514 research outputs found
Scaling and Linear Response in the GOY Turbulence model
The GOY model is a model for turbulence in which two conserved quantities
cascade up and down a linear array of shells. When the viscosity parameter,
, is small the model has a qualitative behavior which is similar to the
Kolmogorov theories of turbulence. Here a static solution to the model is
examined, and a linear stability analysis is performed to obtain response
eigenvalues and eigenfunctions. Both the static behavior and the linear
response show an inertial range with a relatively simple scaling structure. Our
main results are: (i) The response frequencies cover a wide range of scales,
with ratios which can be understood in terms of the frequency scaling
properties of the model. (ii) Even small viscosities play a crucial role in
determining the model's eigenvalue spectrum. (iii) As a parameter within the
model is varied, it shows a ``phase transition'' in which there is an abrupt
change in many eigenvalues from stable to unstable values. (iv) The abrupt
change is determined by the model's conservation laws and symmetries.
This work is thus intended to add to our knowledge of the linear response of
a stiff dynamical systems and at the same time to help illuminate scaling
within a class of turbulence models.Comment: 25 pages, figures on reques
How the viscous subrange determines inertial range properties in turbulence shell models
We calculate static solutions of the 'GOY' shell model of turbulence and do a
linear stability analysis. The asymptotic limit of large Reynolds numbers is
analyzed. A phase diagram is presented which shows the range of stability of
the static solution. We see an unexpected oscillatory dependence of the
stability range upon , where is the viscosity. This effect
depends upon the discrete structure of the shell model and goes to zero as the
separation between the shells is brought to zero. These findings show how
viscous effects play a role in determining inertial properties of shell models
and give some hints for understanding the effects of viscous dissipation upon
real turbulence.Comment: Physica D, in pres
Bias driven coherent carrier dynamics in a two-dimensional aperiodic potential
We study the dynamics of an electron wave-packet in a two-dimensional square
lattice with an aperiodic site potential in the presence of an external uniform
electric field. The aperiodicity is described by at lattice sites
, with being a rational number, and and
tunable parameters, controlling the aperiodicity. Using an exact
diagonalization procedure and a finite-size scaling analysis, we show that in
the weakly aperiodic regime (), a phase of extended states
emerges in the center of the band at zero field giving support to a macroscopic
conductivity in the thermodynamic limit. Turning on the field gives rise to
Bloch oscillations of the electron wave-packet. The spectral density of these
oscillations may display a double peak structure signaling the spatial
anisotropy of the potential landscape. The frequency of the oscillations can be
understood using a semi-classical approach.Comment: 16 pages, to appear in Phys. Lett.
Clostridium difficile sortase recognizes a (S/P)PXTG sequence motif and can accommodate diaminopimelic acid as a substrate for transpeptidation
AbstractCovalent attachment of surface proteins to the cell wall of Gram-positive bacteria requires a sortase-mediated transpeptidation reaction. In almost all Gram-positive bacteria, the housekeeping sortase, sortase A, recognizes the canonical recognition sequence LPXTG (X=any amino acid). The human pathogen Clostridium difficile carries a single putative sortase gene (cd2718) but neither transpeptidation activity nor specificity of CD2718 has been investigated. We produced recombinant CD2718 and examined its transpeptidation activity in vitro using synthetic peptides and MALDI-ToF(-ToF) MS analysis. We demonstrate that CD2718 has sortase activity with specificity for a (S/P)PXTG motif and can accommodate diaminopimelic acid as a substrate for transpeptidation
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