41,103 research outputs found
Effect of particle inertia on the turbulence in a suspension
We propose a one-fluid analytical model for a turbulently flowing dilute
suspension, based on modified Navier-Stokes equation with a -dependent
effective density of suspension, , and an additional damping
term , representing the fluid-particle friction
(described by Stokes law). The statistical description of turbulence within the
model is simplified by a modification of the usual closure procedure based on
the Richardson-Kolmogorov picture of turbulence with a differential
approximation for the energy transfer term. The resulting ordinary differential
equation for the energy budget is solved analytically for various important
limiting cases and numerically in the general case. In the inertial interval of
scales we describe analytically two competing effects: the energy suppression
due to the fluid particle friction and the energy enhancement during the
cascade process due to decrease of the effective density of the small scale
motions. An additional suppression or enhancement of the energy density may
occur in the viscous subrange, caused by the variation of the extent of the
inertial interval due to the combined effect of the fluid-particle friction and
the decrease of the kinematic viscosity of the suspensions. The analytical
description of the complicated interplay of these effects supported by
numerical calculations is presented. Our findings allow one to rationalize the
qualitative picture of the isotropic homogeneous turbulence of dilute
suspensions as observed in direct numerical simulations.Comment: 21 pages, 5 figues,included, PRE, submitte
The influence of temperature dynamics and dynamic finite ion Larmor radius effects on seeded high amplitude plasma blobs
Thermal effects on the perpendicular convection of seeded pressure blobs in
the scrape-off layer of magnetised fusion plasmas are investigated. Our
numerical study is based on a four field full-F gyrofluid model, which entails
the consistent description of high fluctuation amplitudes and dynamic finite
Larmor radius effects. We find that the maximal radial blob velocity increases
with the square root of the initial pressure perturbation and that a finite
Larmor radius contributes to highly compact blob structures that propagate in
the poloidal direction. An extensive parameter study reveals that a smooth
transition to this compact blob regime occurs when the finite Larmor radius
effect strength, defined by the ratio of the magnetic field aligned component
of the ion diamagnetic to the vorticity, exceeds unity.
The maximal radial blob velocities agree excellently with the inertial velocity
scaling law over more than an order of magnitude. We show that the finite
Larmor radius effect strength affects the poloidal and total particle transport
and present an empirical scaling law for the poloidal and total blob
velocities. Distinctions to the blob behaviour in the isothermal limit with
constant finite Larmor radius effects are highlighted
Nonlinear damping of slab modes and cosmic ray transport
By applying recent results for the slab correlation time scale onto cosmic
ray scattering theory, we compute cosmic ray parallel mean free paths within
the quasilinear limit. By employing these results onto charged particle
transport in the solar system, we demonstrate that much larger parallel mean
free paths can be obtained in comparison to previous results. A comparison with
solar wind observations is also presented to show that the new theoretical
results are much closer to the observations than the previous results
Acceleration statistics of finite-sized particles in turbulent flow: the role of Faxen forces
The dynamics of particles in turbulence when the particle-size is larger than
the dissipative scale of the carrier flow is studied. Recent experiments have
highlighted signatures of particles finiteness on their statistical properties,
namely a decrease of their acceleration variance, an increase of correlation
times -at increasing the particles size- and an independence of the probability
density function of the acceleration once normalized to their variance. These
effects are not captured by point particle models. By means of a detailed
comparison between numerical simulations and experimental data, we show that a
more accurate model is obtained once Faxen corrections are included.Comment: 10 pages, 4 figure
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