8 research outputs found
Implicit particle methods and their connection with variational data assimilation
The implicit particle filter is a sequential Monte Carlo method for data
assimilation that guides the particles to the high-probability regions via a
sequence of steps that includes minimizations. We present a new and more
general derivation of this approach and extend the method to particle smoothing
as well as to data assimilation for perfect models. We show that the
minimizations required by implicit particle methods are similar to the ones one
encounters in variational data assimilation and explore the connection of
implicit particle methods with variational data assimilation. In particular, we
argue that existing variational codes can be converted into implicit particle
methods at a low cost, often yielding better estimates, that are also equipped
with quantitative measures of the uncertainty. A detailed example is presented
A random map implementation of implicit filters
Implicit particle filters for data assimilation generate high-probability
samples by representing each particle location as a separate function of a
common reference variable. This representation requires that a certain
underdetermined equation be solved for each particle and at each time an
observation becomes available. We present a new implementation of implicit
filters in which we find the solution of the equation via a random map. As
examples, we assimilate data for a stochastically driven Lorenz system with
sparse observations and for a stochastic Kuramoto-Sivashinski equation with
observations that are sparse in both space and time
Topological character of hydrodynamic screening in suspensions of hard spheres: an example of universal phenomenon
Although in the case of polymer solutions the existence of hydrodynamic
screening is considered as established, use of the same methods for suspensions
of hard spheres so far have failed to produce similar results. In this work we
reconsider this problem. Using superposition of topological, combinatorial and
London-style qualitative arguments, we prove the existence of screening in
suspensions. We show that the nature of hydrodynamic screening in suspensions
is analogous to that known for the Meissner effect in superconductors. The
extent of screening depends on volume fraction of hard spheres. The zero volume
fraction limit corresponds to the normal state. The case of finite volume
fractions-to the mixed state typical for superconductors of the second kind.
Such a state is becoming fully "superconducting" at some critical volume
fraction for which the (zero frequency) relative viscosity diverges. Our
analytical results describing this divergence are in accord with known scaling
results obtained by Brady and Bicerano et al which are well supported by
experimental data. We provide theoretical explanation of the divergence of
relative viscosity in terms of a topological-type transition which
mathematically can be made isomorphic to the more familiar Bose-Einstein
condensation transition. Because of this, the methods developed in this work
are not limited to suspensions only. In concluding section we mention other
applications of the developed formalism ranging from turbulence and
magnetohydrodynamics to high temperature superconductors, QCD, string models,
etc.Comment: 49 page
Contested Governance: Understanding Justice Interventions in Post-Qaddafi Libya
This contribution reflects upon the nexus between transitional justice and peacebuilding through a study of how transitional justice practices in post-Qadhafi Libya interacted with broader efforts to establish governance institutions in the aftermath of Libya’s 2011 armed conflict. It argues that dominant practices of transitional justice, promoted by external actors, prescribed narrow state-centric justice interventions that were ill-suited for a polity in which the state was highly contested. In fact, transitional justice proved divisive in Libya because attempts to project state-centric liberal justice practices were limited by their targeting of weak institutions that lacked local legitimacy and their inability to reconcile alternative normative frameworks that challenge the modern state. In addition, the weakness of Libya’s state institutions allowed thuwwar, or revolutionary armed groups, to dictate an exclusionary form of justice known as political isolation. Drawn from fieldwork conducted in Libya, this contribution provides lessons for both peacebuilding and transitional justice practice that call for a rethinking of teleological notions of transition and greater engagement with notions and concepts that fall outside dominant practices