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A lifting gas or lighter-than-air gas is a gas that has a density lower than normal atmospheric gases and rises above them as a result. It is required for aerostats to create buoyancy, particularly in lighter-than-air aircraft, which include free balloons, moored balloons, and airships. Only certain lighter than air gases are suitable as lifting gases. Dry air has a density of about 1.29 g/L (gram per liter) at standard conditions for temperature and pressure (STP) and an average molecular mass of 28.97 g/mol, and so lighter-than-air gases have a density lower than this. Gases used for lifting Hot air Heated atmospheric air is frequently used in recreational ballooning. According to the ideal gas law, an amount of gas (and also a mixture of gases such as air) expands as it is heated. As a result, a certain volume of gas has a lower density as the temperature is higher. The temperature of the hot air in the envelope will vary depending upon the ambient

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Suppressed convective rainfall by agricultural expansion in southeastern Burkina Faso

Natalie Claire Ceperley, Gabriel George Katul, Theophile Mande, Marc Parlange, Scott W. Tyler

With the green economy being promoted as a path to sustainable development and food security within the African continent, the influx of agricultural land is proliferating at a rapid pace often replacing natural savannah forests. Where agriculture is primarily rainfed, the possible adverse impacts of agricultural land influx on rainfall occurrences in water-limited areas such as West Africa warrant attention. Using field observations complemented by model calculations in southeastern Burkina Faso, the main causes of a 10-30% suppressed daytime rainfall recorded over agricultural fields when referenced to natural savannah forests are examined. Measurements and model runs reveal that the crossing of the mixed layer height and lifting condensation levels, a necessary condition for cloud formation and subsequent rainfall occurrence, was 30% more frequent above the natural savannah forest. This increase in crossing statistics was primarily explained by increases in measured sensible heat flux above the savannah forest rather than differences in lifting condensation heights.

American Geophysical Union2015

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Design of Compact Bearingless Disc Drive Systems

Douglas Martins Araujo, Juan Patricio Tomás Peralta Fierro, Yves Perriard

Inflating costs and emerging applications are spawning interests toward smaller and faster electric drives. The challenges of miniaturization make magnetic bearings more appealing, as they provide intrinsic high-speed capability and need virtually zero-maintenance. These advantages come at the cost of more sensors, which require a seamless integration especially at small geometrical scale. In the magnetic levitation domain, slice drives are particularly interesting, given their stabilizing passive characteristics that simplify the control of levitation and render the system simpler. These drives are already suitable for compressor applications, where control of all degrees of freedom is not necessary. In this article, the design process for a bearingless disc drive with a high degree of system integration is proposed. Two motor topologies, namely slotted and slotless drives, are rated by their passive stability, active properties, and power losses. Two sensor systems, fundamental for magnetic bearing commissioning, are proposed. They are simple to industrialize and offer a low footprint, thus not interfering with the drive design process. Ultimately, an optimization example is presented. For the presented scenario, drives with intermediate rotor size are the optimal contenders. Specifically among them, the slotless variant proves the best, given lower electrical losses and lighter mass compared to the slotted variant. However, the slotted becomes interesting if a stiffer magnetic bearing is needed.

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