1,637 research outputs found
Periodicity and criticality in the Olami-Feder-Christensen model of earthquakes
Characteristic versus critical features of earthquakes are studied on the
basis of the Olami-Feder-Christensen model. It is found that the local
recurrence-time distribution exhibits a sharp -function-like peak
corresponding to rhythmic recurrence of events with a fixed ``period'' uniquely
determined by the transmission parameter of the model, together with a
power-law-like tail corresponding to scale-free recurrence of events. The model
exhibits phenomena closely resembling the asperity known in seismology
Seismic cycles, size of the largest events, and the avalanche size distribution in a model of seismicity
We address several questions on the behavior of a numerical model recently
introduced to study seismic phenomena, that includes relaxation in the plates
as a key ingredient. We make an analysis of the scaling of the largest events
with system size, and show that when parameters are appropriately interpreted,
the typical size of the largest events scale as the system size, without the
necessity to tune any parameter. Secondly, we show that the temporal activity
in the model is inherently non-stationary, and obtain from here justification
and support for the concept of a "seismic cycle" in the temporal evolution of
seismic activity. Finally, we ask for the reasons that make the model display a
realistic value of the decaying exponent in the Gutenberg-Richter law for
the avalanche size distribution. We explain why relaxation induces a systematic
increase of from its value observed in the absence of
relaxation. However, we have not been able to justify the actual robustness of
the model in displaying a consistent value around the experimentally
observed value .Comment: 11 pages, 10 figure
Translating Public Policy: Enhancing the Applicability of Social Impact Techniques for Grassroots Community Groups
This paper reports on an exploratory action research study designed to understand how grassroots community organisations engage in the measurement and reporting of social impact and how they demonstrate their social impact to local government funders. Our findings suggest that the relationships between small non-profit organisations, the communities they serve or represent and their funders are increasingly driven from the top down formalised practices. Volunteer-run grassroots organisations can be marginalized in this process. Members may lack awareness of funders strategic approaches or the formalized auditing and control requirements of funders mean grassroots organisations lose capacity to define their programs and projects. We conclude that, to help counter this trend, tools and techniques which open up possibilities for dialogue between those holding power and those seeking support are essentia
Entrainment by turbulent fountains
This is the author accepted manuscript. The final version is available from Cambridge University Press via https://doi.org/10.1017/jfm.2016.16Experimental measurements of entrainment by turbulent fountains from circular sources in quiescent uniform environments are presented. Our results span almost four orders of magnitude in the source Froude number (0.004 ≤ Fr₀ ≤ 25) and thereby encompass the entrainment across all classes of fountain behaviour identified to date. We identify scalings for the entrained volume flux Qᴇ, in terms of Fr₀ and the source volume flux Q₀, within a number of distinct Froude-number bands corresponding to each class of fountain. Additionally we identify a distinct class of new behaviour, as yet unreported, for Fr₀ ≲ 0.1
Relating quanta conservation and compartmental epidemiological models of airborne disease outbreaks in buildings
We investigate the underlying assumptions and limits of applicability of several documented models for outbreaks of airborne disease inside buildings by showing how they may each be regarded as special cases of a system of equations which combines quanta conservation and compartmental epidemiological modelling. We investigate the behaviour of this system analytically, gaining insight to its behaviour at large time. We then investigate the characteristic timescales of an indoor outbreak, showing how the dilution rate of the space, and the quanta generation rate, incubation rate and removal rate associated with the illness may be used to predict the evolution of an outbreak over time, and may also be used to predict the relative performances of other indoor airborne outbreak models. The model is compared to a more commonly used model, in which it is assumed the environmental concentration of infectious aerosols adheres to a quasi-steady-state, so that the the dimensionless quanta concentration is equal to the the infectious fraction. The model presented here is shown to approach this limit exponentially to within an interval defined by the incubation and removal rates. This may be used to predict the maximum extent to which a case will deviate from the quasi steady state condition
What you do Matters: Demostrate your Community Impact
A 'tool kit' for small grass roots organisations seeking to report their social impact to their funding organisation
Network of recurrent events for the Olami-Feder-Christensen model
We numerically study the dynamics of a discrete spring-block model introduced
by Olami, Feder and Christensen (OFC) to mimic earthquakes and investigate to
which extent this simple model is able to reproduce the observed spatiotemporal
clustering of seismicty. Following a recently proposed method to characterize
such clustering by networks of recurrent events [Geophys. Res. Lett. {\bf 33},
L1304, 2006], we find that for synthetic catalogs generated by the OFC model
these networks have many non-trivial statistical properties. This includes
characteristic degree distributions -- very similar to what has been observed
for real seismicity. There are, however, also significant differences between
the OFC model and earthquake catalogs indicating that this simple model is
insufficient to account for certain aspects of the spatiotemporal clustering of
seismicity.Comment: 11 pages, 16 figure
Asperity characteristics of the Olami-Feder-Christensen model of earthquakes
Properties of the Olami-Feder-Christensen (OFC) model of earthquakes are
studied by numerical simulations. The previous study indicated that the model
exhibits ``asperity''-like phenomena, {\it i.e.}, the same region ruptures many
times near periodically [T.Kotani {\it et al}, Phys. Rev. E {\bf 77}, 010102
(2008)]. Such periodic or characteristic features apparently coexist with
power-law-like critical features, {\it e.g.}, the Gutenberg-Richter law
observed in the size distribution. In order to clarify the origin and the
nature of the asperity-like phenomena, we investigate here the properties of
the OFC model with emphasis on its stress distribution. It is found that the
asperity formation is accompanied by self-organization of the highly
concentrated stress state. Such stress organization naturally provides the
mechanism underlying our observation that a series of asperity events repeat
with a common epicenter site and with a common period solely determined by the
transmission parameter of the model. Asperity events tend to cluster both in
time and in space
Functional Electrical Stimulation mediated by Iterative Learning Control and 3D robotics reduces motor impairment in chronic stroke
Background: Novel stroke rehabilitation techniques that employ electrical stimulation (ES) and robotic technologies are effective in reducing upper limb impairments. ES is most effective when it is applied to support the patients’ voluntary effort; however, current systems fail to fully exploit this connection. This study builds on previous work using advanced ES controllers, and aims to investigate the feasibility of Stimulation Assistance through Iterative Learning (SAIL), a novel upper limb stroke rehabilitation system which utilises robotic support, ES, and voluntary effort. Methods: Five hemiparetic, chronic stroke participants with impaired upper limb function attended 18, 1 hour intervention sessions. Participants completed virtual reality tracking tasks whereby they moved their impaired arm to follow a slowly moving sphere along a specified trajectory. To do this, the participants’ arm was supported by a robot. ES, mediated by advanced iterative learning control (ILC) algorithms, was applied to the triceps and anterior deltoid muscles. Each movement was repeated 6 times and ILC adjusted the amount of stimulation applied on each trial to improve accuracy and maximise voluntary effort. Participants completed clinical assessments (Fugl-Meyer, Action Research Arm Test) at baseline and post-intervention, as well as unassisted tracking tasks at the beginning and end of each intervention session. Data were analysed using t-tests and linear regression. Results: From baseline to post-intervention, Fugl-Meyer scores improved, assisted and unassisted tracking performance improved, and the amount of ES required to assist tracking reduced. Conclusions: The concept of minimising support from ES using ILC algorithms was demonstrated. The positive results are promising with respect to reducing upper limb impairments following stroke, however, a larger study is required to confirm this
Rate- and State-Dependent Friction Law and Statistical Properties of Earthquakes
In order to clarify how the statistical properties of earthquakes depend on
the constitutive law characterizing the stick-slip dynamics, we make an
extensive numerical simulation of the one-dimensional spring-block model with
the rate- and state-dependent friction law. Both the magnitude distribution and
the recurrence-time distribution are studied with varying the constitutive
parameters characterizing the model. While a continuous spectrum of seismic
events from smaller to larger magnitudes is obtained, earthquakes described by
this model turn out to possess pronounced ``characteristic'' features.Comment: Minor revisions are made in the text and in the figures. Accepted for
publication in Europhys. Letter
- …