E. Martín

Drift instability of standing Faraday waves

We consider the weakly nonlinear evolution of the Faraday waves produced in a vertically vibrated two-dimensional liquid layer, at small viscosity. It is seen that the surface wave evolves to a drifting standing wave, namely a wave that is standing in a moving reference frame. This wave is determined up to a spatial phase, whose calculation requires consideration of the associated mean flow. This is just the streaming flow generated in the boundary layer attached to the lower plate supporting the liquid. A system of equations is derived for the coupled slow evolution of the spatial phase and the streaming flow. These equations are numerically integrated to show that the simplest reflection symmetric steady state (the usual array of counter-rotating eddies below the surface wave) becomes unstable for realistic values of the parameters. The new states include limit cycles (the array of eddies oscillating laterally), drifted standing waves (patterns that are standing in a uniformly propagating reference frame) and some more complex attractors.

The effect of surface contamination on the drift instability of standing Faraday waves

We consider the effect of surface contamination, modelled by Marangoni elasticity with insoluble surfactant and surface viscosity, in promoting drift instabilities in spatially uniform standing Faraday waves. It is seen that contamination enhances drift instabilities that lead to various steadily propagating and (both standing and propagating) oscillatory patterns. In particular, steadily propagating waves appear to be quite robust, as a seminal experiment by Douady, Fauve & Thual (1989).

Finite element simulation for ultraviolet excimer laser processing of patterned Si/SiGe/Si(100) heterostructures

Ultraviolet (UV) Excimer laser assisted processing is an alternative strategy for producing patterned silicon germanium heterostructures. We numerically analyzed the effects caused by pulsed 193 Excimer laser radiation impinging on patterned amorphous hydrogenated silicon (a-Si:H) and germanium (a-Ge:H) bilayers deposited on a crystalline silicon substrate [Si(100)]. The proposed two dimensional axisymmetric numerical model allowed us to estimate the temperature and concentration gradients caused by the laser induced rapid melting and solidification processes. Energy density dependence of maximum melting depth and melting time evolution as well as three dimensional temperature and element distribution have been simulated and compared with experimentally obtained results.

Condensation of superheated R134a and R437A inside a vertical tube

An experimental study is performed to investigate condensation of superheated R134a and R437A inside a vertical tube. Experimental tests have been performed at the normal operating conditions found in a small power vapor compression refrigeration system. The experimental setup is described and the experimental procedure and data reduction method are explained. Experimental results of the condensation heat transfer coefficient are reported and discussed for different pressures, vapor mass flow rates, and vapor degrees of superheating. Different approximate methods to evaluate the condensate heat transfer coefficient of the zeotropic refrigerant mixture R437A are discussed. The experimental heat transfer coefficients are also compared with those obtained using different correlations; it is found that the values are well predicted with the Chen correlation.

Heat transfer model for quenching by submerging

In quenching by submerging the workpiece is cooled due to vaporization, convective flow and interaction of both mechanisms. The dynamics of these phenomena is very complex and the corresponding heat fluxes are strongly dependent on local flow variables such as velocity of fluid and vapor fraction. This local dependence may produce very different cooling rates along the piece, responsible for inappropriate metallurgical transformations, variability of material properties and residual stresses. In order to obtain an accurate description of cooling during quenching, a mathematical model of heat transfer is presented here. The model is based on the drift-flux mixture-model for multiphase flows, including an equation of conservation of energy for the liquid phase and specific boundary conditions that account for evaporation and presence of vapor phase on the surface of the piece. The model was implemented on Comsol Multiphysics software. Generation of appropriate initial and boundary conditions, as well as numerical resolution details, is briefly discussed. To test the model, a simple flow condition was analyzed. The effect of vapor fraction on heat transfer is assessed. The presence of the typical vapor blanket and its collapse can be recovered by the model, and its effect on the cooling rates on different parts of the piece is analyzed. Comparisons between numerical results and data from literature are made.

Analysis of the thermo-mechanical deformations in a hot forging tool by numerical simulation

Although programs have been developed for the design of tools for hot forging, its design is still largely based on the experience of the tool maker. This obliges to build some test matrices and correct their errors to minimize distortions in the forged piece. This phase prior to mass production consumes time and material resources, which makes the final product more expensive. The forging tools are usually constituted by various parts made of different grades of steel, which in turn have different mechanical properties and therefore suffer different degrees of strain. Furthermore, the tools used in the hot forging are exposed to a thermal field that also induces strain or stress based on the degree of confinement of the piece. Therefore, the mechanical behaviour of the assembly is determined by the contact between the different pieces. The numerical simulation allows to analyse different configurations and anticipate possible defects before tool making, thus, reducing the costs of this preliminary phase. In order to improve the dimensional quality of the manufactured parts, the work presented here focuses on the application of a numerical model to a hot forging manufacturing process in order to predict the areas of the forging die subjected to large deformations. The thermo-mechanical model developed and implemented with free software (Code-Aster) includes the strains of thermal origin, strains during forge impact and contact effects. The numerical results are validated with experimental measurements in a tooling set that produces forged crankshafts for the automotive industry. The numerical results show good agreement with the experimental tests. Thereby, a very useful tool for the design of tooling sets for hot forging is achieved.

Streaming Flow Effects in the Nearly Inviscid Faraday Instability

We study the weakly nonlinear evolution of Faraday waves in a two dimensional container that is vertically vibrated. It is seen that the surface wave evolves to a drifting standing wave, namely a wave that is standing in a moving reference frame. In the small viscosity limit, the evolution of the surface waves is coupled to a non-oscillatory mean flow that develops in the bulk of the container. A system of equations is derived for the coupled slow evolution of the spatial phase of the surface wave and the streaming flow. These equations are numerically integrated to show that the simplest reflection symmetric steady state (the usual array of counter rotating eddies below the surface wave) becomes unstable for realistic values of the parameters. The new states include limit cycles, drifted standing waves and some more complex attractors.We also consider the effect of surface contamination, modelled by Marangoni elasticity with insoluble surfactant, in promoting drift instabilities in spatially uniform standing Faraday waves. It is seen that contamination enhances drift instabilities that lead to various steadily propagating and (both standing and propagating) oscillatory patterns. In particular, steadily propagating waves appear to be quite robust, as in the experiment by Douady et al., Europhysics Letters, pp. 309-315, 1989