41 research outputs found
Permeability Estimates of Self-Affine Fracture Faults Based on Generalization of the Bottle Neck Concept
We propose a method for calculating the effective permeability of
two-dimensional self-affine permeability fields based on generalizing the
one-dimensional concept of a bottleneck. We test the method on fracture faults
where the local permeability field is given by the cube of the aperture field.
The method remains accurate even when there is substantial mechanical overlap
between the two fracture surfaces. The computational efficiency of the method
is comparable to calculating a simple average and is more than two orders of
magnitude faster than solving the Reynolds equations using a finite-difference
scheme
Understanding Terrorist Organizations with a Dynamic Model
Terrorist organizations change over time because of processes such as
recruitment and training as well as counter-terrorism (CT) measures, but the
effects of these processes are typically studied qualitatively and in
separation from each other. Seeking a more quantitative and integrated
understanding, we constructed a simple dynamic model where equations describe
how these processes change an organization's membership. Analysis of the model
yields a number of intuitive as well as novel findings. Most importantly it
becomes possible to predict whether counter-terrorism measures would be
sufficient to defeat the organization. Furthermore, we can prove in general
that an organization would collapse if its strength and its pool of foot
soldiers decline simultaneously. In contrast, a simultaneous decline in its
strength and its pool of leaders is often insufficient and short-termed. These
results and other like them demonstrate the great potential of dynamic models
for informing terrorism scholarship and counter-terrorism policy making.Comment: To appear as Springer Lecture Notes in Computer Science v2:
vectorized 4 figures, fixed two typos, more detailed bibliograph
The developmental dynamics of terrorist organizations
We identify robust statistical patterns in the frequency and severity of
violent attacks by terrorist organizations as they grow and age. Using
group-level static and dynamic analyses of terrorist events worldwide from
1968-2008 and a simulation model of organizational dynamics, we show that the
production of violent events tends to accelerate with increasing size and
experience. This coupling of frequency, experience and size arises from a
fundamental positive feedback loop in which attacks lead to growth which leads
to increased production of new attacks. In contrast, event severity is
independent of both size and experience. Thus larger, more experienced
organizations are more deadly because they attack more frequently, not because
their attacks are more deadly, and large events are equally likely to come from
large and small organizations. These results hold across political ideologies
and time, suggesting that the frequency and severity of terrorism may be
constrained by fundamental processes.Comment: 28 pages, 8 figures, 4 tables, supplementary materia
Quantitative imaging of concentrated suspensions under flow
We review recent advances in imaging the flow of concentrated suspensions,
focussing on the use of confocal microscopy to obtain time-resolved information
on the single-particle level in these systems. After motivating the need for
quantitative (confocal) imaging in suspension rheology, we briefly describe the
particles, sample environments, microscopy tools and analysis algorithms needed
to perform this kind of experiments. The second part of the review focusses on
microscopic aspects of the flow of concentrated model hard-sphere-like
suspensions, and the relation to non-linear rheological phenomena such as
yielding, shear localization, wall slip and shear-induced ordering. Both
Brownian and non-Brownian systems will be described. We show how quantitative
imaging can improve our understanding of the connection between microscopic
dynamics and bulk flow.Comment: Review on imaging hard-sphere suspensions, incl summary of
methodology. Submitted for special volume 'High Solid Dispersions' ed. M.
Cloitre, Vol. xx of 'Advances and Polymer Science' (Springer, Berlin, 2009);
22 pages, 16 fig
Operational resilience: concepts, design and analysis
Building resilience into today’s complex infrastructures is critical to the daily functioning of society andits ability to withstand and recover from natural disasters, epidemics, and cyber-threats. This studyproposes quantitative measures that capture and implement the definition of engineering resilienceadvanced by the National Academy of Sciences. The approach is applicable across physical, information,and social domains. It evaluates the critical functionality, defined as a performance function of time setby the stakeholders. Critical functionality is a source of valuable information, such as the integratedsystem resilience over a time interval, and its robustness. The paper demonstrates the formulation ontwo classes of models: 1) multi-level directed acyclic graphs, and 2) interdependent coupled networks.For both models synthetic case studies are used to explore trends. For the first class, the approach isalso applied to the Linux operating system. Results indicate that desired resilience and robustness levelsare achievable by trading off different design parameters, such as redundancy, node recovery time, andbackup supply available. The nonlinear relationship between network parameters and resilience levelsconfirms the utility of the proposed approach, which is of benefit to analysts and designers of complexsystems and networks
Optimizing Distribution of Pandemic Influenza Antiviral Drugs
We provide a data-driven method for optimizing pharmacy-based distribution of antiviral drugs during an influenza pandemic in terms of overall access for a target population and apply it to the state of Texas, USA. We found that during the 2009 influenza pandemic, the Texas Department of State Health Services achieved an estimated statewide access of 88% (proportion of population willing to travel to the nearest dispensing point). However, access reached only 34.5% of US postal code (ZIP code) areas containing <1,000 underinsured persons. Optimized distribution networks increased expected access to 91% overall and 60% in hard-to-reach regions, and 2 or 3 major pharmacy chains achieved near maximal coverage in well-populated areas. Independent pharmacies were essential for reaching ZIP code areas containing <1,000 underinsured persons. This model was developed during a collaboration between academic researchers and public health officials and is available as a decision support tool for Texas Department of State Health Services at a Web-based interface