15,640 research outputs found
A stochastic filter for fluid motion tracking
In this paper we present a method for the tracking of fluid flows velocity fields. The technique we propose is formalized within sequential Bayesian filter framework. The filter we propose here combines an ItĂ´ diffusion process coming from a stochastic formulation of the vorticity-velocity form of Navier-Stokes equation and discrete measurements extracted from an image sequence. The resulting tracker provides robust and consistent estimations of instantaneous motion fields along the whole image sequence. In order to handle a state space of reasonable dimension for the s-tochastic filtering problem, we represent the motion field as a combination of adapted basis functions. The used basis functions ensue from a mollification of Biot-Savart integral and a discretization of the vorticity and divergence maps of the fluid vector field. The efficiency of the method is demonstrated on a long real world sequence showing a vortex launch at tip of airplane wing. 1
The FDF or LES/PDF method for turbulent two-phase flows
In this paper, a new formalism for the filtered density function (FDF)
approach is developed for the treatment of turbulent polydispersed two-phase
flows in LES simulations. Contrary to the FDF used for turbulent reactive
single-phase flows, the present formalislm is based on Lagrangian quantities
and, in particular, on the Lagrangian filtered mass density function (LFMDF) as
the central concept. This framework allows modeling and simulation of particle
flows for LES to be set in a rigorous context and various links with other
approaches to be made. In particular, the relation between LES for particle
simulations of single-phase flows and Smoothed Particle Hydrodynamics (SPH) is
put forward. Then, the discussion and derivation of possible subgrid stochastic
models used for Lagrangian models in two-phase flows can set in a clear
probabilistic equivalence with the corresponding LFMDF.Comment: 11 pages, proceedings of the 13 europena turbulence conference,
submitted to JPC
Residence times of receptors in dendritic spines analyzed by simulations in empirical domains
Analysis of high-density superresolution imaging of receptors reveal the
organization of dendrites at the nano-scale resolution. We present here
simulations in empirical live cell images, which allows converting local
information extracted from short range trajectories into simulations of long
range trajectories. Based on these empirical simulations, we compute the
residence time of an AMPA receptor (AMPAR) in dendritic spines that accounts
for receptors local interactions and geometrical organization. We report here
that depending on the type of the spine, the residence time varies from one to
five minutes. Moreover, we show that there exists transient organized
structures, previously described as potential wells that can regulate the
trafficking of AMPARs to dendritic spines.Comment: 19 page
Some issues concerning Large-Eddy Simulation of inertial particle dispersion in turbulent bounded flows
The problem of an accurate Eulerian-Lagrangian modeling of inertial particle
dispersion in Large Eddy Simulation (LES) of turbulent wall-bounded flows is
addressed. We run Direct Numerical Simulation (DNS) for turbulent channel flow
at shear Reynolds numbers equal to 150 and 300 and corresponding a-priori and
a-posteriori LES on differently coarse grids. We then tracked swarms of
different inertia particles and we examined the influence of filtering and of
Sub-Grid Scale (SGS) modeling for the fluid phase on particle velocity and
concentration statistics. We also focused on how particle preferential
segregation is predicted by LES. Results show that even ``well-resolved'' LES
is unable to reproduce the physics as demonstrated by DNS, both for particle
accumulation at the wall and for particle preferential segregation. Inaccurate
prediction is observed for the entire range of particles considered in this
study, even when the particle response time is much larger than the flow
timescales not resolved in LES. Both a-priori and a-posteriori tests indicate
that recovering the level of fluid and particle velocity fluctuations is not
enough to have accurate prediction of near-wall accumulation and local
segregation. This may suggest that reintroducing the correct amount of
higher-order moments of the velocity fluctuations is also a key point for SGS
closure models for the particle equation. Another important issue is the
presence of possible flow Reynolds number effects on particle dispersion. Our
results show that, in small Reynolds number turbulence and in the case of heavy
particles, the shear fluid velocity is a suitable scaling parameter to quantify
these effects
Lagrangian Structure Functions in Turbulence: A Quantitative Comparison between Experiment and Direct Numerical Simulation
A detailed comparison between data from experimental measurements and
numerical simulations of Lagrangian velocity structure functions in turbulence
is presented. By integrating information from experiments and numerics, a
quantitative understanding of the velocity scaling properties over a wide range
of time scales and Reynolds numbers is achieved. The local scaling properties
of the Lagrangian velocity increments for the experimental and numerical data
are in good quantitative agreement for all time lags. The degree of
intermittency changes when measured close to the Kolmogorov time scales or at
larger time lags. This study resolves apparent disagreements between experiment
and numerics.Comment: 13 RevTeX pages (2 columns) + 8 figures include
Slip-velocity of large neutrally-buoyant particles in turbulent flows
We discuss possible definitions for a stochastic slip velocity that describes
the relative motion between large particles and a turbulent flow. This
definition is necessary because the slip velocity used in the standard drag
model fails when particle size falls within the inertial subrange of ambient
turbulence. We propose two definitions, selected in part due to their
simplicity: they do not require filtration of the fluid phase velocity field,
nor do they require the construction of conditional averages on particle
locations. A key benefit of this simplicity is that the stochastic slip
velocity proposed here can be calculated equally well for laboratory, field,
and numerical experiments. The stochastic slip velocity allows the definition
of a Reynolds number that should indicate whether large particles in turbulent
flow behave (a) as passive tracers; (b) as a linear filter of the velocity
field; or (c) as a nonlinear filter to the velocity field. We calculate the
value of stochastic slip for ellipsoidal and spherical particles (the size of
the Taylor microscale) measured in laboratory homogeneous isotropic turbulence.
The resulting Reynolds number is significantly higher than 1 for both particle
shapes, and velocity statistics show that particle motion is a complex
non-linear function of the fluid velocity. We further investigate the nonlinear
relationship by comparing the probability distribution of fluctuating
velocities for particle and fluid phases
A particle filter to reconstruct a free-surface flow from a depth camera
We investigate the combined use of a Kinect depth sensor and of a stochastic
data assimilation method to recover free-surface flows. More specifically, we
use a Weighted ensemble Kalman filter method to reconstruct the complete state
of free-surface flows from a sequence of depth images only. This particle
filter accounts for model and observations errors. This data assimilation
scheme is enhanced with the use of two observations instead of one classically.
We evaluate the developed approach on two numerical test cases: a collapse of a
water column as a toy-example and a flow in an suddenly expanding flume as a
more realistic flow. The robustness of the method to depth data errors and also
to initial and inflow conditions is considered. We illustrate the interest of
using two observations instead of one observation into the correction step,
especially for unknown inflow boundary conditions. Then, the performance of the
Kinect sensor to capture temporal sequences of depth observations is
investigated. Finally, the efficiency of the algorithm is qualified for a wave
in a real rectangular flat bottom tank. It is shown that for basic initial
conditions, the particle filter rapidly and remarkably reconstructs velocity
and height of the free surface flow based on noisy measurements of the
elevation alone
Statistical properties of an ideal subgrid-scale correction for Lagrangian particle tracking in turbulent channel flow
One issue associated with the use of Large-Eddy Simulation (LES) to
investigate the dispersion of small inertial particles in turbulent flows is
the accuracy with which particle statistics and concentration can be
reproduced. The motion of particles in LES fields may differ significantly from
that observed in experiments or direct numerical simulation (DNS) because the
force acting on the particles is not accurately estimated, due to the
availability of the only filtered fluid velocity, and because errors accumulate
in time leading to a progressive divergence of the trajectories. This may lead
to different degrees of inaccuracy in the prediction of statistics and
concentration. We identify herein an ideal subgrid correction of the a-priori
LES fluid velocity seen by the particles in turbulent channel flow. This
correction is computed by imposing that the trajectories of individual
particles moving in filtered DNS fields exactly coincide with the particle
trajectories in a DNS. In this way the errors introduced by filtering into the
particle motion equations can be singled out and analyzed separately from those
due to the progressive divergence of the trajectories. The subgrid correction
term, and therefore the filtering error, is characterized in the present paper
in terms of statistical moments. The effects of the particle inertia and of the
filter type and width on the properties of the correction term are
investigated.Comment: 15 pages,24 figures. Submitted to Journal of Physics: Conference
Serie
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Cytoplasmic Flow and Mixing Due to Deformation of Motile Cells.
The cytoplasm of a living cell is a dynamic environment through which intracellular components must move and mix. In motile, rapidly deforming cells such as human neutrophils, bulk cytoplasmic flow couples cell deformation to the transport and dispersion of cytoplasmic particles. Using particle-tracking measurements in live neutrophil-like cells, we demonstrate that fluid flow associated with the cell deformation contributes to the motion of small acidic organelles, dominating over diffusion on timescales above a few seconds. We then use a general physical model of particle dispersion in a deforming fluid domain to show that transport of organelle-sized particles between the cell periphery and the bulk can be enhanced by dynamic deformation comparable to that observed in neutrophils. Our results implicate an important mechanism contributing to organelle transport in these motile cells: cytoplasmic flow driven by cell shape deformation
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