2D CFD Modeling of H-Darrieus Wind Turbines Using a Transition Turbulence Model

AbstractIn the present paper, the authors describe the strategy to develop a 2D CFD model of H-Darrieus Wind Turbines. The model was implemented in ANSYS Fluent solver to predict wind turbines performance and optimize its geometry. As the RANS Turbulence Modeling plays a strategic role for the prediction of the flowfield around wind turbines, different Turbulence Models were tested. The results demonstrate the good capabilities of the Transition SST turbulence model compared to the classical fully turbulent models. The SST Transition model was calibrated modifying the local correlation parameters through a series of CFD tests on aerodynamic coefficients of wind turbines airfoils. The results of the tests were implemented in the 2D model of the wind turbine. The computational domain was structured with a rotating ring mesh and the unsteady solver was used to capture the dynamic stall phenomena and unsteady rotational effects. Both grid and time step were optimized to reach independent solutions. Particularly a high quality 2D mesh was obtained using the ANSYS Meshing tool while a Sliding Mesh Model was used to simulate rotation. Spatial discretization algorithm, interpolation scheme, pressure - velocity coupling and turbulence boundary condition were optimized also.The 2D CFD model was calibrated and validated comparing the numerical results with two different type of H-Darrieus experimental data, available in scientific literature. A good agreement between numerical and experimental data was found.The present work represents the basis to develop an accurate 3D CFD unsteady model and may be used to validate the simplest 1D models and support wind tunnel experiments

HAWT Design and Performance Evaluation: Improving the BEM Theory Mathematical Models☆

Abstract This paper presents an improved numerical code based on BEM theory, implemented to evaluate the performance of a HAWT (Horizontal Axis Wind Turbines). This numerical code is a 1-D code characterized by fast processing times and reliable results. The critical aspects of the codes based on the BEM theory are widely known in scientific literature. In this paper, the authors explain how to resolve these aspects. One of these is represented by the radial flow along the blades. Radial flow is a 3-D flow, but can be dealt with inside a 1-D code only using a mathematical expedient. This expedient was tested and validated for the Riso test turbine LM 8.2 (with the NACA 63 x -2xx airfoil series along the blades). Radial flow along the blades is taken into account, thus increasing the experimental C L distribution in the stalled aerodynamic region, based on CFD 3D results. The mathematical equation adopted to describe the C L distribution of the NACA 63 x -2xx airfoil is a fifth order logarithmic polynomial. With this numerical code, the mechanical power curve of the Riso test turbine has been calculated, and then compared with the experimental curve found in scientific literature

Evaluation of the Radial Flow Effects on Micro HAWTs through the Use of a Transition CFD 3D Model – part I: State of the Art and Numerical Model Review

Abstract The radial flow along a rotating blade is a fluid dynamic behavior that specifically affects the flow field of HAWTs. The physical effects of such flow on the rotor performance are not yet fully understood due to the complexity of the phenomenon and its high dependence on three dimensionality and Reynolds numbers. In the first part of this paper the authors reviewed the State of the Ar tof physics and modeling of radial flows. Some researchers have proposed empirical models to take into account the centrifugal pumping inside 1D codes. It was found in general, that the radial flow acts on the blades, increasing the forces and delaying the stall. Compared to a simple 2D condition, the aerodynamic coefficients are hence increased. Obviously, this phenomenon is heavily dependent on rotational speed as the centrifugal force increases with the square of the angular velocity and only linearly with the radial distance. So, due to higher rotational speed, the aerodynamics of mini and micro rotors is mostly influenced by the radial flow rather than the large rotors. The combined effects of both transitional and radial flow were evaluated in the present work using an accurate CFD 3D model as there was no specific literature in this particular field. This model, developed by the authors, was based on a RANS, four equations, transition turbulence model and it was calibrated and validated on a suitably designed micro rotor. The rotor was tested in the subsonic wind tunnel owned by the University of Catania. A review of the modeling and validation strategy is presented in the first part of this paper while the extrapolated data and the post-processing is presented in the second part, thus finding results of significant interest

Evaluation of the Radial Flow Effects on Micro HAWTs through the Use of a Transition CFD 3D Model – Part II: Post-processing and Comparison of the Results☆

Abstract The importance and the complexity of the phenomena related to the development of radial flows is demonstrated in the first part of this paper. In order to further study the radial flow effects and to extend the analysis to laminar and transitional flows, the authors used a CFD 3D model, validated in the wind tunnel owned by the University of Catania. In the second part of this paper, the authors describe the strategy which was used to post-process the simulation results. Furthermore, a comparison of the results was made. Several simulations were first carried out at various wind and rotational speeds. Angles of Attack and aerodynamic coefficients were evaluated on cylindrical surfaces at different radial stations using the ANSYS Fluent Solver and ANSYS Post. Local velocities and forces, related to the sectional airfoil, were obtained in each cylindrical surface along with pressure coefficient distributions. In this way, it was possible to demonstrate the close relationship between radial flows and the strong depressurization of the suction side of the blade. Moreover, the results proved that the increase of lift and drag coefficients is linked to rotational speed and Angle of Attack as well. The radial effects were found to be enforced by laminar and transitional flows related to low Reynolds numbers. This will affect both design and analysis of wind rotor performance, more so than that which was originally suggested by empirical stall delay models

Numerical and experimental analysis of micro HAWTs designed for wind tunnel applications

In this paper the authors describe a design and optimization process of micro HAWTs using a numerical and experimental approach. An in-house 1D BEM model was used to obtain a first geometrical draft. It allowed to quickly optimize blade geometry to maximize energy production as well. As these models are quite sensitive to airfoil coefficients, above all at low Reynolds numbers, an accurate 3D CFD model was developed to support and validate the 1D BEM design, analyzing and fixing the discrepancies between model output. The 3D CFD model was developed and optimized using ANSYS Fluent solver and a RANS transition turbulence model. This allowed to correctly reproduce the transition and stall phenomena that characterize the aerodynamic behavior of micro wind turbines, solving the issues related to low Reynolds flows. The procedure was completed, thus building two micro HAWTs with different scales, testing them in the subsonic wind tunnel of the University of Catania. Wind tunnel features, experimental set-up and testing procedures are presented in the paper. Through the comparison of numerical CFD and experimental test results, a good compatibility was found. This allowed the authors to analyze and compare numerical calculation results and verify blockage effects on the prototypes as well

Transition turbulence model calibration for wind turbine airfoil characterization through the use of a Micro-Genetic Algorithm

Abstract The aerodynamic characterization of airfoils is of crucial importance for the design and optimization of wind turbines. The present paper tries to provide an engineering methodology for the improvement of the accuracy and reliability of 2D airfoil computational fluid dynamics models, by coupling the ANSYS Fluent solver and a Micro-Genetic Algorithm. The modeling strategy provided includes meshing optimization, solver settings, comparison between different turbulence models and, mainly, the calibration of the local correlation parameters of the transition turbulence model by Menter, which was found to be the most accurate model for the simulation of transitional flows. Specifically, the Micro-Genetic Algorithm works by generating populations of the missing local correlation parameters. In doing so, it is possible to search for the minimization of the error in lift calculations. For each specific Reynolds number, the calibration was carried out only at the Angle of Attack where the lift drop occurs and the airfoil completely stalls. This new idea allowed for a relatively rapid and good calibration as demonstrated by the experimental–numerical comparisons presented in this paper. Only the experimental stall angle and the relative lift coefficient were, therefore, necessary for obtaining a good calibration. The calibration was made using the widely known S809 profile data. The correlation parameters, obtained as so, were subsequently used for testing on the NACA 0018 airfoil with satisfactory results. Therefore, the calibration obtained using the S809 airfoil data appeared to be reliable and may be used for the simulation of other airfoils. This can be done without the need for further wind tunnel experimental data or recalibrations. The proposed methodology will, therefore, be of essential help in obtaining accurate aerodynamic coefficients data. This will drastically improve the capabilities of the 1D design codes at low Reynolds numbers thanks to the possibility of generating accurate databases of 2D airfoil aerodynamic coefficients. The advantages of the proposed calibration will be helpful in the generation of more accurate 3D wind turbine models as well. The final objective of the paper was thus to obtain a fine and reliable calibration of the transition turbulence model by Menter. This was specifically made for an accurate prediction of the aerodynamic coefficients of any airfoil at low Reynolds numbers and for the improvements of 3D rotor models

Heat Exchange Numerical Modeling of a Submarine Pipeline for Crude Oil Transport

Abstract The present paper deals with a real issue of the Exxon-Mobil refinery in Augusta (Sicily). The crude oil, which is transported by oil tankers, is transferred through a submarine pipeline where it remains for a long time. In order to predict the transient temperature of the pipe, two numerical approaches were developed. The simplest one was a conductive model, based on the Finite Element Method, implemented by using the ANSYS Thermal FEM software for a first approximation solution. After having carried out an accurate grid resolution study and having evaluated the thermal error, a prediction of thermal profiles and heat fluxes was obtained. Thanks to the axisymmetrics of the physical problem, only a limited portion of the 3D pipe was modelled. The second approach was instead based on the use of a more accurate CFD Finite Volume Model, developed in ANSYS Fluent. In this case, in order to have reasonable calculation time and thanks to the aforementioned axisymetrics, the problem was carried out in 2D. Moreover, both grid and time step sensitivity was evaluated. Accurate buoyancy and turbulence models as well as viscosity and density temperature dependence models were used in order to obtain the most accurate physical modelling. The CFD model was developed basing on codes validated in the scientific literature. The comparison between FEM conductive and CFD results demonstrated the superior accuracy of the CFD, thanks to an accurate modelling of the internal convective motions

on the wind turbine wake mathematical modelling

Abstract The present paper deals with a study on the wind turbine wake mathematical modelling as well as experimental validation by means of wind tunnel experiments. In particular, different wind turbine wake's equations were implemented and results compared with experimental data. Therefore, an experimental setup was implemented in the wind tunnel test section with a small-scale wind turbine, while velocity deficit was measured. A design of experiment based on three parameters variation was defined: wind velocity, turbine rotational speed and distance from the wind turbine rotor. In the same experimental conditions simulations were carried out by means of three 1D equations. In particular, Jensen, Larsen and Frandsen equations were studied. Comparing theoretical and experimental results, it is evident that Larsen mathematical model is in good agreement with experimental data, while Jensen and Frandsen mathematical models are able to identify only mean and peak velocity deficit, respectively

PM10 Dispersion Modeling by Means of CFD 3D and Eulerian–Lagrangian Models: Analysis and Comparison with Experiments☆

Abstract This research deals with the analysis of the dispersion of PM10 by using fluid-dynamic simulation framework. Firstly, an experimental campaign was made in a wind tunnel. A cylindrical emitter of PM10 was characterized in terms of PM10 mass flow rate and outlet velocity. It was positioned in the wind tunnel chamber where several sensors were also placed downwind. The use of different sensor configurations allowed the evaluation of the PM10 concentrations in several locations. The experimental campaign was reproduced in ANSYS-Fluent, by recreating in Design-Model, a 3D geometries of the test case. Different calculation grids were tested in order to find the proper balance between computing time and accuracy. The CFD 3D model was based on the Eulerian approach for the continuous phase and Lagrangian approach for the dispersion phase setting the DPM for the evaluation and dispersion of particulate matters. The turbulence was solved by using a k-ɛ RANS approach and a quite advanced unsteady DES model. Several simulations were carried out by varying the flow inlet velocities in configurations with and without obstacles. The results obtained from the post-processing phase were then compared with the experimental campaign. With obstacles a PM concentration increment is observed at all imposed air velocity because of recirculation phenomena generated around the obstacles

Oscillating Water Column Wave Energy Converter by Means of Straight-bladed Darrieus Turbine☆

Abstract The present paper deals with a preliminary study on an Oscillating Water Column Wave Energy Converter (OWCWEC). The energy conversion is based on a straight-bladed Darrieus type wind turbine. The design of the turbine for maximum power coefficient is discussed. A physical laboratory scale OWC wave energy converter model was built to measure velocity field in the column. The air column was built using transparent materials to allow Particle Image Velocimetry measurements. Velocity field around air turbine rotor was measured by means of PIV. The measured velocities with and without the air turbine are used as inputs in the design procedure and to calibrate and test mathematical models. Moreover, design criteria were obtained using experimental and mathematical results