730 research outputs found
Passive scalar intermittency in compressible flow
A compressible generalization of the Kraichnan model (Phys. Rev. Lett. 72,
1016 (1994)) of passive scalar advection is considered. The dynamical role of
compressibility on the intermittency of the scalar statistics is investigated
for the direct cascade regime. Simple physical arguments suggest that an
enhanced intermittency should appear for increasing compressibility, due to the
slowing down of Lagrangian trajectory separations. This is confirmed by a
numerical study of the dependence of intermittency exponents on the degree of
compressibility, by a Lagrangian method for calculating simultaneous N-point
tracer correlations.Comment: 4 pages, 3 figures Revised version, accepted for publication in PRE -
Rapid communication
About coherent structures in random shell models for passive scalar advection
A study of anomalous scaling in models of passive scalar advection in terms
of singular coherent structures is proposed. The stochastic dynamical system
considered is a shell model reformulation of Kraichnan model. We extend the
method introduced in \cite{DDG99} to the calculation of self-similar instantons
and we show how such objects, being the most singular events, are appropriate
to capture asymptotic scaling properties of the scalar field. Preliminary
results concerning the statistical weight of fluctuations around these optimal
configurations are also presented.Comment: 4 pages, 2 postscript figures, submitted to PR
Isotropy vs anisotropy in small-scale turbulence
The decay of large-scale anisotropies in small-scale turbulent flow is
investigated. By introducing two different kinds of estimators we discuss the
relation between the presence of a hierarchy for the isotropic and the
anisotropic scaling exponents and the persistence of anisotropies. Direct
measurements from a channel flow numerical simulation are presented.Comment: 7 pages, 2 figure
Acceleration statistics of heavy particles in turbulence
We present the results of direct numerical simulations of heavy particle
transport in homogeneous, isotropic, fully developed turbulence, up to
resolution (). Following the trajectories of up
to 120 million particles with Stokes numbers, , in the range from 0.16 to
3.5 we are able to characterize in full detail the statistics of particle
acceleration. We show that: ({\it i}) The root-mean-squared acceleration
sharply falls off from the fluid tracer value already at quite
small Stokes numbers; ({\it ii}) At a given the normalised acceleration
increases with consistently
with the trend observed for fluid tracers; ({\it iii}) The tails of the
probability density function of the normalised acceleration
decrease with . Two concurrent mechanisms lead to the above results:
preferential concentration of particles, very effective at small , and
filtering induced by the particle response time, that takes over at larger
.Comment: 10 pages, 3 figs, 2 tables. A section with new results has been
added. Revised version accepted for pubblication on Journal of Fluid
Mechanic
The decay of homogeneous anisotropic turbulence
We present the results of a numerical investigation of three-dimensional
decaying turbulence with statistically homogeneous and anisotropic initial
conditions. We show that at large times, in the inertial range of scales: (i)
isotropic velocity fluctuations decay self-similarly at an algebraic rate which
can be obtained by dimensional arguments; (ii) the ratio of anisotropic to
isotropic fluctuations of a given intensity falls off in time as a power law,
with an exponent approximately independent of the strength of the fluctuation;
(iii) the decay of anisotropic fluctuations is not self-similar, their
statistics becoming more and more intermittent as time elapses. We also
investigate the early stages of the decay. The different short-time behavior
observed in two experiments differing by the phase organization of their
initial conditions gives a new hunch on the degree of universality of
small-scale turbulence statistics, i.e. its independence of the conditions at
large scales.Comment: 9 pages, 17 figure
Lagrangian statistics of particle pairs in homogeneous isotropic turbulence
We present a detailed investigation of the particle pair separation process
in homogeneous isotropic turbulence. We use data from direct numerical
simulations up to Taylor's Reynolds number 280 following the evolution of about
two million passive tracers advected by the flow over a time span of about
three decades. We present data for both the separation distance and the
relative velocity statistics. Statistics are measured along the particle pair
trajectories both as a function of time and as a function of their separation,
i.e. at fixed scales. We compare and contrast both sets of statistics in order
to gain an insight into the mechanisms governing the separation process. We
find very high levels of intermittency in the early stages, that is, for travel
times up to order ten Kolmogorov time scales. The fixed scale statistics allow
us to quantify anomalous corrections to Richardson diffusion in the inertial
range of scales for those pairs that separate rapidly. It also allows a
quantitative analysis of intermittency corrections for the relative velocity
statistics.Comment: 16 pages, 16 figure
Potential of hydrogen addition to natural gas or ammonia as a solution towards low- or zero-carbon fuel for the supply of a small turbocharged SI engine
Nowadays there is an increasing interest in carbon-free fuels such as ammonia and hydrogen. Those fuels, on one hand, allow to drastically reduce CO2 emissions, helping to comply with the increasingly stringent emission regulations, and, on the other hand, could lead to possible advantages in performances if blended with conventional fuels. In this regard, this work focuses on the 1D numerical study of an internal combustion engine supplied with different fuels: pure gasoline, and blends of methane-hydrogen and ammonia-hydrogen. The analyses are carried out with reference to a downsized turbocharged two-cylinder engine working in an operating point representative of engine operations along WLTC, namely 1800 rpm and 9.4 bar of BMEP. To evaluate the potential of methane-hydrogen and ammonia-hydrogen blends, a parametric study is performed. The varied parameters are air/fuel proportions (from 1 up to 2) and the hydrogen fraction over the total fuel. Hydrogen volume percentages up to 60% are considered both in the case of methane-hydrogen and ammonia-hydrogen blends. Model predictive capabilities are enhanced through a refined treatment of the laminar flame speed and chemistry of the end gas to improve the description of the combustion process and knock phenomenon, respectively. After the model validation under pure gasoline supply, numerical analyses allowed to estimate the benefits and drawbacks of considered alternative fuels in terms of efficiency, carbon monoxide, and pollutant emissions
Anomalous and dimensional scaling in anisotropic turbulence
We present a numerical study of anisotropic statistical fluctuations in
homogeneous turbulent flows. We give an argument to predict the dimensional
scaling exponents, (p+j)/3, for the projections of p-th order structure
function in the j-th sector of the rotational group. We show that measured
exponents are anomalous, showing a clear deviation from the dimensional
prediction. Dimensional scaling is subleading and it is recovered only after a
random reshuffling of all velocity phases, in the stationary ensemble. This
supports the idea that anomalous scaling is the result of a genuine inertial
evolution, independent of large-scale behavior.Comment: 4 pages, 3 figure
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
- …