Electrical devices, especially embedded ones, are becoming more and more compact and require higher power consumption. In fact, all these electrical components produce heat that must be evacuated to maintain an efficient operation. In this context, the electrofluidodynamics team at the Pprime Institute works on developing electrohydrodynamic (EHD) systems that have shown numerous advantages for both spatial and terrestrial applications. In order to achieve an optimized performance, EHD systems use eco–friendly dielectric liquids, such as Hydrofluoroethers (HFE), whose electrical behavior is not well defined yet. The objective of the current work is to characterize the variations of the electrical properties of HFE 7000 and 7100 in function of temperature. Several methods are used to analyze the electrical behavior of these liquids at low voltage. The first method is in accordance with the guidelines of the IEC 61620 standard and the second one is based on dielectric spectroscopy.
The study is then extended to the thermal behavior under high voltage. The analysis of the current–voltage characteristics makes it possible to highlight the three typical zones of the electrical behavior: ohmic, quasi–ohmic and injection, and thus to define the limits of the conduction and injection (or ion–drag) regimes. The study at high voltage also addresses, in compliance with the IEC 60156 standard, the problem of the dielectric strength of the two HFEs at different temperatures in both gas and liquid phases. Finally, an innovative preliminary study on the Kerr effect in HFE–7100 is conducted. It shows
that this electro–optical effect can be used to study the development of charged layers at the HFE/electrode interfaces. In conclusion, the results obtained in this work contribute to the understanding of the electrical behavior of HFEs. This is necessary to improve and optimize the performance of EHD systems working with these liquids.
Effect of free surface on the hydrodynamics of plates in cross-flow
Cylinders and flat plates oriented normal to the flow are typical bluff bodies, characterized with large regions of separated flow and a significant pressure drag component. The study of forces generated by bluff bodies has been one of the oldest problems in fluid mechanics, studied initially using free streamline theory and the modified hodograph plane theory. These canonical bodies are also used extensively in several industrial sectors, ranging from oil & gas, transportation to energy. There is a good amount of literature on the interaction between cylinders and boundaries, but very little information is available related to the interaction between flat plates and deformable boundaries. This lack of information is even more apparent when plate aspect ratio is considered as well. With this in mind, this talk focuses on hydrodynamics of plates in cross-flow near the free surface over a range of aspect ratios, Reynolds numbers, and submergence depths. Later, a few strategies for drag control, namely structural flexibility and strategic porosity are also presented.
Trees are the largest living beings that have ever lived on Earth, with more than one hundred meters high, masses of several thousand tons, and ages of several thousand years. In order to achieve this, among other functional conditions, trees must cope with all kinds of mechanical loads. On that line of thought, this lecture presents some of the most important strategies of growth, anatomy and morphology that plants and trees use, from a biomimetic approach, to meet structural requirements. The idea is to highlight how species of the plant kingdom can serve as biomechanic models in engineering, architecture and industrial design applications. Special emphasis will be placed on the relationship between the structure and function of some plant systems to study materials and technological solutions, or simply to wonder with the wisdom of nature.