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The present is the period of time that is occurring now. The present is contrasted with the past, the period of time that has already occurred, and the future, the period of time that has yet to occur. It is sometimes represented as a hyperplane in space-time, typically called "now", although modern physics demonstrates that such a hyperplane cannot be defined uniquely for observers in relative motion. The present may also be viewed as a duration. Historiography Contemporary history describes the historical timeframe immediately relevant to the present time and is a certain perspective of modern history. Philosophy and religion Philosophy of time Philosophy of time "The present" raises the question: "How is it that all sentient beings experience now at the same time?" There is no logical reason why this should be the case and no easy answer to the question. In Buddhism Buddhism and many of its associated paradigms emphasize the importance of

Energy Efficient Cruise Ships HVAC Loads

Othmane Tamri

The present work is a Master semester project created collaterally to the mechanical engineering major (SGM) at the Swiss federal institute of technology (EPFL). The project aim is the comprehension, assessment and optimization of an HVAC energy system which is located on a cruise ship. To approach the subject, a first analysis of the real field data from the ship is made. Then, an approximation of the energy demand of the HVAC ystem on board is carried out using Matlab. The real consumed HVAC energy is compared with the theoretical results. Based on these results, optimization options regarding the HVAC systems and their possible implementation are discussed.

2014

Performance and User Acceptance of GDM Filters for HWTS in a Rural Community of the Occupied Palestinian Territories

Alexis Baron

As everybody knows, access to safe water is vital for human beings. It is also considered as a human right to have access to safe drinking water, free from pathogens that can cause illness and death. According to the World Health Organization (WHO), unsafe water is still a part of the daily life of millions of people on Earth. As it is not always possible to build centralized treatment facilities, household water treatment and safe storage (HWTS) is a promising alternative for many remote areas in developing countries. It is in this perspective that the Italian NGO named Cesvi targets unserved communities of South Hebron in Palestine. It has the goal to provide HWTS units to households. These HWTS units are composed of a gravity-driven membrane (GDM) filter. The water is pushed through the membranes by the pressure of the water column. It can allow high bacterial removal and requires no energy and low maintenance. That’s why it is a very interesting technology for household scale. An valuation of the technical performance and the user acceptance of 9 HWTS units set up in the community of Khirbet Ghuwein was the main focus of the present study. The results have shown that most of the filters have a log-removal value (LRV) equal to 2 or more, while LRVs of at least 4 are obtained in the laboratory. However, this corresponds to a comprehensive protection according to the WHO international scheme to evaluate household water treatment technologies. The calculated removal efficiency of the filter can be influenced by the concentration of bacteria in the raw water, how the system is operated and the quality of the internal connections of the system. Nevertheless, it allows to observe the removal of total coliforms but also of E. coli, even if the latter were not very present in water. In contrast, GDM filters allow a high removal of suspended particles. Finally, the data do not allow to judge the safety of the water produced, as the HWTS unit seems to be sensitive and favorable to recontamination and regrowth of coliforms. The HWTS units are globally used by the households even if their use tends to decrease over time. Most of the users seem satisfied even if they are bored by cleaning the HWTS unit and think that it takes too many operational efforts. In addition, most of them would not be willing to buy one if it was not provided by the NGO. It seems that they used the filters without realizing the real purpose of them. It is very important in this kind of projects to accompany the delivery of filters with a workshop explaining the interest of such HWTS units and the proper way to use and maintain them. Despite the results that are not as conclusive as expected, the application of GDM filters in easy-to-use HWTS units is very promising but it still needs studies, applications, and improvements to allow a significant impact on the field.

2019

Exploiting Marginal Stability in Slow-Fast Quasilinear Dynamical Systems

Alessia Ferraro

The accurate investigation of many geophysical phenomena via direct numerical simulations is computationally not possible nowadays due to the huge range of spatial and temporal scales to be resolved. Therefore advances in this field rely on the development of new theoretical tools and numerical algorithms. In this work we investigate a new mathematical formalism that exploits the property of quasi-linear systems to self-tune towards marginally stable states. The inspiration for this study comes from the asymptotic analysis of strongly stratified flows, performed by Chini et al.. The application of multi-scale analysis to this problem, justified by the presence of scale separation, yields to a simplified quasi-linear model. In this reduced description small-scale instabilities evolve linearly about a large-scale hydrostatic field (whose evolution is fully non-linear) and modify it via a feedback term. From the only assumption of scale separation, two extremely interesting features of this model arise. First the presence of the coupling term between the two dynamics and second the quasi-linearity of the dynamics. The first aspect, generally not present in the hydrostatic approximation, can capture the non-local energy transfer between the small and the large scales, which is key for the quantification of the mixing efficiency in the deep ocean. The second aspect, namely the quasi-linearity, is suggestive of the self-organisation of the dynamics about marginally-stable states. This results in a coupled evolution where the fast dynamics adapts (is slaved) to the mean field, maintaining the marginal stability of the latter. The low-dimensional evolution that arises, enables the integration of the reduced system on temporal scales comparable to the characteristic time scale of the slow dynamics, making this novel approach highly suited to the investigation of the stratified flow problem.Building upon the results obtained by Chini et al. in the present work we extend this methodology addressing three different aspects of the reduced model.As a first case we investigate the twofold nature of the fluctuation feedback, which is not sign-definite and might lead to intense bursting events where the fluctuations exhibit positive growth rates on a fast time scale. In this scenario the scale separation is temporarily lost and the two dynamics have to be co-evolved until a new marginally stable manifold can be approached. Here we propose three different co-evolution techniques and test their efficacy on a one-dimensional model problem.The second aspect we address is the presence of a finite scale separation between the two dynamics. We develop an algorithm that carefully identifies the validity regions of the quasi-linear reduction and determines the relevance of the fluctuation feedback w.r.t. the characteristic time scale of the slow dynamics and the growth rate of the fluctuations.As a third case we derive an efficient extension of the original methodology to two-dimensional model problems, deriving an evolution equation for the wavenumber of the fastest growing mode, which then get slaved to the mean dynamics.Eventually the methodologies derived in the context of the two model problems are applied and discussed for the strongly stratified flow problem.

EPFL2022