A volume-preserving sharpening approach for the propagation of sharp phase boundaries in multiphase lattice Boltzmann simulations

Lattice Boltzmann models that recover a macroscopic description of multiphase flow of immiscible liquids typically represent the boundaries between phases using a scalar function, the phase field, that varies smoothly over several grid points. Attempts to tune the model parameters to minimise the thicknesses of these interfaces typically lead to the interfaces becoming fixed to the underlying grid instead of advecting with the fluid velocity. This phenomenon, known as lattice pinning, is strikingly similar to that associated with the numerical simulation of conservation laws coupled to stiff algebraic source terms. We present a lattice Boltzmann formulation of the model problem proposed by LeVeque and Yee [J. Comput. Phys. 86, 187] to study the latter phenomenon in the context of computational combustion, and offer a volume-conserving extension in multiple space dimensions. Inspired by the random projection method of Bao and Jin [J. Comput. Phys. 163, 216] we further generalise this formulation by introducing a uniformly distributed quasi-random variable into the term responsible for the sharpening of phase boundaries. This method is mass conserving and the statistical average of this method is shown to significantly delay the onset of pinning

Conflict and Computation on Wikipedia: a Finite-State Machine Analysis of Editor Interactions

What is the boundary between a vigorous argument and a breakdown of relations? What drives a group of individuals across it? Taking Wikipedia as a test case, we use a hidden Markov model to approximate the computational structure and social grammar of more than a decade of cooperation and conflict among its editors. Across a wide range of pages, we discover a bursty war/peace structure where the systems can become trapped, sometimes for months, in a computational subspace associated with significantly higher levels of conflict-tracking "revert" actions. Distinct patterns of behavior characterize the lower-conflict subspace, including tit-for-tat reversion. While a fraction of the transitions between these subspaces are associated with top-down actions taken by administrators, the effects are weak. Surprisingly, we find no statistical signal that transitions are associated with the appearance of particularly anti-social users, and only weak association with significant news events outside the system. These findings are consistent with transitions being driven by decentralized processes with no clear locus of control. Models of belief revision in the presence of a common resource for information-sharing predict the existence of two distinct phases: a disordered high-conflict phase, and a frozen phase with spontaneously-broken symmetry. The bistability we observe empirically may be a consequence of editor turn-over, which drives the system to a critical point between them.Comment: 23 pages, 3 figures. Matches published version. Code for HMM fitting available at http://bit.ly/sfihmm ; time series and derived finite state machines at bit.ly/wiki_hm

State Agency Sea-Level Rise Action Plan for California

Numerous state agencies are dedicated to helping California prepare for sea-level rise (SLR). Our vision is to make all of California, including its coastlines, inland areas, and bays and estuaries, resilient to the impacts of SLR, such as flooding, erosion, and habitat degradation and loss. This occurs through thoughtful and coordinated plans and projects aimed to protect and conserve our unique biodiversity, public health and safety, infrastructure, outdoor access, and coastal economy. This must be done through an equitable process, inclusive of all Californians, that prioritizes environmental and climate justice with an emphasis on partnerships with California Native American tribes and supporting vulnerable communities. In 2020, the California Natural Resources Agency, CalEPA and over 15 departments and agencies developed SLR Principles, to guide unified, effective action toward SLR resilience for California’s coastal communities, ecosystems, and economies. This action is centered on seven principles: Best Available Science Partnerships Alignment Communications Local Support Coastal Resilience Projects Equity Though these Principles create a direction for how to align state actions and decision points around SLR, an actionable path was needed. The Ocean Protection Council (OPC) was charged with working with agency partners to create a statewide, collaborative Action Plan that would carry out these principles. The result is this document, the SLR Action Plan, a five-year plan to make advances toward coastal resilience through comprehensive, coordinated, and collaborative work. The actions in this plan address urgent needs by identifying proposed new and ongoing work that will be leveraged upon in the next five years. This Action Plan includes over 80 trackable actions, covering both a regional and statewide scope. This work plan is the first of its kind, and sets California up for a safe, equitable, and resilient future. The pace of implementation will depend upon the feasibility and availability of resources and competing priorities

On a poroviscoelastic model for cell crawling

In this paper a minimal, one–dimensional, two–phase, viscoelastic, reactive, flow model for a crawling cell is presented. Two–phase models are used with a variety of constitutive assumptions in the literature to model cell motility. We use an upper–convected Maxwell model and demonstrate that even the simplest of two–phase, viscoelastic models displays features relevant to cell motility. We also show care must be exercised in choosing parameters for such models as a poor choice can lead to an ill–posed problem. A stability analysis reveals that the initially stationary, spatially uniform strip of cytoplasm starts to crawl in response to a perturbation which breaks the symmetry of the network volume fraction or network stress. We also demonstrate numerically that there is a steady travelling–wave solution in which the crawling velocity has a bell–shaped dependence on adhesion strength, in agreement with biological observation