrary-Shaped Particles
Aerosol Science and Technology, 2014Co-Authors: Thaseem Thajudeen, Benjamin Hunt, Christopher J. HoganAbstract:We extend the equations for the dimensionless Collision kernel and filtration efficiency, attained previously via mean first-passage time (MFPT) calculations, to particles of arbitrary shape. Specifically, we show that the regression equations for the dimensionless Collision Rate found considering particle-fiber Collisions driven by simultaneous diffusion and interception remain valid for non-spherical particles, provided that an appropriate Collision length scale for the non-spherical particle (L) is defined and incorpoRated into the definitions of the dimensionless Collision Rate (H) and the diffusive Knudsen number (KnD). Regression equations are provided to calculate this length scale for quasifractal aggregates of varying fractal dimension, as well as cylinders. MFPT calculations reveal that, over ∼5 orders of magnitude in H, these regression equations for the Collision length are valid. Furthermore, using the previously attained proportionality between the predicted dimensionless Collision Rate and ...
-
the single fiber Collision Rate and filtration efficiency for nanoparticles i the first passage time calculation approach
Aerosol Science and Technology, 2014Co-Authors: Benjamin Hunt, Thaseem Thajudeen, Christopher J. HoganAbstract:We describe an approach to filtration-efficiency calculations as an alternative to the traditional depth filtration theory. The new approach involves linking the single-fiber efficiency to the Collision Rate coefficient/kernel between nanoparticles and fibers, and correspondingly inferring the Collision kernel via dimensionless mean first-passage time (MFPT) calculations. This method has the advantage of easily incorporating the influences of particle diffusion, inertia, and particle size; therefore, all filtration mechanisms can be considered simultaneously. Through non-dimensionalization of the equation of motion for a particle in MFPT calculations (the Langevin equation), it is shown that both the single-fiber efficiency Ef and dimensionless particle-fiber Collision kernel, H, are functions of the ratio of particle radius to filter-fiber radius, R, the solid volume fraction in the filter, Vf, the ratio of particle persistence distance to the particle-filter Collision distance, KnD (the diffusive Knudse...
-
the single fiber Collision Rate and filtration efficiency for nanoparticles i the first passage time calculation approach
Aerosol Science and Technology, 2014Co-Authors: Benjamin Hunt, Thaseem Thajudeen, Christopher J. HoganAbstract:We describe an approach to filtration-efficiency calculations as an alternative to the traditional depth filtration theory. The new approach involves linking the single-fiber efficiency to the Collision Rate coefficient/kernel between nanoparticles and fibers, and correspondingly inferring the Collision kernel via dimensionless mean first-passage time (MFPT) calculations. This method has the advantage of easily incorporating the influences of particle diffusion, inertia, and particle size; therefore, all filtration mechanisms can be considered simultaneously. Through non-dimensionalization of the equation of motion for a particle in MFPT calculations (the Langevin equation), it is shown that both the single-fiber efficiency Ef and dimensionless particle-fiber Collision kernel, H, are functions of the ratio of particle radius to filter-fiber radius, R, the solid volume fraction in the filter, Vf, the ratio of particle persistence distance to the particle-filter Collision distance, KnD (the diffusive Knudse...
-
The Collision Rate of Nonspherical Particles and Aggregates for all Diffusive Knudsen Numbers
Aerosol Science and Technology, 2012Co-Authors: Thaseem Thajudeen, Ranganathan Gopalakrishnan, Christopher J. HoganAbstract:We examine theoretically and numerically Collisions of arbitrarily shaped particles in the mass transfer transition regime, where ambiguities remain regarding the Collision Rate coefficient (Collision kernel). Specifically, we show that the dimensionless Collision kernel for arbitrarily shaped particles, H, depends solely on a correctly defined diffusive Knudsen number (KnD , in contrast with the traditional Knudsen number), and to determine the diffusive Knudsen number, it is necessary to calculate two combined size parameters for the colliding particles: the Smoluchowski radius, which defines the Collision Rate in the continuum (KnD →0) regime, and the projected area, which defines the Collision Rate in the free molecular (KnD →∞) regime. Algorithms are provided to compute these parameters. Using mean first passage time calculations with computationally geneRated quasifractal (statistically fractal) aggregates, we find that with correct definitions of H and KnD , the H(KnD) relationship found valid for ...
Alain Pumir - One of the best experts on this subject based on the ideXlab platform.
-
Collision Rate of ice crystals with water droplets in turbulent flows
Journal of Fluid Mechanics, 2018Co-Authors: Aurore Naso, Emmanuel Lévêque, Jennifer Jucha, Alain PumirAbstract:Riming, the process whereby ice crystals get coated by impacting supercooled liquid droplets, is one of the dominant processes leading to precipitation in mixed-phase clouds. How a settling crystal collides with very small water droplets has been mostly studied in laminar conditions. The present numerical study aims at providing further insight on how turbulent flow motion affects the riming of ice crystals. We model the crystals as narrow oblate ellipsoids, smaller than the Kolmogorov elementary scale. By neglecting the effect of fluid inertia on the motion of the crystals and droplets, and using direct numerical simulations of the Navier–Stokes equations in a modeRately turbulent regime, over a range of kinetic energy dissipation $1~\text{cm}^{2}~\text{s}^{-3}\lesssim \unicode[STIX]{x1D700}\lesssim 256~\text{cm}^{2}~\text{s}^{-3}$ , we determine the Collision Rate between disk-shaped ice crystals and very small liquid water droplets. Whereas differential settling plays the dominant role in determining the Collision Rate at small turbulence intensity, the role of turbulence becomes more important at the large values of $\unicode[STIX]{x1D700}$ simulated, an effect that can be partly attributed to the increased role of inertia. We always find that Collisions occur with a large probability on the rim of the ellipsoids, a phenomenon that can be explained to a large extent by kinematic considerations. The difference in the settling velocity of crystals and droplets induces a strong asymmetry in the probability of Collision between the faces of the ellipsoids. Our results shed light on the physical mechanisms involved in the riming of ice crystals in clouds.
-
Collision Rate for suspensions at large stokes numbers comparing navier stokes and synthetic turbulence
Journal of Turbulence, 2015Co-Authors: Michel Voskuhle, Alain Pumir, Emmanuel Lévêque, Michael WilkinsonAbstract:The use of simplified models of turbulent flows provides an appealing possibility to study the Collision Rate of turbulent suspensions, especially in conditions relevant to astrophysics, which require large timescale separations. To check the validity of such approaches, we used a direct numerical simulation (DNS) velocity field, which satisfies the Navier–Stokes equations (although it neglects the effect of the suspended particles on the flow field), and a kinematic simulation (KS) velocity field, which is a random field designed so that its statistics are in accord with the Kolmogorov theory for fully-developed turbulence. In the limit where the effects of particle inertia (characterised by the Stokes number) are negligible, the Collision Rates from the two approaches agree. As the Stokes number St increases, however, we show that the DNS Collision Rate exceeds the KS Collision Rate by orders of magnitude. We propose an explanation for this phenomenon and explore its consequences. We discuss the collisi...
-
Collision Rate for suspensions at large Stokes numbers – comparing Navier–Stokes and synthetic turbulence
Journal of Turbulence, 2014Co-Authors: Michel Voßkuhle, Alain Pumir, Emmanuel Lévêque, Michael WilkinsonAbstract:The use of simplified models of turbulent flows provides an appealing possibility to study the Collision Rate of turbulent suspensions, especially in conditions relevant to astrophysics, which require large timescale separations. To check the validity of such approaches, we used a direct numerical simulation (DNS) velocity field, which satisfies the Navier–Stokes equations (although it neglects the effect of the suspended particles on the flow field), and a kinematic simulation (KS) velocity field, which is a random field designed so that its statistics are in accord with the Kolmogorov theory for fully-developed turbulence. In the limit where the effects of particle inertia (characterised by the Stokes number) are negligible, the Collision Rates from the two approaches agree. As the Stokes number St increases, however, we show that the DNS Collision Rate exceeds the KS Collision Rate by orders of magnitude. We propose an explanation for this phenomenon and explore its consequences. We discuss the collisi...
-
Collision Rate for suspensions at large Stokes numbers - comparing Navier-Stokes and synthetic turbulence
Journal of Turbulence, 2014Co-Authors: Michel Voßkuhle, Alain Pumir, Emmanuel Lévêque, Michael WilkinsonAbstract:The use of simplified models of turbulent flows provides an appealing possibility to study the Collision Rate of turbulent suspensions, especially in conditions relevant to astrophysics, which require large time scale separations. To check the validity of such approaches, we used a direct numerical simulation (DNS) velocity field, which satisfies the Navier-Stokes equations (although it neglects the effect of the suspended particles on the flow field), and a kinematic simulation (KS) velocity field, which is a random field designed so that its statistics are in accord with the Kolmogorov theory for fully-developed turbulence. In the limit where the effects of particle inertia (characterised by the Stokes number) are negligible, the Collision Rates from the two approaches agree. As the Stokes number St increases, however, we show that the DNS Collision Rate exceeds the KS Collision Rate by orders of magnitude. We propose an explanation for this phenomenon and explore its consequences. We discuss the Collision Rate $R$ for particles in high Reynolds number flows at large Stokes number, and present evidence that $R\propto \sqrt{{\rm St}}$.
-
Prevalence of the sling effect for enhancing Collision Rates in turbulent suspensions
Journal of Fluid Mechanics, 2014Co-Authors: Michel Voßkuhle, Alain Pumir, Emmanuel Lévêque, Michael WilkinsonAbstract:Turbulence facilitates Collisions between particles suspended in a turbulent flow. Two effects have been proposed that can enhance the Collision Rate at high turbulence intensities: ‘preferential concentration’ (a clustering phenomenon) and the ‘sling effect’ (arising from the formation of caustic folds in the phase space of the suspended particles). We have determined numerically the Collision Rate of small heavy particles as a function of their size and densities. The dependence on particle densities allows us to quantify the contribution of the sling effect to the Collision Rate. Our results demonstRate that the sling effect provides the dominant mechanism to the enhancement of the Collision Rate of particles, when inertia becomes significant.
Lian-ping Wang - One of the best experts on this subject based on the ideXlab platform.
-
effects of turbulence on the geometric Collision Rate of sedimenting droplets part 1 results from direct numerical simulation
New Journal of Physics, 2008Co-Authors: Orlando Ayala, Lian-ping Wang, Bogdan Rosa, Wojciech W. GrabowskiAbstract:There have been relatively few studies of turbulent Collision Rate of sedimenting droplets in the context of cloud physics, for which both the gravitational settling and inertial effects must be simultaneously considered. In this study, direct numerical simulations (DNS) were used to study the geometric Collision Rates of cloud droplets. Both Stokes drag law and a nonlinear drag law were considered, but the droplet–droplet local aerodynamic interactions were not included. Typical droplet and turbulence parameters of convective clouds were used to determine the flow dissipation Rate , characteristic Stokes numbers, and the nondimensional terminal velocities. DNS results from a large number of runs covering the range from 10 to 400 cm2 s− 3 and droplet sizes from 10 to 60 μm in radius are presented. These results show that air turbulence can increase the geometric Collision kernel by up to 47%, relative to geometric Collision by differential sedimentation. This is due to both a modeRate enhancement of the radial relative velocity between droplets and a modeRate level of pair nonuniform concentration due to local droplet clustering. The turbulence enhancements increase with the flow dissipation Rate and flow Reynolds number. Comparisons with related DNS studies show that our results confirm and extend the previous findings. The mean settling velocity of droplets in a turbulent flow was also obtained, showing that a maximum increase relative to the terminal velocity occurs for 20 μm cloud droplets. This agrees with a previous theory based on simple vortex flows and confirms the importance of a new nondimensional parameter τp3g2/ν for sedimenting droplets, where τp is the droplet inertial response time, g is the gravitational acceleration and ν is the air kinematic viscosity. Limitations of DNS and future directions are also noted.
-
Theoretical Formulation of Collision Rate and Collision Efficiency of Hydrodynamically Interacting Cloud Droplets in Turbulent Atmosphere
Journal of the Atmospheric Sciences, 2005Co-Authors: Lian-ping Wang, Orlando Ayala, Scott E. Kasprzak, Wojciech W. GrabowskiAbstract:A methodology for conducting direct numerical simulations (DNSs) of hydrodynamically interacting droplets in the context of cloud microphysics has been developed and used to validate a new kinematic formulation capable of describing the Collision Rate and Collision efficiency of cloud droplets in turbulent air. The theoretical formulation is formally the same as the formulation recently developed for geometrical Collision Rate of finite-inertia, nonsettling particles. It is shown that its application to hydrodynamically interacting droplets requires corrections because of a nonoverlap requirement. An approximate method for correcting the kinematic properties has been developed and validated against DNS data. The formulation presented here is more general and accuRate than previously published formulations that, in most cases, are some extension to the description of hydrodynamic–gravitational Collision. General dynamic and kinematic representations of the properly defined Collision efficiency in a turbulent flow have been discussed. In addition to augmenting the geometric Collision Rate, air turbulence has been found to enhance the Collision efficiency because, in a turbulent flow, hydrodynamic interactions become less effective in reducing the average relative radial velocity. The level of increase in the Collision efficiency depends on the flow dissipation Rate. For example, the Collision efficiency between droplet so f 20 and 25m in radii is increased by 59% and 10% by air turbulence at dissipation Rates of 400 and 100 cm 2 s 3 , respectively. It is also shown that hydrodynamic interactions lead to higher droplet concentration fluctuations. The formulation presented here sepaRates the effect of turbulence on Collision efficiency from the previously observed effect of turbulence on the geometric Collision Rate.
-
on the Collision Rate of small particles in isotropic turbulence ii finite inertia case
Physics of Fluids, 1998Co-Authors: Lian-ping Wang, Anthony S Wexler, Yong ZhouAbstract:Numerical experiments have been performed to study the geometric Collision Rate of finite-size particles with zero inertia (i.e., fluid elements) in isotropic turbulence. The turbulent flow was geneRated by the pseudospectral method. We argue that the formulation of Saffman and Turner [J. Fluid Mech. 1, 16 (1956)] for the average Collision kernel is correct only under the assumptions that the particles are kept in the system after Collision and allowed to overlap in space. This was confirmed, for the first time, by numerical experiments to within a numerical uncertainty as small as 1%. Finite corrections to the Saffman and Turner result must be made if one applies the theory to actual coagulation process where particles are not allowed to overlap before Collision and particles are removed from a given size group after Collision. This is due to the fact that Saffman and Turner assumed a uniform, time-independent concentration field in their formulation of the average Collision kernel, while in the actual m...
-
on the Collision Rate of small particles in isotropic turbulence ii finite inertia case
Physics of Fluids, 1998Co-Authors: Lian-ping Wang, Anthony S Wexle, Yong ZhouAbstract:Numerical experiments have been performed to study the geometric Collision Rate of finite-size particles with zero inertia (i.e., fluid elements) in isotropic turbulence. The turbulent flow was geneRated by the pseudospectral method. We argue that the formulation of Saffman and Turner [J. Fluid Mech. 1, 16 (1956)] for the average Collision kernel is correct only under the assumptions that the particles are kept in the system after Collision and allowed to overlap in space. This was confirmed, for the first time, by numerical experiments to within a numerical uncertainty as small as 1%. Finite corrections to the Saffman and Turner result must be made if one applies the theory to actual coagulation process where particles are not allowed to overlap before Collision and particles are removed from a given size group after Collision. This is due to the fact that Saffman and Turner assumed a uniform, time-independent concentration field in their formulation of the average Collision kernel, while in the actual modeling of population evolution the particle number concentration changes in time and may be locally nonuniform as a result of a biased removal process due to spatially nonuniform coagulation Rates. However, the quantitative level of the deviations from the Saffman and Turner result remain to be explained. Numerical experiments in simple shear flow were also conducted to elaboRate our findings.
Renwei Mei - One of the best experts on this subject based on the ideXlab platform.
-
On the Collision Rate of small particles in turbulent flows
Journal of Fluid Mechanics, 1999Co-Authors: Renwei MeiAbstract:A theoretical framework is developed to predict the Rate of geometric Collision and the Collision velocity of small size inertialess particles in general turbulent flows. The present approach evaluates the Collision Rate for small size, inertialess particles in a given instantaneous flow field based on the local eigenvalues of the Rate-of-strain tensor. An ensemble average is then applied to the instantaneous Collision Rate to obtain the average Collision Rate. The Collision Rates predicted by Smoluchowski for laminar shear flow and by Saffman & Turner for isotropic turbulence are recovered. The Collision velocities presently predicted in both laminar shear flow and isotropic turbulence agree well with the results from numerical simulations for particle Collision in both flows. The present theory for evaluating the Collision Rate and the Collision velocity is also applied to a rapidly sheared homogeneous turbulence to assess the effect of strong anisotropy on the Collision Rate
-
Particle Collision Rate in fluid flows
Physics of Fluids, 1998Co-Authors: Renwei MeiAbstract:The classical result of Smoluchowski [Z. Phys. Chem. 92, 129 (1917)] for the Collision Rate of monodisperse particles in a laminar shear flow is shown to be inaccuRate due to the inclusion of the self-Collision. In the present work we extend Smoluchowski’s result by excluding the self-Collision in the counting of Collision pairs. A numerical simulation for particle Collisions in a laminar shear flow at very low concentration is carried out to validate the extended result of Smoluchowski. Good agreement for the Collision Rate between the numerical simulation and the prediction based on the extended expression is obtained.
Thaseem Thajudeen - One of the best experts on this subject based on the ideXlab platform.
-
The Single-Fiber Collision Rate and Filtration Efficiency for Nanoparticles II: Extension to Arbitrary-Shaped Particles
Aerosol Science and Technology, 2014Co-Authors: Thaseem Thajudeen, Benjamin Hunt, Christopher J. HoganAbstract:We extend the equations for the dimensionless Collision kernel and filtration efficiency, attained previously via mean first-passage time (MFPT) calculations, to particles of arbitrary shape. Specifically, we show that the regression equations for the dimensionless Collision Rate found considering particle-fiber Collisions driven by simultaneous diffusion and interception remain valid for non-spherical particles, provided that an appropriate Collision length scale for the non-spherical particle (L) is defined and incorpoRated into the definitions of the dimensionless Collision Rate (H) and the diffusive Knudsen number (KnD). Regression equations are provided to calculate this length scale for quasifractal aggregates of varying fractal dimension, as well as cylinders. MFPT calculations reveal that, over ∼5 orders of magnitude in H, these regression equations for the Collision length are valid. Furthermore, using the previously attained proportionality between the predicted dimensionless Collision Rate and ...
-
the single fiber Collision Rate and filtration efficiency for nanoparticles i the first passage time calculation approach
Aerosol Science and Technology, 2014Co-Authors: Benjamin Hunt, Thaseem Thajudeen, Christopher J. HoganAbstract:We describe an approach to filtration-efficiency calculations as an alternative to the traditional depth filtration theory. The new approach involves linking the single-fiber efficiency to the Collision Rate coefficient/kernel between nanoparticles and fibers, and correspondingly inferring the Collision kernel via dimensionless mean first-passage time (MFPT) calculations. This method has the advantage of easily incorporating the influences of particle diffusion, inertia, and particle size; therefore, all filtration mechanisms can be considered simultaneously. Through non-dimensionalization of the equation of motion for a particle in MFPT calculations (the Langevin equation), it is shown that both the single-fiber efficiency Ef and dimensionless particle-fiber Collision kernel, H, are functions of the ratio of particle radius to filter-fiber radius, R, the solid volume fraction in the filter, Vf, the ratio of particle persistence distance to the particle-filter Collision distance, KnD (the diffusive Knudse...
-
the single fiber Collision Rate and filtration efficiency for nanoparticles i the first passage time calculation approach
Aerosol Science and Technology, 2014Co-Authors: Benjamin Hunt, Thaseem Thajudeen, Christopher J. HoganAbstract:We describe an approach to filtration-efficiency calculations as an alternative to the traditional depth filtration theory. The new approach involves linking the single-fiber efficiency to the Collision Rate coefficient/kernel between nanoparticles and fibers, and correspondingly inferring the Collision kernel via dimensionless mean first-passage time (MFPT) calculations. This method has the advantage of easily incorporating the influences of particle diffusion, inertia, and particle size; therefore, all filtration mechanisms can be considered simultaneously. Through non-dimensionalization of the equation of motion for a particle in MFPT calculations (the Langevin equation), it is shown that both the single-fiber efficiency Ef and dimensionless particle-fiber Collision kernel, H, are functions of the ratio of particle radius to filter-fiber radius, R, the solid volume fraction in the filter, Vf, the ratio of particle persistence distance to the particle-filter Collision distance, KnD (the diffusive Knudse...
-
The Collision Rate of Nonspherical Particles and Aggregates for all Diffusive Knudsen Numbers
Aerosol Science and Technology, 2012Co-Authors: Thaseem Thajudeen, Ranganathan Gopalakrishnan, Christopher J. HoganAbstract:We examine theoretically and numerically Collisions of arbitrarily shaped particles in the mass transfer transition regime, where ambiguities remain regarding the Collision Rate coefficient (Collision kernel). Specifically, we show that the dimensionless Collision kernel for arbitrarily shaped particles, H, depends solely on a correctly defined diffusive Knudsen number (KnD , in contrast with the traditional Knudsen number), and to determine the diffusive Knudsen number, it is necessary to calculate two combined size parameters for the colliding particles: the Smoluchowski radius, which defines the Collision Rate in the continuum (KnD →0) regime, and the projected area, which defines the Collision Rate in the free molecular (KnD →∞) regime. Algorithms are provided to compute these parameters. Using mean first passage time calculations with computationally geneRated quasifractal (statistically fractal) aggregates, we find that with correct definitions of H and KnD , the H(KnD) relationship found valid for ...