Marcelo Reggio

A finite-volume method for the Euler equations on arbitrary Lagrangian-Eulerian grids

Abstract This study presents a finite-volume method for the solution of 2-D/axisymmetric Euler equations using triangular moving grids. The flow simulation is carried out using Roes approximate Riemann solver. The importance of the implicit treatment of the space conversation laws, based on geometric analysis, is emphasized. The procedure for reconstructing Roes method for moving meshes is described and validated.

A staggered control volume scheme for unstructured triangular grids

SUMMARY The purpose of this work is to introduce and validate a new staggered control volume method for the simulation of 2D=axisymmetric incompressible flows. The present study introduces a numerical procedure for solving the Navier‐Stokes equations using the primitive variable formulation. The proposed method is an extension of the staggered grid methodology to unstructured triangular meshes for a control volume approach which features ease of handling of irregularly shaped domains. Two alternative elements are studied: transported scalars are stored either at the sides of an element or at its vertices, while the pressure is always stored at the centre of an element. Two interpolation functions were investigated for the integration of the momentum equations: a skewed mass-weighted upwind function and a flow-oriented exponential shape function. The momentum equations are solved over the covolume of a side or of a vertex and the pressure‐velocity coupling makes use of a localized linear reconstruction of the discontinuous pressure field surrounding an element in order to obtain the pressure gradient terms. The pressure equation is obtained through a discretization of the continuity equation which uses the triangular element itself as the control volume. The method is applied to the simulation of the following test cases: backward-facing step flow, flow over a two-dimensional obstacle and flow in a pipe with sudden contraction of cross-sectional area. All numerical investigations are compared with experimental data from the literature. A grid convergence and error analysis study is also carried out for flow in a driven cavity. Results compared favourably with experimental data and so the new control volume scheme is deemed well suited for the prediction of incompressible flows in complex geometries. # 1997 John Wiley & Sons, Ltd.

Progress and investigation on lattice Boltzmann modeling of multiple immiscible fluids or components with variable density and viscosity ratios

Lattice Boltzmann models for simulating multiphase flows are relatively new, and much work remains to be done to demonstrate their ability to solve fundamental test cases before they are considered for engineering problems. From this perspective, a hydrodynamic lattice Boltzmann model for simulating immiscible multiphase flows with high density and high viscosity ratios, up to O(1000)O(1000) and O(100)O(100) respectively, is presented and validated against analytical solutions. The method is based on a two phase flow model with operators extended to handle N immiscible fluids. The current approach is O(N)O(N) in computational complexity for the number of different gradient approximations. This is a major improvement, considering the O(N2)O(N2) complexity found in most works. A sequence of systematic and essential tests have been conducted to establish milestones that need to be met by the proposed approach (as well as by other methods). First, the method is validated qualitatively by demonstrating its ability to address the spinodal decomposition of immiscible fluids. Second, the model is quantitatively verified for the case of multilayered planar interfaces. Third, the multiphase Laplace law is studied for the case of three fluids. Fourth, a quality index is developed for the three-phase Laplace–Young’s law, which concerns the position of the interfaces between the fluids resulting from the different surface tensions. The current model is compatible with the analytical solution, and is shown to be first order accurate in terms of this quality index. Finally, the multilayered Couette’s flow is studied. In this study, numerical results can recover the analytical solutions for all the selected test cases, as long as unit density ratios are considered. For high density and high viscosity ratios, the analytical solution is recovered for all tests, except that of the multilayered Couette’s flow. Numerical results and a discussion are presented for this unsuccessful test case. It is believed that other LB models may have the same problem in addressing the simulation of multiphase flows with variable density ratios.

Modelling and simulation of nozzle ablation in high-voltage circuit-breakers

In the present paper, a new approach to modelling the ablation phenomenon using computational fluid dynamics (CFD) tools coupled with a physical radiation model is proposed, implemented and validated for application in circuit-breaker arc studies. The present work shows that the principal characteristics of ablation-dominated arcs can be simulated effectively using CFD tools if the radiation incident at the wall boundaries can be predicted accurately.

Assessment Study of K-ɛ Turbulence Models and Near-Wall Modeling for Steady State Swirling Flow Analysis in Draft Tube Using Fluent

Abstract The accuracy of k-ε turbulence models for the swirling flow in the Turbine 99 draft tube is the subject of this work. The relation of the first-order upwind, second-order upwind, third-order upwind (QUICK) and Power Law schemes used with these models has been studied. As the turbulent flows are significantly affected by the presence of walls, the wall function and the near-wall models were tested for modeling the near-wall region. Two different grid concentrations near the wall y+1 and y+50 were used to study the flow behavior for case 1 of Turbine 99 Workshop III. Discussion is based on graphical results and by comparing numerical simulations and experiments in operational mode T (close to best efficiency). The results of this study indicate a very good representation of the flow at different cross sections by the RNG turbulence model but a poor level of convergence.

NUMERICAL SOLUTION OF REACTIVE LAMINAR FLOW BY A CONTROL-VOLUME BASED FINITE-ELEMENT METHOD AND THE VORTICITY-STREAMFUNCTION FORMULATION

A numerical procedure has been developed to solve laminar reactive flows. It makes use of a special interpolation Junction defined in a local flow-oriented coordinate system. Based on a control volume finite-element and an unstructured triangular domain discretization, it can handle irregularly shaped domains. The velocity-pressure coupling is addressed via the vorticity-streamfunction formulation and the disc retized flow equations are solved in a coupled way. The procedure has been successfully applied to simulate reactive and nonre-active flows. It was found to be accurate, stable, and fast. Comparison with experiments shows that it correctly stimulates the physics of the flows.

Numerical Study of Flow Around Iced Wind Turbine Airfoil

Abstract This investigation analyzes the impact of ice accretion on the aerodynamic coefficients of a wind turbine airfoil. Three blade sections located at different radial positions were analyzed. Numerical simulations were conducted over a two-dimensional clean and ice-accreted NACA 63–415 airfoil at various angles of attack. The results for pressure, lift, and drag coefficients were inspected at an angle of 13° for which experimental data were available. The streamlines around the clean and iced airfoil were also inspected, in order to evaluate the ice impact on lift and drag. The simulations were carried out using the commercial package FLUENT, and turbulence was addressed with the SST k - ω) model.

Time-accurate local time stepping method based on flux updating

measured profiles of velocity components U and V, the turbulent kinetic energy k, and the dissipation rate of the turbulent energy £, which is deduced from the equilibrium relation 8 = (0.3&)//. The values of the mixing length / are calculated from the data using its definition in terms of the mean-velocity gradient and the shear stress. The boundary conditions are that U at the wake edges is equal to the measured edge velocity Ue measured in the experiment, k and e satisfy the zero-gradient conditions, Ue dk/dx = e and Ue d£/dx = Ce2e/&, where Ce2 is one of the model constants. The configuration of the flow for which the calculation is made is depicted in Fig. 1. The wake-generating model is a flexible plate whose shape is varied to produce different pressure gradients on the upper and lower sides of the plate controlling the properties of the initial wake. The characteristics of the test flow at the trailing edge including the boundary layer thickness, the friction coefficient Cy, and the momentum thickness Reynolds number Re§ are shown in Table 1. The details of the experiments and the results are given in Nakayama and Kreplin. The step size in the calculation is initially taken about 0.5 x 10~ times the momentum thickness 0, at the trailing edge and is doubled at every 50th step until the step size of 0.26, is reached. About 1000 integration steps are needed to cover a distance of about 400, from the trailing edge, which is considered a near wake region.

Assessment of turbulence models for flow simulation around a wind turbine airfoil

This investigation focuses on the application of the computational fluid dynamics tool FLUENT to the study of flows over the NACA 63-415 airfoil at various angles of attack. With the aim of selecting the most suitable turbulence model to simulate flow around ice-accreted airfoils, this work concentrates on assessing the prediction capabilities of various turbulence models on clean airfoils at the large angles of attack that cause highly separated flows to occur. The study was undertaken by conducting simulations with the one-equation Spalart-Allmaras (SA) model, the two-equation RNG k-e and SST k-ω models, and the Reynolds stress model (RSM). Domain discretization was carried out using general quadrilateral grids generated with GAMBIT, the FLUENT preprocessing tool. Comparisons were made with available experimental data.

CFD Analysis of Ventilation Flow for a Scale Model Hydro-Generator

In 2006, the first Computational Fluid Dynamics (CFD) simulations of the ventilation of specific hydro-generator components were performed at the Hydro-Quebec Research Institute (IREQ) and lately the entire ventilation circuit is being investigated. Due to the complexity of flow calculations, a validation process is necessary and for this reason a 1:4 scale model of a hydro-generator has been built at IREQ to get experimental data by means of particle image velocimetry (PIV). This paper presents 2D and 3D simulation results for the scale model obtained with a commercial CFD code and addresses the challenges associated with the application of CFD to hydro-generators. In particular, the effect of rotor-stator interface (RSI) types and configuration is analyzed to determine the approach that best suits this application. Two-dimensional calculations show that the steady state multiple frames of reference (MFR) solution is highly sensitive to the type (frozen rotor (FR) vs. mixing plane (MP)) and location of the RSI. A parametric study is performed where each interface configuration is compared to the transient case results. The MFR-FR interface model produces results that may vary significantly depending on the relative rotor position and the radial location of the RSI in the air gap. The MFR-MP interface model appears to be more coherent with reference values obtained from a transient case, since the radial velocity profiles in the stator are similar. Furthermore with an appropriate radial positioning of the interface, the windage losses are within 20%. Simulations of the complete 3D ventilation circuit revealed a maximum variation of 10% in both total ventilation flow rate and total windage losses, between the RSI configurations studied. However, the relative flow distributions, normalized with respect to the total flow rate, are unaffected by changes in RSI configuration. This paper focuses mainly on sensitivity studies to numerical settings, but this comparison still requires experimental validation before any final conclusions can be made.Copyright