Ivan Dobrev

Hybrid rotor models for the numerical optimisation of wind turbine farms

Two hybrid models are developed to predict the horizontal axis wind turbine aerodynamic performance. A three-dimensional Navier-Stokes code is coupled to a blade element method (BEM). This combination is applied in the two cases of models; actuator disc and actuator line. So to validate these models, computations are carried out in the case of NREL S809 Phase II wind turbine. The results are compared to experimental data from the NREL database and to other numerical computations. The computed aerodynamic performance by the two models is found to be in good agreement with experimental and numerical reference.

Experimental study of yawed inflow around wind turbine rotor

In this article, we present an experimental study in a wind tunnel of a three-bladed, Rutland 503 model, horizontal axis yawed wind turbine. Power measurement and an exploration downstream wake of the turbine using particle image velocimetry measurements are performed. The variation of power coefficient as a function of rotational velocity is presented for different yaw angles. The results show a loss of power from the wind turbine when the yaw angle increases. The velocity field of the downstream wake of the rotor is presented in an azimuth plane, which passes through the symmetry axis of the rotor. The instantaneous velocity field is measured and recorded to allow for obtaining the averaged velocity field. The results also show variations in the wake downstream due to decelerating flow caused by the yawed turbine rotor. Analysis of this data shows that the active control of yaw angles could be an advantage to preserve the power from the wind turbine and that details near rotor wake are important for wake theories and to predict the performance of wind turbines as well.

Experimental and numerical study of flow around a wind turbine rotor

An improved model of an actuator surface is proposed, representing the flow around a wind turbine. This model was developed in conjunction with a Navier-Stokes solver using a blade element method for the calculation of power and wake development. Blades have been replaced with thin surfaces, and a boundary condition of ‘pressure discontinuity’ has been applied with rotor inflow and blade-section characteristics. The proposed improvement consists of applying tangential body forces along the chord, in addition to normal body forces resulting from pressure discontinuity along the blade cross-section. The proposed model has been validated for the flow around a horizontal-axis wind turbine. The results obtained from the proposed model are compared with the experimental results obtained from PIV-wind tunnel techniques. The comparison has displayed the necessity of the proposed model for accurate reproduction of the wake behind rotor. The rapidity of calculation, in comparison to full-geometry modelling, appears to be promising for wind farm simulations.

NUMERICAL STUDY ON THE INFLUENCE OF WAKE CONTRACTION ON THE COMPUTATIONAL ACCURACY AND RAPIDITY OF A MODEL OF A ROTOR IN HOVER

In the present article, the authors are performing a comparison study between two vortex models for the numerical computation of the static thrust of a model helicopter rotor in hover. The aim of this study is to access the rapidity and accuracy of the vortex models, comprised of a series of vortex rings and a single semi-infinite vortex cylinder. The first model has a cylindrical arrangement of the vortex elements, while the second model has a contracting arrangement of its vortex elements. Studies are performed for the optimal positioning and spacing of the vortex elements, in order to provide both rapid computation and good agreement with the experimental data, obtained from a wind tunnel test of the model rotor. The results of the study show a slight increase in the required computational time for the case of the contracting wake, while offering a significantly better accuracy than the cylindrical wake. Another interesting result from the numerical study is that in the case of the contracting wake model there is a reduction of the spacing (compression) between the vortex rings in the near wake, which results in the reduction of the overall length of the vortex system trailing downstream of the rotor.

INFLUENCE OF THE NUMBER OF VORTEX ELEMENTS ON THE STABILITY AND ACCURACY OF THE NUMERICAL SIMULATION FOR A ROTOR IN HOVER

Numerical simulations based on vortex models are very sensible from both the number of vortex elements and the initial conditions. The aim of this study is to evaluate the influence of the number of the vortex elements, modelling the wake, on the stability and accuracy of the numerical solution for the performance of a rotor in hover. For each computational case the number of the vortex elements varies from the number of blades per rotor and from the number of the emitted vortices per blade, which are taken into account. It was observed that larger series of vortex rings provide more accurate results, while a quicker convergence of the numerical solution is achieved with a smaller number of vortex elements. The use of time-stepping predictor-corrector scheme of second order, contributed for an increased stability of the simulation of the convection and propagation of the free-wake in the computational domain. The numerical results are compared against previously obtained experimental data for a model rotor.

3D unsteady flow analysis around a rotor blade of horizontal axis wind turbine-Rutland 503

The unsteady aerodynamics computational fluid dynamics simulations of the flow-field around a three-bladed Rutland 503 horizontal axis wind turbine rotor have been conducted in this paper to provide information needed to predict the three-dimensional aerodynamics behavior. In the CFD analysis, the variation of wind velocity between 9.3m/s and 15m/s is investigated. The Detached Eddy Simulation method is used with the turbulence model SST. A multiblock structured mesh is used for the discretization of the study domain. The solutions are obtained by using the Solver Fluent 6.3 which uses the method of finites volumes. The results of simulations obtained are compared with the experimental data [1] obtained in Wind Tunnel Laboratory LMA Arts and Metiers Paris-Tech in order to validate these results. Results of the numerical simulation in three dimensions of the unsteady field of flow around the blade are presented.