George Pechlivanoglou

Aeroelastic simulation of multi-MW wind turbines using a free vortex model coupled to a geometrically exact beam model

The current paper investigates the aeroelastic modelling of large, flexible multi- MW wind turbine blades. Most current performance prediction tools make use of the Blade Element Momentum (BEM) model, based upon a number of simplifying assumptions that hold only under steady conditions. This is why a lifting line free vortex wake (LLFVW) algorithm is used here to accurately resolve unsteady wind turbine aerodynamics. A coupling to the structural analysis tool BeamDyn, based on geometrically exact beam theory, allows for time-resolved aeroelastic simulations with highly deflected blades including bend-twist, coupling. Predictions of blade loading and deformation for rigid and flexible blades are analysed with reference to different aerodynamic and structural approaches. The emergency shutdown procedure is chosen as an examplary design load case causing large deflections to place emphasis on the influence of structural coupling and demonstrate the necessity of high fidelity structural models.

Modern methods for investigating the stability of a pitching floating platform wind turbine

The QBlade implementation of the Lifting Line Free Vortex Wake method(LLFVW) was tested in conditions analogous to floating platform motion. Comparisons against two independent test cases, using a variety of simulation methods show excellent agreement in thrust forces, rotor power, blade forces and rotor plane induction. Along with the many verifications already undertaken in literature, it seems that the code performs solidly even in these challenging cases. Further to this, the key steps are presented from a new formulation of the instantaneous aerodynamic thrust damping of a wind turbine rotor. A test case with harmonic platform motion and collective pitch is used to demonstrate how combining such tools can lead to better understanding of aeroelastic stability.

The Unsteady Aerodynamic Response of an Airfoil with Microtabs and it's Implications for Aerodynamic Damping

In this study, experimental airfoil data was taken in the Laminar Wind Tunnel of the Technical University of Berlin. A NACA0021 airfoil was tested with various sizes and positions of static microtabs while the foil underwent pitching oscillations. The resulting data was analysed using the novel Hilbert Transform method to yield the instantaneous aerodynamic damping. From this analysis it was possible to determine that in light and deep stall, microtabs increase the cycle averaged aerodynamic damping but in the case of light stall, the microtabs amplify the sub-cycle negative damping regions as well. This yields the conclusion that sub-cycle actuation of the microtab in light stall could further optimise the aerodynamic damping. The aforementioned effects get stronger with increasing tab size and as the microtabs are positioned further forward.

Three-Dimensional Aerodynamic Analysis of a Darrieus Wind Turbine Blade Using Computational Fluid Dynamics and Lifting Line Theory

Due to the rapid progress in high-performance computing and the availability of increasingly large computational resources, Navier-Stokes computational fluid dynamics (CFD) now offers a cost-effective, versatile and accurate means to improve the understanding of the unsteady aerodynamics of Darrieus wind turbines and deliver more efficient designs. In particular, the possibility of determining a fully resolved flow field past the blades by means of CFD offers the opportunity to both further understand the physics underlying the turbine fluid dynamics and to use this knowledge to validate lower-order models, which can have a wider diffusion in the wind energy sector, particularly for industrial use, in the light of their lower computational burden. In this context, highly spatially and temporally refined time-dependent three-dimensional Navier-Stokes simulations were carried out using more than 16,000 processor cores per simulation on an IBM BG/Q cluster in order to thoroughly investigate the three-dimensional unsteady aerodynamics of a single blade in Darrieus-like motion. Particular attention was payed to tip losses, dynamic stall, and blade/wake interaction. CFD results are compared with those obtained with an open-source code based on the Lifting Line Free Vortex Wake Model (LLFVW). At present, this approach is the most refined method among the “lower-fidelity” models and, as the wake is explicitly resolved in contrast to BEM-based methods, LLFVW analyses provide three-dimensional flow solutions. Extended comparisons between the two approaches are presented and a critical analysis is carried out to identify the benefits and drawbacks of the two approaches.

Advanced Medium-Order Modelling of a Wind Turbine Wake with a Vortex Particle Method Integrated within a Multilevel Code

The current paper describes an aerodynamic model for treatment of wind turbine wakes. For accurate treatment of the wake evolution for the near wake, along with interaction with local flow features, a model with low numerical diffusion has been chosen, a vortex particle method, which inherently allows treatment also of shearing effects and viscous diffusion. Treatment of blade loading is facilitated with the use of a lifting-line model. Details of correct specification of distributed and shed vortical elements in the blade wake are provided. Reduction of the computation cost has been achieved by implementing the model within a multilevel framework. In addition the model has been highly parallelised, so that relatively quick simulations at high fidelity can be achieved on the order of seconds. The ability of the model to produce results of comparable accuracy to CFD is demonstrated by comparison to the MEXICO test rotor.