Coefficient de traînée | EPFL Graph Search

En dynamique des fluides, le coefficient de traînée, dont le symbole normalisé est Cx, CA ou CD (c_\mathrm d en anglais, c_\mathrm w en allemand) fait partie de la famille des coefficients aérodynamiques. C'est un nombre sans dimension qui est utilisé pour quantifier la traînée ou résistance d'un objet dans un fluide (comme, par exemple, l'air ou l'eau). Il est toujours associé à une surface particulière (selon le contexte, appelée maître-couple, surface alaire ou plus généralement surface de référence). La définition adimensionnelle actuelle du coefficient de traînée a été proposée par Ludwig Prandtl sur une idée de . Un objet mobile se déplaçant dans un fluide pesant subit de la part de ce fluide une distribution de pression et un frottement visqueux (ou friction) dont la résultante s'oppose à sa marche. La composante de cette résultante selon la direction du mobile est appelée traînée. L'intensité de la force de traînée est exprimée en fonction de la vit

Single Crystal Growth of Constantan by Vertical Bridgman Method

This project deals with aspects involved in the growth of single-crystals of Constantan. Constantan is an alloy of Nickel in Copper, whose technological interest is that resistance is independent of temperature over a very large range of temperatures (hence the name Constantan). This, along with other suitable properties such as high strain sensitivity, high resistivity and low coefficient of thermal expansion makes it the principal component of all modern strain gauges used. However, Constantan also represents exactly the critical composition of Nickel in Copper (45% by weight Ni in Cu), which is the border between ferromagnetism and paramagnetism at low temperature. Such a transition in magnetic states at low (zero) temperature is called a quantum phase transition, and results in a uniquely scaling excitation spectrum. The eventual purpose of growing these single crystals is to validate our hypothesis that in fact the technologically relevant properties of Constantan are the result of quantum critical fluctuations. This can be investigated by a technique called inelastic neutron scattering.

2011

Simulation of benchmark and industrial unsteady compressible turbulent fluid flows

Michele Mossi

Large-eddy simulation (LES) is a very promising technique for the numerical computation of unsteady turbulent flows, and seems to be the perfect tool to simulate the compressible air flow around a high-speed train in a tunnel, providing unsteady results for aerodynamic and aeroacoustic analysis. To look into this possible future application of LES, two major lines of investigation are pursued in this thesis: first, the study of the effective ability of shock-capturing schemes to predict fundamental turbulent phenomena; second, the analysis of the aerodynamic phenomena induced by a high-speed train in a tunnel. The numerical simulation of compressible flows requires the use of shock-capturing schemes. These schemes can be relatively dissipative and mask the subgrid-scale contribution introduced in a large-eddy simulation to account for the unresolved turbulence scales. To estimate their effective dissipation and their ability to resolve turbulence phenomena, shock-capturing schemes widely used for aeronautical applications, from second- to fifth-order space accuracy, are employed here for simulating well-known fundamental flows in subsonic and supersonic regimes. Direct and large-eddy numerical simulations are undertaken for the inviscid and viscous Taylor-Green vortex decay problem, the freely decaying homogeneous and isotropic turbulence, and the fully developed channel flow. For all the turbulent flows investigated, several turbulence statistics are computed and the numerical dissipation of the schemes tested is interpreted in terms of subgrid-scale dissipation in a LES context, yielding an equivalent Smagorinsky or dynamic coefficient. This coefficient is for all schemes of the same order of magnitude as the commonly accepted values in LES for the subgrid-scale term. On the grounds of this analysis and of the comparisons of the turbulence statistics with accurate data obtained in the literature, the addition of explicit subgrid-scale models to the shock-capturing schemes tested is not recommended. It is thus concluded that the use of the LES technique for compressible turbulent flows is not yet suitable for industrial applications. The aerodynamic phenomena generated by a high-speed train travelling in a tunnel are also discussed, their importance on the design of high-speed lies is pointed out, and the analysis tools commonly employed for their study are reviewed. To study numerically the three-dimensional, compressible and turbulent air flow around a high-speed train accelerating in a tunnel, by accounting for the unsteady effects at inlet and outlet boundaries due to the propagation of pressure waves generated at the train departure, new coupling conditions between one-dimensional and three-dimensional domains are developed. These conditions are applied successfully to the numerical simulation of the unsteady wake developing behind two- and three-dimensional vehicles, where the averaged Navier-Stokes equations are solved with the turbulence modelling approach. The influence on the wake of the length of the vehicle tail is also discussed and results of multi-dimensional simulations are compared with one-dimensional data.

EPFL1999

A novel tethered-sphere add-on to enhance grid turbulence

Peter Monkewitz, Richard Vonlanthen

The new turbulence generator consists of a standard uniform grid with tethered spheres attached to its nodes and is capable of producing approximately twice the turbulence energy per unit pressure drop coefficient C (p) than the same bare grid without the spheres. At the same time, the Reynolds number Re-lambda based on the Taylor microscale is also amplified by a factor of roughly 2, and the turbulence anisotropy is reduced to a constant level of 10% at all downstream distances without further flow conditioning after the grid. The new grid's simple design makes it suitable for a variety of fluid-flow facilities, in particular smaller water tunnels. Its performance in comparison with the plain grid is documented by measurements of the streamwise decay of turbulence energy and velocity spectra in the Re-lambda range of 50-100.

Springer-Verlag2011

Simulation of benchmark and industrial unsteady compressible turbulent fluid flows

Michele Mossi

EPFL1999