Today, Vortex Generators (VGs) are becoming an integral part of a Wind Turbine blade design. However, the challenges involved in the computation of the flow around VGs are yet to be dealt with in a ...satisfactory manner. A large number of VG models for Reynolds Averaged Navier Stokes (RANS) solvers has been proposed and, among them, the Bender–Anderson–Yagle (BAY) model (ASME Pap. FEDSM99-6919) is one of the most popular, due to its ease of use and relatively low requirements for user input. In the present paper a thorough investigation on the performance and application of the BAY model for aerodynamic VG flows is presented. A fully resolved RANS simulation is validated against experiments and then used as a benchmark for the BAY model simulations. A case relevant to wind turbines is examined, which deals with the flow past a wind turbine airfoil at Reynolds number 0.87e6. When the grid related errors are excluded, it is found that the generated vortices are weaker in the BAY model simulations than in the fully resolved computation. The latter finding is linked to an inherent deficiency of the model, which is first found in this study and which is explained in detail.
•Most detailed analysis of the BAY model to date.•First time the BAY model was applied to a Fully Resolved grid.•Model inherent deficiency highlighted.•Identification of areas and ways to improve the model.
A consistent two dimensional vortex type aerodynamic model for VAWTs is presented alongside with its validation against measured data. The flow solver assumes incompressible and inviscid conditions. ...It combines a source-vorticity panel formulation for the blades and a vortex blob representation of their wakes. By construction the model accounts for the effects of curvature of the relative to the blade inflow while blade vortex interactions are modelled by locally correcting the position of the wake vortices when they impinge on the blade. In order to get realistic loading estimations, lift and drag are corrected using a modified version of the ONERA model in which only the contribution of the separated generalised circulation is considered. Comparisons against wind tunnel tests on model rotors as well as full scale, field measurements on a 12kW VAWT indicate that the model predicts well the aerodynamic loads on the blades and the power output of the rotor.
Abstract
A reduced order model of a floating wind turbine with 22 degrees of freedom has been implemented and validated in comparison to the multibody, hydro-servo-aero-elastic code hGAST. The ...multi-blade Coleman transformation is applied and Floquet stability analysis is performed. This allows on one hand, to verify the actual Floquet implementation against the standard Coleman-based eigenvalue analysis in cases the transformed dynamic system has constant coefficients and on the other hand to analyze the effects of different anisotropies on stability. Mass imbalance of the rotor and yaw misalignment have been specifically addressed. In the mass imbalance scenario, the homogeneous results are not substantially altered while in the wind yaw scenario, there is slight change of the flapwise natural frequencies and of the tower lateral frequency as well as of the corresponding damping characteristics.
Since the introduction of the 3rd edition of the IEC Standard 61400-1, designers of wind turbines are required to apply statistical extrapolation techniques, to estimate the extreme (ultimate) load ...values corresponding to fifty-year return period. In the present paper, the certification procedure is assessed under the uncertainty of the material properties using simulated load time series of the NREL 5MW Reference Wind Turbine. The uncertainty of the material properties is introduced in the elastic properties of the composite blades by using input data from the OptiDAT composite material database. Comparison of the estimated extreme loads and deflections of the blades as well as maximum stresses, also in connection to the Tsai Wu failure criterion, is performed for different material sets. It is found that the variability of the material properties does not affect the estimated ultimate flapwise moments (difference <1.5%) but affects the maximum flapwise deflection (differences ∼8%). It is concluded that for the levels of variation considered in the composite material properties, the coefficient of variation of the extreme stresses and the Tsai Wu failure criterion are in the order of 2% and 8% respectively.
The present paper presents a comprehensive method for the structural design and verification of a floater, accounting for both ultimate limit state (ULS) and fatigue limit state (FLS). Three software ...are properly combined, a 3D Finite Element Method (FEM) solver, a coupled time domain hydro-servo-aero-elastic tool and a frequency domain hydrodynamic solver. The detailed analysis is performed in the 3D FEM solver through static load cases for ULS and frequency domain stochastic analysis for FLS, while the other tools provide the environmental excitation. The method is used for the detailed design and verification of the INNWIND 10MW tri-spar concrete floater that is assessed in terms of capacity ratios (for ULS) and damage ratios (for FLS), estimated at the most critical connecting positions.
This paper describes a new aeroelastic numerical model, which combines a Navier–Stokes CFD solver with an elastic model and two coupling schemes for the study of the aeroelastic behaviour of wind ...turbine blades undergoing classical flutter. The basic characteristics of the aerodynamic and elastic models are presented together with the coupling schemes. The present model is evaluated by comparing with previous numerical results and the corresponding linear analytical solutions. Consequently, a parametric study is carried out. Conclusions are drawn about the ability of the model to handle the aeroelastic behaviour of an airfoil and about the most appropriate coupling scheme in terms of predicting the modal damping and the flutter limiting point. The present study shows that the predictions are only slightly affected by the coupling or the space discretization scheme and mainly by the turbulence model used.
Gaia Data Release 1 Brown, A G A; Vallenari, A; Prusti, T ...
Astronomy and astrophysics (Berlin),
11/2016, Letnik:
595
Journal Article, Web Resource
Recenzirano
Odprti dostop
Context. At about 1000 days after the launch of Gaia we present the first Gaia data release, Gaia DR1, consisting of astrometry and photometry for over 1 billion sources brighter than magnitude 20.7. ...Aims. A summary of Gaia DR1 is presented along with illustrations of the scientific quality of the data, followed by a discussion of the limitations due to the preliminary nature of this release. Methods. The raw data collected by Gaia during the first 14 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into an astrometric and photometric catalogue. Results. Gaia DR1 consists of three components: a primary astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the Hipparcos and Tycho-2 catalogues - a realisation of the Tycho-Gaia Astrometric Solution (TGAS) - and a secondary astrometric data set containing the positions for an additional 1.1 billion sources. The second component is the photometric data set, consisting of mean G-band magnitudes for all sources. The G-band light curves and the characteristics of ~3000 Cepheid and RR Lyrae stars, observed at high cadence around the south ecliptic pole, form the third component. For the primary astrometric data set the typical uncertainty is about 0.3 mas for the positions and parallaxes, and about 1 mas yr super(-1) for the proper motions. A systematic component of ~0.3 mas should be added to the parallax uncertainties. For the subset of ~94000 Hipparcos stars in the primary data set, the proper motions are much more precise at about 0.06 mas yr super(-1). For the secondary astrometric data set, the typical uncertainty of the positions is ~10 mas. The median uncertainties on the mean G-band magnitudes range from the mmag level to ~0.03 mag over the magnitude range 5 to 20.7. Conclusions. Gaia DR1 is an important milestone ahead of the next Gaia data release, which will feature five-parameter astrometry for all sources. Extensive validation shows that Gaia DR1 represents a major advance in the mapping of the heavens and the availability of basic stellar data that underpin observational astrophysics. Nevertheless, the very preliminary nature of this first Gaia data release does lead to a number of important limitations to the data quality which should be carefully considered before drawing conclusions from the data.
The paper presents a general three dimensional hydro-elastic tool for the analysis of different types of fishing nets and aquaculture facilities. Flexible net strands are modeled by non-linear truss ...elements having two nodes. Hydrodynamic loads due to relative motion of the net with the surrounding fluid are computed using the Morison equation. The coupled hydrodynamic–elastodynamic equations are solved using finite element (FE) approximations. Furthermore, experimental data are presented for the drag resistance of a purse seine net, commonly used as fishing tool in the Mediterranean sea. The measurements were conducted in the towing tank of NTUA on a sample of a net. The net was tested in three configurations: vertical, horizontal and inclined at 45°. The derived drag coefficients are compared to predictions of the FEM developed model. The vertical submergence behavior of the seine in calm water is also examined, both experimentally and theoretically. Moreover, the shooting phase of the purse-seine fishing is simulated with the aim to investigate the diving behavior of the net. The flow shading effect of neighboring strands is identified as a critical parameter for the consistent predictions of the diving behavior.
► We developed a general three dimensional hydro-elastic tool for the analysis of fishing nets. ► The coupled hydrodynamic–elastodynamic equations are solved using the finite element method. ► We conducted experiments for the drag resistance of a purse seine net. ► The derived drag coefficients are compared to predictions of the FEM developed model. ► The flow shadow effect of neighboring strands is identified as a critical parameter.
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