89 research outputs found
Lagrangian velocity and acceleration correlations of large inertial particles in a closed turbulent flow
We investigate the response of large inertial particle to turbulent
fluctuations in a inhomogeneous and anisotropic flow. We conduct a Lagrangian
study using particles both heavier and lighter than the surrounding fluid, and
whose diameters are comparable to the flow integral scale. Both velocity and
acceleration correlation functions are analyzed to compute the Lagrangian
integral time and the acceleration time scale of such particles. The knowledge
of how size and density affect these time scales is crucial in understanding
partical dynamics and may permit stochastic process modelization using two-time
models (for instance Saw-ford's). As particles are tracked over long times in
the quasi totality of a closed flow, the mean flow influences their behaviour
and also biases the velocity time statistics, in particular the velocity
correlation functions. By using a method that allows for the computation of
turbulent velocity trajectories, we can obtain unbiased Lagrangian integral
time. This is particularly useful in accessing the scale separation for such
particles and to comparing it to the case of fluid particles in a similar
configuration
Do finite size neutrally buoyant particles cluster?
We investigate the preferential concentration of particles which are
neutrally buoyant but with a diameter significantly larger than the dissipation
scale of the carrier flow. Such particles are known not to behave as flow
tracers (Qureshi et al., Phys. Re. Lett. 2007) but whether they do cluster or
not remains an open question. For this purpose, we take advantage of a new
turbulence generating apparatus, the Lagrangian Exploration Module which
produces homogeneous and isotropic turbulence in a closed water flow. The flow
is seeded with neutrally buoyant particles with diameter 700\mum, corresponding
to 4.4 to 17 times the turbulent dissipation scale when the rotation frequency
of the impellers driving the flow goes from 2 Hz to 12 Hz, and spanning a range
of Stokes numbers from 1.6 to 24.2. The spatial structuration of these
inclusions is then investigated by a Voronoi tesselation analysis, as recently
proposed by Monchaux et al. (Phys. Fluids 2010), from images of particle
concentration field taken in a laser sheet at the center of the flow. No matter
the rotating frequency and subsequently the Reynolds and Stokes numbers, the
particles are found not to cluster. The Stokes number by itself is therefore
shown to be an insufficient indicator of the clustering trend in particles
laden flows
Impact of trailing wake drag on the statistical properties and dynamics of finite-sized particle in turbulence
We study by means of an Eulerian-Lagrangian model the statistical properties
of velocity and acceleration of a neutrally-buoyant finite-sized particle in a
turbulent flow statistically homogeneous and isotropic. The particle equation
of motion, beside added mass and steady Stokes drag, keeps into account the
unsteady Stokes drag force - known as Basset-Boussinesq history force - and the
non-Stokesian drag based on Schiller-Naumann parametrization, together with the
finite-size Faxen corrections. We focus on the case of flow at low
Taylor-Reynolds number, Re_lambda ~ 31, for which fully resolved numerical data
which can be taken as a reference are available (Homann & Bec 651 81-91 J.
Fluid Mech. (2010)). Remarkably, we show that while drag forces have always
minor effects on the acceleration statistics, their role is important on the
velocity behavior. We propose also that the scaling relations for the particle
velocity variance as a function of its size, which have been first detected in
fully resolved simulations, does not originate from inertial-scale properties
of the background turbulent flow but it is likely to arise from the
non-Stokesian component of the drag produced by the wake behind the particle.
Furthermore, by means of comparison with fully resolved simulations, we show
that the Faxen correction to the added mass has a dominant role in the particle
acceleration statistics even for particle with size in the inertial range.Comment: 9 pages, 9 figure
Large spheres motion in a non homogeneous turbulent flow
We investigate the dynamics of very large particles freely advected in a
turbulent von Karman flow. Contrary to other experiments for which the particle
dynamics is generally studied near the geometrical center of the flow, we track
the particles in the whole experiment volume. We observe a strong influence of
the mean structure of the flow that generates an unexpected large-scale
sampling effect for the larger particles studied; contrary to neutrally buoyant
particles of smaller yet finite sizes that exhibit no preferential
concentration in homogeneous and isotropic turbulence (Fiabane et al., Phys.
Rev. E 86(3), 2012). We find that particles whose diameter approaches the flow
integral length scale explore the von Karman flow non-uniformly, with a higher
probability to move in the vicinity of two tori situated near the poloidal
neutral lines. This preferential sampling is quite robust with respect to
changes of any varied parameters: Reynolds number, particle density and
particle surface roughness
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
Tracking the dynamics of translation and absolute orientation of a sphere in a turbulent flow
We study the 6-dimensional dynamics -- position and orientation -- of a large
sphere advected by a turbulent flow. The movement of the sphere is recorded
with 2 high-speed cameras. Its orientation is tracked using a novel, efficient
algorithm; it is based on the identification of possible orientation
`candidates' at each time step, with the dynamics later obtained from
maximization of a likelihood function. Analysis of the resulting linear and
angular velocities and accelerations reveal a surprising intermittency for an
object whose size lies in the integral range, close to the integral scale of
the underlying turbulent flow
Rotational intermittency and turbulence induced lift experienced by large particles in a turbulent flow
The motion of a large, neutrally buoyant, particle, freely advected by a
turbulent flow is determined experimentally. We demonstrate that both the
translational and angular accelerations exhibit very wide probability
distributions, a manifestation of intermittency. The orientation of the angular
velocity with respect to the trajectory, as well as the translational
acceleration conditioned on the spinning velocity provide evidence of a lift
force acting on the particle.Comment: 4 page, 4 figure
Diffusiophoresis at the macroscale
Diffusiophoresis, a ubiquitous phenomenon that induces particle transport
whenever solute concentration gradients are present, was recently observed in
the context of microsystems and shown to strongly impact colloidal transport
(patterning and mixing) at such scales. In the present work, we show
experimentally that this nanoscale mechanism can induce changes in the
macroscale mixing of colloids by chaotic advection. Rather than the decay of
the standard deviation of concentration, which is a global parameter commonly
employed in studies of mixing, we instead use multiscale tools adapted from
studies of chaotic flows or intermittent turbulent mixing: concentration
spectra and second and fourth moments of the probability density functions of
scalar gradients. Not only can these tools be used in open flows, but they also
allow for scale-by-scale analysis. Strikingly, diffusiophoresis is shown to
affect all scales, although more particularly the small ones, resulting in a
change of scalar intermittency and in an unusual scale bridging spanning more
than seven orders of magnitude. By quantifying the averaged impact of
diffusiophoresis on the macroscale mixing, we explain why the effects observed
are consistent with the introduction of an effective P\'eclet number.Comment: 13 page
Measurement of particle and bubble accelerations in turbulence
4 pagesWe use an extended laser Doppler technique to track optically the velocity of individual particles in a high Reynolds number turbulent flow. The particle sizes are of the order of the Kolmogorov scale and the time resolution, 30 microseconds, resolves the fastest scales of the fluid motion. Particles are tracked for mean durations of the order of 10 Kolmogorov time scales. The fastest scales of the particle motion are resolved and the particle acceleration is measured. For neutrally buoyant particles, our measurement matches the performance of the silicon strip detector technique introduced at Cornell University~\cite{Voth,MordantCornell}. This reference dynamics is then compared to that of slightly heavier solid particles (density 1.4) and to air bubbles. We observe that the acceleration variance strongly depends on the particle density: bubbles experience higher accelerations than fluid particles, while heavier particles have lower accelerations. We find that the probability distribution functions of accelerations normalized to the variance are very close although the air bubbles have a much faster dynamics
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