Smoothed particle hydrodynamics simulations of evaporation and explosive boiling of liquid drops in microgravity.

The rapid evaporation and explosive boiling of a van der Waals (vdW) liquid drop in microgravity is simulated numerically in two-space dimensions using the method of smoothed particle hydrodynamics. The numerical approach is fully adaptive and incorporates the effects of surface tension, latent heat, mass transfer across the interface, and liquid-vapor interface dynamics. Thermocapillary forces are modeled by coupling the hydrodynamics to a diffuse-interface description of the liquid-vapor interface. The models start from a nonequilibrium square-shaped liquid of varying density and temperature. For a fixed density, the drop temperature is increased gradually to predict the point separating normal boiling at subcritical heating from explosive boiling at the superheat limit for this vdW fluid. At subcritical heating, spontaneous evaporation produces stable drops floating in a vapor atmosphere, while at near-critical heating, a bubble is nucleated inside the drop, which then collapses upon itself, leaving a smaller equilibrated drop embedded in its own vapor. At the superheat limit, unstable bubble growth leads to either fragmentation or violent disruption of the liquid layer into small secondary drops, depending on the liquid density. At higher superheats, explosive boiling occurs for all densities. The experimentally observed wrinkling of the bubble surface driven by rapid evaporation followed by a Rayleigh-Taylor instability of the thin liquid layer and the linear growth of the bubble radius with time are reproduced by the simulations. The predicted superheat limit (T(s)≈0.96) is close to the theoretically derived value of T(s)=1 at zero ambient pressure for this vdW fluid.

Diffuse-interface modeling of liquid-vapor coexistence in equilibrium drops using smoothed particle hydrodynamics.

We study numerically liquid-vapor phase separation in two-dimensional, nonisothermal, van der Waals (vdW) liquid drops using the method of smoothed particle hydrodynamics (SPH). In contrast to previous SPH simulations of drop formation, our approach is fully adaptive and follows the diffuse-interface model for a single-component fluid, where a reversible, capillary (Korteweg) force is added to the equations of motion to model the rapid but smooth transition of physical quantities through the interface separating the bulk phases. Surface tension arises naturally from the cohesive part of the vdW equation of state and the capillary forces. The drop models all start from a square-shaped liquid and spinodal decomposition is investigated for a range of initial densities and temperatures. The simulations predict the formation of stable, subcritical liquid drops with a vapor atmosphere, with the densities and temperatures of coexisting liquid and vapor in the vdW phase diagram closely matching the binodal curve. We find that the values of surface tension, as determined from the Young-Laplace equation, are in good agreement with the results of independent numerical simulations and experimental data. The models also predict the increase of the vapor pressure with temperature and the fitting to the numerical data reproduces very well the Clausius-Clapeyron relation, thus allowing for the calculation of the vaporization pressure for this vdW fluid.

Tratamiento de la hernia inguinal: utilización de la técnica de lichtenstein en 179 casos consecutivos / Treatment of the inguinal hernia: utilization of lichtenstein technic in 179 consecutive cases

El tratamiento quirúrgico moderno de la hernia inguinal se puede resumir a dos principios simples: reforzamiento de la pared posterior del canal inguinal y reparación del anillo inguinal interno. Las técnicas se han modificado en el tiempo de acuerdo a la aparición de fallas, las más comunes el dolor y la recidiva y ambas tienen un origen común que es la creación de tensión, lo cual es una violación de los principios quirúrgicos. Sucesivos autores han tratado de resolver el problema de la tensión y en este sentido podemos mencionar a McVay con su técnica de incisión de relajación y la operación de Shouldice utilizando la sutura en capas anatómicas para reforzar sin aumentar la tensión. sin embargo, no es sino hasta 1986 cuando Lichtenstein, en la segunda edición de su libro, publica el uso de la "Técnica sin tensión", en 300 casos consecutivos para todos los tipos de hernias inguinales. En este trabajo analizaremos los resultados de 179 herniorrafias consecutivas realizadas por el mismo equipo quirúrgico con la técnica sin tensión, en el Hospital de Clínicas Caracas, durante el período 1990-1997