558 research outputs found
Effect of turbulence on collisions of dust particles with planetesimals in protoplanetary disks
Planetesimals in gaseous protoplanetary disks may grow by collecting dust
particles. Hydrodynamical studies show that small particles generally avoid
collisions with the planetesimals because they are entrained by the flow around
them. This occurs when , the Stokes number, defined as the ratio of the
dust stopping time to the planetesimal crossing time, becomes much smaller than
unity. However, these studies have been limited to the laminar case, whereas
these disks are believed to be turbulent. We want to estimate the influence of
gas turbulence on the dust-planetesimal collision rate and on the impact
speeds. We used three-dimensional direct numerical simulations of a fixed
sphere (planetesimal) facing a laminar and turbulent flow seeded with small
inertial particles (dust) subject to a Stokes drag. A no-slip boundary
condition on the planetesimal surface is modeled via a penalty method. We find
that turbulence can significantly increase the collision rate of dust particles
with planetesimals. For a high turbulence case (when the amplitude of turbulent
fluctuations is similar to the headwind velocity), we find that the collision
probability remains equal to the geometrical rate or even higher for , i.e., for dust sizes an order of magnitude smaller than in the laminar
case. We derive expressions to calculate impact probabilities as a function of
dust and planetesimal size and turbulent intensity
Universality of Velocity Gradients in Forced Burgers Turbulence
It is demonstrated that Burgers turbulence subject to large-scale
white-noise-in-time random forcing has a universal power-law tail with exponent
-7/2 in the probability density function of negative velocity gradients, as
predicted by E, Khanin, Mazel and Sinai (1997, Phys. Rev. Lett. 78, 1904). A
particle and shock tracking numerical method gives about five decades of
scaling. Using a Lagrangian approach, the -7/2 law is related to the shape of
the unstable manifold associated to the global minimizer.Comment: 4 pages, 2 figures, RevTex4, published versio
Ergodic properties of a model for turbulent dispersion of inertial particles
We study a simple stochastic differential equation that models the dispersion
of close heavy particles moving in a turbulent flow. In one and two dimensions,
the model is closely related to the one-dimensional stationary Schroedinger
equation in a random delta-correlated potential. The ergodic properties of the
dispersion process are investigated by proving that its generator is
hypoelliptic and using control theory
Caustics in turbulent aerosols
Networks of caustics can occur in the distribution of particles suspended in
a randomly moving gas. These can facilitate coagulation of particles by
bringing them into close proximity, even in cases where the trajectories do not
coalesce. We show that the long-time morphology of these caustic patterns is
determined by the Lyapunov exponents lambda_1, lambda_2 of the suspended
particles, as well as the rate J at which particles encounter caustics. We
develop a theory determining the quantities J, lambda_1, lambda_2 from the
statistical properties of the gas flow, in the limit of short correlation
times.Comment: 4 pages, 3 figure
Dynamics and statistics of heavy particles in turbulent flows
We present the results of Direct Numerical Simulations (DNS) of turbulent
flows seeded with millions of passive inertial particles. The maximum Taylor's
Reynolds number is around 200. We consider particles much heavier than the
carrier flow in the limit when the Stokes drag force dominates their dynamical
evolution. We discuss both the transient and the stationary regimes. In the
transient regime, we study the growt of inhomogeneities in the particle spatial
distribution driven by the preferential concentration out of intense vortex
filaments. In the stationary regime, we study the acceleration fluctuations as
a function of the Stokes number in the range [0.16:3.3]. We also compare our
results with those of pure fluid tracers (St=0) and we find a critical behavior
of inertia for small Stokes values. Starting from the pure monodisperse
statistics we also characterize polydisperse suspensions with a given mean
Stokes.Comment: 13 pages, 10 figures, 2 table
Transition from ergodic to explosive behavior in a family of stochastic differential equations
We study a family of quadratic stochastic differential equations in the
plane, motivated by applications to turbulent transport of heavy particles.
Using Lyapunov functions, we find a critical parameter value
such that when the system is
ergodic and when solutions are not defined for all
times. H\"{o}rmander's hypoellipticity theorem and geometric control theory are
also utilized.Comment: 35 pages, 6 figure
Anisotropic clustering of inertial particles in homogeneous shear flow
Recently, clustering of inertial particles in turbulence has been thoroughly
analyzed for statistically homogeneous isotropic flows. Phenomenologically,
spatial homogeneity of particles configurations is broken by the advection of a
range of eddies determined by the Stokes relaxation time of the particles which
results in a multi-scale distribution of local concentrations and voids. Much
less is known concerning anisotropic flows. Here, by addressing direct
numerical simulations (DNS) of a statistically steady particle-laden
homogeneous shear flow, we provide evidence that the mean shear preferentially
orients particle patterns. By imprinting anisotropy on large scales velocity
fluctuations, the shear indirectly affects the geometry of the clusters.
Quantitative evaluation is provided by a purposely designed tool, the angular
distribution function of particle pairs (ADF), which allows to address the
anisotropy content of particles aggregates on a scale by scale basis. The data
provide evidence that, depending on the Stokes relaxation time of the
particles, anisotropic clustering may occur even in the range of scales where
the carrier phase velocity field is already recovering isotropy. The strength
of the singularity in the anisotropic component of the ADF quantifies the level
of fine scale anisotropy, which may even reach values of more than 30%
direction-dependent variation in the probability to find two close-by particles
at viscous scale separation.Comment: To appear in Journal Fluid Mechanics 200
Superdiffusion of massive particles induced by multi-scale velocity fields
We study drag-induced diffusion of massive particles in scale-free velocity
fields, where superdiffusive behavior emerges due to the scale-free size
distribution of the vortices of the underlying velocity field. The results show
qualitative resemblance to what is observed in fluid systems, namely the
diffusive exponent for the mean square separation of pairs of particles and the
preferential concentration of the particles, both as a function of the response
time.Comment: 5 pages, 5 figures. Accepted for publication in EP
Effect of turbulence on collisions of dust particles with planetesimals in protoplanetary disks
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