618 research outputs found
Swiss Biotech â An Overview of the Industry and the Key Stakeholders 2010
This article presents the stakeholders of the Swiss Biotechnology sector. From academia to industry, from TechTransfer initiatives to state impulse programs, the sector has developed rapidly in the last years. Public Private Partnerships such as Life Science Clusters and collaborations
between industry associations have proven to be an essential part for sustainable success for our national GDP. The author has extensive experience in the various sub-sectors
Shell to shell energy transfer in MHD, Part II: Kinematic dynamo
We study the transfer of energy between different scales for forced
three-dimensional MHD turbulent flows in the kinematic dynamo regime. Two
different forces are examined: a non-helical Taylor Green flow with magnetic
Prandtl number P_M=0.4, and a helical ABC flow with P_M=1. This analysis allows
us to examine which scales of the velocity flow are responsible for dynamo
action, and identify which scales of the magnetic field receive energy directly
from the velocity field and which scales receive magnetic energy through the
cascade of the magnetic field from large to small scales. Our results show that
the turbulent velocity fluctuations are responsible for the magnetic field
amplification in the small scales (small scale dynamo) while the large scale
field is amplified mostly due to the large scale flow. A direct cascade of the
magnetic field energy from large to small scales is also present and is a
complementary mechanism for the increase of the magnetic field in the small
scales. Input of energy from the velocity field in the small magnetic scales
dominates over the energy that is cascaded down from the large scales until the
large-scale peak of the magnetic energy spectrum is reached. At even smaller
scales, most of the magnetic energy input is from the cascading process.Comment: Submitted to PR
Marginally unstable Holmboe modes
Marginally unstable Holmboe modes for smooth density and velocity profiles
are studied. For a large family of flows and stratification that exhibit
Holmboe instability, we show that the modes with phase velocity equal to the
maximum or the minimum velocity of the shear are marginally unstable. This
allows us to determine the critical value of the control parameter R
(expressing the ratio of the velocity variation length scale to the density
variation length scale) that Holmboe instability appears R=2. We then examine
systems for which the parameter R is very close to this critical value. For
this case we derive an analytical expression for the dispersion relation of the
complex phase speed c(k) in the unstable region. The growth rate and the width
of the region of unstable wave numbers has a very strong (exponential)
dependence on the deviation of R from the critical value. Two specific examples
are examined and the implications of the results are discussed.Comment: Submitted to Physics of Fluid
Shell to shell energy transfer in MHD, Part I: steady state turbulence
We investigate the transfer of energy from large scales to small scales in
fully developed forced three-dimensional MHD-turbulence by analyzing the
results of direct numerical simulations in the absence of an externally imposed
uniform magnetic field. Our results show that the transfer of kinetic energy
from the large scales to kinetic energy at smaller scales, and the transfer of
magnetic energy from the large scales to magnetic energy at smaller scales, are
local, as is also found in the case of neutral fluids, and in a way that is
compatible with Kolmogorov (1941) theory of turbulence. However, the transfer
of energy from the velocity field to the magnetic field is a highly non-local
process in Fourier space. Energy from the velocity field at large scales can be
transfered directly into small scale magnetic fields without the participation
of intermediate scales. Some implications of our results to MHD turbulence
modeling are also discussed.Comment: Submitted to PR
Stratified shear flow instabilities at large Richardson numbers
Numerical simulations of stratified shear flow instabilities are performed in
two dimensions in the Boussinesq limit. The density variation length scale is
chosen to be four times smaller than the velocity variation length scale so
that Holmboe or Kelvin-Helmholtz unstable modes are present depending on the
choice of the global Richardson number Ri. Three different values of Ri were
examined Ri =0.2, 2, 20. The flows for the three examined values are all
unstable due to different modes namely: the Kelvin-Helmholtz mode for Ri=0.2,
the first Holmboe mode for Ri=2, and the second Holmboe mode for Ri=20 that has
been discovered recently and it is the first time that it is examined in the
non-linear stage. It is found that the amplitude of the velocity perturbation
of the second Holmboe mode at the non-linear stage is smaller but comparable to
first Holmboe mode. The increase of the potential energy however due to the
second Holmboe modes is greater than that of the first mode. The
Kelvin-Helmholtz mode is larger by two orders of magnitude in kinetic energy
than the Holmboe modes and about ten times larger in potential energy than the
Holmboe modes. The results in this paper suggest that although mixing is
suppressed at large Richardson numbers it is not negligible, and turbulent
mixing processes in strongly stratified environments can not be excluded.Comment: Submitted to Physics of Fluid
The imprint of large-scale flows on turbulence
We investigate the locality of interactions in hydrodynamic turbulence using
data from a direct numerical simulation on a grid of 1024^3 points; the flow is
forced with the Taylor-Green vortex. An inertial range for the energy is
obtained in which the flux is constant and the spectrum follows an approximate
Kolmogorov law. Nonlinear triadic interactions are dominated by their non-local
components, involving widely separated scales. The resulting nonlinear transfer
itself is local at each scale but the step in the energy cascade is independent
of that scale and directly related to the integral scale of the flow.
Interactions with large scales represent 20% of the total energy flux. Possible
explanations for the deviation from self-similar models, the link between these
findings and intermittency, and their consequences for modeling of turbulent
flows are briefly discussed
Water waves over a time-dependent bottom: Exact description for 2D potential flows
Two-dimensional potential flows of an ideal fluid with a free surface are
considered in situations when shape of the bottom depends on time due to
external reasons. Exact nonlinear equations describing surface waves in terms
of the so called conformal variables are derived for an arbitrary time-evolving
bottom parameterized by an analytical function. An efficient numerical method
for the obtained equations is suggested.Comment: revtex4, 7 pages, 19 EPS figures; corrected version with more
numerical result
Effects of anisotropy in geostrophic turbulence
The Boussinesq model of convection in a flat layer with heating from below is
considered. We analyze the effects of anisotropy caused by rapid rotation in
physical and wave spaces and demonstrate the suppression of energy transfer by
rotation. We also examine the structure of the wave triangle in nonlinear
interaction. The range of parameters is adapted to the models of convection in
the geodynamo
Non-local interactions in hydrodynamic turbulence at high Reynolds numbers: the slow emergence of scaling laws
We analyze the data stemming from a forced incompressible hydrodynamic
simulation on a grid of 2048^3 regularly spaced points, with a Taylor Reynolds
number of Re~1300. The forcing is given by the Taylor-Green flow, which shares
similarities with the flow in several laboratory experiments, and the
computation is run for ten turnover times in the turbulent steady state. At
this Reynolds number the anisotropic large scale flow pattern, the inertial
range, the bottleneck, and the dissipative range are clearly visible, thus
providing a good test case for the study of turbulence as it appears in nature.
Triadic interactions, the locality of energy fluxes, and structure functions of
the velocity increments are computed. A comparison with runs at lower Reynolds
numbers is performed, and shows the emergence of scaling laws for the relative
amplitude of local and non-local interactions in spectral space. The scalings
of the Kolmogorov constant, and of skewness and flatness of velocity
increments, performed as well and are consistent with previous experimental
results. Furthermore, the accumulation of energy in the small-scales associated
with the bottleneck seems to occur on a span of wavenumbers that is independent
of the Reynolds number, possibly ruling out an inertial range explanation for
it. Finally, intermittency exponents seem to depart from standard models at
high Re, leaving the interpretation of intermittency an open problem.Comment: 8 pages, 8 figure
Phase transitions and flux-loop metastable states in rotating turbulence
By using direct numerical simulations of up to a record resolution of
512x512x32768 grid points we discover the existence of a new metastable
out-of-equilibrium state in rotating turbulence. We scan the phase space by
varying both the rotation rate (proportional to the inverse of the Rossby
number, ) and the dimensionless aspect ratio, , where and
are the sizes of the domain perpendicular and parallel to the direction of
rotation, respectively. We show the existence of three turbulent phases. For
small but finite , we have a split cascade where the injected
energy is transferred to both large and small scales. For large and
finite there is no inverse cascade and the energy is transferred forward
in Fourier space only. Surprisingly, between these two regimes, a third phase
is observed as reported here for the first time. Consequently, for certain
intervals of and , energy is no longer accumulated at arbitrarily
large scales, rather it stops at some characteristic intermediate length-scales
from where it is then redistributed forward in Fourier space, leading to a
flux-loop mechanism where the flow is out of equilibrium with vanishing net
flux, and non-vanishing heterochiral and homochiral sub-fluxes. The system is
further characterized by the presence of metastability and critical slowing
down, explaining why previous experiments and numerical simulations were not
able to detect this phenomenon, requiring extremely long observation time and
huge computational resources
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