681 research outputs found
Excitation of stellar p-modes by turbulent convection: 1. Theoretical formulation
Stochatic excitation of stellar oscillations by turbulent convection is
investigated and an expression for the power injected into the oscillations by
the turbulent convection of the outer layers is derived which takes into
account excitation through turbulent Reynolds stresses and turbulent entropy
fluctuations. This formulation generalizes results from previous works and is
built so as to enable investigations of various possible spatial and temporal
spectra of stellar turbulent convection. For the Reynolds stress contribution
and assuming the Kolmogorov spectrum we obtain a similar formulation than those
derived by previous authors. The entropy contribution to excitation is found to
originate from the advection of the Eulerian entropy fluctuations by the
turbulent velocity field. Numerical computations in the solar case in a
companion paper indicate that the entropy source term is dominant over Reynold
stress contribution to mode excitation, except at high frequencies.Comment: 14 pages, accepted for publication in A&
Intermittency of velocity time increments in turbulence
We analyze the statistics of turbulent velocity fluctuations in the time
domain. Three cases are computed numerically and compared: (i) the time traces
of Lagrangian fluid particles in a (3D) turbulent flow (referred to as the
"dynamic" case); (ii) the time evolution of tracers advected by a frozen
turbulent field (the "static" case), and (iii) the evolution in time of the
velocity recorded at a fixed location in an evolving Eulerian velocity field,
as it would be measured by a local probe (referred to as the "virtual probe"
case). We observe that the static case and the virtual probe cases share many
properties with Eulerian velocity statistics. The dynamic (Lagrangian) case is
clearly different; it bears the signature of the global dynamics of the flow.Comment: 5 pages, 3 figures, to appear in PR
Melting dynamics of large ice balls in a turbulent swirling flow
We study the melting dynamics of large ice balls in a turbulent von Karman
flow at very high Reynolds number. Using an optical shadowgraphy setup, we
record the time evolution of particle sizes. We study the heat transfer as a
function of the particle scale Reynolds number for three cases: fixed ice balls
melting in a region of strong turbulence with zero mean flow, fixed ice balls
melting under the action of a strong mean flow with lower fluctuations, and ice
balls freely advected in the whole flow. For the fixed particles cases, heat
transfer is observed to be much stronger than in laminar flows, the Nusselt
number behaving as a power law of the Reynolds number of exponent 0.8. For
freely advected ice balls, the turbulent transfer is further enhanced and the
Nusselt number is proportional to the Reynolds number. The surface heat flux is
then independent of the particles size, leading to an ultimate regime of heat
transfer reached when the thermal boundary layer is fully turbulent
A dissipative random velocity field for fully developed fluid turbulence
We investigate the statistical properties, based on numerical simulations and
analytical calculations, of a recently proposed stochastic model for the
velocity field of an incompressible, homogeneous, isotropic and fully developed
turbulent flow. A key step in the construction of this model is the
introduction of some aspects of the vorticity stretching mechanism that governs
the dynamics of fluid particles along their trajectory. An additional further
phenomenological step aimed at including the long range correlated nature of
turbulence makes this model depending on a single free parameter that
can be estimated from experimental measurements. We confirm the realism of the
model regarding the geometry of the velocity gradient tensor, the power-law
behaviour of the moments of velocity increments (i.e. the structure functions),
including the intermittent corrections, and the existence of energy transfers
across scales. We quantify the dependence of these basic properties of
turbulent flows on the free parameter and derive analytically the
spectrum of exponents of the structure functions in a simplified non
dissipative case. A perturbative expansion in power of shows that
energy transfers, at leading order, indeed take place, justifying the
dissipative nature of this random field.Comment: 38 pages, 5 figure
Decay of scalar variance in isotropic turbulence in a bounded domain
The decay of scalar variance in isotropic turbulence in a bounded domain is
investigated. Extending the study of Touil, Bertoglio and Shao (2002; Journal
of Turbulence, 03, 49) to the case of a passive scalar, the effect of the
finite size of the domain on the lengthscales of turbulent eddies and scalar
structures is studied by truncating the infrared range of the wavenumber
spectra. Analytical arguments based on a simple model for the spectral
distributions show that the decay exponent for the variance of scalar
fluctuations is proportional to the ratio of the Kolmogorov constant to the
Corrsin-Obukhov constant. This result is verified by closure calculations in
which the Corrsin-Obukhov constant is artificially varied. Large-eddy
simulations provide support to the results and give an estimation of the value
of the decay exponent and of the scalar to velocity time scale ratio
Measurements of the magnetic field induced by a turbulent flow of liquid metal
Initial results from the Madison Dynamo Experiment provide details of the
inductive response of a turbulent flow of liquid sodium to an applied magnetic
field. The magnetic field structure is reconstructed from both internal and
external measurements. A mean toroidal magnetic field is induced by the flow
when an axial field is applied, thereby demonstrating the omega effect.
Poloidal magnetic flux is expelled from the fluid by the poloidal flow.
Small-scale magnetic field structures are generated by turbulence in the flow.
The resulting magnetic power spectrum exhibits a power-law scaling consistent
with the equipartition of the magnetic field with a turbulent velocity field.
The magnetic power spectrum has an apparent knee at the resistive dissipation
scale. Large-scale eddies in the flow cause significant changes to the
instantaneous flow profile resulting in intermittent bursts of non-axisymmetric
magnetic fields, demonstrating that the transition to a dynamo is not smooth
for a turbulent flow.Comment: 9 pages, 11 figures, invited talk by C. B. Forest at 2005 APS DPP
meeting, resubmitted to Physics of Plasma
Localness of energy cascade in hydrodynamic turbulence, II. Sharp spectral filter
We investigate the scale-locality of subgrid-scale (SGS) energy flux and
inter-band energy transfers defined by the sharp spectral filter. We show by
rigorous bounds, physical arguments and numerical simulations that the spectral
SGS flux is dominated by local triadic interactions in an extended turbulent
inertial-range. Inter-band energy transfers are also shown to be dominated by
local triads if the spectral bands have constant width on a logarithmic scale.
We disprove in particular an alternative picture of ``local transfer by
nonlocal triads,'' with the advecting wavenumber mode at the energy peak.
Although such triads have the largest transfer rates of all {\it individual}
wavenumber triads, we show rigorously that, due to their restricted number,
they make an asymptotically negligible contribution to energy flux and
log-banded energy transfers at high wavenumbers in the inertial-range. We show
that it is only the aggregate effect of a geometrically increasing number of
local wavenumber triads which can sustain an energy cascade to small scales.
Furthermore, non-local triads are argued to contribute even less to the
space-average energy flux than is implied by our rigorous bounds, because of
additional cancellations from scale-decorrelation effects. We can thus recover
the -4/3 scaling of nonlocal contributions to spectral energy flux predicted by
Kraichnan's ALHDIA and TFM closures. We support our results with numerical data
from a pseudospectral simulation of isotropic turbulence with
phase-shift dealiasing. We conclude that the sharp spectral filter has a firm
theoretical basis for use in large-eddy simulation (LES) modeling of turbulent
flows.Comment: 42 pages, 9 figure
Intermittent magnetic field excitation by a turbulent flow of liquid sodium
The magnetic field measured in the Madison Dynamo Experiment shows
intermittent periods of growth when an axial magnetic field is applied. The
geometry of the intermittent field is consistent with the fastest growing
magnetic eigenmode predicted by kinematic dynamo theory using a laminar model
of the mean flow. Though the eigenmodes of the mean flow are decaying, it is
postulated that turbulent fluctuations of the velocity field change the flow
geometry such that the eigenmode growth rate is temporarily positive.
Therefore, it is expected that a characteristic of the onset of a turbulent
dynamo is magnetic intermittency.Comment: 5 pages, 7 figure
Universal long-time properties of Lagrangian statistics in the Batchelor regime and their application to the passive scalar problem
We consider transport of dynamically passive quantities in the Batchelor
regime of smooth in space velocity field. For the case of arbitrary temporal
correlations of the velocity we formulate the statistics of relevant
characteristics of Lagrangian motion. This allows to generalize many results
obtained previously for the delta-correlated in time strain, thus answering the
question of universality of these results.Comment: 11 pages, revtex; added references, typos correcte
ClockBoard: A zoning system for urban analysis
Zones are the building blocks of urban analysis. Fields ranging from demo-graphics to transport planning routinely use zones — spatially contiguous areal units thatbreak-up continuous space into discrete chunks — as the foundation for diverse analysistechniques. Key methods such as origin-destination analysis and choropleth mapping relyon zones with appropriate sizes, shapes and coverage. However, existing zoning systemsare sub-optimal in many urban analysis contexts, for three main reasons: 1) administra-tive zoning systems are often based on somewhat arbitrary factors; 2) zoning systems thatare evidence-based (e.g., based on equal population size) are often highly variable in sizeand shape, reducing their utility for inter-city comparison; and 3) official zoning systemsin many places simply do not exist or are unavailable. We set out to develop a flexible,open and scalable solution to these problems. The result is the zonebuilder project (withR, Rust and Python implementations), which was used to create the ClockBoard zoningsystem. ClockBoard consists of 12 segments emanating from a central place and dividedby concentric rings with radii that increase in line with the triangular number sequence(1, 3, 6 km etc). ‘ClockBoards’ thus create a consistent visual frame of reference for mono-centric cities that is reminiscent of clocks and a dartboard. This paper outlines the designand potential uses of the ClockBoard zoning system in the historical context, and discussesfuture avenues for research into the design and assessment of zoning systems
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