Flame propagation in random media

We introduce a phase-field model to describe the dynamics of a self-sustaining propagating combustion front within a medium of randomly distributed reactants. Numerical simulations of this model show that a flame front exists for reactant concentration $c > c^* > 0$, while its vanishing at $c^*$ is consistent with mean-field percolation theory. For $c > c^*$, we find that the interface associated with the diffuse combustion zone exhibits kinetic roughening characteristic of the Kardar-Parisi-Zhang equation.Comment: 4, LR541

Scaling, Propagation, and Kinetic Roughening of Flame Fronts in Random Media

We introduce a model of two coupled reaction-diffusion equations to describe the dynamics and propagation of flame fronts in random media. The model incorporates heat diffusion, its dissipation, and its production through coupling to the background reactant density. We first show analytically and numerically that there is a finite critical value of the background density, below which the front associated with the temperature field stops propagating. The critical exponents associated with this transition are shown to be consistent with mean field theory of percolation. Second, we study the kinetic roughening associated with a moving planar flame front above the critical density. By numerically calculating the time dependent width and equal time height correlation function of the front, we demonstrate that the roughening process belongs to the universality class of the Kardar-Parisi-Zhang interface equation. Finally, we show how this interface equation can be analytically derived from our model in the limit of almost uniform background density.Comment: Standard LaTeX, no figures, 29 pages; (to appear in J. Stat. Phys. vol.81, 1995). Complete file available at http://www.physics.helsinki.fi/tft/tft.html or anonymous ftp at ftp://rock.helsinki.fi/pub/preprints/tft

First Order Phase Transition in Intermediate Energy Heavy Ion Collisions

We model the disassembly of an excited nuclear system formed as a result of a heavy ion collision. We find that, as the beam energy in central collisions in varied, the dissociating system crosses a liquid-gas coexistence curve, resulting in a first-order phase transition. Accessible experimental signatures are identified: a peak in specific heat, a power-law yield for composites, and a maximum in the second moment of the yield distribution

Statistical Properties of Turbulence: An Overview

We present an introductory overview of several challenging problems in the statistical characterisation of turbulence. We provide examples from fluid turbulence in three and two dimensions, from the turbulent advection of passive scalars, turbulence in the one-dimensional Burgers equation, and fluid turbulence in the presence of polymer additives.Comment: 34 pages, 31 figure

Adaptive phenotypic plasticity in the Midas cichlid fish pharyngeal jaw and its relevance in adaptive radiation

Phenotypic evolution and its role in the diversification of organisms is a central topic in evolutionary biology. A neglected factor during the modern evolutionary synthesis, adaptive phenotypic plasticity, more recently attracted the attention of many evolutionary biologists and is now recognized as an important ingredient in both population persistence and diversification. The traits and directions in which an ancestral source population displays phenotypic plasticity might partly determine the trajectories in morphospace, which are accessible for an adaptive radiation, starting from the colonization of a novel environment. In the case of repeated colonizations of similar environments from the same source population this "flexible stem" hypothesis predicts similar phenotypes to arise in repeated subsequent radiations. The Midas Cichlid (Amphilophus spp.) in Nicaragua has radiated in parallel in several crater-lakes seeded by populations originating from the Nicaraguan Great Lakes. Here, we tested phenotypic plasticity in the pharyngeal jaw of Midas Cichlids. The pharyngeal jaw apparatus of cichlids, a second set of jaws functionally decoupled from the oral ones, is known to mediate ecological specialization and often differs strongly between sister-species. We performed a common garden experiment raising three groups of Midas cichlids on food differing in hardness and calcium content. Analyzing the lower pharyngeal jaw-bones we find significant differences between diet groups qualitatively resembling the differences found between specialized species. Observed differences in pharyngeal jaw expression between groups were attributable to the diet's mechanical resistance, whereas surplus calcium in the diet was not found to be of importance. The pharyngeal jaw apparatus of Midas Cichlids can be expressed plastically if stimulated mechanically during feeding. Since this trait is commonly differentiated - among other traits - between Midas Cichlid species, its plasticity might be an important factor in Midas Cichlid speciation. The prevalence of pharyngeal jaw differentiation across the Cichlidae further suggests that adaptive phenotypic plasticity in this trait could play an important role in cichlid speciation in general. We discuss several possibilities how the adaptive radiation of Midas Cichlids might have been influenced in this respect

Reimagining the Development of Downtown Cahokia Using Remote Sensing Visualizations from the Western Edge of the Grand Plaza

The distribution of mounds, plazas, and defensive palisades associated with Cahokia Mounds State Historic Site (CMSHS) defines the core urban environment of Eastern North America’s first American Indian city. The large mounds surrounding Cahokia’s centrally located Grand Plaza, including the palisades that enclose them, are referred to as Downtown Cahokia. In this portion of the site, archaeologists have identified material culture (e.g., ceramics), earthen fills to level the plaza, and several earthen mound constructions. These findings suggest an occupational history for the area that occurred over the 9th–14th centuries CE, with the emergence of plaza delineation and earthwork construction beginning in the early 11th century CE. In sum, Downtown Cahokia and its Grand Plaza are considered by archaeologists to be a vibrant space characterized by ongoing American Indian transformations to an early metropolitan landscape. We conducted magnetometer and electromagnetic induction surveys at the western edge of the Grand Plaza. When compared with the LiDAR-derived visualizations we generated from this portion of the site, our aerial and terrestrial remote sensing data offered new information on the nature and sequence of monument construction in Downtown Cahokia, as well as architectural changes in domestic and special-use structures. These multi-scalar and complementary remote sensing datasets allowed us, without excavating, to trace important sequences of change in Downtown Cahokia’s history

Reimagining the Development of Downtown Cahokia Using Remote Sensing Visualizations from the Western Edge of the Grand Plaza

The distribution of mounds, plazas, and defensive palisades associated with Cahokia Mounds State Historic Site (CMSHS) defines the core urban environment of Eastern North America’s first American Indian city. The large mounds surrounding Cahokia’s centrally located Grand Plaza, including the palisades that enclose them, are referred to as Downtown Cahokia. In this portion of the site, archaeologists have identified material culture (e.g., ceramics), earthen fills to level the plaza, and several earthen mound constructions. These findings suggest an occupational history for the area that occurred over the 9th–14th centuries CE, with the emergence of plaza delineation and earthwork construction beginning in the early 11th century CE. In sum, Downtown Cahokia and its Grand Plaza are considered by archaeologists to be a vibrant space characterized by ongoing American Indian transformations to an early metropolitan landscape. We conducted magnetometer and electromagnetic induction surveys at the western edge of the Grand Plaza. When compared with the LiDAR-derived visualizations we generated from this portion of the site, our aerial and terrestrial remote sensing data offered new information on the nature and sequence of monument construction in Downtown Cahokia, as well as architectural changes in domestic and special-use structures. These multi-scalar and complementary remote sensing datasets allowed us, without excavating, to trace important sequences of change in Downtown Cahokia’s history

Taehwa Research Forest: A receptor site for severe domestic pollution events in Korea during 2016

During the May–June 2016 International Cooperative Air Quality Field Study in Korea (KORUS-AQ), light synoptic meteorological forcing facilitated Seoul metropolitan pollution outflow to reach the remote Taehwa Research Forest (TRF) site and cause regulatory exceedances of ozone on 24 days. Two of these severe pollution events are thoroughly examined. The first, occurring on 17 May 2016, tracks transboundary pollution transport exiting eastern China and the Yellow Sea, traversing the Seoul Metropolitan Area (SMA), and then reaching TRF in the afternoon hours with severely polluted conditions. This case study indicates that although outflow from China and the Yellow Sea were elevated with respect to chemically unperturbed conditions, the regulatory exceedance at TRF was directly linked in time, space, and altitude to urban Seoul emissions. The second case studied, which occurred on 9 June 2016, reveals that increased levels of biogenic emissions, in combination with amplified urban emissions, were associated with severe levels of pollution and a regulatory exceedance at TRF. In summary, domestic emissions may be causing more pollution than by transboundary pathways, which have been historically believed to be the major source of air pollution in South Korea. The case studies are assessed with multiple aircraft, model (photochemical and meteorological) simulations, in situ chemical sampling, and extensive ground-based profiling at TRF. These observations clearly identify TRF and the surrounding rural communities as receptor sites for severe pollution events associated with Seoul outflow, which will result in long-term negative effects to both human health and agriculture in the affected areas