Luís Eça

Bio: Luís Eça is an academic researcher from Instituto Superior Técnico. The author has contributed to research in topics: Reynolds-averaged Navier–Stokes equations & Turbulence. The author has an hindex of 15, co-authored 50 publications receiving 1035 citations. Previous affiliations of Luís Eça include Higher Technical Institute of Cyprus & Technical University of Lisbon.

A procedure for the estimation of the numerical uncertainty of CFD calculations based on grid refinement studies

Abstract: This paper offers a procedure for the estimation of the numerical uncertainty of any integral or local flow quantity as a result of a fluid flow computation; the procedure requires solutions on systematically refined grids. The error is estimated with power series expansions as a function of the typical cell size. These expansions, of which four types are used, are fitted to the data in the least-squares sense. The selection of the best error estimate is based on the standard deviation of the fits. The error estimate is converted into an uncertainty with a safety factor that depends on the observed order of grid convergence and on the standard deviation of the fit. For well-behaved data sets, i.e. monotonic convergence with the expected observed order of grid convergence and no scatter in the data, the method reduces to the well known Grid Convergence Index. Examples of application of the procedure are included. Estimation of the numerical uncertainty of any integral or local flow quantity.Least squares fits to power series expansions to handle noisy data.Excellent results obtained for manufactured solutions.Consistent results obtained for practical CFD calculations.Reduces to the well known Grid Convergence Index for well-behaved data sets.

A manufactured solution for a two-dimensional steady wall-bounded incompressible turbulent flow

Abstract: This paper presents a manufactured solution (MS), resembling a two-dimensional, steady, wall-bounded, incompressible, turbulent flow for RANS codes verification. The specified flow field satisfies mass conservation, but requires additional source terms in the momentum equations. To also allow verification of the correct implementation of the turbulence models transport equations, the proposed MS exhibits most features of a true near-wall turbulent flow. The model is suited for testing six eddy-viscosity turbulence models: the one-equation models of Spalart and Allmaras and Menter; the standard two-equation k-e model and the low-Reynolds version proposed by Chien; the TNT and BSL versions of the k-ω model.

On the construction of manufactured solutions for one and two‐equation eddy‐viscosity models

Abstract: This paper presents manufactured solutions (MSs) for some well-known eddy-viscosity turbulence models, viz. the Spalart & Allmaras one-equation model and the TNT and BSL versions of the two-equation k-ω model. The manufactured flow solutions apply to two-dimensional, steady, wall-bounded, incompressible, turbulent flows. The two velocity components and the pressure are identical for all MSs, but various alternatives are considered for specifying the eddy-viscosity and other turbulence quantities in the turbulence models. The results obtained for the proposed MSs with a second-order accurate numerical method show that the MSs for turbulence quantities must be constructed carefully to avoid instabilities in the numerical solutions. This behaviour is model dependent: the performance of the Spalart & Allmaras and k-ω models is significantly affected by the type of MS. In one of the MSs tested, even the two versions of the k-ω model exhibit significant differences in the convergence properties.

Simulation of the flow around a circular cylinder at Re=3900 with Partially-Averaged Navier-Stokes equations

Abstract: This study employs Partially-Averaged Navier-Stokes (PANS) equations to simulate the flow around a smooth circular cylinder at Reynolds number 3900. It intends to evaluate the importance of discretization and modelling errors on the accuracy of this mathematical model. Furthermore, the study addresses the effect of the physical resolution, or fraction of turbulence kinetic energy being modelled fk, on the predictions accuracy. To this end, Validation exercises are carried out using five different values of fk which range from typical values for well-resolved Scale-Resolving Simulations (fk ≤ 0.25) to Reynolds-Averaged Navier-Stokes equations ( f k = 1.00 ). Naturally, these exercises require the evaluation of numerical errors, i.e. Verification studies. Consequently, and taking advantage of the ability of PANS to enable the distinction between discretization and modelling errors, spatial and temporal grid refinement studies are carried out to assess the magnitude of the discretization error, as well as its dependence on fk. The outcome confirms the ability of PANS, in combination with fk f k = 1.00 . However, the reduction of fk tends to increase the model dependence on the spatial and temporal resolution. It is demonstrated that similarly to the effect of the spatial and temporal grid resolution on the magnitude of the numerical error, the modelling error diminishes with the physical resolution (fk → 0). The convergence of the predictions with fk is also illustrated.

2D Orthogonal Grid Generation with Boundary Point Distribution Control

Abstract: A numerical method for 2D orthogonal grid generation with control of the boundary point distribution is presented. The method is based on the solution of a system of partial differential equations. The grid cell aspect ratio, the so-called distortion function, is calculated from its definition equation in the entire domain. The method allows the specification of the boundary point distribution in all the boundaries and also the distance of the first grid node to the boundary. The method is successfully applied to several geometries, including geometries with grid singularities and grids around airfoils and cross-sections of ship sterns.

Wave energy utilization: A review of the technologies

Abstract: Sea wave energy is being increasingly regarded in many countries as a major and promising resource. The paper deals with the development of wave energy utilization since the 1970s. Several topics are addressed: the characterization of the wave energy resource; theoretical background, with especial relevance to hydrodynamics of wave energy absorption and control; how a large range of devices kept being proposed and studied, and how such devices can be organized into classes; the conception, design, model-testing, construction and deployment into real sea of prototypes; and the development of specific equipment (air and water turbines, high-pressure hydraulics, linear electrical generators) and mooring systems.

Verification and Validation in Scientific Computing

Abstract: Advances in scientific computing have made modelling and simulation an important part of the decision-making process in engineering, science, and public policy. This book provides a comprehensive and systematic development of the basic concepts, principles, and procedures for verification and validation of models and simulations. The emphasis is placed on models that are described by partial differential and integral equations and the simulations that result from their numerical solution. The methods described can be applied to a wide range of technical fields, from the physical sciences, engineering and technology and industry, through to environmental regulations and safety, product and plant safety, financial investing, and governmental regulations. This book will be genuinely welcomed by researchers, practitioners, and decision makers in a broad range of fields, who seek to improve the credibility and reliability of simulation results. It will also be appropriate either for university courses or for independent study.

New insights into large eddy simulation

Abstract: Fluid dynamic turbulence is one of the most challenging computational physics problems because of the extremely wide range of time and space scales involved, the strong nonlinearity of the governing equations, and the many practical and important applications. While most linear fluid instabilities are well understood, the nonlinear interactions among them makes even the relatively simple limit of homogeneous isotropic turbulence difficult to treat physically, mathematically, and computationally. Turbulence is modeled computationally by a two-stage bootstrap process. The first stage, direct numerical simulation, attempts to resolve the relevant physical time and space scales but its application is limited to diffusive flows with a relatively small Reynolds number (Re). Using direct numerical simulation to provide a database, in turn, allows calibration of phenomenological turbulence models for engineering applications. Large eddy simulation incorporates a form of turbulence modeling applicable when the large-scale flows of interest are intrinsically time dependent, thus throwing common statistical models into question. A promising approach to large eddy simulation involves the use of high-resolution monotone computational fluid dynamics algorithms such as flux-corrected transport or the piecewise parabolic method which have intrinsic subgrid turbulence models coupled naturally to the resolved scales in the computed flow. The physical considerations underlying and evidence supporting this monotone integrated large eddy simulation approach are discussed.

Oscillating-water-column wave energy converters and air turbines: A review

Abstract: The ocean waves are an important renewable energy resource that, if extensively exploited, may contribute significantly to the electrical energy supply of countries with coasts facing the sea. A wide variety of technologies has been proposed, studied, and in some cases tested at full size in real ocean conditions. Oscillating-water-column (OWC) devices, of fixed structure or floating, are an important class of wave energy devices. A large part of wave energy converter prototypes deployed so far into the sea are of OWC type. In an OWC, there is a fixed or floating hollow structure, open to the sea below the water surface, that traps air above the inner free-surface. Wave action alternately compresses and decompresses the trapped air which is forced to flow through a turbine coupled to a generator. The paper presents a comprehensive review of OWC technologies and air turbines. This is followed by a survey of theoretical, numerical and experimental modelling techniques of OWC converters. Reactive phase control and phase control by latching are important issues that are addressed, together with turbine rotational speed control.