Oscilaciones en sistemas hidráulicos

Aplicación de las técnicas de perturbaciones con métodos de escalas múltiples al estudio de los sistemas hidráulicos. En particular, se estudia el sistema de alimentación de una turbina, provisto de un regulador capaz de hacer funcionar la turbina con potencia constante. Se hace especial hincapié en la determinación de los comportamientos transitorios del sistema cuando los parámetros que lo determinan tienen valores cercanos a aquéllos en los que se produce un cambio de estabilidad en el comportamiento estacionario

Transients and limit cycles in simple surge tanks

Transients and limit cycles in simple surge tank

Fenómenos de resonancia en chimeneas de equilibrio

En lo que sigue se analiza el comportamiento oscilatorio del sistema de alimentación de una central hidroeléctrica, protegido del golpe de ariete por una chimenea de equilibrio, cuando la potencia suministrada a la red es pequeña frente a la máxima teórica y oscila con frecuencias cercanas a la de resonancia de la chimenea

Comportamiento no estacionario de las chimeneas de equilibrio en centrales hidroeléctricas

Se exponen a continuación los resultados obtenidos al aplicar técnicas de escalas múltiples, en el estudio de una central hidráulica con chimenea de equilibrio (Surge chamber). Se recogen en primer lugar los resultados bien conocidos de estabilidad lineal de las soluciones estacionarias del sistema. Se analiza a continuación el comportamiento no estacionarlo del mismo en las zonas próximas a las super ficies de cambio de estabilidad en el espacio de los parámetros. Así se describen las oscilaciones para valores de los parámetros cerca del límite de esta bilidad de Thoma, y las que aparecen cuando la potencia de funcionamiento es pequeña frente a la máxima teórica

Experimental results on evaporation waves

We address an experimental investigation of evaporation waves. They are obtained when a liquid contained in a vertical glass tube is suddenly depressurized from a high initial pressure down to the atmospheric one. After the release of pressure, the state of the liquid, which is at ambient pressure and the initial temperature, is well known to be metastable when the corresponding stable state is vapour. For moderately large evaporation rates (moderately large initial to ambient pressure ratios), the vapour-liquid interface ultimately evolves into an evaporation wave in which a highly corrugated front propagates downwards into the liquid with a well defined mean velocity. This mean velocity turns out to be a function of the ratio between the initial and the ambient pressures. In addition, attention to some new phenomena not previously reported is brought

Local Nonlinear Stability of the Steady State in an Isothermal Catalyst

A first-order, irreversible, exothermic reaction in a bounded porous catalyst is considered, with smooth boundary, of one, two, or three dimensions. For small Prater and Nusselt numbers, ,3 and I>, and a large Sherwood number, o, two isothermal models are derived. An analysis of linear stability of the steady states of such models shows that oscillatory instabilities appear for appropriate values of the Damk6hler number if the nondimensional activation energy is larger than 7* and the Lewis number is sufficiently large, where y* = 4 if m = v/,8o-c 1 and y* = (m + 1)2/ m if m > 1. A local Hopf bifurcation analysis is carried out at neutral stability points in order to ascertain whether such bifurcation is subcritical or supercritical

Mechanical Imperfections Effect on the Minimum Stability Limit of Liquid Bridges

The bifurcation to unstable equilibrium shapes in the neighborhood of the minimum volume stability limit of liquid bridges has been described by using the Lyapunov–Schmidt technique. Prior to the bifurcation analysis, the stability limits of axisymmetric liquid bridges (both that of maximum and that of minimum volume) have been analytically calculated when the liquid bridge supports are two circular, coaxial disks. The interface shapes have been parametrically described and the parameters corresponding to the marginally stable shapes have been determined in terms of elliptic variables. Bifurcation equations have been obtained analytically describing the behavior near the critical points previously calculated and the effect of small axisymmetric imperfections has been considered. The considered imperfections are inequality in the diameter of the supporting disks, small body forces due to an axial gravity, and liquid bridge rotation as a solid bod

On the Stability Limit Change due to Imperfections

The minimum volume stability limit of axisymmetric liquid bridges has been obtained in the past for zero Bond number, no solid body rotation and equal disks. When gravity, rotation or different disk diameters are considered, only a few numerical results are available and the singular case of small values of these parameters have been only partly considered. An analytical study using the Lyapunov-Schmidt method considering small values of gravity, disk diameter ratio and rotation rate is presented and the variation of the stability limit determined

Esquemas ENO para las ecuaciones de Euler en mallas no estructuradas. Parte 1: Descripción del método

En el presente trabajo, que ha sido dividido en dos partes, se desarrollan esquemas numéricos de primer y segundo orden para resolver las ecuaciones de Euler en dos dimensiones alrededor de perfiles aerodinámicos. En la primera parte, a continuación, se describe el método numérico em¬pleado. En él, el dominio espacial se discretiza utilizando una malla no estructurada donde las celdas son triángulos. El esquema hace uso de métodos de volúmenes finitos de primer orden y de segundo orden a partir de los cuales se obtienen ecuaciones de evolución para los valores me¬dios en cada celda. En una segunda parte se presentarán, como aplicación al diseño aerodinámi¬co, casos de prueba de perfiles aerodinámicos en régimen transónico y subsónico

Unsteady residual distribution schemes for transition prediction

In this work, the unsteady simulation of the Navier–Stokes equations is carried out by using a Residual Distribution Schemes (RDS) methodology. This algorithm has a compact stencil (cell-based computations) and uses a finite element like method to compute the residual over the cell. The RDS method has been successfully proven in steady Navier–Stokes computation but its application to fully unsteady configurations is still not closed, because some of the properties of the steady counterpart can be lost. Here, we proposed a numerical solution for unsteady problems that is fully compatible with the original approach. In order to check the method, we chose a very demanding test case, namely the numerical simulation of a Tollmien–Schlichting (TS) wave in a 2D boundary layer. The evolution of this numerical perturbation is accurately computed and checked against theoretical results