259 research outputs found
Vorticity statistics in the two-dimensional enstrophy cascade
We report the first extensive experimental observation of the two-dimensional
enstrophy cascade, along with the determination of the high order vorticity
statistics. The energy spectra we obtain are remarkably close to the Kraichnan
Batchelor expectation. The distributions of the vorticity increments, in the
inertial range, deviate only little from gaussianity and the corresponding
structure functions exponents are indistinguishable from zero. It is thus shown
that there is no sizeable small scale intermittency in the enstrophy cascade,
in agreement with recent theoretical analyses.Comment: 5 pages, 7 Figure
Pemodelan Inundasi (Banjir Rob) Di Pesisir Kota Semarang Dengan Menggunakan Model Hidrodinamika
Fenomena banjir rob yang terjadi di Semarang sudah sangat memprihatinkan, karena banjir ini tidak hanya terjadi pada daerah pesisir pantai saja tetapi sudah menggenangi wilayah pariwisata, pemukiman, maupun industri. Tujuan penelitian ini adalah untuk menghitung jarak dan luas genangan rob di pesisir Kota Semarang akibat kondisi pasang surut pada saat purnama dan perbani.Metode yang digunakan dalam penelitian ini adalah metode kuantitatif. Penelitian dibagi menjadi tiga tahap, yaitu pengumpulan data sekunder, pengambilan data lapangan, dan pengolahan data. Pengambilan data lapangan dilakukan di perairan Kota Semarang yang dilaksanakan dari tanggal 5 - 20 Juli 2010 pada koordinat 6052'30”- 6057'30” LS dan 110019'45”-110027'30” BT. Pengolahan data serta proses pemodelan dilakukan di BPPT, Yogyakarta. Berdasarkan hasil penelitian diketahui bahwa arus di daerah penelitian didominasi oleh arus pasang surut sebesar 74,3%, dimana pergerakannya condong dari arah utara ke selatan dengan kecepatan rata-rata terhadap kedalaman mencapai 0,024 m/s. Sedangkan dari hasil analisis banjir rob (inundasi), jarak terjauh terjadi di kecamatan Genuk yang mencapai 4,295 km dari garis pantai pada saat pasang purnama dan jarak terdekat terjadi di kecamatan Semarang Barat yang mencapai 488,93 m dari garis pantai pada saat pasang perbani. Selain itu berdasarkan hasil penelitian didapat luas banjir maksimal terjadi di kecamatan Tugu dengan luas 3450,1 Ha pada saat pasang purnama. Sedangkan wilayah yang paling sedikit terendam terjadi di kecamatan Gayamsari dengan luas 71,228 Ha pada saat pasang perbani. Hasil verifikasi bathimetri mempunyai tingkat kebenaran sekitar 73,69%, pasut sekitar 84,87%, vektor u dan v arus lapangan sekitar 93,58% dan model sekitar 92%
Global Hopf bifurcation in the ZIP regulatory system
Regulation of zinc uptake in roots of Arabidopsis thaliana has recently been
modeled by a system of ordinary differential equations based on the uptake of
zinc, expression of a transporter protein and the interaction between an
activator and inhibitor. For certain parameter choices the steady state of this
model becomes unstable upon variation in the external zinc concentration.
Numerical results show periodic orbits emerging between two critical values of
the external zinc concentration. Here we show the existence of a global Hopf
bifurcation with a continuous family of stable periodic orbits between two Hopf
bifurcation points. The stability of the orbits in a neighborhood of the
bifurcation points is analyzed by deriving the normal form, while the stability
of the orbits in the global continuation is shown by calculation of the Floquet
multipliers. From a biological point of view, stable periodic orbits lead to
potentially toxic zinc peaks in plant cells. Buffering is believed to be an
efficient way to deal with strong transient variations in zinc supply. We
extend the model by a buffer reaction and analyze the stability of the steady
state in dependence of the properties of this reaction. We find that a large
enough equilibrium constant of the buffering reaction stabilizes the steady
state and prevents the development of oscillations. Hence, our results suggest
that buffering has a key role in the dynamics of zinc homeostasis in plant
cells.Comment: 22 pages, 5 figures, uses svjour3.cl
Experimental study of Taylor's hypothesis in a turbulent soap film
An experimental study of Taylor's hypothesis in a quasi-two-dimensional
turbulent soap film is presented. A two probe laser Doppler velocimeter enables
a non-intrusive simultaneous measurement of the velocity at spatially separated
points. The breakdown of Taylor's hypothesis is quantified using the cross
correlation between two points displaced in both space and time; correlation is
better than 90% for scales less than the integral scale. A quantitative study
of the decorrelation beyond the integral scale is presented, including an
analysis of the failure of Taylor's hypothesis using techniques from
predictability studies of turbulent flows. Our results are compared with
similar studies of 3D turbulence.Comment: 27 pages, + 19 figure
Scaling and universality in turbulent convection
Anomalous correlation functions of the temperature field in two-dimensional
turbulent convection are shown to be universal with respect to the choice of
external sources. Moreover, they are equal to the anomalous correlations of the
concentration field of a passive tracer advected by the convective flow itself.
The statistics of velocity differences is found to be universal, self-similar
and close to Gaussian. These results point to the conclusion that temperature
intermittency in two-dimensional turbulent convection may be traced back to the
existence of statistically preserved structures, as it is in passive scalar
turbulence.Comment: 4 pages, 6 figure
Magnetized strangelets at finite temperature
The main properties of magnetized strangelets, namely, their energy per
baryon, radius and electric charge, are studied. Temperature effects are also
taken into account in order to study their stability compared to the 56Fe
isotope and non-magnetized strangelets using the liquid drop model. Massive
quarks are considered with the aim to have a more realistic description for
strangelets in the astrophysical context and the environment of heavy ion
colliders, playing also an important role in the thermodynamical quantities of
the quark gas. It is concluded that the presence of a magnetic field tends to
stabilize more the strangelets, even when temperature effects are taken into
account. Magnetized strangelets in a paired superconductor phase (magnetized
color flavor locked phase) are also discussed. It is shown that they are more
stable than ordinary magnetized strangelets for typical gap values of the order
of O(100) MeV.Comment: 10 pages, 10 figures, discussion extended, new references adde
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
Universal decay of scalar turbulence
The asymptotic decay of passive scalar fields is solved analytically for the
Kraichnan model, where the velocity has a short correlation time. At long
times, two universality classes are found, both characterized by a distribution
of the scalar -- generally non-Gaussian -- with global self-similar evolution
in time. Analogous behavior is found numerically with a more realistic flow
resulting from an inverse energy cascade.Comment: 4 pages, 3 Postscript figures, submitted to PR
Dynamical equations for high-order structure functions, and a comparison of a mean field theory with experiments in three-dimensional turbulence
Two recent publications [V. Yakhot, Phys. Rev. E {\bf 63}, 026307, (2001) and
R.J. Hill, J. Fluid Mech. {\bf 434}, 379, (2001)] derive, through two different
approaches that have the Navier-Stokes equations as the common starting point,
a set of steady-state dynamic equations for structure functions of arbitrary
order in hydrodynamic turbulence. These equations are not closed. Yakhot
proposed a "mean field theory" to close the equations for locally isotropic
turbulence, and obtained scaling exponents of structure functions and an
expression for the tails of the probability density function of transverse
velocity increments. At high Reynolds numbers, we present some relevant
experimental data on pressure and dissipation terms that are needed to provide
closure, as well as on aspects predicted by the theory. Comparison between the
theory and the data shows varying levels of agreement, and reveals gaps
inherent to the implementation of the theory.Comment: 16 pages, 23 figure
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