Blind Test 2 calculations for two in-line model wind turbines where the downstream turbine operates at various rotational speeds (original) (raw)
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Journal of Physics: Conference Series, 2014
This is a report on data presented at the "Blind test 3" Workshop organized jointly by Nowitech and Norcowe in Bergen, December 2013. A number of research groups were invited to predict the performances and the wake development behind two model wind turbines that have been extensively tested at the Department of Energy and Process Engineering, NTNU. The turbines were arranged in-line, but slightly offset so that the wake of the upstream turbine only affected roughly half the area swept by the second rotor. This is a common event in most wind parks and produces flow fields that are both complicated and harmful for the downstream turbine. Contributions were received from five different groups using a range of methods, from fully resolved Reynolds averaged Computational Fluid Dynamics (CFD) models to Large Eddy Simulations (LES). The range of results was large but the overall trend is that the current methods predict the power generation as well as the thrust force reasonably well. But there is a large uncertainty in the prediction of the turbulence field in the wake.
Large eddy simulation of the wind turbine wake characteristics in the numerical wind tunnel model
Journal of Wind Engineering and Industrial Aerodynamics, 2013
Large Eddy Simulation of NREL Phase VI wind turbine was performed in a virtual wind tunnel (24.4m by 36.6m) in order to achieve a better understanding of the turbine wake characteristics. For this purpose, ANSYS-Fluent package was used to run the simulation using the dynamic Smagorinsky-Lilly model. For the purpose of validation, the pressure distribution at different span-wise sections along the turbine blade and the power produced by the wind turbine were compared with the published experimental results for the NREL phase VI rotor tested in the NASA wind tunnel with the same dimensions as in the model and a good agreement was found between the two. The airflow immediately behind the wind turbine was observed to be a system of intense and stable rotating helical vortices, which determined the dynamics of the far-wake. The system of vortices in the near-wake became unstable and broke down due to wake instability at a distance of five rotor diameters downstream of the wind turbine. This was defined as the boundary between the near-and far-wake regions. The collapsed spiral wake was found to spread in all directions in the far-wake resulting in the formation of the two pairs of counter-rotating vortices which caused the gradual increase of turbulence in these regions. The turbulence intensity in the wake was observed to increase immediately behind the turbine with a maximum of 12.12% at a distance of three rotor diameters downstream of the turbine, after which a gradual decrease in the turbulence intensity was observed in the near-wake regions due to wake instability. However, in the far-wake regions, due to counter-rotating vortices formed by the wake instability, the turbulence intensity showed a tendency to increase intensity. Finally the time-averaged wake velocities from the LES, with and without the blockage corrections, were compared with WAsP and a comparatively good agreement for the axial velocity predictions was observed in the far-wake. Blockage correction; Numerical wind tunnel number of researchers have used CFD, based on the RANS equations to acquire comparatively fast results (Menter et al, 2006; Potsdam and Mavriplis, 2009; Sørensen et al, 2002b). Others have used LES to simulate the wake flows without the turbines, combined with the actuator line and disc methodologies (Wu and Porté-Agel, 2011). However, the approach of "return to the basics" as proposed by Vermeer et al is highly valuable, in that it provides the opportunity to study the aerodynamics of the wind turbines in controlled environments like wind tunnels. The objective of this investigation is to achieve a better understanding of the turbulent wake characteristics behind the wind turbine (NREL Phase VI wind turbine) that was tested in the NASA Ames 24.4 m by 36.6 m wind tunnel. For this, LES was carried out using the commercial CFD code, ANSYS FLUENT 13. The results of the LES have been compared with the aerodynamics of the wind turbine blade that were obtained experimentally by the NREL (Hand and Simms, 2001;. Particular emphasis has been placed on the study of the distribution of the overall wake structure, time-averaged axial velocity (corresponding to velocity deficit) and the increased turbulence intensity in cross-sectional planes perpendicular to the axis of the wake at uniform incoming velocity. The study will be useful for the designers to plan and design future wind farms for the purpose of improvement of the overall wind farm efficiency and the fatigue life of the wind turbines. It also provides understanding of the turbulent wake characteristics of a wind turbine and the much needed results required for validation of turbine wake models. For this reason, the results of LES were also compared with the simple equations provided by WAsP to estimate the wake velocities in the far-wake.
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.
Evaluation of RANS/actuator disk modelling of wind turbine wake flow using wind tunnel measurements
International Journal of Engineering Systems Modelling and Simulation, 2013
Wake modelling plays a central role in wind farm planning through the evaluation of losses, the prediction of the energy yield, and the estimation of turbine loads. These models must be reasonably accurate -to minimise financial risk -and yet economical so that many configurations can be tested within reasonable time. While many such models have been proposed, an especially attractive approach is based on the solution of the Reynolds-averaged Navier-Stokes equations with two-equation turbulence closure and an actuator disk representation of the rotor. The validity of this approach and its inherent limitations however remain to be fully understood. To this end, detailed wind tunnel measurements in the wake of a porous disk (with similar aerodynamic properties as a turbine rotor) immersed in a uniform flow are compared with the predictions of several closures. Agreement with measurements is found to be excellent for all models. This unexpected result seems to derive from a fundamental difference in the turbulent nature of the homogeneous wind tunnel flow and that of the atmospheric boundary layer.
32nd ASME Wind Energy Symposium, 2014
When using an actuator-line representation of a wind turbine for computational fluid dynamics, it is common practice to volumetrically project the line force onto the flow field to create a body force in the fluid momentum equation. The objective of this study is to investigate how different volumetric projection techniques of the body force created by an actuator-line wind turbine rotor model affect the generated wake characteristics and blade loads in a turbine-turbine interaction problem. Two techniques for the body-force projection width are used, and they are based on either i) the grid spacing, or ii) the combination of grid spacing and an equivalent elliptic blade planform. An array of two NREL 5-MW turbines separated by seven rotor diameters is simulated within a large-eddy simulation solver subject to offshore neutral and moderately-convective atmospheric boundary-layer inflow. Power, thrust, and bending moment histories of both turbines, the statistics of angle of attack and blade loads over 2000 sec, variations in the mean and fluctuating velocity components, and turbulent kinetic energy and selected Reynolds stresses along vertical and spanwise sampling locations in the wake are analyzed. Comparisons for the different techniques of determining the body-force projection width of the actuator-line method are made and their effect on different physical quantities are assessed.
Wind turbine wake interactions; results from blind tests
Journal of Physics: Conference Series, 2015
Results from three "Blind test" Workshops on wind turbine wake modeling are presented. While the first "Blind test" (BT1, 2011) consisted of a single model turbine located in a large wind tunnel, the complexity was increased for each new test in order to see how various models performed. Thus the next "Blind test" (BT2, 2012) had two turbines mounted in-line. This is a crucial test for models intended to predict turbine performances in a wind farm. In the last "Blind test" (BT3, 2013) the two turbines were again mounted in-line, but offset sideways so that the rotor of the downstream turbine only intersected half the wake from the upstream turbine. This case produces high dynamic loads and strong asymmetry in the wake. For each "Blind test" the turbine geometry and wind tunnel environment was specified and the participants were asked to predict the turbine performances, as well as the wake development to five diameters downstream of the second turbine. For the first two tests axisymmetry could be assumed if the influence of the towers was neglected. This was not possible in BT3 and therefore only fully 3D methods could be applied. In all tests the prediction scatter was surprisingly high.
Exploration and numerical simulation of wind turbine wake
In this article, the flow behind a horizontal axis wind turbine (HAWT) is investigated and the obtained data is compared to the results of numerical simulation. The aim is to test reliability of random averaged Navier-Stokes (RANS) solver to model the wake behind a wind turbine. The experimental investigations are carried out by means of the 2D PIV measurements. The flow field is obtained in rotating frame of reference in which the rotor appears fixed by means of the phase-locked technique. Explorations are carried out in different azimuth planes. Because of large dimensions of the flow field, each azimuth plane is divided into several windows. For each window, the instantaneous velocity field is measured and stored successively to enable obtaining the averaged velocity field. Then, the flow in each azimuthal plane is reconstructed by stitching the averaged velocity field of these windows. Finally, the 3D velocity field is reconstituted by treating the results of images resulting from the different explored azimuth planes. These results are compared with RANS calculations. In general the numerical results show agreement with experiment, but some inconsistency concerning obtained power is revealed.
Numerical investigation of the wake interaction between two model wind turbines with span-wise offset Sasan Sarmast, Hamid Sarlak Chivaee, Stefan Ivanell et al. Numerical analysis of the tip and root vortex position in the wake of a wind turbine S Ivanell, J N Sørensen, R Mikkelsen et al. Reduced order model of the inherent turbulence of wind turbine wakes inside an infinitely long row of turbines S J Andersen, J N Sørensen and R Mikkelsen The relationship between loads and power of a rotor and an actuator disc Gijs A M van Kuik Flow performance of highly loaded axial fan with bowed rotor blades L Chen, X J Liu, A L Yang et al.
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.