5,196 research outputs found
The Operating Diagram for a Two-Step Anaerobic Digestion Model
The Anaerobic Digestion Model No. 1 (ADM1) is a complex model which is widely
accepted as a common platform for anaerobic process modeling and simulation.
However, it has a large number of parameters and states that hinder its
analytical study. Here, we consider the two-step reduced model of anaerobic
digestion (AM2) which is a four-dimensional system of ordinary differential
equations. The AM2 model is able to adequately capture the main dynamical
behavior of the full anaerobic digestion model ADM1 and has the advantage that
a complete analysis for the existence and local stability of its steady states
is available. We describe its operating diagram, which is the bifurcation
diagram which gives the behavior of the system with respect to the operating
parameters represented by the dilution rate and the input concentrations of the
substrates. This diagram, is very useful to understand the model from both the
mathematical and biological points of view
Are rotating strange quark stars good sources of gravitational waves?
We study the viscosity driven (Jacobi-like) bar mode instability of rapidly
rotating strange stars in general relativity. A triaxial, "bar shaped" compact
star could be an efficient source of continuous wave gravitational radiation in
the frequency range of the forthcoming interferometric detectors. We locate the
secular instability point along several constant baryon mass sequences of
uniformly rotating strange stars described by the MIT bag model. Contrary to
neutron stars, strange stars with T/|W| (the ratio of the rotational kinetic
energy to the absolute value of the gravitational potential energy) much lower
than the corresponding value for the mass-shed limit can be secularly unstable
to bar mode formation if shear viscosity is high enough to damp out any
deviation from uniform rotation. The instability develops for a broad range of
gravitational masses and rotational frequencies of strange quark stars. It
imposes strong constraints on the lower limit of the frequency at the innermost
stable circular orbit around rapidly rotating strange stars. The above results
are robust for all linear self-bound equations of state assuming the growth
time of the instability is faster than the damping timescale. We discuss
astrophysical scenarios where triaxial instabilities (r-mode and viscosity
driven instability) could be relevant in strange stars described by the
standard MIT bag model of normal quark matter. Taking into account actual
values of viscosities in strange quark matter and neglecting the magnetic field
we show that Jacobi-like instability cannot develop in any astrophysicaly
interesting temperature windows. The main result is that strange quark stars
described by the MIT bag model can be accelerated to very high frequency in Low
Mass X-ray binaries if the strange quark mass is ~ 200 MeV or higher.Comment: 15 pages, 10 figures, to appear in Astronomy and Astrophysic
Modeling, Analysis, and Optimization Issues for Large Space Structures
Topics concerning the modeling, analysis, and optimization of large space structures are discussed including structure-control interaction, structural and structural dynamics modeling, thermal analysis, testing, and design
Statistical characteristics of irreversible predictability time in regional ocean models
Probabilistic aspects of regional ocean model predictability is analyzed using the probability density function (PDF) of the irreversible predictability time (IPT) (called τ-PDF) computed from an unconstrained ensemble of stochastic perturbations in initial conditions, winds, and open boundary conditions. Two-attractors (a chaotic attractor and a small-amplitude stable limit cycle) are found in the wind-driven circulation. Relationship between attractor's residence time and IPT determines the τ-PDF for the short (up to several weeks) and intermediate (up to two months) predictions. The τ-PDF is usually non-Gaussian but not multi-modal for red-noise perturbations in initial conditions and perturbations in the wind and open boundary conditions. Bifurcation of τ-PDF occurs as the tolerance level varies. Generally, extremely successful predictions (corresponding to the τ-PDF's tail toward large IPT domain) are not outliers and share the same statistics as a whole ensemble of predictions
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