16,341 research outputs found
Lattices of hydrodynamically interacting flapping swimmers
Fish schools and bird flocks exhibit complex collective dynamics whose
self-organization principles are largely unknown. The influence of
hydrodynamics on such collectives has been relatively unexplored theoretically,
in part due to the difficulty in modeling the temporally long-lived
hydrodynamic interactions between many dynamic bodies. We address this through
a novel discrete-time dynamical system (iterated map) that describes the
hydrodynamic interactions between flapping swimmers arranged in one- and
two-dimensional lattice formations. Our 1D results exhibit good agreement with
previously published experimental data, in particular predicting the
bistability of schooling states and new instabilities that can be probed in
experimental settings. For 2D lattices, we determine the formations for which
swimmers optimally benefit from hydrodynamic interactions. We thus obtain the
following hierarchy: while a side-by-side single-row "phalanx" formation offers
a small improvement over a solitary swimmer, 1D in-line and 2D rectangular
lattice formations exhibit substantial improvements, with the 2D diamond
lattice offering the largest hydrodynamic benefit. Generally, our
self-consistent modeling framework may be broadly applicable to active systems
in which the collective dynamics is primarily driven by a fluid-mediated
memory
Roll Resonance for a Gravity-gradient Satellite
Roll and attitude stability for gravity gradient satellite
Optimal Power Cost Management Using Stored Energy in Data Centers
Since the electricity bill of a data center constitutes a significant portion
of its overall operational costs, reducing this has become important. We
investigate cost reduction opportunities that arise by the use of uninterrupted
power supply (UPS) units as energy storage devices. This represents a deviation
from the usual use of these devices as mere transitional fail-over mechanisms
between utility and captive sources such as diesel generators. We consider the
problem of opportunistically using these devices to reduce the time average
electric utility bill in a data center. Using the technique of Lyapunov
optimization, we develop an online control algorithm that can optimally exploit
these devices to minimize the time average cost. This algorithm operates
without any knowledge of the statistics of the workload or electricity cost
processes, making it attractive in the presence of workload and pricing
uncertainties. An interesting feature of our algorithm is that its deviation
from optimality reduces as the storage capacity is increased. Our work opens up
a new area in data center power management.Comment: Full version of Sigmetrics 2011 pape
Emergence of the fuzzy horizon through gravitational collapse
For a large enough Schwarzschild black hole, the horizon is a region of space
where gravitational forces are weak; yet it is also a region leading to
numerous puzzles connected to stringy physics. In this work, we analyze the
process of gravitational collapse and black hole formation in the context of
light-cone M theory. We find that, as a shell of matter contracts and is about
to reveal a black hole horizon, it undergoes a thermodynamic phase transition.
This involves the binding of D0 branes into D2's, and the new phase leads to
large membranes of the size of the horizon. These in turn can sustain their
large size through back-reaction and the dielectric Myers effect - realizing
the fuzzball proposal of Mathur and the Matrix black hole of M(atrix) theory.
The physics responsible for this phenomenon lies in strongly coupled 2+1
dimensional non-commutative dynamics. The phenomenon has a universal character
and appears generic.Comment: 24 pages, 4 figures; v2: minor clarifications, citations adde
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Understanding the Chlorine Isotopic Compositions of Apatites in Lunar Basalts
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Structurally Embedded Electrical Systems Using Ultrasonic Consolidation (UC)
Current research has demonstrated the use of Ultrasonic Consolidation (UC) to embed
several USB-based sensors into aluminum, and is working toward embedding suites of
sensors, heaters and other devices, connected via USB hubs, which can be monitored and
controlled using an embedded USB capable processor. Additionally, the research has
shown that electronics can be embedded at room temperature, but with some inter-layer
delamination between the ultrasonically bonded aluminum layers. Embedding sensors
and electronics at 300o
F to overcome the delamination issues resulted in optimal
bonding, and the sensors used thus far have functioned normally. Future investigation
will explore other UC parameter combinations to ascertain the quality of embedding at
lower temperatures.Mechanical Engineerin
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