11,906 research outputs found
Encapsulated formulation of the Selective Frequency Damping method
We present an alternative "encapsulated" formulation of the Selective
Frequency Damping method for finding unstable equilibria of dynamical systems,
which is particularly useful when analysing the stability of fluid flows. The
formulation makes use of splitting methods, which means that it can be wrapped
around an existing time-stepping code as a "black box". The method is first
applied to a scalar problem in order to analyse its stability and highlight the
roles of the control coefficient and the filter width in the
convergence (or not) towards the steady-state. Then the steady-state of the
incompressible flow past a two-dimensional cylinder at , obtained with
a code which implements the spectral/hp element method, is presented
Anharmonic Torsional Stiffness of DNA Revealed under Small External Torques
DNA supercoiling plays an important role in a variety of cellular processes.
The torsional stress related with supercoiling may be also involved in gene
regulation through the local structure and dynamics of the double helix. To
check this possibility steady torsional stress was applied to DNA in the course
of all-atom molecular dynamics simulations. It is found that small static
untwisting significantly reduces the torsional persistence length () of
GC-alternating DNA. For the AT-alternating sequence a smaller effect of the
opposite sign is observed. As a result, the measured values are similar
under zero stress, but diverge with untwisting. The effect is traced to
sequence-specific asymmetry of local torsional fluctuations, and it should be
small in long random DNA due to compensation. In contrast, the stiffness of
special short sequences can vary significantly, which gives a simple
possibility of gene regulation via probabilities of strong fluctuations. These
results have important implications for the role of local DNA twisting in
complexes with transcription factors.Comment: 8 pages, 5 figures, to appear in Phys. Rev. Let
On the role of chemical synapses in coupled neurons with noise
We examine the behavior in the presence of noise of an array of Morris-Lecar
neurons coupled via chemical synapses. Special attention is devoted to
comparing this behavior with the better known case of electrical coupling
arising via gap junctions. In particular, our numerical simulations show that
chemical synapses are more efficient than gap junctions in enhancing coherence
at an optimal noise (what is known as array-enhanced coherence resonance): in
the case of (nonlinear) chemical coupling, we observe a substantial increase in
the stochastic coherence of the system, in comparison with (linear) electrical
coupling. We interpret this qualitative difference between both types of
coupling as arising from the fact that chemical synapses only act while the
presynaptic neuron is spiking, whereas gap junctions connect the voltage of the
two neurons at all times. This leads in the electrical coupling case to larger
correlations during interspike time intervals which are detrimental to the
array-enhanced coherence effect. Finally, we report on the existence of a
system-size coherence resonance in this locally coupled system, exhibited by
the average membrane potential of the array.Comment: 7 pages, 7 figure
The Macroeconomic Effects of Oil Shocks: Why are the 2000s So Different from the 1970s?
We characterize the macroeconomic performance of a set of industrialized economies in the aftermath of the oil price shocks of the 1970s and of the last decade, focusing on the differences across episodes. We examine four different hypotheses for the mild effects on inflation and economic activity of the recent increase in the price of oil: (a) good luck (i.e. lack of concurrent adverse shocks), (b) smaller share of oil in production, (c) more flexible labor markets, and (d) improvements in monetary policy. We conclude that all four have played an important role.
"Observability and Overcoming Coordination Failure in Organizations"
Many organizations suffer poor performance because its members fail to coordinate on efficient patterns of behavior. In previous research, we have shown that financial incentives can be used to find a way out of such performance traps. Here we examine the sensitivity of this result to the ability of people to observe others' choices. Our experiments are set in a corporate environment where subjects' payoffs depend on coordinating at high effort levels; the underlying game being played repeatedly by the employees of an experimental firm is a weak-link game. Treatments vary along two dimensions. First, subjects either start with low financial incentives for coordination, which typically leads to coordination failure, and then are switched to higher incentives or start with high incentives, which typically yield effective coordination, and are switched to low incentives. Second, as the key treatment variable, subjects either observe the effort levels chosen by all employees in their experimentaIncentives, Coordination, Observation, Experiments, Organizations
It's What You Say Not What You Pay
We study manager-employee interactions in experiments set in a corporate environment where payoffs depend on employees coordinating at high effort levels; the underlying game being played repeatedly by employees is a weak-link game. In the absence of managerial intervention subjects invariably slip into coordination failure. To overcome a history of coordination failure, managers have two instruments at their disposal, increasing employees' financial incentives to coordinate and communication with employees. We find that communication is a more effective tool than incentive changes for leading organizations out of performance traps. Examining the content of managers' communication, the most effective messages specifically request a high effort, point out the mutual benefits of high effort, and imply that employees are being paid well.Change, Incentives, Coordination, Communication, Experiments, Organizations
The macroeconomic effects of oil price shocks: Why are the 2000s so different from the 1970s?
We characterize the macroeconomic performance of a set of industrialized economies in the aftermath of the oil price shocks of the 1970s and of the last decade, focusing on the differences across episodes. We examine four different hypotheses for the mild effects on inflation and economic activity of the recent increase in the price of oil: (a) good luck (i.e. lack of concurrent adverse shocks), (b) smaller share of oil in production, (c) more flexible labor markets, and (d) improvements in monetary policy. We conclude that all four have played an important role.Great moderation, supply shocks, stagflation, monetary policy, real wage rigidities
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