47 research outputs found
Velocity gradients statistics along particle trajectories in turbulent flows: the refined similarity hypothesis in the Lagrangian frame
We present an investigation of the statistics of velocity gradient related
quantities, in particluar energy dissipation rate and enstrophy, along the
trajectories of fluid tracers and of heavy/light particles advected by a
homogeneous and isotropic turbulent flow. The Refined Similarity Hypothesis
(RSH) proposed by Kolmogorov and Oboukhov in 1962 is rephrased in the
Lagrangian context and then tested along the particle trajectories. The study
is performed on state-of-the-art numerical data resulting from numerical
simulations up to Re~400 with 2048^3 collocation points. When particles have
small inertia, we show that the Lagrangian formulation of the RSH is well
verified for time lags larger than the typical response time of the particle.
In contrast, in the large inertia limit when the particle response time
approaches the integral-time-scale of the flow, particles behave nearly
ballistic, and the Eulerian formulation of RSH holds in the inertial-range.Comment: 7 pages, 7 figures; Physical Review E (accepted Dec 7, 2009
Non-Oberbeck-Boussinesq effects in two-dimensional Rayleigh-Benard convection in glycerol
We numerically analyze Non-Oberbeck-Boussinesq (NOB) effects in
two-dimensional Rayleigh-Benard flow in glycerol, which shows a dramatic change
in the viscosity with temperature. The results are presented both as functions
of the Rayleigh number (Ra) up to (for fixed temperature difference
between the top and bottom plates) and as functions of
"non-Oberbeck-Boussinesqness'' or "NOBness'' () up to 50 K (for fixed
Ra). For this large NOBness the center temperature is more than 5 K
larger than the arithmetic mean temperature between top and bottom plate
and only weakly depends on Ra. To physically account for the NOB deviations of
the Nusselt numbers from its Oberbeck-Boussinesq values, we apply the
decomposition of into the product of two effects, namely
first the change in the sum of the top and bottom thermal BL thicknesses, and
second the shift of the center temperature as compared to . While
for water the origin of the deviation is totally dominated by the second
effect (cf. Ahlers et al., J. Fluid Mech. 569, pp. 409 (2006)) for glycerol the
first effect is dominating, in spite of the large increase of as compared
to .Comment: 6 pages, 7 figure
Copepods encounter rates from a model of escape jump behaviour in turbulence
A key ecological parameter for planktonic copepods studies is their
interspecies encounter rate which is driven by their behaviour and is strongly
influenced by turbulence of the surrounding environment. A distinctive feature
of copepods motility is their ability to perform quick displacements, often
dubbed jumps, by means of powerful swimming strokes. Such a reaction has been
associated to an escape behaviour from flow disturbances due to predators or
other external dangers. In the present study, the encounter rate of copepods in
a developed turbulent flow with intensity comparable to the one found in
copepods' habitat is numerically investigated. This is done by means of a
Lagrangian copepod (LC) model that mimics the jump escape reaction behaviour
from localised high-shear rate fluctuations in the turbulent flows. Our
analysis shows that the encounter rate for copepods of typical perception
radius of ~ {\eta}, where {\eta} is the dissipative scale of turbulence, can be
increased by a factor up to ~ 100 compared to the one experienced by passively
transported fluid tracers. Furthermore, we address the effect of introducing in
the LC model a minimal waiting time between consecutive jumps. It is shown that
any encounter-rate enhancement is lost if such time goes beyond the dissipative
time-scale of turbulence, {\tau}_{\eta}. Because typically in the ocean {\eta}
~ 0.001m and {\tau}_{\eta} ~ 1s, this provides stringent constraints on the
turbulent-driven enhancement of encounter-rate due to a purely mechanical
induced escape reaction.Comment: 11 pages, 10 figure
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
Lagrangian filtered density function for LES-based stochastic modelling of turbulent dispersed flows
The Eulerian-Lagrangian approach based on Large-Eddy Simulation (LES) is one
of the most promising and viable numerical tools to study turbulent dispersed
flows when the computational cost of Direct Numerical Simulation (DNS) becomes
too expensive. The applicability of this approach is however limited if the
effects of the Sub-Grid Scales (SGS) of the flow on particle dynamics are
neglected. In this paper, we propose to take these effects into account by
means of a Lagrangian stochastic SGS model for the equations of particle
motion. The model extends to particle-laden flows the velocity-filtered density
function method originally developed for reactive flows. The underlying
filtered density function is simulated through a Lagrangian Monte Carlo
procedure that solves for a set of Stochastic Differential Equations (SDEs)
along individual particle trajectories. The resulting model is tested for the
reference case of turbulent channel flow, using a hybrid algorithm in which the
fluid velocity field is provided by LES and then used to advance the SDEs in
time. The model consistency is assessed in the limit of particles with zero
inertia, when "duplicate fields" are available from both the Eulerian LES and
the Lagrangian tracking. Tests with inertial particles were performed to
examine the capability of the model to capture particle preferential
concentration and near-wall segregation. Upon comparison with DNS-based
statistics, our results show improved accuracy and considerably reduced errors
with respect to the case in which no SGS model is used in the equations of
particle motion
Non-Oberbeck-Boussinesq effects in turbulent thermal convection in ethane close to the critical point
As shown in earlier work (Ahlers et al., J. Fluid Mech. 569, p.409 (2006)),
non-Oberbeck Boussinesq (NOB) corrections to the center temperature in
turbulent Rayleigh-Benard convection in water and also in glycerol are governed
by the temperature dependences of the kinematic viscosity and the thermal
diffusion coefficient. If the working fluid is ethane close to the critical
point the origin of non-Oberbeck-Boussinesq corrections is very different, as
will be shown in the present paper. Namely, the main origin of NOB corrections
then lies in the strong temperature dependence of the isobaric thermal
expansion coefficient \beta(T). More precisely, it is the nonlinear
T-dependence of the density \rho(T) in the buoyancy force which causes another
type of NOB effect. We demonstrate that through a combination of experimental,
numerical, and theoretical work, the latter in the framework of the extended
Prandtl-Blasius boundary layer theory developed in Ahlers et al., J. Fluid
Mech. 569, p.409 (2006). The latter comes to its limits, if the temperature
dependence of the thermal expension coefficient \beta(T) is significant.Comment: 18 pages, 15 figures, 3 table
Universal intermittent properties of particle trajectories in highly turbulent flows
We present a collection of eight data sets, from state-of-the-art experiments
and numerical simulations on turbulent velocity statistics along particle
trajectories obtained in different flows with Reynolds numbers in the range
. Lagrangian structure functions from all data sets
are found to collapse onto each other on a wide range of time lags, revealing a
universal statistics, and calling for a unified theoretical description.
Parisi-Frisch Multifractal theory, suitable extended to the dissipative scales
and to the Lagrangian domain, is found to capture intermittency of velocity
statistics over the whole three decades of temporal scales here investigated.Comment: 5 pages, 1 figure; content changed, references update
A high Gas6 level in plasma predicts venous thromboembolism recurrence, major bleeding and mortality in the elderly: a prospective multicenter cohort study.
Essentials Predictive ability of pro-hemostatic Gas6 for recurrent venous thromboembolism (VTE) is unknown. We measured Gas6 levels in 864 patients with VTE over 3 years. High Gas6 (> 157%) at diagnosis is associated with VTE recurrence, major bleeding and mortality. Gas6 plasma levels measured 12 months after the index VTE are discriminatory for VTE recurrence. SUMMARY: Background Growth arrest-specific gene 6 (Gas6) is a prohemostatic protein with an unknown predictive ability for recurrent venous thromboembolism (VTE). In the elderly, VTE results in higher mortality but does not have a higher rate of recurrence than in younger patients. Consequently, anticoagulation management in the elderly is challenging. Objective To prospectively investigate the performance of Gas6 in predicting VTE recurrence, major bleeding and mortality in the elderly. Methods Consecutive patients aged ≥ 65 years with acute VTE were followed for a period of 3 years. Primary outcomes were symptomatic VTE recurrence, major bleeding, and mortality. Plasma Gas6 was measured with ELISA. Results Gas6 levels were measured in 864 patients at the time of the index VTE (T1) and, in 70% of them, also 12 months later (T2). The Gas6 level at T1 was discriminatory for VTE recurrence (C-statistic, 0.56; 95% confidence interval [CI] 0.51-0.62), major bleeding (0.60, 95% CI 0.55-0.65) and mortality (0.69, 95% CI 0.65-0.73) up to 36 months. VTE recurrence up to 24 months after T2 was discriminated by the Gas6 level at T2 (0.62, 95% CI 0.54-0.71). High Gas6 levels (> 157%) and continuous Gas6 levels at T1 were associated with VTE recurrence up to 6 months and 12 months, respectively. Conclusions In elderly patients, a high Gas6 level is associated with higher risks of VTE recurrence, major bleeding, and death. These findings support further studies to assess the performance of Gas6 in adjusting the length of anticoagulation