Piston (mécanique) | EPFL Graph Search

En mécanique, un piston est une pièce rigide de section généralement circulaire coulissant dans un cylindre de forme complémentaire. Le déplacement du piston entraîne une variation de volume de la chambre, partie située entre le piston et le cylindre. Un piston permet la conversion d'une pression en un travail, ou réciproquement. Applications thumb|upright|Un piston permet la conversion d'une pression en un travail ou réciproquement. Les pistons sont présents dans de nombreuses applications mécaniques. La plus courante est le moteur à combustion interne, notamment dans l'automobile. On trouve également un ou plusieurs pistons dans les compresseurs, les pompes, les vérins, les détendeurs, les régulateurs, les distributeurs, les valves, les amortisseurs, mais aussi les seringues médicales ou les instruments de musique à pistons. On distingue deux types de pistons : les pistons à simple effet, où la pression n'agit que sur une face ( seringues médicales), et les pistons à

Propriétés hors-équilibre d'une paroi adiabatique

Séverine Pache

We study the evolution of a system composed of N non-interacting particles of mass m distributed in a cylinder of length L. The cylinder is separated into two parts by an adiabatic piston of a mass M ≫ m. The length of the cylinder is a fix parameter and can be finite or infinite (in this case N is infinite). For the infinite case we carry out a perturbative analysis using Boltzmann's equation based on a development of the velocity distribution of the piston in function of a small dimensionless parameter ε = √(m/M). The non-stationary case is solved up to the order ε ;; our analysis shows that the system tends exponentially fast towards a stationary state where the piston has an average velocity V. The characteristic time scale for this relaxation is proportional to the mass of the piston (τ0 = M/A where A is the cross-section of the piston). We show that for equal pressures the collisions of the particles induce asymmetric fluctuations of the velocity of the piston which leads to a macroscopic movement of the piston in the direction of the higher temperature. In the case of the finite model a perturbative approach based on Liouville's equation (using the parameter α = 2m/(M + m)) shows that the evolution towards thermal equilibrium happens on two well separated time scales. The first relaxation step is a fast, deterministic and adiabatic evolution towards a state of mechanical equilibrium with approximately equal pressures but different temperatures. The movement of the piston is more or less damped. This damping qualitatively depends on whether the ratio R = Mgas/M between the total mass of the gas and the mass of the piston is small (R < 2) or large (R > 4). The second part of the evolution is much slower ; the typical time scales are proportional to the mass of the piston. There is a stochastic evolution including heat transfer leading to thermal equilibrium. A microscopic analysis yields the relation XM(t) = L(1/2 - ξ(at)) where the function ξ is independent of M. Using the hypothesis of homogeneity (i.e. the values of the densities, pressures and temperatures at the surface of the piston can be replaced by their respective average values) introduced in the previous analysis the observed damping does not show up. This can be explained by shock waves propagating between the piston and the walls at the extremities of the cylinder. In order to study the behaviour of the system there is hence a need to adequately describe the non-equilibrium fluids around the piston. We carry out an analysis of the infinite case, based on the perturbative approach introduced earlier. In this case the initial conditions are chosen in such a manner that the piston on average stays at the origin. It is shown that it is possible to describe the evolution of the fluids in such a way that it is coherent with the two laws of thermodynamics and the phenomenological relationships. Finally we study the case of a constant velocity of the piston in a finite cylinder. Such a condition and elastic collisions allow us to derive an explicit expression for the distribution of the fluids and hence for the hydrodynamics fields. This expression reveals the presence of shock waves between the piston and the extremities of the cylinder.

EPFL2004

Evolution of the vorticity distribution in vortex rings

Guillaume de Guyon-Crozier

Vortex rings are very efficient at transporting fluid on long distances and can generate large forces, either thrust or drag. These abilities are influenced by the vorticity distribution within the vortex. Previous work on vortices produced by piston-cylinders showed that the vorticity distribution reaches a steady state when the vortex separates from the apparatus. First, we experimentally investigate the evolution of the vorticity distribution independently of the vortex separation. The vortices are created by impulsively accelerating cones immersed in water. In this configuration, the self-induced velocity of the vortex is directed towards the cone and there is no separation. Particle image velocimetry is carried on at Reynolds numbers around 30000. The vorticity distribution is quantified using the non-dimensional energy of the vortex, which is the energy with respect to the impulse and circulation. After three convective times, the volume of fluid recirculating within the vortex ring is filled with vortical fluid and the non-dimensional energy to a value around 0.3. The vorticity produced on the cone circumvents the vortex and a portion of the vortex volume is lost via tail-shedding. The translational velocity of the vortex ring linearly depends on its circulation and non-dimensional energy. This velocity, relative to the cone, also converges after three convective times and is found to be a more reliable scaling parameter than energy or circulation. It consistently reaches values around 0.9. In a second part, we present models to predict the vortex growth in the wake of disks and cones. Two models are developed. The first model reduces the vortex ring to a core of constant vorticity density. The translational velocity of the vortex is deduced and its trajectory integrated. The model accurately predicts the maximum circulation of the vortex. A second model, based on axisymmetric discrete vortex methods, predicts the growth, vorticity distribution and tail-shedding of the vortex. A third model is developed to explain why the non-dimensional energy consistently converges to values around 0.3. Based on the self similar roll-up of inviscid shear layers, a non-dimensional energy of 0.33 is computed for vortices formed by impulsively accelerated disks or pistons. The model also predicts that a linear acceleration profile leads to a more uniform vorticity distribution, decreasing the non-dimensional energy to 0.18. This result indicates that the vorticity distribution can be controlled by varying the velocity profile of the vortex generator. Another control option is to use permeable disks. We impulsively accelerated perforated disks and observed the vortex formation. A portion of the incoming flow bleeds through the disk and does not circulate around the disk edge, resulting in a lower vorticity maximum. The vortex ring has a more uniform vorticity distribution, as well as a more elongated shape. The non-dimensional energy is brought down to 0.14. Finally, vortex rings have a great potential to transport fluid on long distances, such as extinguishing powder. Their resilience to vortical perturbations is critical for the transport and depends on the vorticity distribution within the vortex. Simulations with nested contour methods are performed to assess that resilience. Vortices with lower non-dimensional energy shed less vortical volume when facing perturbations and qualify as better candidates for fluid transport.

EPFL2021

Non-equilibrium effects during the sudden vaporization of saturated water / Vaporization of superheated water under sudden depressurization

Daniel Favrat

The vaporization of an entrapped mass of saturated water suddenly allowed to expand is studied both experimentally and theoretically. In the experimental set-up, the water is initially pressurized by a piston at the closed end of a cylinder and heated to the onset of vaporization, at which time the expansion is initiated by releasing the gas counterpressure acting on the back face of the piston. The initial depressurization rate is mainly controlled by the gas pressure releasing mechanism, which, in the present study, consists of either a fast opening electric valve or a diaphragm breaker. Pressure measurements along the path of the piston indicate a significant pressure undershoot when compared to the numerical finite element predictions based on a one-dimensional, homogeneous equilibrium flow. Pressure undershoots up to 290 kPa for a initial water pressure of 600 kPa have been measured and are caused by non-equilibrium vaporization delays which affect the whole expansion range tested (volume ratio up to 50). For the sake of comparison, reference is being made to the Simpson and Silver non-equilibrium prediction model. The vaporization process is illustrated by a high-speed movie taken through a transparent cylinder. A sharp separation front appears between the steam and water phases as the light high quality steam migrates faster towards the face of the moving piston.