646 research outputs found

    Ecological Invasion, Roughened Fronts, and a Competitor's Extreme Advance: Integrating Stochastic Spatial-Growth Models

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    Both community ecology and conservation biology seek further understanding of factors governing the advance of an invasive species. We model biological invasion as an individual-based, stochastic process on a two-dimensional landscape. An ecologically superior invader and a resident species compete for space preemptively. Our general model includes the basic contact process and a variant of the Eden model as special cases. We employ the concept of a "roughened" front to quantify effects of discreteness and stochasticity on invasion; we emphasize the probability distribution of the front-runner's relative position. That is, we analyze the location of the most advanced invader as the extreme deviation about the front's mean position. We find that a class of models with different assumptions about neighborhood interactions exhibit universal characteristics. That is, key features of the invasion dynamics span a class of models, independently of locally detailed demographic rules. Our results integrate theories of invasive spatial growth and generate novel hypotheses linking habitat or landscape size (length of the invading front) to invasion velocity, and to the relative position of the most advanced invader.Comment: The original publication is available at www.springerlink.com/content/8528v8563r7u2742

    Evolution of local computing time in parallel modeling of mobile networks

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    Introduction: The study concerns the properties of a parallel discrete-event simulation (PDES) model, namely a simple mobile network model known as a personal communication service (PCS) model. In this type of parallel computing, each process has its own computation time, known as local virtual time. The local virtual times change during the simulation process, forming a complex profile similar to the surface growth profile in physics.Methods: We apply the scaling theory of statistical physics to study the properties of the PCS model. We construct a simple local virtual time evolution algorithm for the PCS model and compare this theoretical time evolution model to a standard parallel mobile network implementation in Rensselaer’s Optimistic Simulation System (ROSS).Results: We show that the value of the critical exponent for the mobile network system is close to the value in the theoretical local virtual time profile model. A roughening transition is found in the LVT–PCS model, which belongs to the universality class of directed percolation in dimension 2 + 1.Discussion: We believe that the analogies we found can be useful for preliminary analyses of scalability, process desynchronization, and possible deadlocks in a wide class of parallel discrete-event simulation models

    Towards a solution of the closure problem for convective atmospheric boundary-layer turbulence

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    We consider the closure problem for turbulence in the dry convective atmospheric boundary layer (CBL). Transport in the CBL is carried by small scale eddies near the surface and large plumes in the well mixed middle part up to the inversion that separates the CBL from the stably stratified air above. An analytically tractable model based on a multivariate Delta-PDF approach is developed. It is an extension of the model of Gryanik and Hartmann [1] (GH02) that additionally includes a term for background turbulence. Thus an exact solution is derived and all higher order moments (HOMs) are explained by second order moments, correlation coefficients and the skewness. The solution provides a proof of the extended universality hypothesis of GH02 which is the refinement of the Millionshchikov hypothesis (quasi- normality of FOM). This refined hypothesis states that CBL turbulence can be considered as result of a linear interpolation between the Gaussian and the very skewed turbulence regimes. Although the extended universality hypothesis was confirmed by results of field measurements, LES and DNS simulations (see e.g. [2-4]), several questions remained unexplained. These are now answered by the new model including the reasons of the universality of the functional form of the HOMs, the significant scatter of the values of the coefficients and the source of the magic of the linear interpolation. Finally, the closures 61 predicted by the model are tested against measurements and LES data. Some of the other issues of CBL turbulence, e.g. familiar kurtosis-skewness relationships and relation of area coverage parameters of plumes (so called filling factors) with HOM will be discussed also
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