256 research outputs found
Effective diffusivity of passive scalars in rotating turbulence
We use direct numerical simulations to compute turbulent transport
coefficients for passive scalars in turbulent rotating flows. Effective
diffusion coefficients in the directions parallel and perpendicular to the
rotations axis are obtained by studying the diffusion of an imposed initial
profile for the passive scalar, and calculated by measuring the scalar average
concentration and average spatial flux as a function of time. The Rossby and
Schmidt numbers are varied to quantify their effect on the effective diffusion.
It is find that rotation reduces scalar diffusivity in the perpendicular
direction. The perpendicular diffusion can be estimated from mixing length
arguments using the characteristic velocities and lengths perpendicular to the
rotation axis. Deviations are observed for small Schmidt numbers, for which
turbulent transport decreases and molecular diffusion becomes more significant.Comment: 10 pages, 13 figures. Slightly modified version to address referees'
comment
Inverse cascade behavior in freely decaying two-dimensional fluid turbulence
We present results from an ensemble of 50 runs of two-dimensional
hydrodynamic turbulence with spatial resolution of 2048^2 grid points, and from
an ensemble of 10 runs with 4096^2 grid points. All runs in each ensemble have
random initial conditions with same initial integral scale, energy, enstrophy,
and Reynolds number. When both ensemble- and time-averaged, inverse energy
cascade behavior is observed, even in the absence of external mechanical
forcing: the energy spectrum at scales larger than the characteristic scale of
the flow follows a k^(-5/3) law, with negative flux, together with a k^(-3) law
at smaller scales, and a positive flux of enstrophy. The source of energy for
this behavior comes from the modal energy around the energy containing scale at
t=0. The results shed some light into connections between decaying and forced
turbulence, and into recent controversies in experimental studies of
two-dimensional and magnetohydrodynamic turbulent flows.Comment: 7 pages, 6 figure
Tridimensional to bidimensional transition in magnetohydrodynamic turbulence with a guide field and kinetic helicity injection
We study the transition in dimensionality of a three-dimensional
magnetohydrodynamic flow forced only mechanically, when the strength of a
magnetic guiding field is gradually increased. We use numerical simulations to
consider cases in which the mechanical forcing injects (or not) helicity in the
flow. As the guiding field is increased, the strength of the magnetic field
fluctuations decrease as a power law of the guiding field intensity. We show
that for strong enough guiding fields, the helical magnetohydrodynamic flow can
become almost two-dimensional. In this case, the mechanical energy can undergo
a process compatible with an inverse cascade, being transferred preferentially
towards scales larger than the forcing scale. The presence of helicity changes
the spectral scaling of the small magnetic field fluctuations, and affects the
statistics of the velocity field and of the velocity gradients. Moreover, at
small scales the dynamics of the flow becomes dominated by a direct cascade of
helicity, which can be used to derive scaling laws for the velocity field.Comment: 11 pages, 11 figure
Turbulence comes in bursts in stably stratified flows
There is a clear distinction between simple laminar and complex turbulent
fluids. But in some cases, as for the nocturnal planetary boundary layer, a
stable and well-ordered flow can develop intense and sporadic bursts of
turbulent activity which disappear slowly in time. This phenomenon is
ill-understood and poorly modeled; and yet, it is central to our understanding
of weather and climate dynamics. We present here a simple model which shows
that in stably stratified turbulence, the stronger bursts can occur when the
flow is expected to be more stable. The bursts are generated by a rapid
non-linear amplification of energy stored in waves, and are associated with
energetic interchanges between vertical velocity and temperature (or density)
fluctuations. Direct numerical simulations on grids of 2048^3 points confirm
this somewhat paradoxical result of measurably stronger events for more stable
flows, displayed not only in the temperature and vertical velocity derivatives,
but also in the amplitude of the fields themselves
The spatio-temporal spectrum of turbulent flows
Identification and extraction of vortical structures and of waves in a
disorganised flow is a mayor challenge in the study of turbulence. We present a
study of the spatio-temporal behavior of turbulent flows in the presence of
different restitutive forces. We show how to compute and analyse the
spatio-temporal spectrum from data stemming from numerical simulations and from
laboratory experiments. Four cases are considered: homogeneous and isotropic
turbulence, rotating turbulence, stratified turbulence, and water wave
turbulence. For homogeneous and isotropic turbulence, the spectrum allows
identification of sweeping by the large scale flow. For rotating and for
stratified turbulence, the spectrum allows identification of the waves, precise
quantification of the energy in the waves and in the turbulent eddies, and
identification of physical mechanisms such as Doppler shift and wave absorption
in critical layers. Finally, in water wave turbulence the spectrum shows a
transition from gravity-capillary waves to bound waves as the amplitude of the
forcing is increased.Comment: Added new references and analysi
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