215 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
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
Helicity dynamics in stratified turbulence in the absence of forcing
A numerical study of decaying stably-stratified flows is performed.
Relatively high stratification and moderate Reynolds numbers are considered,
and a particular emphasis is placed on the role of helicity (velocity-vorticity
correlations). The problem is tackled by integrating the Boussinesq equations
in a periodic cubical domain using different initial conditions: a non-helical
Taylor-Green (TG) flow, a fully helical Beltrami (ABC) flow, and random flows
with a tunable helicity. We show that for stratified ABC flows helicity
undergoes a substantially slower decay than for unstratified ABC flows. This
fact is likely associated to the combined effect of stratification and large
scale coherent structures. Indeed, when the latter are missing, as in random
flows, helicity is rapidly destroyed by the onset of gravitational waves. A
type of large-scale dissipative "cyclostrophic" balance can be invoked to
explain this behavior. When helicity survives in the system it strongly affects
the temporal energy decay and the energy distribution among Fourier modes. We
discover in fact that the decay rate of energy for stratified helical flows is
much slower than for stratified non-helical flows and can be considered with a
phenomenological model in a way similar to what is done for unstratified
rotating flows. We also show that helicity, when strong, has a measurable
effect on the Fourier spectra, in particular at scales larger than the buoyancy
scale for which it displays a rather flat scaling associated with vertical
shear
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