263 research outputs found
On the third order structure function for rotating 3D homogeneous turbulent flow
A form for the two-point third order structure function has been calculated
for three dimensional homogeneous incompressible slowly rotating turbulent
fluid. It has been argued that it may possibly hint at the initiation of the
phenomenon of two-dimensionalisation of the 3D incompressible turbulence owing
to rotation.Comment: This revised version corrects some serious flaws in the discussions
after the equation (2) and the equation (13) of the earlier version. Some
typos are also correcte
Dimensional transition in rotating turbulence
In this work we investigate, by means of direct numerical simulations, how
rotation affects the bi-dimensionalization of a turbulent flow. We study a thin
layer of fluid, forced by a two-dimensional forcing, within the framework of
the "split cascade" in which the injected energy flows both to small scales
(generating the direct cascade) and to large scale (to form the inverse
cascade). It is shown that rotation reinforces the inverse cascade at the
expense of the direct one, thus promoting bi-dimensionalization of the flow.
This is achieved by a suppression of the enstrophy production at large scales.
Nonetheless, we find that, in the range of rotation rates investigated,
increasing the the vertical scale causes a reduction of the flux of the inverse
cascade. Our results suggest that, even in rotating flows, the inverse cascade
may eventually disappear when the vertical scale is sufficiently large with
respect to the forcing scale
Scaling analysis and simulation of strongly stratified turbulent flows
International audienceDirect numerical simulations of stably and strongly stratified turbulent flows with Reynolds number Re " 1 and horizontal Froude number Fh Gt; 1 are presented. The results are interpreted on the basis of a scaling analysis of the governing equations. The analysis suggests that there are two different strongly stratified regimes according to the parameter R = ReFh2. When R " 1, viscous forces are nimportant and lv scales as lv ~ U/N (U is a characteristic horizontal velocity and N is the Brunt - Väis¨alä frequency) so that the dynamics of the flow is inherently three-dimensional but strongly anisotropic. When R " 1, vertical viscous shearing is important so that lv ~ lh/Re1/2 (lh is a characteristic horizontal length scale). The parameter R is further shown to be related to the buoyancy Reynolds number and proportional to (lO/?) 4/3, where lO is the Ozmidov length scale and ? the Kolmogorov length scale. This implies that there are simultaneously two distinct ranges in strongly stratified turbulence when R " 1: the scales larger than lO are strongly influenced by the stratification while those between lO and ? are weakly affected by stratification. The direct numerical simulations with forced large-scale horizontal two-dimensional motions and uniform stratification cover a wide Re and Fh range and support the main parameter controlling strongly stratified turbulence being R. The numerical results are in good agreement with the scaling laws for the vertical length scale. Thin horizontal layers are observed independently of the value of R but they tend to be smooth for R > 1, while for R > 1 small-scale three-dimensional turbulent disturbances are increasingly superimposed. The dissipation of kinetic energy is mostly due to vertical shearing for R > 1 but tends to isotropy as R increases above unity. When R > 1, the horizontal and vertical energy spectra are very steep while, when R > 1, the horizontal spectra of kinetic and potential energy exhibit an pproximate kh-5/3-power-law range and a clear forward energy cascade is observed. © 2007 Cambridge University Press
Consent and privacy in telemedicine.
The electronic broadcast of a medical interview, or a video tele-consultation (VTC), challenges many of our traditional concepts of privacy and confidentiality. The nature of a doctor-patient relationship changes dramatically when the open airwaves carry the personal histories, images, and concerns of a patient. Discussions of telemedicine often allude to inherent ethical concerns yet there are no established guidelines for the ethical conduct of a VTC
Comment on "Reinterpreting aircraft measurement in anisotropic scaling turbulence" by Lovejoy et al. (2009)
Recently, Lovejoy et al. (2009) argued that the steep ~k−3 atmospheric kinetic energy spectrum at synoptic scales (≥1000 km) observed by aircraft is a spurious artefact of aircraft following isobars instead of isoheights. Without taking into account the earth's rotation they hypothesise that the horizontal atmospheric energy spectrum should scale as k−5/3 at all scales. We point out that the approximate k−3-spectrum at synoptic scales has been observed by a number of non-aircraft means since the 1960s and that general circulation models and other current models have successfully produced this spectrum. We also argue that the vertical movements of the aircraft are far too small to cause any strong effect on the measured spectrum at synoptic scales
Adaptation to dislodgement risk on wave-swept rocky shores in the snail Littorina saxatilis
The periwinkle Littorina saxatilis has repeatedly evolved both a small, fragile and globose "wave ecotype" confined to wave-swept shores and a large, robust and elongated "crab ecotype" found in nearby crab-rich but less-exposed shores. This phenotypic divergence is assumed to reflect, in some part, local adaptation to wave exposure, but this hypothesis has received incomplete experimental testing. Here, we report a test of the prediction that the wave ecotype has a higher capacity to resist water flow than the crab ecotype. We sampled snails along a crab-wave transect and measured their resistance to dislodgement in a high-speed water flume with water speeds that match those of breaking waves in a range of relevant field conditions. Snails from the wave environment were consistently more resistant to water flow than snails from the crab environment and high resistance was positively correlated with the surface area of the foot and the area of the outer aperture contour both relative to shell size, and to the extent of lateral shell compression. In a separate experiment, we found that snails raised in still water in a common garden showed higher resistance to water flow if originating from a wave environment than from a crab environment, and this was true both at juvenile (2 weeks) and adult (10 months) developmental stages. This result suggests genetic control of a distinct "wave adapted" phenotype, likely to be maintained under strong divergent selection between the two adjacent habitats
Unified Multifractal Description of Velocity Increments Statistics in Turbulence: Intermittency and Skewness
The phenomenology of velocity statistics in turbulent flows, up to now,
relates to different models dealing with either signed or unsigned longitudinal
velocity increments, with either inertial or dissipative fluctuations. In this
paper, we are concerned with the complete probability density function (PDF) of
signed longitudinal increments at all scales. First, we focus on the symmetric
part of the PDFs, taking into account the observed departure from scale
invariance induced by dissipation effects. The analysis is then extended to the
asymmetric part of the PDFs, with the specific goal to predict the skewness of
the velocity derivatives. It opens the route to the complete description of all
measurable quantities, for any Reynolds number, and various experimental
conditions. This description is based on a single universal parameter function
D(h) and a universal constant R*.Comment: 13 pages, 3 figures, Extended version, Publishe
Vortical and Wave Modes in 3D Rotating Stratified Flows: Random Large Scale Forcing
Utilizing an eigenfunction decomposition, we study the growth and spectra of
energy in the vortical and wave modes of a 3D rotating stratified fluid as a
function of . Working in regimes characterized by moderate
Burger numbers, i.e. or , our results
indicate profound change in the character of vortical and wave mode
interactions with respect to . As with the reference state of
, for the wave mode energy saturates quite quickly
and the ensuing forward cascade continues to act as an efficient means of
dissipating ageostrophic energy. Further, these saturated spectra steepen as
decreases: we see a shift from to scaling for
(where and are the forcing and dissipation scales,
respectively). On the other hand, when the wave mode energy
never saturates and comes to dominate the total energy in the system. In fact,
in a sense the wave modes behave in an asymmetric manner about .
With regard to the vortical modes, for , the signatures of 3D
quasigeostrophy are clearly evident. Specifically, we see a scaling
for and, in accord with an inverse transfer of energy, the
vortical mode energy never saturates but rather increases for all . In
contrast, for and increasing, the vortical modes contain a
progressively smaller fraction of the total energy indicating that the 3D
quasigeostrophic subsystem plays an energetically smaller role in the overall
dynamics.Comment: 18 pages, 6 figs. (abbreviated abstract
Recent Developments in Understanding Two-dimensional Turbulence and the Nastrom-Gage Spectrum
Two-dimensional turbulence appears to be a more formidable problem than
three-dimensional turbulence despite the numerical advantage of working with
one less dimension. In the present paper we review recent numerical
investigations of the phenomenology of two-dimensional turbulence as well as
recent theoretical breakthroughs by various leading researchers. We also review
efforts to reconcile the observed energy spectrum of the atmosphere (the
spectrum) with the predictions of two-dimensional turbulence and
quasi-geostrophic turbulence.Comment: Invited review; accepted by J. Low Temp. Phys.; Proceedings for
Warwick Turbulence Symposium Workshop on Universal features in turbulence:
from quantum to cosmological scales, 200
Statistical geometry in scalar turbulence
A general link between geometry and intermittency in passive scalar
turbulence is established. Intermittency is qualitatively traced back to events
where tracer particles stay for anomalousy long times in degenerate geometries
characterized by strong clustering. The quantitative counterpart is the
existence of special functions of particle configurations which are
statistically invariant under the flow. These are the statistical integrals of
motion controlling the scalar statistics at small scales and responsible for
the breaking of scale invariance associated to intermittency.Comment: 4 pages, 5 figure
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