Pression | EPFL Graph Search

La pression est une grandeur physique qui traduit les échanges de quantité de mouvement dans un système thermodynamique, et notamment au sein d'un solide ou d'un fluide. Elle est définie classiquement comme l'intensité de la force qu'exerce un fluide par unité de surface. C'est une grandeur scalaire (ou tensorielle) intensive. Dans le Système international d'unités elle s'exprime en pascals, de symbole Pa. L'analyse dimensionnelle montre que la pression est homogène à une force surfacique ( ) comme à une énergie volumique ( ). On appelle parfois « pression exercée » (par un fluide sur une paroi) la force (dite « force pressante ») qu'il exerce par unité d'aire de la paroi, mais il s'agit alors d'une grandeur vectorielle définie localement, alors que la pression est une grandeur scalaire définie en tout point du fluide. Histoire de la notion de pression Chronologie de la thermodynamique et de la physique statistique thumb|180px|« Pompe à air » de Boyle La notion de pres

Modeling the breaking-off of icebergs from tidewater glaciers

Qimin Wang

Ice calving is the breakaway process of icebergs which plays an important role in the dy- namics and the mass balance of ice sheets. Our project is to use Akantu (An open source object-oriented Finite Element library by LSMS) to reproduce the fracturing stages of the Bowdoin glacier in Greenland. Studies were first made on the mechanisms of ice calving and the provided data, to extract the key factors for our simulation. The HPC (high performance computing) simulation model was built by Python scripts, including preprocessing mesh file, applying arbitrary loading profiles, defining boundary conditions and initial conditions, pass- ing message through the simulation process, etc. The simulation scheme has been modified to better analysis the impact of water pressure and velocity field. Simulation acceleration was also implemented by amplifying the velocity field, so that the damage initiation and propagation could be explored faster. Some preliminary results about the relationship of system energy and damage propagation was analyzed. In the end, we proposed some study direction for the future researchers on the similar subject.

2019

Theoretical study of electronic transport in TCV ELMy H-mode

Gaël Induni

High-confinement mode (H-mode) is a promising reference scenario for ITER. But we are still facing major issues because of instabilities. They expel periodically some of the energy, which can damage the device. These instabilities are called the edge localized modes (ELM) and are not yet fully theoretically understood. The present work is a study on the profiles evolution in between ELMs and on the ELM effects. This may help to have a better understanding of the conditions before the ELM. We use the simulations as theoretical tool. For the purpose of the simulations, we build an H-mode χ e profile according to a standard L-mode one that we truncate at the edge to create a transport barrier. This gives a good agreement with the experimental data. Several scaling laws were successfully used. The first one is the energy confinement time scaling which was used for the thermal diffusivity to scale the temperature profile. A scaling between the core and pedestal energies was found recently. It was used to compute the pedestal χ e to scale the temperature pedestal, which was successful. Finally, we used a scaling for transport barriers which links the density gradient length to that of the temperature to compute the density in the pedestal. It was already found to be good in TCV electron internal transport barriers and in ASDEX Upgrade H-mode pedestals. Looking at the MHD stability parameters, it was found that for our reference case, ELMs are not likely to be triggered by the time evolution of the pressure gradient and the current density profiles in our model, as these are only varying significantly during the first millisecond after the crash, and are almost constant during the long remaining time until the next crash. Studying different cases, we investigate the behavior of the plasma when replacing the edge heating by central one to observe the influence of the heating profile, but no significant difference was found, neither in the MHD stability parameters. Further we change the particle diffusion coefficient to compare the dynamic behavior of the density. Slowing down the density dynamic behavior also slows down the pressure one, this can be seen on the MHD stability parameters. We also vary the ELM period to compare to the change due to the variation of the particle diffusivity. It was found that there may be a sort of relation between the particle diffusivity and the ELM period at least for the density, since both cases change the density recovery time with respect to the ELM period. A last case considered is doubling the radial ELM interaction range. This is done in order to observe the difference to the reference simulation that takes the density top of pedestal as ELM range, and to compare the spatial range influenced by the MHD activity and the one by the transport improvement. It was found that the MHD stability parameters in the pedestal exhibit a different behavior with the pressure gradient starting to increase very fast.

2011

Aerodynamic study of a Hyperloop pod equipped with compressor to overcome the Kantrowitz limit

Yohei Sato

This paper describes an investigation of the aerodynamic performance of a Hyperloop pod equipped with an axial compressor using CFD simulation. The compressor is expected to reduce the drag if the operational speed of the pod exceeds the Kantrowitz Limit (KL). To evaluate the effectiveness of the compressor, two types of pod have been designed: those with and those without a compressor. A validation study was undertaken for supersonic flow around a cylindrical body in a wind tunnel. Several combinations of pod Mach number and blockage ratios were examined. The axial compressor was modeled with the fan interface model and our original method, which we call the source terms model. Results are similar, but our model has the advantage that it can be applied to higher pressure ratios without inducing numerical instabilities. The power consumption was reduced owing to the compressor by decreasing the pressure accumulated in front of the pod. The effectiveness of the compressor was shown to be greater for conditions well above the KL, with a maximum power reduction of 47.5%. Simulations performed half-scale, and at a different system pressure, showed that the effectiveness of the compressor was not greatly influenced by these parameters.

2021

Theoretical study of electronic transport in TCV ELMy H-mode

Gaël Induni

2011