Snowflakes in the atmospheric surface layer: observation of particle–turbulence dynamics

2017 ◽  
Vol 814 ◽  
pp. 592-613 ◽  
Author(s):  
Andras Nemes ◽  
Teja Dasari ◽  
Jiarong Hong ◽  
Michele Guala ◽  
Filippo Coletti
Keyword(s):  
Large Scale ◽  
Isotropic Turbulence ◽  
Volume Fraction ◽  
Response Times ◽  
Field Measurements ◽  
Stokes Number ◽  
Inertial Particles ◽  
Natural Setting ◽  
Scale Particle ◽  
Particle Imaging

We report on optical field measurements of snow settling in atmospheric turbulence at $Re_{\unicode[STIX]{x1D706}}=940$. It is found that the snowflakes exhibit hallmark features of inertial particles in turbulence. The snow motion is analysed in both Eulerian and Lagrangian frameworks by large-scale particle imaging, while sonic anemometry is used to characterize the flow field. Additionally, the snowflake size and morphology are assessed by digital in-line holography. The low volume fraction and mass loading imply a one-way interaction with the turbulent air. Acceleration probability density functions show wide exponential tails consistent with laboratory and numerical studies of homogeneous isotropic turbulence. Invoking the assumption that the particle acceleration has a stronger dependence on the Stokes number than on the specific features of the turbulence (e.g. precise Reynolds number and large-scale anisotropy), we make inferences on the snowflakes’ aerodynamic response time. In particular, we observe that their acceleration distribution is consistent with that of particles of Stokes number in the range $St=0.1{-}0.4$ based on the Kolmogorov time scale. The still-air terminal velocities estimated for the resulting range of aerodynamic response times are significantly smaller than the measured snow particle fall speed. This is interpreted as a manifestation of settling enhancement by turbulence, which is observed here for the first time in a natural setting.

An Investigation of Crossing Trajectory Effects in Turbulent Shear Flow (Keynote)

2005 ◽  
Author(s):  
Lionel Thomas ◽  
Benoiˆt Oesterle´
Keyword(s):  
Shear Flow ◽  
Isotropic Turbulence ◽  
Fluid Velocity ◽  
Stokes Number ◽  
Turbulent Shear ◽  
Inertial Particles ◽  
Turbulent Shear Flow ◽  
Velocity Magnitude ◽  
Dispersed Particles ◽  
Lagrangian Tracking

The dispersion of small inertial particles moving in a homogeneous, hypothetically stationary, shear flow is investigated using both theoretical analysis and numerical simulation, under one-way coupling approximation. In the theoretical approach, the previous studies are extended to the case of homogeneous shear flow with a corresponding anisotropic spectrum. As it is impossible to obtain a closed theoretical solution without some drastic simplifications, the motion of dispersed particles is also investigated using kinematic simulation where random Fourier modes are generated according to a prescribed anisotropic spectrum with a superimposed linear mean fluid velocity profile. The combined effects of particle Stokes number and dimensionless drift velocity (magnitude and direction) are investigated by computing the statistics from Lagrangian tracking of a large number of particles in many flow field realizations, and comparison is made between the observed effects in shear flow and in isotropic turbulence.

On the dynamics of turbulence near a free surface

2017 ◽  
Vol 821 ◽  
pp. 248-265 ◽  
Author(s):  
Oscar Flores ◽  
James J. Riley ◽  
Alexander R. Horner-Devine
Keyword(s):  
Free Surface ◽  
Vertical Velocity ◽  
Large Scale ◽  
Isotropic Turbulence ◽  
Field Measurements ◽  
Inertial Range ◽  
Stratified Flows ◽  
Range Type ◽  
Spectral Behaviour ◽  
Vertical Shearing

We report on direct numerical simulations to examine the spectral behaviour of turbulence close to and at a flat, stress-free surface. We find, consistent with field measurements near such a free surface, that an inertial-range type of behaviour is obtained for the horizontal components of the velocity at and near the stress-free surface, at horizontal wavelengths for which the vertical velocity is much smaller than the horizontal components. At approximately an integral length scale from the stress-free surface, the flow has adjusted back to more classical isotropic turbulence. The behaviour of the turbulence near the stress-free surface is similar to that observed recently for strongly stratified flows, and we argue that the causes of that behaviour are the same in both flows: the suppression of the large-scale vertical velocity and the allowance of strong vertical shearing of the horizontal velocity leading to a downscale transfer of energy and to the development of the$-5/3$spectra for the horizontal velocities.

Reynolds number scaling of inertial particle statistics in turbulent channel flows

2014 ◽  
Vol 758 ◽  
Author(s):  
Matteo Bernardini
Keyword(s):  
Reynolds Number ◽  
Turbulent Flows ◽  
Large Scale ◽  
Spatial Organization ◽  
Reynolds Numbers ◽  
Stokes Number ◽  
Wall Region ◽  
Inertial Particles ◽  
Turbulent Channel Flows ◽  
Strong Segregation

AbstractThe effect of the Reynolds number on the behaviour of inertial particles in wall-bounded turbulent flows is investigated through large-scale direct numerical simulations (DNS) of particle-laden canonical channel flow spanning almost a decade in the friction Reynolds number, from $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}\mathit{Re}_{\tau } = 150$ to $\mathit{Re}_{\tau } = 1000$. Lagrangian particle tracking is used to study the motion of six different particle sets, described by a Stokes number in the range $\mathit{St} = 1\text {--}1000$. At all Reynolds numbers a strong segregation in the near-wall region is observed for particles characterized by intermediate Stokes number, in the range $\mathit{St} =10\text {--}100$. The wall-normal concentration profiles of such particles collapse in inner scaling, thus suggesting the independence of the turbophoretic drift from the large-scale outer motions. This observation is also supported by the spatial organization of the suspended phase in the inner layer, which is found to be universal with the Reynolds number. The deposition rate coefficient increases with $\mathit{Re}_{\tau }$ for a given $\mathit{St}$. Suitable inner and outer scalings are proposed to collapse the deposition curves across the available ranges of Reynolds and Stokes numbers for the different deposition regimes.

Direct numerical simulation of turbulence modulation by particles in compressible isotropic turbulence

2017 ◽  
Vol 832 ◽  
pp. 438-482 ◽  
Author(s):  
Qi Dai ◽  
Kun Luo ◽  
Tai Jin ◽  
Jianren Fan
Keyword(s):  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Rotational Motion ◽  
Isotropic Turbulence ◽  
Volume Fraction ◽  
Stokes Number ◽  
Systematic Investigation ◽  
Turbulence Modulation ◽  
Physical Mechanisms ◽  
Preferential Concentration

In this paper, a systematic investigation of turbulence modulation by particles and its underlying physical mechanisms in decaying compressible isotropic turbulence is performed by using direct numerical simulations with the Eulerian–Lagrangian point-source approach. Particles interact with turbulence through two-way coupling and the initial turbulent Mach number is 1.2. Five simulations with different particle diameters (or initial Stokes numbers, $St_{0}$) are conducted while fixing both their volume fraction and particle densities. The underlying physical mechanisms responsible for turbulence modulation are analysed through investigating the particle motion in the different cases and the transport equations of turbulent kinetic energy, vorticity and dilatation, especially the two-way coupling terms. Our results show that microparticles ($St_{0}\leqslant 0.5$) augment turbulent kinetic energy and the rotational motion of fluid, critical particles ($St_{0}\approx 1.0$) enhance the rotational motion of fluid, and large particles ($St_{0}\geqslant 5.0$) attenuate turbulent kinetic energy and the rotational motion of fluid. The compressibility of the turbulence field is suppressed for all the cases, and the suppression is more significant if the Stokes number of particles is close to 1. The modifications of turbulent kinetic energy, the rotational motion and the compressibility are all related with the particle inertia and distributions, and the suppression of the compressibility is attributed to the preferential concentration and the inertia of particles.

Statistical properties of particle segregation in homogeneous isotropic turbulence

2011 ◽  
Vol 686 ◽  
pp. 338-351 ◽  
Author(s):  
Elena Meneguz ◽  
Michael W. Reeks
Keyword(s):  
Particle Concentration ◽  
Isotropic Turbulence ◽  
Concentration Field ◽  
Threshold Value ◽  
Stokes Number ◽  
Statistical Properties ◽  
Inertial Particles ◽  
Concentration Changes ◽  
Non Gaussian ◽  
Good Agreement

AbstractA full Lagrangian method (FLM) is used in direct numerical simulations (DNS) of incompressible homogeneous isotropic and statistically stationary turbulent flow to measure the statistical properties of the segregation of small inertial particles advected with Stokes drag by the flow. Qualitative good agreement is observed with previous kinematic simulations (KS) (IJzermans, Meneguz & Reeks, J. Fluid Mech., vol. 653, 2010, pp. 99–136): in particular, the existence of singularities in the particle concentration field and a threshold value for the particle Stokes number $\mathit{St}$ above which the net compressibility of the particle concentration changes sign (from compression to dilation). A further KS analysis is carried out by examining the distribution in time of the compression of an elemental volume of particles, which shows that it is close to Gaussian as far as the third and fourth moments but non-Gaussian (within the uncertainties of the measurements) for higher-order moments when the contribution of singularities in the tails of the distribution increasingly dominates the statistics. Measurements of the rate of occurrence of singularities show that it reaches a maximum at $\mathit{St}\ensuremath{\sim} 1$, with the distribution of times between singularities following a Poisson process. Following the approach used by Fevrier, Simonin & Squires (J. Fluid Mech., vol. 553, 2005, pp. 1–46), we also measured the random uncorrelated motion (RUM) and mesoscopic components of the compression for $\mathit{St}= 1$ and show that the non-Gaussian highly intermittent part of the distribution of the compression is associated with the RUM component and ultimately with the occurrence of singularities. This result is consistent with the formation of caustics (Wilkinson et al. Phys. Fluids, vol. 19, 2007, p. 113303), where the activation of singularities precedes the crossing of trajectories (RUM).

Coherent clusters of inertial particles in homogeneous turbulence

2017 ◽  
Vol 833 ◽  
pp. 364-398 ◽  
Author(s):  
Lucia Baker ◽  
Ari Frankel ◽  
Ali Mani ◽  
Filippo Coletti
Keyword(s):  
Isotropic Turbulence ◽  
Stokes Number ◽  
Inertial Particles ◽  
Gravitational Acceleration ◽  
Self Similarity ◽  
Integral Scale ◽  
Heavy Particles ◽  
Diagram Method ◽  
Vorticity Vector ◽  
Definition Of

Despite the widely acknowledged significance of turbulence-driven clustering, a clear topological definition of particle cluster in turbulent dispersed multiphase flows has been lacking. Here we introduce a definition of coherent cluster based on self-similarity, and apply it to distributions of heavy particles in direct numerical simulations of homogeneous isotropic turbulence, with and without gravitational acceleration. Clusters show self-similarity already at length scales larger than twice the Kolmogorov length, as indicated by the fractal nature of their surface and by the power-law decay of their size distribution. The size of the identified clusters extends to the integral scale, with average concentrations that depend on the Stokes number but not on the cluster dimension. Compared to non-clustered particles, coherent clusters show a stronger tendency to sample regions of high strain and low vorticity. Moreover, we find that the clusters align themselves with the local vorticity vector. In the presence of gravity, they tend to align themselves vertically and their fall speed is significantly different from the average settling velocity: for moderate fall speeds they experience stronger settling enhancement than non-clustered particles, while for large fall speeds they exhibit weakly reduced settling. The proposed approach for cluster identification leverages the Voronoï diagram method, but is also compatible with other tessellation techniques such as the classic box-counting method.

On the role of gravity and shear on inertial particle accelerations in near-wall turbulence

2010 ◽  
Vol 658 ◽  
pp. 229-246 ◽  
Author(s):  
V. LAVEZZO ◽  
A. SOLDATI ◽  
S. GERASHCHENKO ◽  
Z. WARHAFT ◽  
L. R. COLLINS
Keyword(s):  
Isotropic Turbulence ◽  
Response Times ◽  
Quantitative Agreement ◽  
Wall Turbulence ◽  
Wall Region ◽  
Inertial Particles ◽  
Inertial Particle ◽  
Homogeneous And Isotropic Turbulence ◽  
Near Wall

Recent experiments in a turbulent boundary layer by Gerashchenko et al. (J. Fluid Mech., vol. 617, 2008, pp. 255–281) showed that the variance of inertial particle accelerations in the near-wall region increased with increasing particle inertia, contrary to the trend found in homogeneous and isotropic turbulence. This behaviour was attributed to the non-trivial interaction of the inertial particles with both the mean shear and gravity. To investigate this issue, we perform direct numerical simulations of channel flow with suspended inertial particles that are tracked in the Lagrangian frame of reference. Three simulations have been carried out considering (i) fluid particles, (ii) inertial particles with gravity and (iii) inertial particles without gravity. For each set of simulations, three particle response times were examined, corresponding to particle Stokes numbers (in wall units) of 0.9, 1.8 and 11.8. Mean and r.m.s. profiles of particle acceleration computed in the simulation are in qualitative (and in several cases quantitative) agreement with the experimental results, supporting the assumptions made in the simulations. Furthermore, by comparing results from simulations with and without gravity, we are able to isolate and quantify the significant effect of gravitational settling on the phenomenon.

The role of collective effects on settling velocity enhancement for inertial particles in turbulence

2018 ◽  
Vol 846 ◽  
pp. 1059-1075 ◽  
Author(s):  
P. D. Huck ◽  
C. Bateson ◽  
R. Volk ◽  
A. Cartellier ◽  
M. Bourgoin ◽  
...  
Keyword(s):  
Settling Velocity ◽  
Volume Fraction ◽  
Stokes Number ◽  
Collective Effects ◽  
Local Concentration ◽  
Inertial Particles ◽  
Particle Accumulation ◽  
Order Of Magnitude ◽  
Conditional Averaging

A particle-laden homogeneous isotropic turbulent flow is studied experimentally to understand the role of collective effects (e.g. particle–particle aerodynamic interactions caused by local particle accumulation) on the settling velocity of inertial particles (Stokes number: $0.3<St<0.6$). Conditional averaging of the particle vertical velocity on the local concentration identifies three settling regimes: modest enhancement by single particle–turbulence interactions in low concentration regions, rapid settling velocity increases in clusters at intermediate concentrations and saturation of the settling enhancement at large concentrations. The latter effect, associated with four-way coupling, displays qualitative agreement with simulations in the literature and is a new experimental observation. Fluctuations up to an order of magnitude larger than the background volume fraction are measured using Voronoï analysis. A model is developed following a classic volume-averaged multiphase flow methodology to provide an interpretation of the three settling regimes and quantitative predictions consistent with the measurements.

Effect of Nanoscale rods on the Kinetics of Phase-Separating Multi-Component Fluids

2004 ◽  
Vol 856 ◽  
Author(s):  
Michael J.A. Hore ◽  
Mohamed Laradji
Keyword(s):  
Phase Separation ◽  
Large Scale ◽  
Volume Fraction ◽  
Three Dimensions ◽  
Domain Growth ◽  
Slowing Down ◽  
Nanoparticles Volume Fraction ◽  
Scale Particle ◽  
Dynamics Simulations ◽  
Kinetics Of

ABSTRACTUsing large scale particle dynamics simulations, we investigated the effect of nanoscale rods on the dynamics of phase separation dynamics of two-component fluids in three dimensions. We found that when the nanoparticles interact more attractively with one of the two segregating component, they lead to a reduction of the rate of domain growth, and that this decrease is intensified as the nanoparticles volume fraction is increased. Furthermore, our results show that nanorods are much more effective in slowing down the kinetics than nanosphres. The dramatic effect of nanorods on the dynamics of phase separation of multi-component fluids, as opposed to nanospheres, implies that they may be used as an efficacious emulsifying agent of multi-component polymer blends.

scholarly journals Dense gas effects in inviscid homogeneous isotropic turbulence

2016 ◽  
Vol 800 ◽  
pp. 140-179 ◽  
Author(s):  
L. Sciacovelli ◽  
P. Cinnella ◽  
C. Content ◽  
F. Grasso
Keyword(s):  
Speed Of Sound ◽  
Large Scale ◽  
Isotropic Turbulence ◽  
Volume Fraction ◽  
Numerical Study ◽  
Dense Gas ◽  
Homogeneous Isotropic Turbulence ◽  
Perfect Gas ◽  
Strong Compression ◽  
Strong Expansion

A detailed numerical study of the influence of dense gas effects on the large-scale dynamics of decaying homogeneous isotropic turbulence is carried out by using the van der Waals gas model. More specifically, we focus on dense gases of the Bethe–Zel’dovich–Thompson type, which may exhibit non-classical nonlinearities in the transonic and supersonic flow regimes, under suitable thermodynamic conditions. The simulations are based on the inviscid conservation equations, solved by means of a ninth-order numerical scheme. The simulations rely on the numerical viscosity of the scheme to dissipate energy at the finest scales, while leaving the larger scales mostly unaffected. The results are systematically compared with those obtained for a perfect gas. Dense gas effects are found to have a significant influence on the time evolution of the average and root mean square (r.m.s.) of the thermodynamic properties for flows characterized by sufficiently high initial turbulent Mach numbers (above 0.5), whereas the influence on kinematic properties, such as the kinetic energy and the vorticity, are smaller. However, the flow dilatational behaviour is very different, due to the non-classical variation of the speed of sound in flow regions where the dense gas is characterized by a value of the fundamental derivative of the gas dynamics (a measure of the variation of the speed of sound in isentropic compressions) smaller than one or even negative. The most significant differences between the perfect and the dense gas case are found for the repartition of dilatation levels in the flow field. For the perfect gas, strong compressions occupy a much larger volume fraction than expansion regions, leading to probability distributions of the velocity divergence highly skewed toward negative values. For the dense gas, the volume fractions occupied by strong expansion and compression regions are much more balanced; moreover, strong expansion regions are characterized by sheet-like structures, unlike the perfect gas which exhibits tubular structures. In strong compression regions, where compression shocklets may occur, both the dense and the perfect gas exhibit sheet-like structures. This suggests the possibility that expansion eddy shocklets may appear in the dense gas. This hypothesis is also supported by the fact that, in dense gas, vorticity is created with equal probability in strong compression and expansion regions, whereas for a perfect gas, vorticity is more likely to be created in the strong compression ones.

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