138 research outputs found
Particle Resuspension in Turbulent Boundary Layers and the Influence of Non-Gaussian Removal Forces
The work described is concerned with the way micron-size particles attached
to a surface are resuspended when exposed to a turbulent flow. An improved
version of the Rock'n'Roll model (Reeks and Hall, 2001) is developed where this
model employs a stochastic approach to resuspension involving the rocking and
rolling of a particle about surface asperities arising from the moments of the
fluctuating drag forces acting on the particle close to the surface. In this
work, the model is improved by using values of both the streamwise fluid
velocity andacceleration close to the wall obtained from Direct Numerical
Simulation (DNS) of turbulentchannel flow. Using analysis and numerical
calculations of the drag force on a sphere near a wall in shear flow (O'Neill
(1968) and Lee and Balachandar (2010)) these values are used to obtain the
joint distribution of the moments of the fluctuating drag force and its time
derivative acting on a particle attached to a surface. In so doing the
influence of highly non-Gaussian forces (associated with the sweeping and
ejection events in a turbulent boundary layer) on short and long term
resuspension rates is examined for a sparse monolayer coverage of particles,
along with the dependence of the resuspension upon the timescale of the
particle motion attached to the surface, the ratio of the rms/ mean of the
removal force and the distribution of adhesive forces. Model predictions of the
fraction resuspended are compared with experimental results.Comment: 31 pages 21 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
Turbulent thermal diffusion in a multi-fan turbulence generator with the imposed mean temperature gradient
We studied experimentally the effect of turbulent thermal diffusion in a
multi-fan turbulence generator which produces a nearly homogeneous and
isotropic flow with a small mean velocity. Using Particle Image Velocimetry and
Image Processing techniques we showed that in a turbulent flow with an imposed
mean vertical temperature gradient (stably stratified flow) particles
accumulate in the regions with the mean temperature minimum. These experiments
detected the effect of turbulent thermal diffusion in a multi-fan turbulence
generator for relatively high Reynolds numbers. The experimental results are in
compliance with the results of the previous experimental studies of turbulent
thermal diffusion in oscillating grids turbulence (Buchholz et al. 2004;
Eidelman et al. 2004). We demonstrated that turbulent thermal diffusion is an
universal phenomenon. It occurs independently of the method of turbulence
generation, and the qualitative behavior of particle spatial distribution in
these very different turbulent flows is similar. Competition between turbulent
fluxes caused by turbulent thermal diffusion and turbulent diffusion determines
the formation of particle inhomogeneities.Comment: 9 pages, 9 figure, REVTEX4, Experiments in Fluids, in pres
Turbulent Diffusion and Turbulent Thermal Diffusion of Aerosols in Stratified Atmospheric Flows
The paper analyzes the phenomenon of turbulent thermal diffusion in the Earth
atmosphere, its relation to the turbulent diffusion and its potential impact on
aerosol distribution. This phenomenon was predicted theoretically more than 10
years ago and detected recently in the laboratory experiments. This effect
causes a non-diffusive flux of aerosols in the direction of the heat flux and
results in formation of long-living aerosol layers in the vicinity of
temperature inversions. We demonstrated that the theory of turbulent thermal
diffusion explains the GOMOS aerosol observations near the tropopause (i.e.,
the observed shape of aerosol vertical profiles with elevated concentrations
located almost symmetrically with respect to temperature profile). In
combination with the derived expression for the dependence of the turbulent
thermal diffusion ratio on the turbulent diffusion, these measurements yield an
independent method for determining the coefficient of turbulent diffusion at
the tropopause. We evaluated the impact of turbulent thermal diffusion to the
lower-troposphere vertical profiles of aerosol concentration by means of
numerical dispersion modelling, and found a regular upward forcing of aerosols
with coarse particles affected stronger than fine aerosols.Comment: 19 pages, 10 figure
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