49,873 research outputs found
Minimizing the excitation of parasitic modes of vibration in slender power ultrasonic devices
The design of slender power ultrasonic devices can often be challenging due to the excitation of parasitic modes of vibration during operation. The excitation of these modes is known to manifest from behaviors such as modal coupling which if not controlled or designed out of the system can, under operational conditions, lead to poor device performance and device failure. However, a report published by the authors has indicted that the excitation of these modes of vibration could be minimized through device design, specifically careful location of the piezoceramic stack. This paper illustrates that it is possible, through piezoceramic stack position, to minimize modal coupling between a parasitic mode and the tuned longitudinal mode of vibration for slender ultrasonic devices
Cooperation in public goods games: stay, but not for too long
Cooperation in repeated public goods game is hardly achieved, unless
contingent behavior is present. Surely, if mechanisms promoting positive
assortment between cooperators are present, then cooperators may beat
defectors, because cooperators would collect greater payoffs. In the context of
evolutionary game theory, individuals that always cooperate cannot win the
competition against defectors in well-mixed populations. Here, we study the
evolution of a population where fitness is obtained in repeated public goods
games and players have a fixed probability of playing the next round. As a
result, the group size decreases during the game. The population is well-mixed
and there are only two available strategies: always cooperate (ALLC) or always
defect (ALLD). Through numerical calculation and analytical approximations we
show that cooperation can emerge if the players stay playing the game, but not
for too long. The essential mechanism is the interaction between the transition
from strong to weak altruism, as the group size decreases, and the existence of
an upper limit to the number of rounds representing limited time availability
Challenges to Self-Acceleration in Modified Gravity from Gravitational Waves and Large-Scale Structure
With the advent of gravitational-wave astronomy marked by the aLIGO GW150914
and GW151226 observations, a measurement of the cosmological speed of gravity
will likely soon be realized. We show that a confirmation of equality to the
speed of light as indicated by indirect Galactic observations will have
important consequences for a very large class of alternative explanations of
the late-time accelerated expansion of our Universe. It will break the dark
degeneracy of self-accelerated Horndeski scalar-tensor theories in the
large-scale structure that currently limits a rigorous discrimination between
acceleration from modified gravity and from a cosmological constant or dark
energy. Signatures of a self-acceleration must then manifest in the linear,
unscreened cosmological structure. We describe the minimal modification
required for self-acceleration with standard gravitational-wave speed and show
that its maximum likelihood yields a 3-sigma poorer fit to cosmological
observations compared to a cosmological constant. Hence, equality between the
speeds challenges the concept of cosmic acceleration from a genuine
scalar-tensor modification of gravity.Comment: 5 pages; v2 updated with newer data; v3 extended titl
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