Ecological networks: Pursuing the shortest path, however narrow and crooked

International audienceRepresenting data as networks cuts across all sub-disciplines in ecology and evolutionary biology. Besides providing a compact representation of the interconnections between agents, network analysis allows the identification of especially important nodes, according to various metrics that often rely on the calculation of the shortest paths connecting any two nodes. While the interpretation of a shortest paths is straightforward in binary, unweighted networks, whenever weights are reported, the calculation could yield unexpected results. We analyzed 129 studies of ecological networks published in the last decade that use shortest paths, and discovered a methodological inaccuracy related to the edge weights used to calculate shortest paths (and related centrality measures), particularly in interaction networks. Specifically, 49% of the studies do not report sufficient information on the calculation to allow their replication, and 61% of the studies on weighted networks may contain errors in how shortest paths are calculated. Using toy models and empirical ecological data, we show how to transform the data prior to calculation and illustrate the pitfalls that need to be avoided. We conclude by proposing a five-point checklist to foster best-practices in the calculation and reporting of centrality measures in ecology and evolution studies. The last two decades have witnessed an exponential increase in the use of graph analysis in ecological and conservation studies (see refs. 1,2 for recent introductions to network theory in ecology and evolution). Networks (graphs) represent agents as nodes linked by edges representing pairwise relationships. For instance, a food web can be represented as a network of species (nodes) and their feeding relationships (edges) 3. Similarly, the spatial dynamics of a metapopulation can be analyzed by connecting the patches of suitable habitat (nodes) with edges measuring dispersal between patches 4. Data might either simply report the presence/absence of an edge (binary, unweighted networks), or provide a strength for each edge (weighted networks). In turn, these weights can represent a variety of ecologically-relevant quantities, depending on the system being described. For instance, edge weights can quantify interaction frequency (e.g., visitation networks 5), interaction strength (e.g., per-capita effect of one species on the growth rate of another 3), carbon-flow between trophic levels 6 , genetic similarity 7 , niche overlap (e.g., number of shared resources between two species 8), affinity 9 , dispersal probabilities (e.g., the rate at which individuals of a population move between patches 10), cost of dispersal between patches (e.g., resistance 11), etc. Despite such large variety of ecological network representations, a common task is the identification of nodes of high importance, such as keystone species in a food web, patches acting as stepping stones in a dispersal network , or genes with pleiotropic effects. The identification of important nodes is typically accomplished through centrality measures 5,12. Many centrality measures has been proposed, each probing complementary aspects of node-to-node relationships 13. For instance, Closeness centrality 14,15 highlights nodes that are "near" to all othe

KEROMIX - Stabile schadstoffarme Verbrennung. Teilprojekt: Airblast-Zerstaeubung Abschlussbericht

Within the project KEROMIX the sub-project 'Airblast-Zerstaeubung' has been conducted at the Institut fuer Thermische Stroemungsmaschinen (ITS) at the University of Karlsruhe. This sub-project included the experimental and numerical investigation of two airblast-atomizers which differ in swirl generation and prefilming process. For the experimental investigations a fully transparent model combustor has been developed and build up, which allows to analyze the reacting twophase-flow by means of optical measurement techniques at combustor pressures up to 8 bar. Using laser light sheet and endoscopic techniques several possibilities to improve the atomization performance are found. Visualization of the flame luminescence at elevated pressure show that at higher combustor inlet temperature and pressure the flame stabilizes inside the atomizer which results in increased NOx emissions. By iterative modifications of several components of the atomizer the atomization performance is increased significantly and the stabilization of the flame inside the atomizer can be avoided. The theoretical part of the project includes the numerical simulation of the gas-phase, the droplet-phase and the fluid dynamics of the shear driven liquid wall film. The simulation of the liquid film flow by the ITS code PROFILM yields criterions for a suitable design of the prefilming device. For the simulation of the gas-flow it has been found to be necessary to include the inlet-flow into the swirlers and the cooling holes. Further on an iterative approach is presented to specify correct inlet boundary conditions. The results of the simulation are compared with LDV-data of the non-reacting flow. The swirl intensity as well as the central recirculation zone are predicted correctly. (orig.)Am Institut fuer Thermische Stroemungsmaschinen (ITS) der Universitaet Karlsruhe wurde im Rahmen des Verbundprojekts KEROMIX das Teilprojekt Airblast-Zerstaeubung durchgefuehrt. Dieses beinhaltet die experimentelle und numerische Untersuchung zweier Luftstrom-Zerstaeuber, die sich in der Art der Drallerzeugung sowie der Filmaufbringung unterscheiden. Zur experimentellen Untersuchung der Zerstaeuber ist am ITS eine glaeserne Modellbrennkammer entwickelt und aufgebaut worden, die es ermoeglicht, durch verschiedene optische Messtechniken den Zerstaeubungsvorgang und die Spruehstrahlausbreitung unter reagierenden Bedingungen und bei Brennkammerdruecken bis zu 8 bar detailliert zu analysieren. Durch Laser-Lichtschnitt Untersuchungen und durch endoskopische Visualisierung des Filmzerfalls gelingt es, an beiden Zerstaeubern Moeglichkeiten zur Optimierung der Zerstaeubung zu finden. Im Rahmen der Hochdruckversuche kann an einem Zerstaeuber nachgewiesen werden, dass sich bei hoeheren Brennkammereintrittstemperaturen bzw. -druecken die Flamme innerhalb des Zerstaeubers stabilisiert. Durch mehrfache zielgerichtete Modifikation einzelner Bauteile wird eine wesentliche Verbesserung der Zerstaeubung und eine definierte Stabilisierung der Flamme ausserhalb des Zerstaeubers erreicht. Im theoretischen Teil des Projekts werden die Gas-, Tropfen- und Filmstroemung der beiden experimentell untersuchten Brennkammerkonfigurationen simuliert. Aus den Berechnungen zur Filmstroemung werden eindeutige Kriterien zur Auslegung von Filmlegern abgeleitet. Zur Simulation der Gas- und Tropfenstroemung werden die Einstroembereiche vor den Drallapparaten und Kuehlluftbohrungen mit einbezogen. Zur Bestimmung der Eintrittsbedingungen wird eine iterative Vorgehensweise vorgestellt, die eine sehr gute Annaeherung an die experimentell bestimmten Stroemungsdaten ermoeglicht. Die Drallstroemung in der Brennkammer und insbesondere das zentrale Rezirkulationsgebiet werden richtig berechnet. (orig.)Available from TIB Hannover: F01B1529 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEBundesministerium fuer Bildung und Forschung, Berlin (Germany)DEGerman