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Wall-driven flow in square cavities has been studied extensively, yet it appears some main flow characteristics have not been fully investigated. Previous research on the classic lid-driven cavity (S1) flow has produced the critical Reynolds numbers separating the laminar steady and unsteady flows. Wall-driven cavities with two opposing walls moving at the same speed and the same (S2p) or opposite (S2a) directions have seldom been studied in the literature and no critical Reynolds numbers characterizing transitional flows have ever been investigated. After validating the LBM code for the three configurations studied, extensive numerical simulations have been undertaken to provide approximate ranges for the critical Hopf and Neimark-Sacker bifurcations for the classic and two two-sided cavity configurations. The threshold for transition to chaotic motion is also reported. The symmetries of the solutions are monitored across the various bifurcations for the two-sided wall driven cavities. The mirror-symmetry of the base solution for case S2p is lost at the Hopf bifurcation. The exact same scenario occurs with the pi-rotational symmetry of the base state for case S2a.

The turbulent flow around a circular cylinder has been investigated at R e = 5000 using direct numerical simulations. Low frequency behavior, vortex undulation, vortex splitting, vortex dislocations and three dimensional flow within the wake were found to happen at this flow regime. In order to successfully capture the wake three dimensionality, different span-wise lengths were considered. It was found that a length L Z = 2 π D was enough to capture this behavior, correctly predicting different aspects of the flow such as drag coefficient, Strouhal number and pressure and velocity distributions when compared to experimental values. Two instability mechanisms were found to coexist in the present case study: a global type instability originating in the shear layer, which shows a characteristic frequency, and a convective type instability that seems to be constantly present in the near wake. Characteristics of both types of instabilities are identified and discussed in detail. As suggested by Norberg, a resonance-type effect takes place in the vortex formation region, as the coexistence of both instability mechanisms result in distorted vortex tubes. However, vortex coherence is never lost within the wake.

The final publication is available at Springer via http://dx.doi.org/10.1007/s00466-016-1332-9 In this work, we design an explicit time-stepping solver for the simulation of the incompressible turbulent flow through the combination of VMS methods and artificial compressibility. We evaluate the effect of the artificial compressibility on the accuracy of the explicit formulation for under-resolved LES simulations. A set of benchmarks have been solved, e.g., the 3D Taylor–Green vortex problem in turbulent regimes. The resulting method is proven to be an effective alternative to implicit methods in some application ranges (in terms of problem size and computational resources), providing comparable results with very low memory requirements. As an example, with the explicit approach, we are able to solve accurately the Taylor-Green vortex benchmark in a fine mesh with 5123 cells on a 12 cores 64 GB ram machine.

Venetian blinds (VB) are shading devices of widespread use in residential and corporate buildings. They can reflect or transmit light into buildings and at the same time allow daylighting and exterior views. They can also efficiently block radiative heat from entering the building, and if combined with a heat dissipation system such as forced ventilation, they can improve the thermal performance of double skin façades (DSF). Computational Fluid Dynamics (CFD) has proven to be a useful tool for modeling flow and heat transfer in DSF, including conduction, convection and radiation heat transfer phenomena. The aim of this work is to evaluate, by means of CFD, the influence of several optical, construction and operation parameters of a DSF (such as optical properties of the materials, geometrical relations of the VB or flow stream conditions) in terms of energy savings, measured as a reduction of the solar load entering the building. Results obtained show that parameters such as the proximity of the VB to the exterior skin of the façade or a differentiated surface treatment for the exterior and interior faces of the VB louvers can notably affect the thermal performance of the DSF and hence the heat gains experienced by the building.

Large-eddy simulations (LES) of the flow past a circular cylinder are used to investigate the flow topology and the vortex shedding process at Reynolds numbers Re = 2.5 × 10 5 - 8.5 × 10 5 . This range encompasses both the critical and super-critical regimes. As the flow enters the critical regime, major changes occur which affect the flow configuration. Asymmetries in the flow are found in the critical regime, whereas the wake recovers its symmetry and stabilizes in the super-critical regime. Wake characteristic lengths are measured and compared between the different Reynolds numbers. It is shown that the super-critical regime is characterised by a plateau in the drag coefficient at about C D ≈ 0.22 , and a quasi-stable wake which has a non-dimensional width of d w / D ≈ 0.4 . The periodic nature of the flow is analysed by means of measurements of the unsteady drag and lift coefficients. Power spectra of the lift fluctuations are computed. Wake vortex shedding is found to occur for both regimes investigated, although a jump in frequencies is observed when the flow enters the super-critical regime. In this regime, non-dimensional vortex-shedding frequency is almost constant and equal to St = f vs D / U ref ≈ 0.44 . The analysis also shows a steep decrease in the fluctuating lift when entering the super-critical regime. The combined analysis of both wake topology and vortex shedding complements the physical picture of a stable and highly coherent flow in the super-critical regime.

Various fundamental studies based on a turbulent duct flow have gained popularity including heat transfer, magnetohydrodynamics as well as particle-laden transportation. An accurate prediction on the turbulent flow field is critical for these researches. However, the database of the mean flow and turbulence statistics is fairly insufficient due to the enormous cost of numerical simulation at high Reynolds number. This paper aims at providing available information by conducting several Direct Numerical Simulations (DNS) on turbulent duct flows at Re-tau = 300, 600, 900 and 1200. A quantitative comparison between current and previous DNS results was performed where a good agreement was achieved at Re-tau = 300. However, further comparisons of the present results with the previous DNS results at Re-tau = 600 obtained with much coarser meshes revealed some discrepancies which can be explained by the insufficient mesh resolution. At last, the mean flow and turbulent statistics at higher Re-tau was presented and the effect of Re-tau on the mean flow and flow dynamics was discussed.

Nowadays large part of the time needed to perform a numerical simulation is spent in preprocessing, especially in the geometry cleaning operations and mesh generation. Furthermore, these operations are not easy to automatize because they depend strongly on each geometrical model and they often need human interaction. Many of these operations are needed to obtain a watertight geometry. Even with a clean geometry, classical unstructured meshing methods (like Delaunay or Advancing Front based ones) present critical weak points like the need of a given quality in the boundary mesh or a relatively smooth size transition. These aspects decrease their robustness and imply an extra effort in order to reach the final mesh. Octree based meshers try to relax some of these requirements. In the present work an octree based mesher for unstructured tetrahedra is presented. The proposed mesher ensures the mesh generation avoiding most of the geometry cleaning operations. It is based in the following steps: fit an octree onto the model, refine it following given criteria, apply a tetrahedra pattern to the octree cells and adapt the tetrahedra close to the contours in order to represent accurately the boundary shape. An important and innovative aspect of the proposed algorithm is it ensures the final mesh preserves the topology and the geometric features of the original model. The method uses a Ray Casting based algorithm for the identification of the inner and outer parts of the volumes involved in the model. This technique allows the mesh generation of volumes even with non-watertight boundaries, and also opens the use of the mesher for immersed methods only applying slight modifications to the algorithm. The main advantages of the presented mesher are: robustness, no need for watertight boundaries, independent on the contour mesh quality, preservation of geometrical features (corners and ridges), original geometric topology guaranteed, accurate representation of the contours, valid for immersed methods, and fast performance. A lot of time in the preprocessing part of the numerical simulation is saved thanks to the robustness of the mesher, which allows skipping most of the geometry cleaning operations. A shared memory parallel implementation of the algorithm has been done. The effectiveness of the algorithm and its implementation has been verified by some validation examples., En l'actualitat gran part del temps emprat per córrer una simulació numèrica està dedicat al preprocés, especialment a les operacions de neteja de geometria i generació de malla. A més, aquestes operacions no són fàcils d'automatitzar degut a la seva forta dependència del model geomètric i sovint necessiten d’interacció humana. Moltes d'aquestes operacions són necessàries per aconseguir una definició topológicament hermètica de la geometria. Inclús amb una geometria neta, els mètodes clàssics de mallat (com els basats en Delaunay o avançament frontal) presenten punts febles crítics com la necessitat d'una certa qualitat de les malles de contorn o una transició de mides relativament suau. Aquests aspectes disminueixen la seva robustesa i impliquen un esforç extra a l'hora d'obtenir la malla final. Els mètodes de mallat basats en estructures octree relaxen alguns d'aquests requeriments. En aquest treball es presenta un mallador basat en octree per tetraedres no estructurats. Un dels aspectes claus d'aquest mallador és que garanteix la generació de malla evitant moltes de les operacions de neteja de geometria. Es basa en els següents passos: encaixar un octree al model, refinar-lo seguint certs criteris, aplicar un patró de tetraedres a les cel•les de l'octree i adaptar-los a les zones properes als contorns a fi i efecte de representar acuradament la forma del domini. Un aspecte important i innovador de l'algorisme proposat és que manté la topologia del model a la malla final i preserva les seves característiques geomètriques. El mètode presentat utilitza un algorisme basat en la tècnica Ray Casting per la identificació de les parts interiors i exteriors dels volums del model. Aquesta tècnica permet la generació de malla de volums inclús amb contorns que no tanquen hermèticament, i també obre l’ús del mallador a mètodes “immersed” aplicant només petites modificacions a l'algorisme. Els principals avantatges del mallador presentat són: robustesa, no necessitat de definicions hermètiques dels contorns, independent de la qualitat de la malla de contorn, preservació de característiques geomètriques (cantonades i arestes abruptes), topologia original de la geometria garantida, representació precisa dels contorns, vàlid per mètodes “immersed” i ràpid rendiment. L’ús del mallador estalvia molt de temps en la part del preprocés de la simulació numèrica gràcies a la seva robustesa que permet obviar la majoria d'operacions de neteja de geometria. S'ha dut a terme una implementació paral•lela amb memòria compartida de l'algorisme. L'efectivitat del mateix i la seva implementació ha estat verificada mitjançant exemples de validació.

This paper is devoted to the presentation of a new model for fin-and-tube evaporators, focusing on the solid core simulation and its integration with a quasi-homogeneous two-phase flow model for the in-tube refrigerant flow. Special attention is dedicated to separated in-tube flow patterns (stratified, stratified-wavy), because of their importance in liquid overfeed evaporators and the impact on the solid core temperature distribution. The paper presents the solid core formulation and numerical method, the intube two-phase flow model, and describes the proposed integration algorithm between them. A selected single-tube case is analysed in full detail, showing the impact of stratified flow on the fin and tube temperature distributions. Additional studies are finally presented on fin-tube contact aspects as thermal contact resistance and partial contact.

Flow around cylindrical structures is of relevance for many practical applications. Knowledge of flow-related unsteady loading of such structures is crucial for hydro - and aerodynamic control and design. Recenty, the increase in computer power has allowed detailed numerical studies of this flow at high Reynolds numbers [1]. In order to obtain deeper knowledge of this kind of flow, several symmetry-preserving regularization simulations [2] and a DNS have been performed at ReD = 3900 (ReD = UrefD/v). These are the first published studies with symmetry-preserving regularization models for this flow.

The goal of this project is to explore the mechanics of lipid fluid membranes as found in biological and made-man systems through continuum models and numerical simulations. Traditionally, the focus has been on the equilibrium configurations of vesicles through minimization of the curvature energy subject to constraints. Here, the goal is to describe the time-evolution of out-of-equilibrium vesicle configurations, which are of relevance in biological systems. Towards this goal, an accurate description of the dissipative mechanisms is crucial, in particular the viscous dissipation induced by the 2D flow of lipids on the deforming surface that describes a vesicle [1]. The resulting equations can only be solved analytically in very simple settings. So the problem is solved numerically using a B-Spline description of the membrane vesicle. The dynamics are described using two types of viscosity: the L2 or Willmore viscosity and the inner flow viscosity. By comparing the evolution in time of the two systems, it is stated that the dynamics are clearly different. It is found that one describes better the physics of the system.

Fluid power scientists and engineers have produced a large amount of high quality books so far, which cover a vast amount of technologies involved in this field. Names like H. Merritt, D. McCloy, H.R. Martin, W. Backe, J Watton, K. Edge, M. Ivantysynova, N.D. Manring among many others must be considered as the milestones in this field; their scientific publications and books have inspired generations of engineers. The first author of this book was lucky to be able to closely work for over 10 years with Professor John Watton, and in fact, most of the original research presented in this volume was undertaken at Cardiff University. The present book focuses on several components of fluid mechanics. The first three chapters are designed to give a proper background to the reader regarding the main fluid characteristics, chapter 1, the main fluid mechanics equations, chapter 2, and a strategic background of the Computer Fluid Dynamics (CFD) techniques, chapter 3. It must be kept in mind that nowadays, conventional mechanics, as well as fluid mechanics, are fully immersed in the CFD era, therefore the components design desperately needs the use of this relatively new tool. Chapter 4 introduces original research based on fluid mechanics understanding of relief valves and servovalves, dynamic and stability considerations are being given in both cases, and hints to solve stability problems are provided. Chapter 5 also provides original research on, very likely, the most complex machines in the fluid power field; piston pumps and motors. In fact, chapter 5 focuses on axial piston pumps, although the information gathered in this chapter can be directly extrapolated to other piston pumps and motor configurations. In Chapter 5 the reader will find a thorough mathematical description of how slippers with non vented grooves can be designed, the effect of grooves on pistons is also thoroughly analyzed. The barrel dynamic movements and piston pump pressure dynamics under different operating conditions are also introduced in this chapter. In all cases, the reader will be able to extract ideas of how a proper design shall be obtained. It is important o highlight that all experiments presented in chapter 5 were done by the book’s first author in the Professor John Watton Fluid Power laboratory at Cardiff University UK, this is why Professor John Watton has to be seen as a co-author of this particular chapter. Chapters 6, 7 and 8 are designed to introduce some details which are often forgotten in many publications; these are the use of accumulators, the importance of proper filtration and the use of cartridge valves whenever fluid pressure and flow are overcoming a certain value. It is crucial to realize that accumulators can vastly improve a given circuit efficiency, often saving large amounts of energy. A proper filtration is crucial to increase the components life and prevent system failures. According to the authors’ understanding, the present book may very well be used as a design tool for several Fluid Power components. Manufacturers, Engineers and fluid Power Researchers may get some helpful ideas to improve their products