Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session L16: Focus Sessions: Fluid Dynamics of Atmospheric Clouds IIGeophysical
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Chair: Raymond Shaw, Michigan Tech Room: 603 |
Monday, November 20, 2017 4:05PM - 4:18PM |
L16.00001: Collision between ellipsoids settling in a turbulent flow Alain Pumir, Jennifer Jucha, Aurore Naso, Emmanuel Leveque Nucleation of droplets and ice-crystals, starting from aerosols, leads in clouds to the formation of very small particles, whose size has to grow by orders of magnitude to become rain drops or hail particles. The growth of droplets in warm clouds (in the absence of ice crystals) has recently received much attention.~ ~Here, I will discuss the problem of settling and collision of ice crystals in a turbulent environment. In the temperature range -20C \textless T \textless -10C, the crystals shapes can be approximated as very thing oblate ellipsoids, with a size smaller than the size of the smallest eddies (Kolmogorov scale). Neglecting completely the inertia of the fluid, I will show that the crystals tend to settle predominantly with their face first. The orientation bias of the crystals is particularly strong when turbulence intensity is low. When the flow Reynolds number is moderate, the collisions between crystals results essentially from the difference in the settling velocities, due to the different orientation (the settling velocity depends on the crystal orientation). The role of turbulence in determining the collision rate becomes prevalent when the Reynolds number increases. [Preview Abstract] |
Monday, November 20, 2017 4:18PM - 4:31PM |
L16.00002: Droplet Collisions in the Wind Reece Kearney, Gregory Bewley Particle collisions are important in a variety of natural and industrial processes, including rainfall and combustion. In many cases, colliding particles move in and relative to a turbulent fluid. The motion of the fluid between approaching particles affects the way they collide, or indeed whether they collide at all. In our laboratory experiments, the particles are water droplets between 100 and 300 microns in diameter and they move through an air flow. We observe collisions between them with high speed cameras. We vary the relative velocity between the pair of particles and the air flow between 2 and 5 m/s, so that the Weber number is between 5 and 50. We quantify the effect of the air flow on the trajectories of the approaching particles using Lagrangian particle tracking. [Preview Abstract] |
Monday, November 20, 2017 4:31PM - 4:44PM |
L16.00003: Direct numerical simulation of coalescing droplets in turbulence Melanie Li Sing How, Lance Collins There is a rich body of numerical, experimental and theoretical work looking at the role of turbulence in particle collisions, with a particular emphasis on how it might accelerate the evolution of clouds in the atmosphere. This study is a continuation of that lineage. We perform direct numerical simulations of isotropic turbulence with embedded droplets that, upon collision, coalesce to produce a daughter droplet that conserves the mass and momentum of the parent droplets. ~As a consequence of coalescence, the droplet size distribution evolves over time from its monodisperse initial condition. ~The work is an extension of Reade and Collins (J. Fluid Mech. 415:45-64, 2000), which considered the same problem at a much lower Reynolds number. ~We observe important effects of intermittency at Reynolds numbers that are several-fold higher. ~The collisions do not yet take into account the effect of the lubricating gas layer, which will be the topic of future work. [Preview Abstract] |
Monday, November 20, 2017 4:44PM - 4:57PM |
L16.00004: Turbulence effect on coagulatioinal growth of cloud droplets Xiang-Yu Li, Axel Brandenburg, Gunilla Svensson, Nils Haugen, Igor Rogachevskii Bottleneck problem of cloud droplet growth (a rapid growth of initially small micron-size droplets to the $50 \, \mu$m in radius to form rain) is one of the most challenging problems for cloud physics. Cloud droplet growth is neither dominated by condensation nor gravitational coagulation in the size range of $15 \, \mu \rm{m} \sim 50 \, \mu$m in radius. Turbulence-initiated coagulation is argued to be the mechanism to bridge the size gap. This study investigates the turbulence effect on coagulational growth of cloud droplet. We found that the coagulation rate strongly depends on the small-scale properties of turbulence. The coagulation rate is enhanced with increasing energy dissipation rate, therefore, broaden the size spectra of cloud droplets. Consistent with the previous studies, the coagulation rate is insensitive to the Taylor micro-scale Reynolds number. [Preview Abstract] |
Monday, November 20, 2017 4:57PM - 5:10PM |
L16.00005: Enhancement of coalescence in turbulent clouds Alan Kerstein, Steven Krueger An economical numerical model, called ClusColl, for droplet motions and collisions in turbulent flows has been developed, tested, and applied. In the linear eddy model, 1D turbulent advection of fluid is implemented by rearranging the fluid cells. Each permutation represents an individual turbulent eddy, and is called a ``triplet map." The triplet map captures flow processes as small as the smallest turbulent eddy, but the response of cloud droplets to turbulence has important features at scales as small as the droplet radius. ClusColl includes a 3D triplet map for droplets that captures these additional effects. We have also implemented a collision detection algorithm so that ClusColl can simulate collisions and coalescence between finite-inertia particles. For sedimenting droplets with St $<$ 1, there is fairly good agreement between collision kernels obtained from ClusColl and from direct numerical simulation. Collision and coalescence calculations made using ClusColl suggest that turbulence can significantly accelerate rain formation by droplet clustering and/or by spectral broadening due to entrainment and mixing. We are using ClusColl to investigate the relative roles that entrainment and mixing, droplet inertial effects, and ultragiant nuclei play in warm rain initiation. [Preview Abstract] |
Monday, November 20, 2017 5:10PM - 5:23PM |
L16.00006: Stochastic Theory for the Clustering of Rapidly Settling, Low-Inertia Particle Pairs in Isotropic Turbulence - I Vijay Gupta, Sarma Rani, Donald Koch A stochastic theory is developed to predict the Radial Distribution Function (RDF) of monodisperse, rapidly settling, low-inertia particle pairs in isotropic turbulence. The theory is based on approximating the turbulent flow in a reference frame following an aerosol particle as a locally linear velocity field. In the first version of the theory (referred to as T1), the fluid velocity gradient tensor ``seen" by the primary aerosol particle is further assumed to be Gaussian. Analytical closures are then derived for the drift and diffusive fluxes controling the RDF, in the asymptotic limits of small particle Stokes number ($St = \tau_p/\tau_\eta \ll 1$), and large dimensionless settling velocity ($Sv = g \tau_p/u_\eta \gg 1$). It is seen that the RDF for rapidly settling pairs has an inverse power dependency on pair separation $r$ with an exponent, $c_1$, that is proportional to $St^2$. However, the $c_1$ predicted by T1 for $Sv \gg 1$ particles is higher than the $c_1$ of even non-settling ($Sv = 0$) particles obtained from DNS of particle-laden isotropic turbulence. Thus, the Gaussian velocity gradient in T1 leads to the unphysical effect that gravity enhances pair clustering. To address this inconsistency, a second version (T2) was developed. [Preview Abstract] |
Monday, November 20, 2017 5:23PM - 5:36PM |
L16.00007: Stochastic Theory for the Clustering of Rapidly Settling, Low-Inertia Particle Pairs in Isotropic Turbulence - II Sarma Rani, Vijay Gupta, Donald Koch A stochastic theory is developed to predict the Radial Distribution Function (RDF) of monodisperse, rapidly settling, low-inertia particle pairs in isotropic turbulence. In the second version of the theory (T2), the dimensionless strain-rate and rotation-rate tensors ``seen" by the primary particle are assumed to be Gaussian distributed, where the strain-rate and rotation-rate tensors are non-dimensionlized using the instantaneous dissipation rate and enstrophy, respectively. Accordingly, closure is again derived for the drift flux driving particle clustering, in the asympotic limits of Stokes number $St = \tau_p/\tau_\eta \ll 1$, and settling paramater $Sv = g \tau_p/u_\eta \gg 1$. Only the drift flux differs for T1 and T2, while the diffusive flux remains the same. The RDFs for rapidly settling pairs again show an inverse power dependency on pair separation $r$ with an exponent, $c_1$, that is proportional to $St^2$. However, in contrast to T1, the $c_1$ values predicted by T2 show good qualitative and resonable quantitative agreement with the $c_1$ values obtained from DNS of settling particles in isotropic turbulence. Further, the T2-predicted $c_1$ values are smaller than those obtained from DNS of non-settling particles in isotropic turbulence. [Preview Abstract] |
Monday, November 20, 2017 5:36PM - 5:49PM |
L16.00008: Settling-driven instabilities with phase-change in mammatus clouds Rama Govindarajan, S. Ravichandran, Eckart Meiburg Consider a horizontal band of supersaturated atmosphere with an initially uniform distribution of small but inertial water droplets. We study the instability of such a layer. We show that the settling of the water droplets into the unsaturated air below the moist layer, combined with accompanying evaporative cooling drives the instability. We obtain scaling laws from dimensional analysis and model simulations in 1D, and compare these with results from numerical simulations. We find that the instability takes a lobe-like form, resembling mammatus clouds, at large liquid water content in the layer, and a string-like form resembling the leaky mode of Burns and Meiburg at lower liquid water content. We propose this as a model for how mammatus clouds, which have long fascinated cloudspotters and atmospheric scientists alike, and asperitas clouds, recently designated as a separate cloud species, form. [Preview Abstract] |
Monday, November 20, 2017 5:49PM - 6:02PM |
L16.00009: Small-scale dynamics of settling, bidisperse particles in turbulence Rohit Dhariwal, Andrew D. Bragg We use DNS to investigate the dynamics of settling, bidisperse particles in isotropic turbulence. In agreement with previous studies, we find that without gravity (i.e. $Fr=\infty$, where $Fr$ is the Froude number), bidispersity leads to an enhancement of the relative velocities, and a suppression of their spatial clustering. For $Fr<1$, the relative velocities in the direction of gravity can be dominated by the large differential settling velocities of the bidisperse particles, as expected. However, we also find that gravity can strongly enhance the relative velocities in the ``horizontal" directions (the plane normal to gravity). This non-trivial behavior occurs because fast settling particles experience rapid fluctuations in the fluid velocity field along their trajectory, leading to enhanced particle accelerations and relative velocities. We also find that gravity drastically reduces the clustering of bidisperse particles. These results are strikingly different to the monodisperse case, for which recent results have shown that when $Fr<1$, gravity strongly suppresses the relative velocities in all directions, and can enhance clustering. Finally, we consider the implications of these results for the collision rates of settling, bidisperse particles in turbulence. [Preview Abstract] |
Monday, November 20, 2017 6:02PM - 6:15PM |
L16.00010: Interface-Resolving Simulation of Collision Efficiency of Cloud Droplets Lian-Ping Wang, Cheng Peng, Bodgan Rosa, Ryo Onishi Small-scale air turbulence could enhance the geometric collision rate of cloud droplets while large-scale air turbulence could augment the diffusional growth of cloud droplets. Air turbulence could also enhance the collision efficiency of cloud droplets. Accurate simulation of collision efficiency, however, requires capture of the multi-scale droplet-turbulence and droplet-droplet interactions, which has only been partially achieved in the recent past using the hybrid direct numerical simulation (HDNS) approach. % where Stokes disturbance flow is assumed. The HDNS approach has two major drawbacks: (1) the short-range droplet-droplet interaction is not treated rigorously; (2) the finite-Reynolds number correction to the collision efficiency is not included. In this talk, using two independent numerical methods, we will develop an interface-resolved simulation approach in which the disturbance flows are directly resolved numerically, combined with a rigorous lubrication correction model for near-field droplet-droplet interaction. This multi-scale approach is first used to study the effect of finite flow Reynolds numbers on the droplet collision efficiency in still air. Our simulation results show a significant finite-Re effect on collision efficiency when the droplets are of similar sizes. Preliminary results on integrating this approach in a turbulent flow laden with droplets will also be presented. [Preview Abstract] |
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