1,162 research outputs found
Linear and Nonlinear Evolution and Diffusion Layer Selection in Electrokinetic Instability
In the present work fournontrivial stages of electrokinetic instability are
identified by direct numerical simulation (DNS) of the full
Nernst-Planck-Poisson-Stokes (NPPS) system: i) The stage of the influence of
the initial conditions (milliseconds); ii) 1D self-similar evolution
(milliseconds-seconds); iii) The primary instability of the self-similar
solution (seconds); iv) The nonlinear stage with secondary instabilities. The
self-similar character of evolution at intermediately large times is confirmed.
Rubinstein and Zaltzman instability and noise-driven nonlinear evolution to
over-limiting regimes in ion-exchange membranes are numerically simulated and
compared with theoretical and experimental predictions. The primary instability
which happens during this stage is found to arrest self-similar growth of the
diffusion layer and specifies its characteristic length as was first
experimentally predicted by Yossifon and Chang (PRL 101, 254501 (2008)). A
novel principle for the characteristic wave number selection from the
broadbanded initial noise is established.Comment: 13 pages, 8 figure
Manufacturing Algebra. Part II: aggregation, control and simulation
Manufacturing Algebra provides a set of mathematical entities together with some composition rules, that are specially conceived for modelling and controlling a manufacturing system. Here only the modelling capabilities are outlined together with a simple case study. The scope of the paper is to familiarize the reader with the proposed methodology, and to highlight some peculiarities. Formulation is reduced to a minimum. Among the algebra peculiarities, both manufacturing process and the factory layout are defined in their basic elements, and the link between them is given. Specifically the Manufacturing Model (parts, operations) includes time and space coordinates in order it could be employed by factory elements like Production units and Control Units. This calls for the definition of event and event sequence and of the relevant discrete-event elements and operators. A further peculiarity to be clarified in the second part, is the capability of aggregating algebra elements into higher level components, thus favoring hierarchical description and control of manufacturing system
A streamwise-constant model of turbulent pipe flow
A streamwise-constant model is presented to investigate the basic mechanisms
responsible for the change in mean flow occuring during pipe flow transition.
Using a single forced momentum balance equation, we show that the shape of the
velocity profile is robust to changes in the forcing profile and that both
linear non-normal and nonlinear effects are required to capture the change in
mean flow associated with transition to turbulence. The particularly simple
form of the model allows for the study of the momentum transfer directly by
inspection of the equations. The distribution of the high- and low-speed
streaks over the cross-section of the pipe produced by our model is remarkably
similar to one observed in the velocity field near the trailing edge of the
puff structures present in pipe flow transition. Under stochastic forcing, the
model exhibits a quasi-periodic self-sustaining cycle characterized by the
creation and subsequent decay of "streamwise-constant puffs", so-called due to
the good agreement between the temporal evolution of their velocity field and
the projection of the velocity field associated with three-dimensional puffs in
a frame of reference moving at the bulk velocity. We establish that the flow
dynamics are relatively insensitive to the regeneration mechanisms invoked to
produce near-wall streamwise vortices and that using small, unstructured
background disturbances to regenerate the streamwise vortices is sufficient to
capture the formation of the high- and low-speed streaks and their segregation
leading to the blunting of the velocity profile characteristic of turbulent
pipe flow
Manufacturing Algebra. Part II: aggregation, control and simulation
Manufacturing Algebra provides a set of mathematical entities together with some composition rules, that are specially conceived for modelling and controlling a manufacturing system. Here only the modelling capabilities are outlined together with a simple case study. The scope of the paper is to familiarize the reader with the proposed methodology, and to highlight some peculiarities. Formulation is reduced to a minimum. Among the algebra peculiarities, both manufacturing process and the factory layout are defined in their basic elements, and the link between them is given. Specifically the Manufacturing Model (parts, operations) includes time and space coordinates in order it could be employed by factory elements like Production units and Control Units. This calls for the definition of event and event sequence and of the relevant discrete-event elements and operators. A further peculiarity to be clarified in the second part, is the capability of aggregating algebra elements into higher level components, thus favoring hierarchical description and control of manufacturing systems
A Comparison of Measured Crab and Vela Glitch Healing Parameters with Predictions of Neutron Star Models
There are currently two well-accepted models that explain how pulsars exhibit
glitches, sudden changes in their regular rotational spin-down. According to
the starquake model, the glitch healing parameter, Q, which is measurable in
some cases from pulsar timing, should be equal to the ratio of the moment of
inertia of the superfluid core of a neutron star (NS) to its total moment of
inertia. Measured values of the healing parameter from pulsar glitches can
therefore be used in combination with realistic NS structure models as one test
of the feasibility of the starquake model as a glitch mechanism. We have
constructed NS models using seven representative equations of state of
superdense matter to test whether starquakes can account for glitches observed
in the Crab and Vela pulsars, for which the most extensive and accurate glitch
data are available. We also present a compilation of all measured values of Q
for Crab and Vela glitches to date which have been separately published in the
literature. We have computed the fractional core moment of inertia for stellar
models covering a range of NS masses and find that for stable NSs in the
realistic mass range 1.4 +/- 0.2 solar masses, the fraction is greater than
0.55 in all cases. This range is not consistent with the observational
restriction Q < 0.2 for Vela if starquakes are the cause of its glitches. This
confirms results of previous studies of the Vela pulsar which have suggested
that starquakes are not a feasible mechanism for Vela glitches. The much larger
values of Q observed for Crab glitches (Q > 0.7) are consistent with the
starquake model predictions and support previous conclusions that starquakes
can be the cause of Crab glitches.Comment: 8 pages, including 3 figures and 1 table. Accepted for publication in
Ap
Numerical simulations of two dimensional magnetic domain patterns
I show that a model for the interaction of magnetic domains that includes a
short range ferromagnetic and a long range dipolar anti-ferromagnetic
interaction reproduces very well many characteristic features of
two-dimensional magnetic domain patterns. In particular bubble and stripe
phases are obtained, along with polygonal and labyrinthine morphologies. In
addition, two puzzling phenomena, namely the so called `memory effect' and the
`topological melting' observed experimentally are also qualitatively described.
Very similar phenomenology is found in the case in which the model is changed
to be represented by the Swift-Hohenberg equation driven by an external
orienting field.Comment: 8 pages, 8 figures. Version to appear in Phys. Rev.
Existence and approximation of probability measure solutions to models of collective behaviors
In this paper we consider first order differential models of collective
behaviors of groups of agents based on the mass conservation equation. Models
are formulated taking the spatial distribution of the agents as the main
unknown, expressed in terms of a probability measure evolving in time. We
develop an existence and approximation theory of the solutions to such models
and we show that some recently proposed models of crowd and swarm dynamics fit
our theoretic paradigm.Comment: 31 pages, 1 figur
Model-based aerodynamic-angle attitude control of an atmospheric entry capsule
The paper describes the attitude control system of a low lift-to-drag biconic atmospheric
entry capsule based on the Embedded Model Control methodology. The control structure
derives from the development of the attitude dynamics and kinematics written in terms of
aerodynamic angles, instead of Euler/quaternion kinematics. A detailed development of the
simplified set of equations linking the torques generated by the reaction control system with
the time evolution of the aerodynamic angles is provided. The simplified set of equations
becomes the core of the control algorithm. The bank angle dynamics is shown to be fourthorder
and forced by yaw and roll torques. A dynamic dispatching technique is proposed for
converting fourth-order dynamics into a pair of second order systems. Nonlinear dynamic
inversion and active disturbance rejection are employed to handle gyroscopic torques,
parametric errors and to compensate for angular variation of translational velocity. A bank
reversal logic is designed to reduce the effect of bank reversals on the translational motion.
The performance of the attitude control algorithm has been tested on a high fidelity
simulator and relevant results are presented
Satellite-to-satellite attitude control of a long-distance spacecraft formation for the Next Generation Gravity Mission
The paperpresentsthedesignandsomesimulatedresultsoftheattitudecontrolofasatelliteformation
under studybytheEuropeanSpaceAgencyfortheNextGenerationGravityMission.Theformation
consists oftwospacecraftswhich fly morethan200kmapartatanaltitudefromtheEarth'sgroundof
between 300and400km.Theattitudecontrolmustkeeptheopticalaxesofthetwospacecraftaligned
with amicroradianaccuracy(pointingcontrol).Thisismadepossiblebyspecific opticalsensors
accompanyingtheinter-satellitelaserinterferometer,whichisthemainpayloadofthemission.These
sensors alloweachspacecrafttoactuateautonomousalignmentafterasuitableacquisitionprocedure.
Pointing controlisconstrainedbytheangulardrag-freecontrol,whichisimposedbymissionscience
(Earth gravimetryatalowEarthorbit),andmustzerotheangularaccelerationvectorbelow0.01 ÎĽrad/s2
in thesciencefrequencyband.Thisismadepossiblebyultrafine accelerometersfromtheGOCE-class,
whose measurementsmustbecoordinatedwithattitudesensorstoachievedrag-freeandpointing
requirements.EmbeddedModelControlshowshowcoordinationcanbeimplementedaroundthe
embedded modelsofthespacecraftattitudeandoftheformationframequaternion.Evidenceand
discussion aboutsomecriticalrequirementsarealsoincludedtogetherwithextensivesimulatedresults
of twodifferentformationtypes
- …