Análisis experimental y computacional de modelos termofluidomecánicos para el diseño de ingeniería con metales líquidos

Es importante disponer de una herramienta con la cual diseñar dispositivos de uso industrial y comercial que trabajen con metales líquidos (fuentes de neutrones de alta intensidad, núcleos de sistemas de transmutación nuclear, reactores de fisión de nueva generación, instalaciones de irradiación de materiales o reactores de fusión nuclear). Los códigos CFD (Computational Fluid Dynamics) son una de esas herramientas, y la manera de llevar a cabo su validación es la simulación de experimentos existentes. La turbulencia y la presencia de dos o más fases, son los dos principales problemas a los que tiene que hacer frente un código CFD. La mayoría de los modelos de turbulencia presentes en los códigos CFD se basan en considerar la proporcionalidad directa entre el transporte de cantidad de movimiento turbulento y el transporte turbulento de calor. Precisamente, el coeficiente de difusión del calor turbulento, se asume que sea proporcional a la viscosidad turbulenta a través de una constante empírica, llamada número de Prandtl turbulento. El valor de este número, en los códigos comerciales está entre 0,9 y 0,85 dependiendo del modelo de turbulencia, lo cual significa que en los códigos se asume que el transporte turbulento tanto de cantidad de movimiento como de calor, son prácticamente equivalentes. Esta asunción no es cierta en los flujos de metales líquidos, donde se demuestra que la transmisión de calor por turbulencia es pequeña frente a la transmisión de calor molecular. La solución pasa por aumentar el número de Prandtl turbulento, o abandonar la analogía de Reynolds, en el tratamiento de la turbulencia. Por otro lado, en los metales líquidos la capa límite térmica es más ancha que la de velocidad, y las funciones de pared incluidas en los códigos no satisfacen adecuadamente los flujos turbulentos de los fluidos con bajo número de Prantdl (los metales líquidos). Sí serían adecuados, si el mallado es tal, que la celda más cercana a la pared, está dentro de la subcapa laminar, en la cual la propiedad dominante es la conductividad molecular. En la simulación de flujo multifase los códigos se encuentran con una serie de dificultades, que en el caso de que las densidades de los fluidos que intervienen sean muy diferentes entre sí (como ocurre con los metales líquidos y los gases), serán aún mayores. La modelización de la interfase gas metal líquido, así como el encontrar una correlación válida para los coeficientes de resistencia y sustentación para el movimiento de las burbujas en el seno del metal líquido, son dos de los principales retos en la simulación de este tipo de flujos. Las dificultades no se limitan sólo a la simulación mediante CFD, las medidas experimentales de velocidad de las burbujas y del metal líquido también son complicadas. Hay parámetros que no se pueden definir bien: la trayectoria y la forma de las burbujas entre ellos. En el campo de aplicación industrial de los metales líquidos, los altos valores de los coeficientes de expansión volumétrica y de conductividad térmica hacen que estos fluidos sean muy atractivos en la refrigeración por convección libre en dispositivos de alta densidad de potencia. Tomando como base uno de los diseños de ADS (Accelerator Driven System), y teniendo en cuenta la dificultad que conlleva el uso de múltiples modelos físicos, los cálculos realizados muestran cómo, en caso de fallo eléctrico, la operación de la instalación puede continuar de forma segura. Para la validación de los códigos CFD en su uso como herramienta de diseño, uno de los fenómenos donde cuantitativamente más dificultades encuentran los códigos es en los que aparecen en la modelización de las superficies libres. Un buen ajuste de los modelos multifase y de turbulencia es imprescindible en este tipo de simulaciones. Efectivamente, en la instalación de irradiación de materiales IFMIF, la formación de ondas en la superficie libre del flujo de Litio, es un fenómeno que hay que tratar de evitar, y además se requiere predecir las temperaturas, para ver si hay peligro de ebullición del metal líquido. La simulación llevada a cabo se enfoca al análisis termohidráulico. Variando la velocidad de inyección de Litio desde 10 hasta 20 m/s, se comprueba que las temperaturas máximas quedan alejadas del punto de ebullición del Litio, debido al aumento de presión producido por la fuerza centrífuga. Una de las cuestiones más críticas que se presentan en las fuentes de neutrones sería la refrigeración de la ventana metálica sobre la que incide el haz de protones. La simulación de experimentos como MEGAPIE y TS-1, permite la “visualización” de recirculación en el flujo, de los puntos de estancamiento, de los puntos calientes, etc, y da una fotografía de las zonas críticas del diseño.

Components of a Wind Tunnel Balance: Design and Calibration

Components of a Wind Tunnel Balance: Design and Calibration

Comparison of Mesh Adaptation Using the Adjoint Methodology and Truncation Error Estimates

Mesh adaptation based on error estimation has become a key technique to improve th eaccuracy o fcomputational-fluid-dynamics computations. The adjoint-based approach for error estimation is one of the most promising techniques for computational-fluid-dynamics applications. Nevertheless, the level of implementation of this technique in the aeronautical industrial environment is still low because it is a computationally expensive method. In the present investigation, a new mesh refinement method based on estimation of truncation error is presented in the context of finite-volume discretization. The estimation method uses auxiliary coarser meshes to estimate the local truncation error, which can be used for driving an adaptation algorithm. The method is demonstrated in the context of two-dimensional NACA0012 and three-dimensional ONERA M6 wing inviscid flows, and the results are compared against the adjoint-based approach and physical sensors based on features of the flow field.

Conceptual design of the liquid metal laboratory of the TECHNOFUSION facility

The application of liquid metal technology in fusion devices requires R&D related to many phenomena: interaction between liquid metals and structural material as corrosion, erosion and passivation techniques; magneto-hydrodynamics; free surface fluid-dynamics and any other physical aspect that will be needed for their safe reliable operation. In particular, there is a significant shortage of experimental facilities dedicated to the development of the lithium technology. In the framework of the TECHNOFUSION project, an experimental laboratory devoted to the lithium technology development is proposed, in order to shed some light in the path to IFMIF and the design of chamber's first wall and divertors. The conceptual design foresee a development in two stages, the first one consisting on a material testing loop. The second stage proposes the construction of a mock-up of the IFMIF target that will allow to assess the behaviour of a free-surface lithium target under vacuum conditions. In this paper, such conceptual design is addressed.

Nonlinear description of transversal motion in a laminar boundary layer with streaks

The nonlinear streamwise growth of a spanwise periodic array of steady streaks in a flat plate boundary layer is numerically computed using the well known Reduced Navier- Stokes formulation. It is found that the flow configuration changes substantially when the amplitude of the streaks grows and the nonlinear effects come into play. The transversal motion (in the wall normal-spanwise plane), which is normally not considered, becomes non-negligible in the nonlinear regime, and it strongly distorts the streamwise velocity profiles, which end up being quite different from those predicted by the linear theory. We analyze in detail the resulting flow patterns for the nonlinearly saturated streaks, and compare them with available experimental results.

Comparing linear and non linear wind flow models

Assessing wind conditions on complex terrain has become a hard task as terrain complexity increases. That is why there is a need to extrapolate in a reliable manner some wind parameters that determine wind farms viability such as annual average wind speed at all hub heights as well as turbulence intensities. The development of these tasks began in the early 90´s with the widely used linear model WAsP and WAsP Engineering especially designed for simple terrain with remarkable results on them but not so good on complex orographies. Simultaneously non-linearized Navier Stokes solvers have been rapidly developed in the last decade through CFD (Computational Fluid Dynamics) codes allowing simulating atmospheric boundary layer flows over steep complex terrain more accurately reducing uncertainties. This paper describes the features of these models by validating them through meteorological masts installed in a highly complex terrain. The study compares the results of the mentioned models in terms of wind speed and turbulence intensity.

Instabilities in two-liquid layers subject to a horizontal temperature gradient

We study theoretically the stability of two superposed fluid layers heated laterally. The fluids are supposed to be immiscible, the interface undeformable and of infinite horizontal extension. Combined thermocapillary and buoyancy forces give rise to a basic flow when a temperature difference is applied. The calculations are performed for a melt of GaAs under a layer of molten B2 O3 , a configuration of considerable technological importance. Four dif- ferent flow patterns and five temperature configurations are found for the basic state in this system. A linear stability analysis shows that the basic state may be destabilized by oscilla- tory motions leading to the so-called hydrothermal waves. Depending on the relative height of the two layers these hydrothermal waves propagate parallel or perpendicular to the temperature gradient. This analysis reveals that these perturbations can alter significantly the liquid flow in the liquid-encapsulated crystal growth techniques.

Development of a wake model for wind farms based on an open source CFD solver. Strategies on parabolization and turbulence modeling

Wake effect represents one of the most important aspects to be analyzed at the engineering phase of every wind farm since it supposes an important power deficit and an increase of turbulence levels with the consequent decrease of the lifetime. It depends on the wind farm design, wind turbine type and the atmospheric conditions prevailing at the site. Traditionally industry has used analytical models, quick and robust, which allow carry out at the preliminary stages wind farm engineering in a flexible way. However, new models based on Computational Fluid Dynamics (CFD) are needed. These models must increase the accuracy of the output variables avoiding at the same time an increase in the computational time. Among them, the elliptic models based on the actuator disk technique have reached an extended use during the last years. These models present three important problems in case of being used by default for the solution of large wind farms: the estimation of the reference wind speed upstream of each rotor disk, turbulence modeling and computational time. In order to minimize the consequence of these problems, this PhD Thesis proposes solutions implemented under the open source CFD solver OpenFOAM and adapted for each type of site: a correction on the reference wind speed for the general elliptic models, the semi-parabollic model for large offshore wind farms and the hybrid model for wind farms in complex terrain. All the models are validated in terms of power ratios by means of experimental data derived from real operating wind farms.

Numerical characterization of particle dispersion in the turbulent recirculation zones of sudden expansion pipe flows

The dispersion of solid particles in the turbulent recirculation zones of sudden expansion pipes can be characterized by different Stokes numbers and mean drift parameter and its study is important because this kind of flows appears in many technological applications.

Long wave theory for experimental devices with compressed/expanded surfactant monolayers

Surfactant monolayers are of interest in a variety of phenomena, including thin film dynamics and the formation and dynamics of foams. Measurement of surface properties has received a continuous attention and requires good theoretical models to extract the relevant physico- chemical information from experimental data. A common experimental set up consists in a shallow liquid layer whose free surface is slowly com- pressed/expanded in periodic fashion by moving two slightly immersed solid barriers, which varies the free surface area and thus the surfactant concentration. The simplest theory ignores the fluid dynamics in the bulk fluid, assuming spatially uniform surfactant concentration, which requires quite small forcing frequencies and provides reversible dynamics in the compression/expansion cycles. Sometimes, it is not clear whether depar- ture from reversibility is due to non-equilibrium effects or to the ignored fluid dynamics. Here we present a long wave theory that takes the fluid dynamics and the symmetries of the problem into account. In particular, the validity of the spatially-uniform-surfactant-concentration assumption is established and a nonlinear diffusion equation is derived. This allows for calculating spatially nonuniform monolayer dynamics and uncovering the physical mechanisms involved in the surfactant behavior. Also, this analysis can be considered a good means for extracting more relevant information from each experimental run.

Spatial linear global instability analysis of the HIFiRE-5 elliptic cone model flow

The linear instability of the three-dimensional boundary-layer over the HIFiRE-5 flight test geometry, i.e. a rounded-tip 2:1 elliptic cone, at Mach 7, has been analyzed through spatial BiGlobal analysis, in a effort to understand transition and accurately predict local heat loads on next-generation ight vehicles. The results at an intermediate axial section of the cone, Re x = 8x10 5, show three different families of spatially amplied linear global modes, the attachment-line and cross- ow modes known from earlier analyses, and a new global mode, peaking in the vicinity of the minor axis of the cone, termed \center-line mode". We discover that a sequence of symmetric and anti-symmetric centerline modes exist and, for the basic ow at hand, are maximally amplied around F* = 130kHz. The wavenumbers and spatial distribution of amplitude functions of the centerline modes are documented

Aerodynamic Study of Airfoils with Leading Edge Imperfections at Low Reynolds Number

A study has been made on the influence of the leading edge imperfections in airfoils used in different devices relating their aerodynamic performances. Wind tunnel tests have been made at different Reynolds numbers and angle of attacks in order to show this effect. Later, a quantitative study of the aerodynamic properties has been made based on the different leading edge imperfections and their size.

Análisis mediante cálculo numérico (CFD) del comportamiento de anemómetros de cazoletas = CFD analysis of cup anemometers performance

El presente trabajo consiste en la simulación del flujo aerodinámico alrededor de cazoletas de anemómetros. Para ello se ha utilizado un código numérico o software comercial de análisis numérico de fluidos o CFD (Computational Fluid Dynamics). Este trabajo es un aporte más en la línea de investigación acerca del comportamiento de los anemómetros de cazoletas, que viene llevándose a cabo en el Instituto Universitario de Microgravedad “Ignacio Da Riva” (IDR/UPM). La primera parte de este proyecto consistió en la realización de simulaciones de tipo estacionarias (esto es, con la cazoleta bajo un cierto ángulo de incidencia con respecto al viento pero sin movimiento de rotación). De esta forma se analiza de forma independiente y asilada la cazoleta en cada una de las diferentes posiciones a lo largo de un giro de 360 grados. Así pues, a varios modelos de cazoleta se les fue variando su posición en incrementos de 10 grados desde de la posición angular inicial q = 0º hasta q = 180º, ya que las cazoletas presentan un comportamiento simétrico. La segunda parte de este proyecto se destinó a la realización de otra serie de simulaciones de tipo no estacionarias. Este tipo de simulaciones se realizaron concretamente a sólo un modelo de cazoleta (cónica no porosa). Estas últimas simulaciones, en concreto nueve, se realizaron variando la velocidad angular de la cazoleta respecto a la velocidad del viento.

Estudio paramétrico del conjunto tractor-trailer mediante aplicación de mecánica de fluidos computacional

La mejora del comportamiento aerodinámico de vehículos pesados en la carretera ha adquirido una mayor importancia estos últimos años debido a la crisis y su consecuente aumento del precio de los combustibles. Dado que una reducción en la resistencia aerodinámica del vehículo conlleva un menor empleo de combustible, el objetivo de este proyecto es realizar un estudio paramétrico del conjunto tractor-tráiler mediante la mecánica de fluidos computacional (CFD) para así obtener la geometría que proporciona un menor gasto de combustible cuando ésta es expuesta a viento frontal. La influencia de 3 parámetros, que son la separación existente entre cabina y remolque, la altura del remolque y el radio de curvatura de las aristas frontales de la cabina, es analizada en este estudio dividido en varias etapas que implican el uso de programas específicos como son: la parametrización y creación de las geometrías en 3D que es llevada a cabo mediante CATIA, el mallado del dominio (realizado con Gambit) y resolución de las ecuaciones mediante FLUENT. Finalmente se obtendrá una relación entre la resistencia aerodinámica (representada mediante el coeficiente de arrastre) y la combinación de los 3 parámetros, que nos permitirá decidir que geometría es la óptima. Abstract The improvement of the heavy vehicle’s aerodynamic behavior on the road has gained a great importance for these last years because of the economic crisis and the consequent increase of the price of the fuels. Due to the fact that a reduction in the aerodynamic resistance of the vehicle involves using a smaller amount of fuel, the objective of this project is to carry out a parametric study about the ensemble tractor-trailer by computational fluid dynamics methods (CFD) in order to obtain the geometry which expense of fuel is the lowest when it’s exposed to frontal wind. The influence of the three parameters, which are the space between cab and trailer, the height of the trailer and the curvature of the frontal cab edges, is analysed in this study which is divided into different parts involving specific programs: choosing the parameters and building the geometries, which is done by using CATIA, the mesh is built by Gambit, and the program equations-solver is FLUENT. Finally a ratio between aerodynamic resistance and a combination of the three parameters will be obtained and it will allow us to choose the best geometry.

Lagrangian descriptors: a method for revealing phase space structures of general time dependent dynamical systems

Lagrangian descriptors are a recent technique which reveals geometrical structures in phase space and which are valid for aperiodically time dependent dynamical systems. We discuss a general methodology for constructing them and we discuss a "heuristic argument" that explains why this method is successful. We support this argument by explicit calculations on a benchmark problem. Several other benchmark examples are considered that allow us to assess the performance of Lagrangian descriptors with both finite time Lyapunov exponents (FTLEs) and finite time averages of certain components of the vector field ("time averages"). In all cases Lagrangian descriptors are shown to be both more accurate and computationally efficient than these methods.