Papers by Panagiotis Chaviaropoulos
Notes on Numerical Fluid Mechanics (NNFM), 1997
Volumes 1 to 29, 45 are out of print. The addresses of the Editors and further titles of the seri... more Volumes 1 to 29, 45 are out of print. The addresses of the Editors and further titles of the series are listed at the end of the book.
Journal of Physics: Conference Series
Modern wind turbines are prone to Vortex Induced Vibrations (VIV). In the present work, an engine... more Modern wind turbines are prone to Vortex Induced Vibrations (VIV). In the present work, an engineering semi-empirical framework is proposed that assesses VIV aero-elastic instabilities of wind turbine configurations. The procedure employs engineering tools relying on airfoil polars. It uses the state-of-the-art aero-elastic tool hGAST along with the EUROCODE VIV framework for steel structures extended to wind turbine configurations. The aero-elastic tool provides the missing modal input data (i.e. modal frequencies, total structural plus aerodynamic modal damping and modeshapes) to evaluate the semi-analytical expressions of the displacement and load amplitudes. Numerical results for single- and two-bladed configurations of the NREL 5MW Reference Wind Turbine (RWT) during assembly are presented, assessing turbine loads under the most unfavourable VIV scenarios examined.
Fluids
Design modifications of the blade inner structure, targeted at reducing design-driving extreme lo... more Design modifications of the blade inner structure, targeted at reducing design-driving extreme loads due to storm conditions, are assessed in the present paper. Under survival wind speeds, the lack of sufficient aerodynamic damping in the edgewise direction is responsible for excessive stall-induced vibrations that usually drive wind turbine blade design loads. The modifications considered in the work are (i) a non-symmetric increase in the thickness of the uniaxial and tri-axial material on the suction and pressure side of the blade sections, (ii) a shift in the spar caps in opposite directions and (iii) the ply-angle re-orientation of the laminates on the spar caps. The first two design interventions aim at increasing the damping of the low-damped edgewise modes in the idling rotor, while the third aims at reducing the fatigue and ultimate loads in normal operation. The design parameters in the problem are determined on the basis of a multidisciplinary optimization (MDAO) process,...
Notes on Numerical Fluid Mechanics (NNFM), 1998
ABSTRACT [The articles of this volume will not be reviewed individually.] This volume contains th... more ABSTRACT [The articles of this volume will not be reviewed individually.] This volume contains the contributions to two workshops, one held on problems of optimizing aerodynamic shapes, carried out with numerical tools, and the other on problems of parallelization of numerical solutions of the Navier-Stokes equations for compressible viscous flows, and the implementation of those solutions on parallel machines. The first part of the book deals with various aspects of aerodynamic optimization, such as wing-pylon-nacelle configurations, transonic wing sections, arrangement of ribblets and other design problems. Several test cases are discussed. The various contributions were originated in industry, research institutions, and universities. The second part of the book is devoted to flow computations on parallel machines. Reported are flow computations for a NACA0012 profile with a parallelized solution of the Navier-Stokes equations, viscous flow computations on structured and unstructured grids, implemented on parallel machines, and computations for test cases. The workshops were organized in the frame of the European Computational Aerodynamics Research Project in the European Industrial and Materials Technology Programme. Such workshops are definitely useful and should be fostered. What remains not clear from the book, is how the contributors were selected and how the topics we were chosen. Otherwise, the book contains a lot of information and hints for future work.Reviewer: E.Krause (Aachen)
Journal of Wind Engineering and Industrial Aerodynamics
Journal of Physics: Conference Series, 2014
The concept of a large (~20MW) multi-rotor wind turbine intended for offshore installations is an... more The concept of a large (~20MW) multi-rotor wind turbine intended for offshore installations is analysed with respect to its aerodynamic performance. The effect of closely clustering rotors on a single actuator disk is estimated using two different modelling approaches: a CFD solver in which the rotors are simulated as distinct actuator disks and a vortex based solver in which the blade geometry is exactly considered. In the present work, a system of 7 rotors is simulated with a centre to centre spacing of 1.05D. At nominal conditions (tip speed ratio=9) both models predict an increase in power of ~3% alongside with an increase in thrust of ~1.5%. The analysis of the flow field indicates that in the 7 rotor system the individual wakes merge into one wake at ~2D and that flow recovery starts at approximately the same downstream distance as in the single rotor case. As regards the dynamic implications of the close spacing of the rotors it was found that there is an increase in the loading amplitude ranging from 0.30-2.13% at blade level in rated conditions.
Journal of Wind Engineering and Industrial Aerodynamics, 2012
A 3D Reynolds-averaged Navier-Stokes solver, CRES-flowNS, is used in this paper for the flow simu... more A 3D Reynolds-averaged Navier-Stokes solver, CRES-flowNS, is used in this paper for the flow simulation of four flow cases established during the Bolund hill experiment. CRES-flowNS, which has been developed at CRES, solves the steady state, incompressible Navier-Stokes equations on a curvilinear grid, employing k-o turbulence closure. By generating alternative meshes, grid independent results are achieved that are compared to measured wind speed and turbulence characteristics in the four flow cases. From a methodological point of view, the scope of the present paper is to illustrate the appropriate numerical steps in order to assure reliable atmospheric flow predictions in complex terrain.
Journal of Propulsion and Power, 1994
ABSTRACT The purpose of this article is to present an inverse subsonic inviscid method for the de... more ABSTRACT The purpose of this article is to present an inverse subsonic inviscid method for the design of axisymmetric channels, with rotational flow. The rotational character of the flow is due to prescribed total enthalpy, entropy, and/or swirl gradients along the inlet of the channel. The method is based on a potential function/stream function formulation. The Clebsch transformation is employed to decompose the meridional velocity vector into a potential and a rotational part. The rotational part is shown to be proportional to the total enthalpy gradient, the coefficient of proportionality being the drift function. A body-fitted coordinate transformation is employed to map the sought boundaries on the (phi, psi) space. The governing equation for the magnitude of the meridional velocity component is derived by treating the inverse problem on the (phi, psi) space as a purely geometric one, employing differential geometry principles. The (meridional) velocity equation is coupled in a nonlinear manner with a transport equation for the drift function and with the geometry via the radial coordinate. The integration of the governing equations is performed on an auxiliary computational grid using a simple iterative scheme. The geometry, in particular, is determined by integrating Frenet equations along the grid lines. The present design method has been applied successfully to the ''reproduction'' of two ''real-life'' geometries concerning the annular duct of a two-stage axial compressor as well as a radial one.
Computer Methods in Applied Mechanics and Engineering, 1993
ABSTRACT An irrotational inviscid compressible inverse design method for two-dimensional airfoil ... more ABSTRACT An irrotational inviscid compressible inverse design method for two-dimensional airfoil profiles is described. The potential (φ) and streamfunction (ψ) are used as the independent natural coordinates. The physical space on which the boundaries of the airfoil are sought, is mapped onto the (φ, ψ) space via a body-fitted coordinate transformation. A novel procedure based on differential geometry arguments is employed to derive the governing equations for the inverse problem, by requiring the curvature of the flat 2-D Euclidean space to be zero. An auxiliary coordinate transformation permits the definition of C-type computational grids on the (φ, ψ) plane resulting in a more accurate description of the leading edge region. Geometry is determined by integrating Frenet equations along the grid lines. A two-parameter iterative scheme has been incorporated in the design procedure in order to assure closure of the trailing edge. To validate the method, inverse calculation results are compared with direct, ‘reproduction’, calculation results. The design procedure of a new airfoil shape is also presented.
Torque, 2010
Use of computational fluid dynamic (CFD) methods to predict the power production from wind entire... more Use of computational fluid dynamic (CFD) methods to predict the power production from wind entire wind farms in flat and complex terrain is presented in this paper. Two full 3D Navier-Stokes solvers for incompressible flow are employed that incorporate the k-ε and k-ω turbulence models respectively. The wind turbines (W/Ts) are modelled as momentum absorbers by means of their thrust coefficient using the actuator disk approach. The WT thrust is estimated using the wind speed one diameter upstream of the rotor at hub height. An alternative method that employs an induction-factor based concept is also tested. This method features the advantage of not utilizing the wind speed at a specific distance from the rotor disk, which is a doubtful approximation when a W/T is located in the wake of another and/or the terrain is complex. To account for the underestimation of the near wake deficit, a correction is introduced to the turbulence model. The turbulence time scale is bounded using the general "realizability" constraint for the turbulent velocities. Application is made on two wind farms, a five-machine one located in flat terrain and another 43-machine one located in complex terrain. In the flat terrain case, the combination of the induction factor method along with the turbulence correction provides satisfactory results. In the complex terrain case, there are some significant discrepancies with the measurements, which are discussed. In this case, the induction factor method does not provide satisfactory results.
AIAA Journal, 1993
ABSTRACT The development of a rotational inviscid compressible inverse design method for two-dime... more ABSTRACT The development of a rotational inviscid compressible inverse design method for two-dimensional internal flow configurations is described. Rotationality is due to an incoming entropy gradient, while total enthalpy is considered to be constant throughout the flowfield. The method is based on the potential function-streamfunction formulation. A novel procedure based on differential geometry arguments is employed to derive the governing equation for velocity by requiring the curvature of the two-dimensional Euclidean space to be zero. The velocity equation solved in conjunction with a transport equation for a thermal drift function provide the flowfield without any geometry feedback. An auxiliary orthogonal computational grid adapted to the solution is employed. Geometry is determined by integrating Frenet equations of the grid lines. Inverse calculation results are compared with results of direct reproduction calculations.
Power deficit and efficiency polar in modeling large offshore wind farms are estimated using the ... more Power deficit and efficiency polar in modeling large offshore wind farms are estimated using the CRES-flowNS RANS solver and the amended GCL engineering model. The comparison with measurements for the Horns Rev and Lillgrund wind farms indicates that predictions significantly overestimate the power deficit when the wind sector is narrow (±2.5°). As the size of the sector increases the agreement between predictions and measurements becomes better and for the wide sector of ±15° it can be considered quite satisfactory, even in the cases of incomplete wind turbine rows. The systematic over-prediction for narrow sectors raises the question if the uncertainty in the measurement of the wind direction produces artificially low power losses in the wake center. The efficiency polar of the wind farms for the whole range of wind directions (0-360°) is well estimated using the amended GCL model calibrated with the CRES-flowNS predictions in specific wind directions. Calibration is made in the s...
Wind turbines are aerodynamically driven machines. The energy produced is chiefly associated with... more Wind turbines are aerodynamically driven machines. The energy produced is chiefly associated with the efficiency of their rotors to convert the kinetic energy of wind to mechanical power. Still, mechanical power must be transformed to electrical through a drivetrain, and the whole system should be kept in place through a support structure. The selection of the proper architecture and the design of these three main subsystems (rotor, drive train, support structure) allow for much freedom, but at the end of the day, it is driven by economical aspects. The optimal turbine design for a given site (onshore/offshore) with known external conditions is the one which can produce electricity in the lowest possible cost, usually expressed through a metric called levelized cost of electricity (LCoE). We acknowledge the fact that since a few years, wind farms are subjected to variable market price mechanisms, and the value of the produced electricity is depending on the market specifics, possibly leading to a different optimum than the one suggested by minimum LCoE. As, however, LCoE remains a pure metric for technology assessment, the goal of this chapter is to make the connection between LCoE and technological selections and design aspects, with focus on rotor aerodynamics. Mastering this connection allows for better understanding the critical areas where the emphasis should be placed for improving cost-efficiency of wind turbine designs.
This work is in the context of the FP7 Innwind.EU Project whose objective is the high performance... more This work is in the context of the FP7 Innwind.EU Project whose objective is the high performance innovative design of beyond state-of-the-art 10-20 MW offshore wind turbines. The assessment of innovation necessitates a framework where different designs can be compared against a reference on the basis of key performance indicators (KPIs). Following the European Wind Industrial Initiative the Levelized Cost of Electricity (LCOE) and its driving components are investigated, while quantifying the sensitivity of LCOE to its constituent factors. Methods whereby innovation in design can reduce component cost and lower LCOE are investigated. Targets are set to the LCOE by associating with specific technologies and high Customer Net Present Value (NPV).
Journal of Physics: Conference Series
The present paper investigates the potential to reduce the mass of the blade of the 10MW DTU Refe... more The present paper investigates the potential to reduce the mass of the blade of the 10MW DTU Reference Wind Turbine through build-in, material bend-twist coupling (BTC). It is materialized by introducing an offset angle on the plies of the uni-directional material over the spar caps of the blade. Optimum BTC designs are obtained on the basis of an integrated optimization framework combining an aeroelastic solver for the calculation of the structural loads of the blade and a cross-sectional tool that provides beam-like structural properties of the blade and stresses distributions. The derived designs are verified based on a subset of representative fatigue and ultimate design loads cases of IEC 61400-1. Reduction of the combined bending moment at the root of the blade by 5% and reduction of the blade mass by 10% is achieved with a hybrid model consisting of three span-wise segments having different constant ply offset angles.
Journal of Physics: Conference Series
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Papers by Panagiotis Chaviaropoulos