Simulations of an Offshore Wind Farm Using Large- Eddy Simulation and a Torque-Controlled Actuator Disc Model (original) (raw)

Comparison of Large Eddy Simulations against measurements from the Lillgrund offshore wind farm

2022

Numerical simulation tools such as Large Eddy Simulations (LES) have been extensively used in recent years to simulate and analyze turbine-wake interactions within large wind farms. However, to ensure the reliability of the performance and accuracy of such numerical solvers, validation against field measurements is essential. To this end, a measurement campaign is carried out at the Lillgrund offshore wind farm to gather data for the validation of an in-house LES solver. Flow field data is collected from the farm using three long-range WindScanners, along with turbine performance and load measurements from individual turbines. Turbulent inflow conditions are reconstructed from an existing precursor database using a scaling-andshifting approach, proposed so that the generated inflow statistics match the measurements. Thus, 5 different simulation cases are setup, corresponding to 5 different inflow conditions at the Lillgrund wind farm. Operation of the 48 Siemens 2.3 MW turbines from the Lillgrund wind farm is parameterized in the flow domain using an Aeroelastic Actuator Sector Model (AASM). Time-series turbine performance metrics from the simulated cases are compared against field measurements to evaluate the accuracy of the optimization framework, turbine model and flow solver. In general, results from the numerical solver show good comparison in terms of power production, turbine loading and wake recovery. Nevertheless, larger errors for a few turbines in the wind farm across the simulated cases reveal the need for an improved controller implementation, and possibly a finer simulation grid for capturing wake turbulence. 1 Introduction Recent years have seen the emergence of wind-farm simulation tools that cover the whole chain from flow-coupled aeroelastic models to power-grid models. The complexity of these models ranges from analytical tools, which simplify wake expansion and merging, to complex Computational Fluid Dynamics (CFD) solvers which represent the turbines and their influence on the surrounding flow field. Amongst all these numerical tools, Large Eddy Simulations (LES) feature detailed representation of the turbulent flow in and around large wind farms (Munters and Meyers, 2018; Lin and Porté-Agel, 2019). This increased detail in simulating the physics governing wind-farm flows has facilitated the study of wind-farm aerodynamics and enabled the analysis of phenomena like turbine-wake interactions, gusts, atmospheric stratification and the effect of wind farms on local wind climate (Mehta et al., 2014). Additionally, LES has also been used to investigate and develop coordinated wind-farm control 1

Wind Turbine Modeling for Computational Fluid Dynamics: December 2010 - December 2012

2013

With the shortage of fossil fuels and the increase of environmental awareness, wind energy is becoming more impor tant than ever. As the market for wind energy grows, wind turbines and wind farms are becoming larger. But, there is still more to learn about this technology. For example, current utility-scale turbines extend a significant distance into the atmospheric boundary layer. Therefore, the interaction between the atmospheric boundary layer and the turbines and their wakes needs to be better understood. The turbulent wakes of upstream turbines affect the flow field of the turbines behind them, thus decreasing power production and increasing mechanical loading. With greater knowledge of this type of flow, wind farm developers could plan better-performing, less maintenance-intensive wind farms. Sim ulating this flow using computational fluid dynamics (CFD) is one important way to gain a better understanding of wind farm flows. In this study, we compare the performance of actuator disk and actuator line models in producing wind turbine wakes and the wake-turbine interaction between multiple turbines. We also examine parameters that affect the performance of these models, such as grid resolution, the use of a tip-loss correction, and the way in which the turbine force is projected onto the flow field. We see that as the grid is coarsened, the predicted power decreases. As the width of the Gaussian body force projection function is increased, the predicted power is increased. The ac tuator disk and actuator line models produce similar wake profiles and predict power within 1% of one another when subject to uniform inflow. The actuator line model is able to capture flow structures near the blades such as root and tip vortices, which the actuator disk does not capture, but in the far wake, they look similar. The actuator line model was validated using the wind tunnel experiment conducted at the Norwegian University of Science and Technol ogy, Trondheim. Agreement between the model and the experiments was obtained, with the maximum percentage difference in power coefficients of 25% and 40% for thrust coefficient. The actuator line and actuator disk models were compared when running large-scale wind farm simulations. Normalized power was similar for both models, but dimensional power differed from 1 to 17%. The actuator disk model was able to run approximately three times faster, though. This work shows that actuator models for wind turbine aerodynamics are a viable alternative to using full blade-resolving simulations. However, care must be taken to use the proper grid resolution and force projection to the CFD grid to obtain accurate predictions of aerodynamic forces and, hence, power. More work is needed to determine the best method of body force projection onto the CFD grid.

Modelling wind turbine wakes for wind farms

The simulation of the wakes behind wind turbines is important in predicting energy yields in wind farms, and so plays a role in planning the layout of these farms. As both wind turbines and farms increase in size, wind farm modellers have faced challenges as previously-held assumptions and parameterisations become inadequate – requiring more detailed, less parameterised methods such as those available through computational fluid dynamics. In this article the authors chart the progress of wind turbine wake modelling from analytical methods towards computational fluid dynamics, discussing approaches such as Reynolds-averaged Navier-Stokes and Large Eddy Simulation.

Modeling Wind Turbine Wakes for Wind Farms

Lehr/Alternative, 2016

The simulation of the wakes behind wind turbines is important in predicting energy yields in wind farms, and so plays a role in planning the layout of these farms. As both wind turbines and farms increase in size, wind farm modellers have faced challenges as previously-held assumptions and parameterisations become inadequate-requiring more detailed, less parameterised methods such as those available through computational fluid dynamics. In this article the authors chart the progress of wind turbine wake modelling from analytical methods towards computational fluid dynamics, discussing approaches such as Reynolds-averaged Navier-Stokes and Large Eddy Simulation.

Numerical Computations of Wind Turbine Wakes

Wind Energy, 2007

Numerical simulations of the Navier-Stokes equations are performed to achieve a better understanding of the behaviour of wakes generated by wind turbines. The simulations are performed by combining the in-house developed computer code EllipSys3D with the actuator line and disc methodologies. In the actuator line and disc methods the blades are represented by a line or a disc on which body forces representing the loading are introduced. The body forces are determined by computing local angles of attack and using tabulated aerofoil coefficients. The advantage of using the actuator disc technique is that it is not necessary to resolve blade boundary layers. Instead the computational resources are devoted to simulating the dynamics of the flow structures.

Modelling and measurements of wakes in large wind farms

Journal of Physics: Conference Series, 2007

The paper presents research conducted in the Flow workpackage of the EU funded UPWIND project which focuses on improving models of flow within and downwind of large wind farms in complex terrain and offshore. The main activity is modelling the behaviour of wind turbine wakes in order to improve power output predictions.

Flow and wakes in large wind farms in complex terrain and offshore

European Wind …, 2008

Power losses due to wind turbine wakes are of the order of 10 and 20% of total power output in large wind farms. The focus of this research carried out within the EC funded UPWIND project is wind speed and turbulence modelling for large wind farms/wind turbines in complex terrain and offshore in order to optimise wind farm layouts to reduce wake losses and loads.

Survey of modelling methods for wind turbine wakes and wind farms

Wind Energy, 1999

This article provides an overview and analysis of different wake-modelling methods which may be used as prediction and design tools for both wind turbines and wind farms. We also survey the available data concerning the measurement of wind magnitudes in both single wakes and wind farms, and of loading effects on wind turbines under single-and multiple-wake conditions. The relative merits of existing wake and wind farm models and their ability to reproduce experimental results are discussed. Conclusions are provided concerning the usefulness of the different modelling approaches examined, and dif®cult issues which have not yet been satisfactorily treated and which require further research are discussed.

Large-eddy simulation of turbulent flow past wind turbines/farms: the Virtual Wind Simulator (VWiS)

Wind Energy, 2014

A large-eddy simulation framework, dubbed as the Virtual Wind Simulator (VWiS), for simulating turbulent flow over wind turbines and wind farms in complex terrain is developed and validated. The wind turbines are parameterized using the actuator line model. The complex terrain is represented by the curvilinear immersed boundary method. The predictive capability of the present method is evaluated by simulating two available wind tunnel experimental cases: the flow over a stand-alone turbine and an aligned wind turbine array. Systematic grid refinement studies are carried out, for both single turbine and multi-turbine array cases, and the accuracy of the computed results is assessed through detailed comparisons with wind tunnel experiments. The model is further applied to simulate the flow over an operational utility-scale wind farm. The inflow velocities for this case are interpolated from a mesoscale simulation using a Weather Research and Forecasting (WRF) model with and without adding synthetic turbulence to the WRF-computed velocity fields. Improvements on power predictions are obtained when synthetic turbulence is added at the inlet. Finally the VWiS is applied to simulate a yet undeveloped wind farm at a complex terrain site where wind resource measurements have already been obtained. Good agreement with field measurements is obtained in terms of the time-averaged streamwise velocity profiles. To demonstrate the ability of the model to simulate the interactions of terrain-induced turbulence with wind turbines, eight hypothetical turbines are placed in this area. The computed extracted power underscores the significant effect of site-specific topography on turbine performance.