In many desert ecosystems, vegetation is both patchy and dynamic: vegetated areas are interspersed with patches of bare ground, and both the positioning and the species composition of the vegetated areas exhibit change through time. These characteristics lead to the emergence of multi-scale patterns in vegetation that arise from complex relationships between plants, soils, and transport processes. Previous attempts to probe the causes of spatial complexity and predict responses of desert ecosystems tend to be limited in their focus: models of dynamics have been developed with no consideration of the inherent patchiness in the vegetation, or else models have been developed to generate patterns with no consideration of the dynamics. Here we develop a general modelling framework for the analysis of ecosystem change in deserts that is rooted in the concept of connectivity and is derived from a detailed process-based understanding. We explicitly consider spatial interactions among multiple vegetation types and multiple resources, and our model is formulated to predict responses to a variety of endogenous and exogenous disturbances. The model is implemented in the deserts of the American Southwest both to test hypotheses of the causes of the invasion of woody shrubs, and to test its ability to reproduce observed spatial differences in response to drought in the 20th century. The model's performance leads us to argue that vertical and lateral connectivity are key emergent properties of the ecosystem, which both control its behavior and provide indicators of its state. If this argument is shown to be compatible with field observations, the model presented here will provide a more certain approach toward preventing further degradation of semiarid grasslands. © 2014 by the Ecological Society of America

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Abstract:A dual-fuel engine is a compression ignition (CI) engine where the primary gaseous fuel source is premixed with air as it enters the combustion chamber. This homogenous mixture is ignited by a small quantity of diesel, the 'pilot', that is injected towards the end of the compression stroke. In the present study, a direct-injection CI engine, was fuelled with three different gaseous fuels: methane, propane, and butane. The engine performance at various gaseous concentrations was recorded at 1500 r/min and quarter, half, and three-quarters relative to full a load of 18.7 kW. In order to investigate the combustion performance, a novel three-zone heat release rate analysis was applied to the data. The resulting heat release rate data are used to aid understanding of the performance characteristics of the engine in dual-fuel mode.Data are presented for the heat release rates, effects of engine load and speed, brake specific energy consumption of the engine, and combustion phasing of the three different primary gaseous fuels.Methane permitted the maximum energy substitution, relative to diesel, and yielded the most significant reductions in CO 2 . However, propane also had significant reductions in CO 2 but had an increased diffusional combustion stage which may lend itself to the modern high-speed direct-injection engine.

Traditional techniques for balancing long, flexible, high-speed rotating shafts are inadequate over a full range of shaft\ud speeds. This problem is compounded by limitations within the manufacturing process, which have resulted in increasing\ud problems with lateral vibrations and hence increased the failure rates of bearings in practical applications. There is a need\ud to develop a novel strategy for balancing these coupling shafts that is low cost, robust under typically long-term operating\ud conditions and amenable to on-site remediation. This paper proposes a new method of balancing long, flexible couplings\ud by means of a pair of balancing sleeve arms that are integrally attached to each end of the coupling shaft. Balance\ud corrections are applied to the free ends of the arms in order to apply a corrective centrifugal force to the coupling shaft\ud in order to limit shaft-end reaction forces and to impart a corrective bending moment to the drive shaft that limits shaft\ud deflection. The aim of this paper is to demonstrate the potential of this method, via the mathematical analysis of a plain,\ud simply supported tube with uniform eccentricity and to show that any drive shaft, even with irregular geometry and/or\ud imbalance, can be converted to an equivalent encastre case. This allows for the theoretical possibility of eliminating the\ud first simply supported critical speed, thereby reducing the need for very large lateral critical speed margins, as this\ud requirement constrains design flexibility. Although the analysis is performed on a sub 15 MW gas turbine, it is anticipated\ud that this mechanism would be beneficial on any shaft system with high-flexibility/shaft deflection

As a new alternative to tilting rotors or turbojet vector mechanical oriented nozzles, ACHEON (Aerial Coanda\ud High Efficiency Orienting-jet Nozzle) has enormous advantages because it is free of moving elements and highly\ud effective for Vertical/Short-Take-Off and Landing (V/STOL) aircraft. In this paper, an integrated flight/ thrust vectoring\ud control scheme for a jet powered Unmanned Aerial Vehicle (UAV) with an ACHEON nozzle is proposed to assess its\ud suitability in jet aircraft flight applications. Firstly, a simplified Thrust-Vectoring (TV) population model is built based\ud on CFD simulation data and parameter identification. Secondly, this TV propulsion model is embedded as a jet actuator\ud for a benchmark fixed-wing ‘Aerosonde’ UAV, and then a four “cascaded-loop” controller, based on nonlinear dynamic\ud inversion (NDI), is designed to individually control the angular rates (in the body frame), attitude angles (in the wind\ud frame), track angles (in the navigation frame), and position (in the earth-centered frame) . Unlike previous research on\ud fixed-wing UAV flight controls or TV controls, our proposed four-cascaded NDI control law can not only coordinate\ud surface control and TV control as well as an optimization controller, but can also implement an absolute self-position\ud control for the autopilot flight control. Finally, flight simulations in a high-fidelity aerodynamic environment are\ud performed to demonstrate the effectiveness and superiority of our proposed control scheme