Bernhard Stoevesandt

Modelling complex terrain effects for wind farm layout optimization

The ow over four analytical hill geometries was calculated by CFD RANS simulations. For each hill, the results were converted into numerical models that transform arbitrary undisturbed in ow pro les by rescaling the e ect of the obstacle. The predictions of such models are compared to full CFD results, rst for atmospheric boundary layer ow, and then for a single turbine wake in the presence of an isolated hill. The implementation of the models into the wind farm modelling software flapFOAM is reported, advancing their inclusion into a fully modular wind farm layout optimization routine.

Development and application of a grid generation tool for aerodynamic simulations of wind turbines

This manuscript presents an effort towards the introduction of a new grid generation tool for computational fluid dynamics (CFD) simulations of wind turbines. The tool was developed and designed to considerably reduce the duration and complexity of the grid generation process and, moreover, to enable users to perform an efficient CFD simulation for a complete wind turbine rotor. Furthermore, the tool is fully automatized, thus enabling the user to refine the grid along the desired direction. For the purpose of validation, the quality of the generated grid was checked via a grid independence test. Finally, the grid generator tool was evaluated by means of a series of simulations of the NREL phase VI rotor using a three-dimensional CFD Reynolds averaged Navier–Stokes method. The capability of CFD simulations based on the generated grid in capturing aerodynamic performances (such as the power) and the detailed flow characteristics (e.g. the surface pressure distributions) were investigated under various conditions. The obtained results demonstrate the quality and reliability of the developed grid generation tool.

Simulation and Optimization of an Airfoil with Leading Edge Slat

A gradient-based optimization is used in order to improve the shape of a leading edge slat upstream of a DU 91-W2-250 airfoil. The simulations are performed by solving the Reynolds-Averaged Navier-Stokes equations (RANS) using the open source CFD code OpenFOAM. Gradients are computed via the adjoint approach, which is suitable to deal with many design parameters, but keeping the computational costs low. The implementation is verified by comparing the gradients from the adjoint method with gradients obtained by finite differences for a NACA 0012 airfoil. The simulations of the leading edge slat are validated against measurements from the acoustic wind tunnel of Oldenburg University at a Reynolds number of Re = 6 • 105. The shape of the slat is optimized using the adjoint approach resulting in a drag reduction of 2%. Although the optimization is done for Re = 6 • 105, the improvements also hold for a higher Reynolds number of Re = 7.9 • 106, which is more realistic at modern wind turbines.

2D Numerical Investigation of the Laminar and Turbulent Flow Over Different Airfoils Using OpenFOAM

The aim of this work is to assess the prediction capabilities of the turbulence models and the transition model kkl-ω available in OpenFOAM and to achieve a database of airfoil aerodynamical characteristics. The airfoils chosen for the simulations are FX 79-W- 15A and NACA 63-430, which are widely used in wind turbines. The numerically obtained lift and drag coefficients are compared with available experimental results. A quantitative and qualitative study is conducted to determine the influence of meshing strategies, computational time step together with interpolation and temporal schemes. Two Reynolds Averaged Navier- Stokes models (RANS models) are used, which are the k-ω SST model by Menter and the kkl-ω model (which involves transition modeling) by Walters and Davor.

Aerodynamic Simulation of the MEXICO Rotor

CFD (Computational Fluid Dynamics) simulations are a very promising method for predicting the aerodynamic behavior of wind turbines in an inexpensive and accurate way. One of the major drawbacks of this method is the lack of validated models. As a consequence, the reliability of numerical results is often difficult to assess. The MEXICO project aimed at solving this problem by providing the project partners with high quality measurements of a 4.5 meters rotor diameter wind turbine operating under controlled conditions. The large measurement data-set allows the validation of all kind of aerodynamic models. This work summarizes our efforts for validating a CFD model based on the open source software OpenFoam. Both steady- state and time-accurate simulations have been performed with the Spalart-Allmaras turbulence model for several operating conditions. In this paper we will concentrate on axisymmetric inflow for 3 different wind speeds. The numerical results are compared with pressure distributions from several blade sections and PIV-flow data from the near wake region. In general, a reasonable agreement between measurements the and our simulations exists. Some discrepancies, which require further research, are also discussed.

Optimization of Airfoils Using the Adjoint Approach and the Influence of Adjoint Turbulent Viscosity

The adjoint approach in gradient-based optimization combined with computational fluid dynamics is commonly applied in various engineering fields. In this work, the gradients are used for the design of a two-dimensional airfoil shape, where the aim is a change in lift and drag coefficient, respectively, to a given target value. The optimizations use the unconstrained quasi-Newton method with an approximation of the Hessian. The flow field is computed with a finite-volume solver where the continuous adjoint approach is implemented. A common assumption in this approach is the use of the same turbulent viscosity in the adjoint diffusion term as for the primal flow field. The effect of this so-called “frozen turbulence” assumption is compared to the results using adjoints to the Spalart–Allmaras turbulence model. The comparison is done at a Reynolds number of R e = 2 × 10 6 for two different airfoils at different angles of attack.

The impact of wake models on wind farm layout optimization

Results for nine gradient-based layout optimization runs of a wind farm with 25 turbines in flat terrain are presented, varying three different choices of the underlying wake model and three inflow scenarios. In all cases the AEP is maximised and the constraints are purely geometrical. A single inflow vector, a uniform wind rose and a realistic synthetic wind rose are studied, and the final layouts for the Jensen, the Ainslie and a CFD-based numerical wake model are compared. Prom this an estimate of the average variation of the turbine position due to the different wake models is obtained. All calculations were carried out with the in-house software flapFOAM.

The consideration of atmospheric stability within wind farm AEP calculations

The annual energy production of an existing wind farm including thermal stratification is calculated with two different methods and compared to the average of three years of SCADA data. The first method is based on steady state computational fluid dynamics simulations and the assumption of Reynolds-similarity at hub height. The second method is a wake modelling calculation, where a new stratification transformation model was imposed on the Jensen an Ainslie wake models. The inflow states for both approaches were obtained from one year WRF simulation data of the site. Although all models underestimate the mean wind speed and wake effects, the results from the phenomenological wake transformation are compatible with high-fidelity simulation results.

DES Study of Airfoil Lift Coefficient Sensitivity to Flow Turbulence

The aerodynamic behavior of wind turbines is strongly influenced by the turbulence level. However, the design of the rotor blades is usually based on experimental results of airfoils operating under laminar conditions. This leads to great uncertainties in the design process, which in turn make wind turbines less reliable and cost-effective. In this work a DES numerical study of the flow around a Wortmann FX 79-W-151A airfoil is performed for different turbulence intensities. Special attention is paid to the resulting loads. The simulations are then compared and validated with already available load measurements. The aim of this work is on one hand to gain a better understanding of the aerodynamics of an airfoil working in a turbulent flow. On the other hand, it is also of great interest to see up to which degree the numerical simulations are able to predict the force coefficients.

Multi-scale Analysis of Turbulence in CFD-Simulations

We performed a 3D DNS simulation using spectral/hpmethod on an fx79- w151a airfoil at a Reynolds number of Re=5000 at an angle of attack of α = 12° Due to a separating flow, an inhomogeneous turbulent field evolved in the wake above the trailing edge. Within this field time series of the flow properties have been gathered at selected points. The data of the time series at one point have been analyzed on the back ground of a multipoint correlation method. From statistics of velocity increments Kramers-Moyal coefficients have been estimated.