Performance and wake development behind two in-line and offset model wind turbines – "Blind test" experiments and calculations (original) (raw)
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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.
Renewable Energy, 2014
In this paper we report on the results of the Blind Test 2 workshop, organized by Norcowe and Nowitech in Trondheim, Norway in October 2012. This workshop was arranged in order to find out how well wind turbine simulation models perform when applied to two turbines operating in line. Modelers with a suitable code were given boundary conditions of a wind tunnel test performed in the large wind tunnel facility at the Department of Energy and Process Engineering, at NTNU Trondheim, where two almost identical model turbines with a diameter of about 0.9wm had been tested under various operating conditions. A detailed geometry specification of the models could be downloaded and the modelers were invited to submit the calculation without knowing the experimental results in advance. Nine different contributions from eight institutions were received, representing a wide range of simulation models, such as a LES coupled with an actuator line rotor model, RANS using an actuator disc, U-RANS models applied to fully resolved turbine model geometries, as well as a vortex panel method. The comparison showed a larger than expected scatter on the performance calculation of the upstream turbine (AE20%), and an even higher uncertainty for the downstream turbine, especially at operating conditions close to the runaway point. The modelers were requested to document the wake development downstream of the second turbine, the development behind the first turbine had been the challenge for a previous blind test (see Krogstad and Eriksen [17]). Mean flow calculations reported at X ¼ 1D downstream of the second turbine showed that the models which fully resolved boundary layers on the rotor surface performed best. Including the tower and the hub in the simulation improved the accuracy of the predictions and is essential in capturing the important asymmetries that develop in the wake. These turbine details strongly influence the development near the center of the wake, but are often omitted in simulations in order to incorporate simplifying symmetry conditions in the calculations. Further from the rotor, at X ¼ 4D, the LES simulations coupled to actuator line rotor models performed well and were able to capture the main features of the mean and turbulent flows, while RANS models using actuator disc models showed limitations especially in predicting correctly the turbulent kinetic energy.
Wind Energy Science Discussions
single-turbine wind tunnel experiments. Various aspects of the numerical approach are considered, to try to reduce its need for tuning, improve its accuracy and limit its computational cost. Simulation results are compared to measurements, including rotor and wake quantities. The study includes nor-5 mal operating conditions, as well as wake manipulation by derating, yaw misalignment and cyclic pitching of the blades. Results indicate a good overall matching of simulations with experiments. Low turbulence test cases appear to be more challenging than moderate and high turbulence ones, due to the need for denser grids to limit numerical diffusion and accurately resolve tip-shed vortices in the near wake region. 10 1 Introduction Wind plants are collections of wind turbines, often operating in close proximity of one another. Several complex phenomena take place within a wind farm. First, there is an interaction between the atmospheric boundary layer and the whole wind farm, caused by the smaller scale interaction between the atmospheric flow and each individual wind turbine. Second, within the power plant it-15 self, there is an interaction among upstream and downstream wind turbines through their wakes. In turn, the wake themselves interact with the atmospheric flow and other wakes, interactions that play a central role in determining the overall behavior of the plant. Wakes produced by upstream wind turbines may have a profound influence on the performance of downstream operating machines. In fact, waked turbines experience lower power output and increased loading, compared to clean iso-20 lated conditions. A thorough understanding of these complex phenomena is clearly indispensable for optimizing the layout and operation of wind plants. However, even an optimal layout will still incur in wake interactions, at least in some wind and environmental conditions. To mitigate these effects, a number of control strategies are currently being investigated to optimize the operation of
Evaluation of the effects of turbulence model enhancements on wind turbine wake predictions
Wind Energy, 2011
The modelling of wind turbine wakes is investigated in this paper using a Navier-Stokes solver employing the k-ω turbulence model appropriately modified for atmospheric flows. It is common knowledge that even single wind turbine wake predictions with Computational Fluid Dynamic methods underestimate the near wake deficit, directly contributing to the overestimation of the power of the downstream turbines. For a single wind turbine, alternative modelling enhancements under neutral and stable atmospheric conditions are tested in this paper to account for and eventually correct the turbulence overestimation which is responsible for the faster flow recovery that appears in the numerical predictions. Their effect on the power predictions is evaluated with comparison to existing wake measurements. A second issue addressed in this paper concerns multi wind farm wake predictions, where the estimation of the reference wind speed which is required for the thrust calculation of a turbine located in the wake(s) of other turbines is not obvious. This is overcome by utilizing an induction-factor based concept: According to it, the definition of the induction factor and its relationship with the thrust coefficient are employed to provide an average wind speed value across the rotor disk for the estimation of the axial force. Application is made on a five wind turbines in a row case.
Assessment of blockage effects on the wake characteristics and power of wind turbines
Renewable Energy (2016)
Large Eddy Simulations (LES) are performed in order to study the wake and power characteristics of a horizontal-axis wind turbine in a wind tunnel. Using an actuator line technique, the effect of wind tunnel blockage ratio (defined as the ratio of the rotor swept area to the tunnel cross-sectional area) is investigated for a wide range of tip speed ratios from 1 to 12, and for four blockage ratios (0.2, 0.09, 0.05 and 0.02). The results demonstrate how the blockage effect increases with the tip speed ratio. When the tip speed ratio is close to or above the optimal design value, blockage ratios of larger than 0.05 affect both tangential and normal forces on the blades and therefore on the power and thrust coefficients. At the highest blockage ratio of 0.2, the mean velocity of the wake is also affected significantly, although the effect on the wake mixing rate is less pronounced. Further, the effect of the Reynolds number on the wake development is illustrated and the impact of numerics and subgrid-scale models are investigated by comparing two different LES codes. Finally, the importance of tip loss correction in actuator-line modeling of wind turbines is illustrated using comparative computations.
CFD Investigations of Wake Flow Interactions in a Wind Farm with 14 Wind Turbines
International Journal of Offshore and Polar Engineering, 2020
Because the wake flow interaction phenomenon among wind turbines has a great influence on aerodynamic power output, wind speed deficit turbulence stress, and wake vortex structure, the wake interaction for the optimal arrangement of wind farm has recently attracted increasing attention. This paper presents a validation of aerodynamics for the two offset model wind turbines on the actuator line model and computational fluid dynamics (CFD) technique. The numerical results of the present simulations are compared with those produced by testing on Blind Test 3 and other simulation models. On the basis of the simulations results, the present study shows good agreement with the experimental results. Besides, considering the uniform inflow condition, a numerical method is harnessed to simulate the complex phenomenon of wake interaction in a wind farm containing 14 wind turbines. Large eddy simulations combined with an actuator line model are conducted in the in-house CFD code FOWT-UALM-SJTU solver, which is an extension based on OpenFOAM. The motivation for this work is to create a sound methodology for performing the simulation of large wind farms. To better understand the wake interaction phenomenon, the aerodynamic power coefficients and basic features of both the near and far wake, including the distribution characteristics of the mean wake velocity and vortex structures, are studied in detail.
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.
Experimental investigation of wake effects on wind turbine performance
Renewable Energy, 2011
The wake interference effect on the performance of a downstream wind turbine was investigated experimentally. Two similar model turbines with the same rotor diameter were used. The effects on the performance of the downstream turbine of the distance of separation between the turbines and the amount of power extracted from the upstream turbine were studied. The effects of these parameters on the total power output from the turbines were also estimated. The reduction in the maximum power coefficient of the downstream turbine is strongly dependent on the distance between the turbines and the operating condition of the upstream turbine. Depending on the distance of separation and blade pitch angle, the loss in power from the downstream turbine varies from about 20 to 46% compared to the power output from an unobstructed single turbine operating at its designed conditions. By operating the upstream turbine slightly outside this optimum setting or yawing the upstream turbine, the power output from the downstream turbine was significantly improved. This study shows that the total power output could be increased by installing an upstream turbine which extracts less power than the following turbines. By operating the upstream turbine in yawed condition, the gain in total power output from the two turbines could be increased by about 12%.