42 research outputs found
Intermittency as a consequence of a stationarity constraint on the energy flux
In his seminal work on turbulence, Kolmogorov made use of the stationary
hypothesis to determine the Power Density Spectrum of the velocity field in
turbulent flows. However to our knowledge, the constraints that stationary
processes impose on the fluctuations of the energy flux have never been used in
the context of turbulence. Here we recall that the Power Density Spectra of the
fluctuations of the injected power, the dissipated power and the energy flux
have to converge to a common value at vanishing frequency. Hence, we show that
the intermittent GOY--shell model fulfills these constraints. We argue that
they can be related to intermittency. Indeed, we find that the constraint on
the fluctuations of the energy flux implies a relation between the scaling
exponents that characterize intermittency, which is verified by the GOY--shell
model and in agreement with the She-Leveque formula. It also fixes the
intermittency parameter of the log-normal model at a realistic value. The
relevance of these results for real turbulence is drawn in the concluding
remarks.Comment: 5 pages, 4 figures, 23 reference
Scaling laws of turbulent dynamos
We consider magnetic fields generated by homogeneous isotropic and parity
invariant turbulent flows. We show that simple scaling laws for dynamo
threshold, magnetic energy and Ohmic dissipation can be obtained depending on
the value of the magnetic Prandtl number.Comment: 6 pages, 1 figur
Observation of gravity-capillary wave turbulence
We report the observation of the cross-over between gravity and capillary
wave turbulence on the surface of mercury. The probability density functions of
the turbulent wave height are found to be asymmetric and thus non Gaussian. The
surface wave height displays power-law spectra in both regimes. In the
capillary region, the exponent is in fair agreement with weak turbulence
theory. In the gravity region, it depends on the forcing parameters. This can
be related to the finite size of the container. In addition, the scaling of
those spectra with the mean energy flux is found in disagreement with weak
turbulence theory for both regimes
Wave-vortex interaction
We present an experimental study on the effect of a electromagneticaly
generated vortex flow on parametrically amplified waves at the surface of a
fluid. The underlying vortex flow, generated by a periodic Lorentz force,
creates spatio-temporal fluctuations that interact nonlinearly with the
standing surface waves. We characterize the bifurcation diagram and measure the
power spectrum density (PSD) of the local surface wave amplitude. We show that
the parametric instability threshold increases with increasing intensity of the
vortex flow.Comment: 8 pages, 10 figures, submitted to Phys. Rev.
A simple mechanism for the reversals of Earth's magnetic field
We show that a model, recently used to describe all the dynamical regimes of
the magnetic field generated by the dynamo effect in the VKS experiment [1],
also provides a simple explanation of the reversals of Earth's magnetic field,
despite strong differences between both systems.Comment: update version, with new figure
Fluctuations of energy flux in wave turbulence
We report that the power driving gravity and capillary wave turbulence in a
statistically stationary regime displays fluctuations much stronger than its
mean value. We show that its probability density function (PDF) has a most
probable value close to zero and involves two asymmetric roughly exponential
tails. We understand the qualitative features of the PDF using a simple
Langevin type model.Comment: submitted to PR