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One of the main triggers of traffic congestion on highways is vehicle merging at on-ramps. The development of automated procedures for cooperative vehicle merging is aimed to ensure safety and alleviate congestion problems. In this work, a longitudinal trajectory planning methodology is presented, developed to assist the merging of vehicles on highways; it achieves safe and traffic-efficient merging, while minimizing the engine effort and passenger discomfort through the minimization of acceleration and its first and second derivatives during the merging maneuver. The problem is formulated as a finite-horizon optimal control problem and is solved analytically. This enables the solution to be stored on-board, saving computational time and rendering the methodology suitable for practical applications. The tunable weights, used for taking into account the different optimization criteria, may serve as parameters to match the individual driver’s preferences. The proposed methodology is first developed for a pair of cooperating vehicles, a merging one and its putative leader. Moreover, an alternative solution procedure via a time-variant Linear-Quadratic Regulator approach is also presented. A Model Predictive Control (MPC) scheme is utilized to compensate possible disturbances in the trajectories of the cooperating vehicles, whereby the analytical optimal solution is applied repeatedly in real time, using updated measurements, until the merging procedure is actually finalized. Subsequently, the methodology is generalized for a set of vehicles inside the merging area. Various numerical simulations illustrate the validity and applicability of the method. [ABSTRACT FROM AUTHOR]

The development and comparison of different parallel spatial/angular agglomeration multigrid schemes to accelerate the finite volume method, for the prediction of radiative heat transfer, are reported in this study. The proposed multigrid methodologies are based on the solution of radiative transfer equation with the full approximation scheme coupled with the full multigrid method, considering different types of sequentially coarser spatial and angular resolutions as well as different V-cycle types. The encountered numerical tests, involving highly scattering media and reflecting boundaries, reveal the superiority of the nested scheme along with the V(2,0)-cycle-type strategy, while they highlight the significant contribution of the angular extension of the multigrid technique. [ABSTRACT FROM PUBLISHER]

A parallel spatial/angular agglomeration multigrid methodology, employing the full approximation scheme (FAS), is developed to accelerate the finite-volume method for the prediction of radiative heat transfer in absorbing, emitting, and scattering gray media. The methodology for the spatial, angular, and combined spatial/angular agglomeration multigrid procedure was analyzed in Part I of this study. In this second part the proposed numerical scheme is validated against benchmark test cases, demonstrating its capability for considerably improved computational performance, especially in problems including scattering media and reflecting boundaries. [ABSTRACT FROM AUTHOR]

A parallel spatial/angular agglomeration multigrid scheme is developed to accelerate the finite-volume method (FVM) for the computation of radiative heat transfer in absorbing, emitting, and scattering gray media. The multigrid scheme is based on the solution of the radiative transfer equation (RTE) with the full approximation scheme (FAS) on successively coarser spatial and angular resolutions, derived from the initial finest ones through the fusion of the adjacent control volumes and control angles, respectively. The numerical tests reveal the improvement of efficiency employing the aforementioned technique, especially in cases considering scattering media and reflecting boundaries. [ABSTRACT FROM AUTHOR]

In this work an algorithm using the finite-volume method along with a second-order-accurate spatial/temporal scheme and theh-refinement technique is evaluated in the parallelized computation of radiative heat transfer in absorbing-emitting and scattering gray media. The second-order spatial scheme is based on the reconstruction of radiative intensity's values, jointed with a slope limiter to maintain monotonicity. Additionally, theh-refinement method is incorporated to enrich targeted areas of the mesh during the solution procedure. The numerical results reveal the mitigation of false scattering and consequently the improvement of accuracy, employing these techniques in coarse unstructured hybrid grids. [ABSTRACT FROM AUTHOR]

In this work an algorithm using the finite-volume method is presented for the parallelized computation of radiative heat transfer for absorbing, emitting, and either isotropically or anisotropically scattering gray media. Attention is mainly toward the extension of the finite-volume methodology to hybrid unstructured meshes, using a node-centered median-dual approach, with highly stretched elements at the solid wall regions, and the evaluation of its effectiveness and accuracy in such girds. As the parallelized calculation and the utilization of hybrid meshes are not common, results of the present algorithm against benchmark problems are used to demonstrate their equal potential. [ABSTRACT FROM AUTHOR]

Artificial Neural Networks (ANNs) comprise a powerful tool to approximate the complicated behavior and response of physical systems allowing considerable reduction in computation time during time-consuming optimization runs. In this work, a Radial Basis Function Artificial Neural Network (RBFN) is combined with a Differential Evolution (DE) algorithm to solve a water resources management problem, using an optimization procedure. The objective of the optimization scheme is to cover the daily water demand on the coastal aquifer east of the city of Heraklion, Crete, without reducing the subsurface water quality due to seawater intrusion. The RBFN is utilized as an on-line surrogate model to approximate the behavior of the aquifer and to replace some of the costly evaluations of an accurate numerical simulation model which solves the subsurface water flow differential equations. The RBFN is used as a local approximation model in such a way as to maintain the robustness of the DE algorithm. The results of this procedure are compared to the corresponding results obtained by using the Simplex method and by using the DE procedure without the surrogate model. As it is demonstrated, the use of the surrogate model accelerates the convergence of the DE optimization procedure and additionally provides a better solution at the same number of exact evaluations, compared to the original DE algorithm. [ABSTRACT FROM AUTHOR]

Purpose The purpose of this paper is the presentation of a technique to be integrated in a numerical airfoil optimization scheme, for the exact satisfaction of a strict equality cross-sectional area constraint.Design/methodology/approach An airfoil optimization framework is presented, based on Area-Preserving Free-Form Deformation (AP FFD) technique. A parallel metamodel-assisted differential evolution (DE) algorithm is used as an optimizer. In each generation of the DE algorithm, before the evaluation of the fitness function, AP FFD is applied to each candidate solution, via coupling a classic B-Spline-based FFD with an area correction step. The area correction step is achieved by solving a sub problem, which consists of computing and applying the minimum possible offset to each one of the free-to-move control points of the FFD lattice, subject to the area preservation constraint.Findings The proposed methodology is able to obtain better values of the objective function, compared to both a classic penalty function approach and a generic framework for handling constraints, which suggests the separation of constraints and objectives (separation-sub-swarm), without any loss of the convergence capabilities of the DE algorithm, while it also guarantees an exact area preservation. Due to the linearity of the area constraint in each axis, the extraction of an inexpensive closed-form solution to the sub problem is possible by using the method of Lagrange multipliers.Practical implications AP FFD can be easily incorporated into any 2D shape optimization/design process, as it is a time-saving and easy-to-implement repair algorithm, independent from the nature of the problem at hand.Originality/value The proposed methodology proved to be an efficient tool in facing airfoil design problems, enhancing the rigidity of the optimal airfoil by preserving its cross-sectional area to a predefined value. [ABSTRACT FROM AUTHOR]

The academic computational fluid dynamics code Galatea-I is used in this study for the prediction of the flow over the DLR-F11 high-lift configuration, presented in the Second AIAA High-Lift Prediction Workshop. The proposed solver employs the Reynolds-averaged Navier-Stokes equations, modified appropriately by the artificial compressibility method and combined with the shear-stress transport turbulence model to account for the simulation of incompressible turbulent flows. Attention is mainly directed toward the assessment of the alternative, artificial compressibility methodology against such test cases because the majority of the workshop participants had implemented compressible Reynolds-averaged Navier-Stokes codes with preconditioning matrices instead. To this end, the flow at various angles of attack over the DLR-F11 aircraft model is studied, whereas the obtained results are compared to the available experimental data as well as simulation results produced by reference solvers. A satisfactory agreement is reported, demonstrating the proposed methodology's potential to accurately predict such complex flows and adapt to uncommon areas of use. [ABSTRACT FROM AUTHOR]

RBF-based mesh deformation methods have been recognized during the past years as an essential tool for numerical simulations involving mesh deformation. Nevertheless, they call for excessive memory and computation time requirements, especially for large-scale problems. A remedy to this shortcoming appears to be the selection of a reduced number of surface mesh nodes, to be used as RBF-centers, resulting in decreased dimensions of the system of equations. In this study a methodology for the reduction of the RBF-centers is developed, based on the agglomeration of boundary nodes' control areas. This technique has been adopted from the multigrid methods frequently employed in numerical simulation of fluid flow, heat transfer, etc. The fusion strategy is performed on a topology-preserving framework, while it resembles the advancing front technique, as it begins from regions with surface discontinuities extending successively to the interior. The proposed algorithm was evaluated against benchmark test cases, concerning deformation of the wing of a transonic commercial aircraft and the blade of a wind turbine, revealing its potential for radically improved computational performance of the grid deformation procedure. Moreover, the obtained results confirm its capability to preserve grid quality, despite the notable reduction of RBF-centers. [ABSTRACT FROM AUTHOR]

Different parallel agglomeration multigrid schemes have been developed aiming to improve the computational performance of a compressible and an incompressible academic Computational Fluid Dynamics (CFD) codes, named Galatea and Galatea-I , respectively. Flow prediction is succeeded via the implementation of Reynolds-Averaged Navier–Stokes (RANS) equations combined with appropriate turbulence models on three-dimensional unstructured tetrahedral or hybrid meshes. The sequence of required coarser grids, composed of irregular polyhedral elements, is generated either with the isotropic or directional (full- or semi-coarsening) fusion of neighbouring control volumes on a topology-preserving framework; it resembles the advancing-front technique as it begins from solid wall surfaces and extends successively to the interior domain. The multigrid accelerated approximation of flow and turbulence equations is achieved via the V-cycle implementation of either the Full Approximation Scheme (FAS) or its coupled version with Full Multigrid (FMG) method. Multigrid approaches with different agglomeration and solution strategies have been extensively tested against three- and quasi-three-dimensional test cases, all of them demonstrating their potential for considerably improved efficiency. Their contributions to the reduction of simulations' computation time are analysed, while additionally the differences due to the type of the flow (compressible or incompressible) are thoroughly discussed. [ABSTRACT FROM AUTHOR]

Abstract: In various applications of the advancing front based algorithms, involving complicated three dimensional geometries, the algorithm fails to complete the mesh generation process, as a number of small regions cannot be meshed using the standard procedure. These regions, formed by faces of the current front, are usually disconnected, highly non-convex and cannot be handled with simple actions. A strategy involving different novel and already in use operators is presented in this work, to successfully discretize such regions after the completion of the standard advancing front technique (AFT), in order to improve the robustness of the mesh generation procedure for difficult and complicated geometries. Examples of generated meshes using the proposed methodology are also presented for the validation of the proposed strategy. [Copyright &y& Elsevier]

Purpose – The purpose of this paper is to design an S-duct intake for unmanned aerial vehicles (UAVs) applications with good efficiency in a wide range of operating conditions. Design/methodology/approach – A fully-parametric 3-D CAD model of the intake was constructed in order to produce different intake configurations, within specific geometric constraints, and to study the influence of geometry variation on efficiency. O-type blocking methodology was adopted in order to construct the block-structured mesh of hexahedral elements, used in the simulations. The commercial CFD code ANSYS-CFX was used to compute the flow field inside the flow domain of each case considered. The Reynolds averaged Navier-Stokes (RANS) equations are discretized using an implicit, vertex-based finite volume method, combined with the shear stress transport (SST) two-equation turbulence model and an automatic wall treatment. Findings – By shortening the axial length the flow separation after the first turning becomes more pronounced and the losses are increasing. For very long ducts the increased internal wall area leads to increased wall friction and, consequently, to increased loss production. Originality/value – The adoption of Gerlach-shaped profiles for the design of the S-duct resulted in a low pressure loss level for the optimal shape, although more uniform distribution of total pressure losses resulted for ducts longer than the optimal one, which should be taken into account in the design process. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a software tool for the conceptual design of turbomachinery bladings named “T4T” (Tools for Turbomachinery). It provides the ability to interactively construct parametric 3D blade rows of various types, including for multistage machines. The design procedure is parametric and a variety of different rotating machinery components may be produced. The design parameters used for the blades as well as the hub and shroud surfaces construction correspond to 2D sections. However, the resulting geometries are modeled as NURBS 3D surfaces and can be imported to other CAD or analysis software via standard exchange protocols. The geometry definition is empowered by the object oriented structure of the software and an easy-to-use graphical interface, which makes the design phase a straightforward procedure. The software’s structure and geometric algorithms, the blading design procedure, along with sample designs which demonstrate the capabilities of the software, are presented in this paper. [Copyright &y& Elsevier]

Summary: In the past optimization techniques have been combined with simulation models to determine cost-effective solutions for various environmental management problems. In the present study, a groundwater management problem in a coastal karstic aquifer in Crere, Greece subject to environmental criteria has been studied using classical linear programming and heuristic optimization methodologies. A numerical simulation model of the unconfined coastal aquifer has been first developed to represent the complex non-linear physical system. Then the classical linear programming optimization algorithm of the Simplex method is used to solve the groundwater management problem where the main objective is the hydraulic control of the saltwater intrusion. A piecewise linearization of the non-linear optimization problem is obtained by sequential implementation of the Simplex algorithm and a convergence to the optimal solution is achieved. The solution of the non-linear management problem is also obtained using a heuristic algorithm. A Differential Evolution (DE) algorithm that emulates some of the principles of evolution is used. A comparison of the results obtained by the two different optimization approaches is presented. Finally, a sensitivity analysis is employed in order to examine the influence of the active pumping wells in the evolution of the seawater intrusion front along the coastline. [Copyright &y& Elsevier]

Abstract: In this work a surface generation software named Ge.P.A.S. (for generic parameterized aircraft surface) is presented, designed for the construction of aircraft aerodynamic surfaces. The surface generation procedure is parameterized and different aircraft configurations can be produced in an interactive way. A hierarchical structure of geometric parameters was adopted, resulting in easier manipulation of the shape and a scalable number of control parameters. Additionally, the geometric parameters may serve as design optimization variables in cooperation with an external optimizer. The surface generation is based on the use of NURBS curves and surfaces, which provide the ability to produce complicated geometries with a relative small number of design variables. Standard or user-defined airfoil sections can be used for the wing generation. The surface description is compatible with international input/output standards; IGES and STEP formats are supported for the output files. Consequently, Ge.P.A.S. can serve as a preprocessor for other software packages, which may be used in order to refine the geometry or to generate the grid for numerical simulations. The geometric algorithms, the software features and its basic characteristics are presented in this paper, along with a demonstration of its abilities in sample aircraft configurations. [Copyright &y& Elsevier]

Purpose: This study aims to feature the application of the artificial compressibility method (ACM) for the numerical prediction of two-dimensional (2D) axisymmetric swirling flows. Design/methodology/approach: The respective academic numerical solver, named IGal2D, is based on the axisymmetric Reynolds-averaged Navier–Stokes (RANS) equations, arranged in a pseudo-Cartesian form, enhanced by the addition of the circumferential momentum equation. Discretization of spatial derivative terms within the governing equations is performed via unstructured 2D grid layouts, with a node-centered finite-volume scheme. For the evaluation of inviscid fluxes, the upwind Roe's approximate Riemann solver is applied, coupled with a higher-order accurate spatial reconstruction, whereas an element-based approach is used for the calculation of gradients required for the viscous ones. Time integration is succeeded through a second-order accurate four-stage Runge-Kutta method, adopting additionally a local time-stepping technique. Further acceleration, in terms of computational time, is achieved by using an agglomeration multigrid scheme, incorporating the full approximation scheme in a V-cycle process, within an efficient edge-based data structure. Findings: A detailed validation of the proposed numerical methodology is performed by encountering both inviscid and viscous (laminar and turbulent) swirling flows with axial symmetry. IGal2D is compared against the commercial software ANSYS fluent – by using appropriate metrics and characteristic flow quantities – but also against experimental measurements, confirming the proposed methodology's potential to predict such flows in terms of accuracy. Originality/value: This study provides a robust methodology for the accurate prediction of swirling flows by combining the axisymmetric RANS equations with ACM. In addition, a detailed description of the convective flux Jacobian is provided, filling a respective gap in research literature. [ABSTRACT FROM AUTHOR]

Shrouded wind turbines represent an attractive solution of high potential that could improve significantly the feasibility of renewable energy production at sites characterized by poor wind resources. This work presents the development of a modular optimization scheme for the aerodynamic shape optimization of diffuser-augmented wind turbine (DAWT) shrouds. For the numerical simulation of the incompressible flow field, an axisymmetric RANS solver has been implemented, based on the artificial compressibility method and SST turbulence model. The major features of the RANS solver are demonstrated, while its validity is assessed against both numerical and experimental data. Mesh and geometry parameterization are simultaneously succeeded by employing an in-house developed computational tool, based on the well-known Free-Form Deformation (FFD) technique. The backbone of the optimization framework is formed by a parallel and asynchronous Differential Evolution (DE) algorithm, which is assisted by Artificial Neural Network (ANN) meta-models. The proposed methodology is applied to the design optimization of an axisymmetric shroud (diffuser) for a 15 kW wind turbine, aiming to maximize the mean velocity speed-up ratio and minimize drag, under geometrical constrains. The resulting designs are capable of providing high velocity accelerations, accompanied by considerable reduction in drag and volume. • A 2D axisymmetric RANS solver has been developed and validated. • An optimization scheme for the aerodynamic optimization of DAWT shrouds is proposed. • The mean velocity speed-up ratio over the rotor plane was increased by 23%. • The total drag force on the shroud was decreased by 47%. • Smaller flow separations near the exit plane of the optimal designs were observed. [ABSTRACT FROM AUTHOR]

Traffic congestion on motorways is a serious threat for the economic and social life of modern society as well as for the environment, which calls for drastic and radical solutions. Conventional traffic management measures, currently applied, are valuable, but face limitations. During the last decades, a variety of vehicle automation and communication systems (VACS) have been developed and deployed, and many more are expected to appear in the near future. These systems provide a novel basis for a new generation of traffic management, which exploits emerging vehicle automation functions and connectivity channels to enable sensible traffic flow improvements in terms of efficiency and safety. A number of innovative concepts, tools and results that open up new horizons for traffic management research and practice in presence of VACS have been produced recently in the frame of TRAMAN21, an ERC Advanced Grant. This paper presents a collection of novel problems as well as of related traffic flow modelling, estimation and control developments for motorway traffic that can be used in the evolving traffic environment with VACS. [ABSTRACT FROM AUTHOR]

Artificial neural networks (ANNs) have recently been used to predict the hydraulic head in well locations. In the present work, the particle swarm optimization (PSO) algorithm was used to train a feed-forward multi-layer ANN for the simulation of hydraulic head change at an observation well in the region of Agia, Chania, Greece. Three variants of the PSO algorithm were considered, the classic one with inertia weight improvement, PSO with time varying acceleration coefficients (PSO-TVAC) and global best PSO (GLBest-PSO). The best performance was achieved by GLBest-PSO when implemented using field data from the region of interest, providing improved training results compared to the back-propagation training algorithm. The trained ANN was subsequently used for mid-term prediction of the hydraulic head, as well as for the study of three climate change scenarios. Data time series were created using a stochastic weather generator, and the scenarios were examined for the period 2010–2020.Editor Z.W. Kundzewicz; Associate editor L. SeeCitation Tapoglou, E., Trichakis, I.C., Dokou, Z., Nikolos, I.K., and Karatzas, G.P., 2014. Groundwater-level forecasting under climate change scenarios using an artificial neural network trained with particle swarm optimization. Hydrological Sciences Journal, 59(6), 1225–1239. http://dx.doi.org/10.1080/02626667.2013.838005 [ABSTRACT FROM AUTHOR]