13,326 research outputs found
Simulation of the space station information system in Ada
The Flexible Ada Simulation Tool (FAST) is a discrete event simulation language which is written in Ada. FAST has been used to simulate a number of options for ground data distribution of Space Station payload data. The fact that Ada language is used for implementation has allowed a number of useful interactive features to be built into FAST and has facilitated quick enhancement of its capabilities to support new modeling requirements. General simulation concepts are discussed, and how these concepts are implemented in FAST. The FAST design is discussed, and it is pointed out how the used of the Ada language enabled the development of some significant advantages over classical FORTRAN based simulation languages. The advantages discussed are in the areas of efficiency, ease of debugging, and ease of integrating user code. The specific Ada language features which enable these advances are discussed
Destabilizing Taylor-Couette flow with suction
We consider the effect of radial fluid injection and suction on
Taylor-Couette flow. Injection at the outer cylinder and suction at the inner
cylinder generally results in a linearly unstable steady spiralling flow, even
for cylindrical shears that are linearly stable in the absence of a radial
flux. We study nonlinear aspects of the unstable motions with the energy
stability method. Our results, though specialized, may have implications for
drag reduction by suction, accretion in astrophysical disks, and perhaps even
in the flow in the earth's polar vortex.Comment: 34 pages, 9 figure
Exact on-event expressions for discrete potential systems
The properties of systems composed of atoms interacting though discrete potentials are dictated by a series of events which occur between pairs of atoms. There are only four basic event types for pairwise discrete potentials and the square-well/shoulder systems studied here exhibit them all. Closed analytical expressions are derived for the on-event kinetic energy distribution functions for an atom, which are distinct from the Maxwell-Boltzmann distribution function. Exact expressions are derived that directly relate the pressure and temperature of equilibrium discrete potential systems to the rates of each type of event. The pressure can be determined from knowledge of only the rate of core and bounce events. The temperature is given by the ratio of the number of bounce events to the number of disassociation/association events. All these expressions are validated with event-driven molecular dynamics simulations and agree with the data within the statistical precision of the simulations
Convectively driven shear and decreased heat flux
We report on direct numerical simulations of two-dimensional, horizontally
periodic Rayleigh-B\'enard convection, focusing on its ability to drive
large-scale horizontal flow that is vertically sheared. For the Prandtl numbers
() between 1 and 10 simulated here, this large-scale shear can be induced
by raising the Rayleigh number () sufficiently, and we explore the
resulting convection for up to . When present in our simulations,
the sheared mean flow accounts for a large fraction of the total kinetic
energy, and this fraction tends towards unity as . The shear helps
disperse convective structures, and it reduces vertical heat flux; in parameter
regimes where one state with large-scale shear and one without are both stable,
the Nusselt number of the state with shear is smaller and grows more slowly
with . When the large-scale shear is present with , the
convection undergoes strong global oscillations on long timescales, and heat
transport occurs in bursts. Nusselt numbers, time-averaged over these bursts,
vary non-monotonically with for . When the shear is present with
, the flow does not burst, and convective heat transport is
sustained at all times. Nusselt numbers then grow roughly as powers of ,
but the growth rates are slower than any previously reported for
Rayleigh-B\'enard convection without large-scale shear. We find the Nusselt
numbers grow proportionally to when and to when
. Analogies with tokamak plasmas are described.Comment: 25 pages, 12 figures, 5 video
Three-dimensional stability of the solar tachocline
The three-dimensional, hydrodynamic stability of the solar tachocline is
investigated based on a rotation profile as a function of both latitude and
radius. By varying the amplitude of the latitudinal differential rotation, we
find linear stability limits at various Reynolds numbers by numerical
computations. We repeated the computations with different latitudinal and
radial dependences of the angular velocity. The stability limits are all higher
than those previously found from two-dimensional approximations and higher than
the shear expected in the Sun. It is concluded that any part of the tachocline
which is radiative is hydrodynamically stable against small perturbations.Comment: 6 pages, 8 figures, accepted by Astron. & Astrophy
Self-Similar Blowup Solutions to the 2-Component Camassa-Holm Equations
In this article, we study the self-similar solutions of the 2-component
Camassa-Holm equations% \begin{equation} \left\{ \begin{array} [c]{c}%
\rho_{t}+u\rho_{x}+\rho u_{x}=0
m_{t}+2u_{x}m+um_{x}+\sigma\rho\rho_{x}=0 \end{array} \right. \end{equation}
with \begin{equation} m=u-\alpha^{2}u_{xx}. \end{equation} By the separation
method, we can obtain a class of blowup or global solutions for or
. In particular, for the integrable system with , we have the
global solutions:% \begin{equation} \left\{ \begin{array} [c]{c}%
\rho(t,x)=\left\{ \begin{array} [c]{c}% \frac{f\left( \eta\right)
}{a(3t)^{1/3}},\text{ for }\eta^{2}<\frac {\alpha^{2}}{\xi}
0,\text{ for }\eta^{2}\geq\frac{\alpha^{2}}{\xi}% \end{array} \right.
,u(t,x)=\frac{\overset{\cdot}{a}(3t)}{a(3t)}x
\overset{\cdot\cdot}{a}(s)-\frac{\xi}{3a(s)^{1/3}}=0,\text{ }a(0)=a_{0}%
>0,\text{ }\overset{\cdot}{a}(0)=a_{1}
f(\eta)=\xi\sqrt{-\frac{1}{\xi}\eta^{2}+\left( \frac{\alpha}{\xi}\right)
^{2}}% \end{array} \right. \end{equation}
where with and are
arbitrary constants.\newline Our analytical solutions could provide concrete
examples for testing the validation and stabilities of numerical methods for
the systems.Comment: 5 more figures can be found in the corresponding journal paper (J.
Math. Phys. 51, 093524 (2010) ). Key Words: 2-Component Camassa-Holm
Equations, Shallow Water System, Analytical Solutions, Blowup, Global,
Self-Similar, Separation Method, Construction of Solutions, Moving Boundar
The Renormalized Stress Tensor in Kerr Space-Time: Numerical Results for the Hartle-Hawking Vacuum
We show that the pathology which afflicts the Hartle-Hawking vacuum on the
Kerr black hole space-time can be regarded as due to rigid rotation of the
state with the horizon in the sense that when the region outside the
speed-of-light surface is removed by introducing a mirror, there is a state
with the defining features of the Hartle-Hawking vacuum. In addition, we show
that when the field is in this state, the expectation value of the
energy-momentum stress tensor measured by an observer close to the horizon and
rigidly rotating with it corresponds to that of a thermal distribution at the
Hawking temperature rigidly rotating with the horizon.Comment: 17 pages, 7 figure
Ally Immersion: A New Look at Anti-Racist Work
Multicultural centers exist on most predominately white campuses in one form or another as a primary support system and safe space for Students of Color. With an explosion of literature in recent years centering on how white student affairs professionals can be allies in anti-racist work, the question arises, can white staff at such centers be successful in supporting students? The personal experiences of the Director of the African, Latino/a, Asian, Native American (ALANA) Student Center at The University of Vermont and a white graduate student working at the same center provide the backdrop for this discussion
- …