Collapse of an ecological network in Ancient Egypt

The dynamics of ecosystem collapse are fundamental to determining how and why biological communities change through time, as well as the potential effects of extinctions on ecosystems. Here we integrate depictions of mammals from Egyptian antiquity with direct lines of paleontological and archeological evidence to infer local extinctions and community dynamics over a 6000-year span. The unprecedented temporal resolution of this data set enables examination of how the tandem effects of human population growth and climate change can disrupt mammalian communities. We show that the extinctions of mammals in Egypt were nonrandom, and that destabilizing changes in community composition coincided with abrupt aridification events and the attendant collapses of some complex societies. We also show that the roles of species in a community can change over time, and that persistence is predicted by measures of species sensitivity, a function of local dynamic stability. Our study is the first high-resolution analysis of the ecological impacts of environmental change on predator-prey networks over millennial timescales, and sheds light on the historical events that have shaped modern animal communities

Computational Prediction of Primary Breakup in Fuel Spray Nozzles for Aero-Engine Combustors

[EN] Primary breakup of liquid fuel in the vicinity of fuel spray nozzles as utilized in aero-engine combustors is numerically investigated. As grid based methods exhibit a variety of disadvantages when it comes to the prediction of multiphase flows, the ”Smoothed Particle Hydrodynamics“ (SPH)-method is employed. The eligibility of the method to analyze breakup of fuel has been demonstrated in recent publications by Braun et al, Dauch et al and Koch et al [1, 2, 3, 4]. In the current paper a methodology for the investigation of the two-phase flow in the vicinity of fuel spray nozzles at typical operating conditions is proposed. Due to lower costs in terms of computing time, 2D predictions are desired. However, atomization of fluids is inherently three dimensional. Hence, differences between 2D and 3D predictions are to be expected. In course of this study, predictions in 2D and based on a 3D sector are presented. Differences in terms of gaseous flow, ligament shape and mixing are assessed.This work was performed on the computational resource ForHLR Phase II funded by the Ministry of Science, Research and Arts Baden-Württemberg and DFG (”Deutsche Forschungsgemeinschaft“). In addition the authors would like to thank Rolls-Royce Deutschland Ltd & Co KG for the outstanding cooperation. The authors also are grateful for many lively and fruitful discussions with Simon Holz.Dauch, T.; Braun, S.; Wieth, L.; Chaussonnet, G.; Keller, M.; Koch, R.; Bauer, H. (2017). Computational Prediction of Primary Breakup in Fuel Spray Nozzles for Aero-Engine Combustors. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 806-813. https://doi.org/10.4995/ILASS2017.2017.4693OCS80681

An Application-Oriented Synthetic Network Traffic Generator

Abstract—Design space exploration and detailed anal-ysis in the field of hardware design applies simulation in many variants. A frequently used method is stochastic simulation where systems are stimulated by randomised input. Synthetic traffic traces mainly form the load for stochastic simulation of network computing devices. The generator presented here utilises two well-known models to meet the features of a majority of applications and traffic sources. Based on application-specific pa-rameter sets, the traffic models stochastically generate packet flows which are merged to an aggregated stream. Nevertheless, all packets can always be identified and are not resolved to a data mass representing the load of a link

Three-dimensional SPH simulation of a twin-fluid atomizer operating at high pressure [in press]

In the context of biofuel production, a twin-fluid atomizer is investigated by the means of the Weakly-Compressible Smoothed Particle Hydrodynamics (WCSPH) method. This configuration consists of a round liquid jet discharging at low velocity into a quiescent cavity. The liquid is atomized by a high-speed turbulent co-flow. This configuration has been studied experimentally as well. In order to reflect the experimental conditions, the liquid is a mixture of Glycerol and water and a constant viscosity is set to 200 mPa s. The ambient pressure is 11 bar and the gas velocity is 58.3 m/s, leading to a gas Reynolds number of 137 000 and a Weber number of 1375. The three-dimensional numerical domain consists of the twin-fluid nozzle and a cavity of 30 mm length and 17.4 mm diameter. The spatial resolution is 33 \textmu m, which leads to 208 million of particles. The simulation is run for 45 ms of physical time using 2000 CPU. The results show that the fiber-mode breakup is well captured by the method. The shape and the dynamics of the fragmented liquid lumps are in very good qualitative agreement with the experimental observations. Further quantitative analyses are performed in terms of time average of the liquid phase, and time evolution of the spray characteristics at the exit of the cavity. Finally, due to the Lagrangian nature of the SPH method, the breakup sequence of each liquid elements can be monitored and collected. Hence, the fragmentation spectrum of this configuration is also presented

New Insights in the Primary Breakup Process of Prefilming Airblast Atomizers by SPH Predictions

Smoothed Particles Hydrodynamics (SPH) simulations of the primary breakup process have been conducted using a planar prefilming airblast atomizer geometry that has been investigated experimentally. Despite the fact that a 2D study is conducted, most of the breakup phenomena are captured by the simulations. A variation of the liquid mass flow effects the simulated air flow and spray in the same way as indicated by experimental results. Using this numerical approach new insights in the primary breakup process are obtained, which can be received only difficultly in experiments. The presence of liquid at the trailing edge of the prefilmer is quantified from a side view perspective. In addition, the influence of a variation of the liquid mass flow on the liquid film and the presence of liquid at the trailing edge are characterized. Furthermore, three different wetting modes of the trailing edge are observed: The non-wetting mode which is characterized by one film wave per breakup event as well as the unstable and stable accumulation modes which are related to two film waves per breakup event. Finally, it is demonstrated that the liquid film waves and the breakup of the ligaments are slightly decoupled by the accumulation of liquid at the trailing edge

Analyzing the Interaction of Vortex and Gas–Liquid Interface Dynamics in Fuel Spray Nozzles by Means of Lagrangian-Coherent Structures (2D)

Predictions of the primary breakup of fuel in realistic fuel spray nozzles for aero-engine combustors by means of the SPH method are presented. Based on simulations in 2D, novel insights into the fundamental effects of primary breakup are established by analyzing the dynamics of Lagrangian-coherent structures (LCSs). An in-house visualization and data exploration platform is used in order to retrieve fields of the finite-time Lyapunov exponent (FTLE) derived from the SPH predictions aiming at the identification of time resolved LCSs. The main focus of this paper is demonstrating the suitability of FTLE fields to capture and visualize the interaction between the gas and the fuel flow leading to liquid disintegration. Aiming for a convenient illustration at a high spatial resolution, the analysis is presented based on 2D datasets. However, the method and the conclusions can analoguosly be transferred to 3D. The FTLE fields of modified nozzle geometries are compared in order to highlight the influence of the nozzle geometry on primary breakup, which is a novel and unique approach for this industrial application. Modifications of the geometry are proposed which are capable of suppressing the formation of certain LCSs, leading to less fluctuation of the fuel flow emerging from the spray nozzle

Modeling of the Deformation Dynamics of Single and Twin Fluid Droplets Exposed to Aerodynamic Loads

Droplet deformation and breakup plays a significant role in liquid fuel atomization processes. The droplet behavior needs to be understood in detail, in order to derive simplified models for predicting the different processes in combustion chambers. Therefore, the behavior of single droplets at low aerodynamic loads was investigated using the Lagrangian, mesh-free Smoothed Particle Hydrodynamics (SPH) method. The simulations to be presented in this paper are focused on the deformation dynamics of pure liquid droplets and fuel droplets with water added to the inside of the droplet. The simulations have been run at two different relative velocities. As SPH is relatively new to Computational Fluid Dynamics (CFD), the pure liquid droplet simulations are used to verify the SPH code by empirical correlations available in literature. Furthermore, an enhanced characteristic deformation time is proposed, leading to a good description of the temporal initial deformation behavior for all investigated test cases. In the further course, the deformation behavior of two fluid droplets are compared to the corresponding single fluid droplet simulations. The results show an influence of the added water on the deformation history. However, it is found that, the droplet behavior can be characterized by the pure fuel Weber number

Numerical Modeling of Oil-Jet Lubrication for Spur Gears using Smoothed Particle Hydrodynamics

Understanding and optimizing the lubrication and cooling in aero engine gearbox applications is crucial for a reliable and efficient sub-system design of future aircraft engines. Due to the complex design of gearboxes with its various rotating parts, space and access for experimental investigations are severely limited. Thus, suitable numerical methods need to be developed in order to thoroughly investigate the evolving oil-air two-phase flow in the vicinity of the gear teeth. In this paper, the impingement of a single oil-jet on a single rotating spur gear was analyzed using the Smoothed Particle Hydrodynamics (SPH) method. The study was conducted with a simplified 2D setup under typical operating conditions met in reduction gear units of novel large civil aircraft engines. Results of the predicted oil-air two-phase flow are presented and compared to conventional Volume-of-Fluid (VOF) simulations. The wetting behavior and the impingement depth of the oil-jet between the gear teeth are investigated for varying oil-jet velocities and rotational speeds. In order to capture three-dimensional flow effects, a 3D setup and preliminary results are presented