221 research outputs found
Structures and intermittency in a passive scalar model
A one-dimensional white-in-time passive scalar model is introduced. Strong
and persistent structures are shown to be present. A perturbative expansion for
the scaling exponents is performed around a Gaussian limit of the model. The
resulting predictions are compared with numerical simulations.Comment: 8 pages, 4 figure
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
Multiple-scale analysis and renormalization for pre-asymptotic scalar transport
Pre-asymptotic transport of a scalar quantity passively advected by a
velocity field formed by a large-scale component superimposed to a small-scale
fluctuation is investigated both analytically and by means of numerical
simulations. Exploiting the multiple-scale expansion one arrives at a
Fokker--Planck equation which describes the pre-asymptotic scalar dynamics.
Such equation is associated to a Langevin equation involving a multiplicative
noise and an effective (compressible) drift. For the general case, no explicit
expression for both the effective drift and the effective diffusivity (actually
a tensorial field) can be obtained. We discuss an approximation under which an
explicit expression for the diffusivity (and thus for the drift) can be
obtained. Its expression permits to highlight the important fact that the
diffusivity explicitly depends on the large-scale advecting velocity. Finally,
the robustness of the aforementioned approximation is checked numerically by
means of direct numerical simulations.Comment: revtex4, 12 twocolumn pages, 3 eps figure
Different transport regimes in a spatially-extended recirculating background
Passive scalar transport in a spatially-extended background of roll
convection is considered in the time-periodic regime. The latter arises due to
the even oscillatory instability of the cell lateral boundary, here accounted
for by sinusoidal oscillations of frequency . By varying the latter
parameter, the strength of anticorrelated regions of the velocity field can be
controled and the conditions under which either an enhancement or a reduction
of transport takes place can be created. Such two ubiquitous regimes are
triggered by a small-scale(random) velocity field superimposed to the
recirculating background. The crucial point is played by the dependence of
Lagrangian trajectories on the statistical properties of the small-scale
velocity field, e.g. its correlation time or its energy.Comment: 9 pages Latex; 5 figure
Intermittency in passive scalar advection
A Lagrangian method for the numerical simulation of the Kraichnan passive
scalar model is introduced. The method is based on Monte--Carlo simulations of
tracer trajectories, supplemented by a point-splitting procedure for coinciding
points. Clean scaling behavior for scalar structure functions is observed. The
scheme is exploited to investigate the dependence of scalar anomalies on the
scaling exponent of the advecting velocity field. The three-dimensional
fourth-order structure function is specifically considered.Comment: 4 pages, 5 figure
Large-scale effects on meso-scale modeling for scalar transport
The transport of scalar quantities passively advected by velocity fields with
a small-scale component can be modeled at meso-scale level by means of an
effective drift and an effective diffusivity, which can be determined by means
of multiple-scale techniques. We show that the presence of a weak large-scale
flow induces interesting effects on the meso-scale scalar transport. In
particular, it gives rise to non-isotropic and non-homogeneous corrections to
the meso-scale drift and diffusivity. We discuss an approximation that allows
us to retain the second-order effects caused by the large-scale flow. This
provides a rather accurate meso-scale modeling for both asymptotic and
pre-asymptotic scalar transport properties. Numerical simulations in model
flows are used to illustrate the importance of such large-scale effects.Comment: 19 pages, 8 figure
Local log-law of the wall: numerical evidences and reasons
Numerical studies performed with a primitive equation model on
two-dimensional sinusoidal hills show that the local velocity profiles behave
logarithmically to a very good approximation, from a distance from the surface
of the order of the maximum hill height almost up to the top of the boundary
layer. This behavior is well known for flows above homogeneous and flat
topographies (``law-of-the-wall'') and, more recently, investigated with
respect to the large-scale (``asymptotic'') averaged flows above complex
topography. Furthermore, this new-found local generalized law-of-the-wall
involves effective parameters showing a smooth dependence on the position along
the underlying topography. This dependence is similar to the topography itself,
while this property does not absolutely hold for the underlying flow, nearest
to the hill surface.Comment: 9 pages, Latex, 2 figure
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