28 research outputs found
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
Force measurements on rising bubbles
The dynamics of millimeter sized air bubbles rising through still water are investigated using precise ultrasound velocity measurements combined with high speed video. From measurements of speed and three dimensional tra jectories we deduce the forces on the bubble which give rise to planar zigzag and spiraling motion
An instrumented tracer for Lagrangian measurements in Rayleigh-B\'enard convection
We have developed novel instrumentation for making Lagrangian measurements of
temperature in diverse fluid flows. A small neutrally buoyant capsule is
equipped with on-board electronics which measure temperature and transmit the
data via a wireless radio frequency link to a desktop computer. The device has
80 dB dynamic range, resolving milli-Kelvin changes in temperature with up to
100 ms sampling time. The capabilities of these "smart particles" are
demonstrated in turbulent thermal convection in water. We measure temperature
variations as the particle is advected by the convective motion, and analyse
its statistics. Additional use of cameras allow us to track the particle
position and to report here the first direct measurement of Lagrangian heat
flux transfer in Rayleigh-B{\'e}nard convection. The device shows promise for
opening new research in a broad variety of fluid systems.Comment: 14 page
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
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
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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Pair Dispersion in Turbulence: The Subdominant Role of Scaling
The mixing properties of turbulent flows are, at first order, related to the dynamics of separation of particle pairs. Scaling laws for the evolution in time of the mean distance between particle pairs (t) have been proposed since the pioneering work of Richardson.We analyze amodel which shares some featureswith 3D experimental and numerical turbulence, and suggest that pure scaling laws are only subdominant. The dynamics is dominated by a very wide distribution of ‘‘delay times’’ t_d, the duration for which particle pairs remain together before their separation increases significantly. The delay time distribution is exponential for small separations and evolves towards a flat distribution at large separations. The observed (t) behavior is best understood as an average over separations that individually follow the Richardson-Obukhov scaling, r^2 propto t^3, but each only after a fluctuating time delay t_d,where t_d is distributed uniformly
Transition from hydrodynamic turbulence to magnetohydrodynamic turbulence in von Kármán flows
International audienceThe influence of an externally applied magnetic field on flow turbulence is investigated in liquid-gallium von-Ka ́rma ́n (VK) swirling flows. Time-resolved measurements of global variables (such as the flow power consumption) and local recordings of the induced magnetic field are made. From these measurements, an effective Reynolds number is introduced as Rmeff = Rm(1−α√N), so as to take into account the influence of the interaction parameter N. This effective magnetic Reynolds number leads to unified scalings for both global variables and the locally induced magnetic field. In addition, when the flow rotation axis is perpendicular to the direction of the applied magnetic field, significant flow and induced magnetic field fluctuations are observed at low interaction parameter values, but corresponding to an Alfve'n speed vA of the order of the fluid velocity fluctuations urms. This strong increase in the flow fluctuations is attributed to chaotic changes between hydrodynamic and magnetohydrodynamic velocity profiles
Dynamo Regimes with a Nonhelical Forcing
A three-dimensional numerical computation of magnetohydrodynamic dynamo behavior is described. The dynamo is mechanically forced with a driving term of the Taylor-Green type. The magnetic field development is followed from negligibly small levels to saturated values that occur at magnetic energies comparable to the kinetic energies. Although there is locally a nonzero helicity density, there is no overall integrated helicity in the system. Persistent oscillations are observed in the saturated state for not-too-large mechanical Reynolds numbers, oscillations in which the kinetic and magnetic energies vary out of phase but with no reversal of the magnetic field. The flow pattern exhibits considerable geometrical structure in this regime. As the Reynolds number is increased, the oscillations disappear and the energies become more nearly stationary, but retain some unsystematically fluctuating turbulent time dependence. The regular geometrical structure of the fields gives way to a more spatially disordered distribution. The injection and dissipation scales are identified, and the different components of energy transfer in Fourier space are analyzed, particularly in the context of clarifying the role played by different flow scales in the amplification of the magnetic field. We observe that small and large scales interact and contribute to the dynamo process
Guidelines for the use and interpretation of assays for monitoring autophagy (3rd edition)
In 2008 we published the first set of guidelines for standardizing research in autophagy. Since then, research on this topic has continued to accelerate, and many new scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Accordingly, it is important to update these guidelines for monitoring autophagy in different organisms. Various reviews have described the range of assays that have been used for this purpose. Nevertheless, there continues to be confusion regarding acceptable methods to measure autophagy, especially in multicellular eukaryotes. For example, a key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers or volume of autophagic elements (e.g., autophagosomes or autolysosomes) at any stage of the autophagic process versus those that measure fl ux through the autophagy pathway (i.e., the complete process including the amount and rate of cargo sequestered and degraded). In particular, a block in macroautophagy that results in autophagosome accumulation must be differentiated from stimuli that increase autophagic activity, defi ned as increased autophagy induction coupled with increased delivery to, and degradation within, lysosomes (inmost higher eukaryotes and some protists such as Dictyostelium ) or the vacuole (in plants and fungi). In other words, it is especially important that investigators new to the fi eld understand that the appearance of more autophagosomes does not necessarily equate with more autophagy. In fact, in many cases, autophagosomes accumulate because of a block in trafficking to lysosomes without a concomitant change in autophagosome biogenesis, whereas an increase in autolysosomes may reflect a reduction in degradative activity. It is worth emphasizing here that lysosomal digestion is a stage of autophagy and evaluating its competence is a crucial part of the evaluation of autophagic flux, or complete autophagy. Here, we present a set of guidelines for the selection and interpretation of methods for use by investigators who aim to examine macroautophagy and related processes, as well as for reviewers who need to provide realistic and reasonable critiques of papers that are focused on these processes. These guidelines are not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to monitor autophagy. Along these lines, because of the potential for pleiotropic effects due to blocking autophagy through genetic manipulation it is imperative to delete or knock down more than one autophagy-related gene. In addition, some individual Atg proteins, or groups of proteins, are involved in other cellular pathways so not all Atg proteins can be used as a specific marker for an autophagic process. In these guidelines, we consider these various methods of assessing autophagy and what information can, or cannot, be obtained from them. Finally, by discussing the merits and limits of particular autophagy assays, we hope to encourage technical innovation in the field