A 3D CFD numerical study of the bubble generation process into a bubble Tjunction generator and its comparison with experimental data: Part I

This work presents a 3D numerical study of the bubble generation process into a bubble generator obtained with the commercial Computational Fluid Dynamics solver ANSYS Fluent v15.0.7, and its comparison with experimental data reproducing the same conditions. The bubble generator is formed by two perpendicular capillaries in which liquid and gas are injected at perpendicular directions into a 1 mm internal diameter capillary T-junction with a total length of 10 mm. The fluids used in experiments and CFD simulations are air and water, both of them considered incompressible and isothermal, at a room temperature of 25º. A total of 23 different cases are studied for different injection conditions, and results between numerical simulations and experiments are compared. In this first part of the analysis, we focus on the flow pattern regimes and the dynamics of the bubble generation process. In addition to the new numerical simulations presented here, a new model has been used to predict the bubble generation frequency and tested with both experimental and numerical data. Results on bubble generation frequency are also presented by means of the non-dimensional Strouhal number. Same types of patterns, bubble and slug flow regimes, are obtained in simulations and experiments. In order to perform an exhaustive validation and comparison of numerical simulations with experimental data, several parameters have been selected: bubble velocity, volumetric void fraction, bubble generation frequency, Strouhal number and bubble equivalent diameter. Numerical simulations agree qualitatively, but not always quantitatively, with experimental results.Peer ReviewedPostprint (published version

Vortex formation above prismatic-shaped cliffs; an experimental and numerical investigation

Peer ReviewedPostprint (published version

Influence of contact angle boundary condition on CFD simulation of T-Junction

“This is a post-peer-review, pre-copyedit version of an article published in Microgravity Science and Technology. The final authenticated version is available online at: https://doi.org/10.1007/s12217-018-9605-x ”.In this work, we study the influence of the contact angle boundary condition on 3D CFD simulations of the bubble generation process occurring in a capillary T-junction. Numerical simulations have been performed with the commercial Computational Fluid Dynamics solver ANSYS Fluent v15.0.7. Experimental results serve as a reference to validate numerical results for four independent parameters: the bubble generation frequency, volume, velocity and length. CFD simulations accurately reproduce experimental results both from qualitative and quantitative points of view. Numerical results are very sensitive to the gas-liquid-wall contact angle boundary conditions, confirming that this is a fundamental parameter to obtain accurate CFD results for simulations of this kind of problems.Peer ReviewedPostprint (published version

Impact study of contact angle boundary conditions for CFD simulation of two-phase flow generation in a T-junction

Gas – liquid two - phase flows dominated by capillary forces occur in countless modern industrial applications. Some examples can be found in normal gravity (mixing process, chemical reactions, emulsion technology, materials synthesis, medical science, persona l care products, etc), as well as in space - based systems (life - support systems for human exploration, thermal management systems, propulsion systems, chemical contactors, space bioreactors, etc). A better understanding and an improvement of two - phase flows generation techniques are thus mandatory for the optimal development of these technologies, both in normal and reduced - gravity environments. In the past, great efforts have been conducted to fulfil this goal, and several methods have been proposed, aiming to generate and control two - phase flows in a very accurate way. T-junction bubble generators have arisen as an efficient method providing trains of bubbles, immersed into a continuous liquid, with small dispersion in bubble size.Peer ReviewedPostprint (published version

High-order discontinuous Galerkin methods for incompressible flows

Aquesta tesi doctoral proposa formulacions de Galerkin discontinu (DG) d'alt ordre per fluxos viscosos incompressibles. Es desenvolupa un nou mètode de DG amb penalti interior (IPM-DG), que condueix a una forma feble simètrica i coerciva pel terme de difusió, i que permet assolir una aproximació espacial d'alt ordre. Aquest mètode s'aplica per resoldre les equacions de Stokes i Navier-Stokes. L'espai d'aproximació de la velocitat es descompon dins de cada element en una part solenoidal i una altra irrotacional, de manera que es pot dividir la forma dèbil IPM-DG en dos problemes desacoblats. El primer permet el càlcul de les velocitats i de les pressions híbrides, mentre que el segon calcula les pressions en l'interior dels elements. Aquest desacoblament permet una reducció important del número de graus de llibertat tant per velocitat com per pressió. S'introdueix també un paràmetre extra de penalti resultant en una formulació DG alternativa per calcular les velocitats solenoidales, on les pressions no apareixen. Les pressions es poden calcular com un post-procés de la solució de les velocitats. Es contemplen altres formulacions DG, com per exemple el mètode Compact Discontinuous Galerkin, i es comparen al mètode IPM-DG. Es proposen mètodes implícits de Runge-Kutta d'alt ordre per problemes transitoris incompressibles, permetent obtenir esquemes incondicionalment estables i amb alt ordre de precisió temporal. Les equacions de Navier-Stokes incompressibles transitòries s'interpreten com un sistema de Equacions Algebraiques Diferencials, és a dir, un sistema d'equacions diferencials ordinàries corresponent a la equació de conservació del moment, més les restriccions algebraiques corresponent a la condició d'incompressibilitat. Mitjançant exemples numèrics es mostra l'aplicabilitat de les metodologies proposades i es comparen la seva eficiència i precisió.This PhD thesis proposes divergence-free Discontinuous Galerkin formulations providing high orders of accuracy for incompressible viscous flows. A new Interior Penalty Discontinuous Galerkin (IPM-DG) formulation is developed, leading to a symmetric and coercive bilinear weak form for the diffusion term, and achieving high-order spatial approximations. It is applied to the solution of the Stokes and Navier-Stokes equations. The velocity approximation space is decomposed in every element into a solenoidal part and an irrotational part. This allows to split the IPM weak form in two uncoupled problems. The first one solves for velocity and hybrid pressure, and the second one allows the evaluation of pressures in the interior of the elements. This results in an important reduction of the total number of degrees of freedom for both velocity and pressure. The introduction of an extra penalty parameter leads to an alternative DG formulation for the computation of solenoidal velocities with no presence of pressure terms. Pressure can then be computed as a post-process of the velocity solution. Other DG formulations, such as the Compact Discontinuous Galerkin method, are contemplated and compared to IPM-DG. High-order Implicit Runge-Kutta methods are then proposed to solve transient incompressible problems, allowing to obtain unconditionally stable schemes with high orders of accuracy in time. For this purpose, the unsteady incompressible Navier-Stokes equations are interpreted as a system of Differential Algebraic Equations, that is, a system of ordinary differential equations corresponding to the conservation of momentum equation, plus algebraic constraints corresponding to the incompressibility condition. Numerical examples demonstrate the applicability of the proposed methodologies and compare their efficiency and accuracy

High-Order discontinuous Galerkin methods for incompressible flows

The spatial discretization of the unsteady incompressible Navier-Stokes equations is stated as a system of Differential Algebraic Equations (DAEs), corresponding to the conservation of momentum equation plus the constraint due to the incompressibility condition. Runge-Kutta methods applied to the solution of the resulting index-2 DAE system are analyzed, allowing a critical comparison in terms of accuracy of semi-implicit and fully implicit Runge-Kutta methods. Numerical examples, considering a discontinuous Galerkin Interior Penalty Method with piecewise solenoidal approximations, demonstrate the applicability of the approach, and compare its performance with classical methods for incompressible flows.Peer ReviewedPostprint (published version

A critical comparison of two discontinuous galerkin methods for the navier-stokes equations using solenoidal aproximations

This paper compares two methods to solve incompressible problems, in particular the Navier-Stokes equations, using a discontinuous polynomial interpolation that is exactly divergence-free in each element. The first method is an Interior Penalty Method, whereas the second method follows the Compact Discontinuous Galerkin approach for the diffusive part of the problem. In both cases the Navier-Stokes equations are then solved using a fractional-step method, using an implicit method for the diffusion part and a semiimplicit method for the convection. Numerical examples compare the efficiency and the accuracy of the two proposed methods.Peer Reviewe

Advanced methodology for wind resource assessment near hydroelectric dams in complex mountainous areas

To increase renewable energy generation in some hydroelectric dams, a solution consisting in installing wind turbines close to dams is proposed. Indeed, dam surroundings are prone to benefit from wind speed-up effect, extra wind generation associated with thermal winds, and existing electrical infrastructure. Identifying the most suitable locations for turbines, that is, areas of relatively high-speed and low-turbulence wind, is fundamental to maximize this complementary power. Easy accessibility to turbines and minimum distance to dam electrical infrastructure are also essential to reduce the costs. Thus, a methodology is proposed to improve wind resource assessment in complex mountainous areas. First, potentially interesting dams are chosen using statistical local wind data. Second, weighted results of wind speed and turbulence intensity, considering all wind directions are presented based on CFD simulations. Finally, wind power density and annual energy production maps are generated, along with accessibility maps, to identify suitable sites. The Camarasa dam in the north-east of the Iberian Peninsula is chosen as case study to show and test the proposed methodology. Error estimations are provided, along with validation against Wind Atlas data and WAsP simulations.Peer ReviewedPostprint (published version

Estimation of flight CO2 emissions and travel time with analytical models

Flights negatively impact the environment and the aviation industry faces significant challenges to decarbonise. This is particularly relevant for short- and medium-haul flights, as operations in these ranges can be less fuel efficient per passenger transported and other transport alternatives could be considered, e.g. rail. The quantification of this environmental impact is usually reliant on estimations of CO2 emissions. Currently available models tend to focus on simple linear relationships between distance and/or time and emissions, which do not differentiate by operation type, or tend to be based on fuel consumption models, which require detailed flight and aircraft parameters on a flight by flight basis, limiting the possibility of network level analysis. We present analytical models calculating gate-to-gate CO2 emissions and travel time based on flight distance and on the number of available seats. These models enable the comparison of the environmental impact of aviation with other means of transport, as operations are translated into passenger transported and time required. Emissions estimated by these models are in line with other CO2 calculators. The models present the advantage of being sensitive to the number of available seats, a parameter generally not explicitly considered. Their applicability is shown in practical examples: analysis of emissions and time of routes, which could potentially be complemented by high-speed rail and night trains.Objectius de Desenvolupament Sostenible::9 - Indústria, Innovació i InfraestructuraObjectius de Desenvolupament Sostenible::12 - Producció i Consum ResponsablesObjectius de Desenvolupament Sostenible::13 - Acció per al ClimaObjectius de Desenvolupament Sostenible::13 - Acció per al Clima::13.2 - Incorporar mesures relatives al canvi climàtic en les polítiques, les estratègies i els plans nacionalsPostprint (published version

Delay assignment optimization strategies at pre-tactical and tactical levels

—This paper compares different optimization strategies for the minimization of flight and passenger delays at two levels: pre-tactical, with on-ground delay at origin, and tactical, with airborne delay close to the destination airport. The optimization model is based on the ground holding problem and uses various cost functions. The scenario considered takes place in a busy European airport and includes realistic values of traffic. Uncertainty is introduced in the model for the passenger allocation, minimum time required for turnaround and tactical uncertainty. Performance of the various optimization processes is presented and compared to ratio by schedule results.Postprint (published version