Compressibilité | EPFL Graph Search

La compressibilité est une caractéristique d'un corps quantifiant sa variation relative de volume sous l'effet d'une pression appliquée. La compressibilité est une grandeur intensive homogène avec l'inverse d'une pression, elle s'exprime en (Pa étant le pascal). Cette définition doit être complétée car sous l'effet d'une compression les corps ont tendance à s'échauffer. On définit donc une compressibilité isotherme, pour un corps restant à température constante, et une compressibilité isentropique (ou adiabatique), pour un corps restant à entropie constante. Les deux coefficients ainsi définis sont reliés aux capacités thermiques du corps par la relation de Reech. Le deuxième principe de la thermodynamique induit que la compressibilité d'un corps stable ne peut être que positive : le volume d'un corps doit diminuer lorsque l'on augmente la pression. Une compressibilité négative ou nulle induit un corps instable, à moins que cette instabilité ne soit compensée par d'autres phénomè

Experimental Contribution to the Mechanics of Herringbone Grooved Journal Air Bearings

Ivan Ognjanovic

Gas Lubricated Herringbone Grooved Journal Bearings (HGJB) have long been used in equipment such as navigation systems, optical scanners for laser printers and high speed cameras and disk spindle for flyhead testers. All such applications are characterized by a relatively small size (journal diameter

EPFL2011

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

Online Measurement of Oil Concentrations of R134a–Oil Mixtures with a Density Flowmeter

Daniel Favrat, John Richard Thome

A very high accuracy, straight vibrating tube type of density flow meter has been used online to measure oil concentraiton of flowing R-134a/oil mixtures. The calibrations covered oil concentrations from 0-6 wt.% oil over the temperature range from -9.4 to 5.9°C. The oil concentrations were correlated statistically as a function of density, temperature and liquid compressibility to an average error of 0.09 wt.% oil with a 95% confidence limit of 0.21 wt.% oil. In addition, a simplified method not requiring calibration tests was developed for general industrial application of the density flowmeter to any refrigerant-oil mixture combination, with an average error of 0.22 wt.% oil and 95% confidence limit of 0.67 wt.% oil for the present data set.

1995

Simulation of benchmark and industrial unsteady compressible turbulent fluid flows

Michele Mossi

EPFL1999