Actuator Line Method Research Articles

Tip-vortex instabilities of two in-line wind turbines

The hydrodynamic stability of a vortex system behind two in-line wind turbines operating at low tip-speed ratios is investigated using the actuator-line method in conjunction with the spectral-element flow solver Nek5000. To this end, a simplified setup with two identical wind turbine geometries rotating at the same tip-speed ratio is simulated and compared with a single turbine wake. Using the rotating frame of reference, a steady solution is obtained, which serves as a base state to study the growth mechanisms of induced perturbations to the system. It is shown that, already in the steady state, the tip vortices of the two turbines interact with each other, exhibiting the so-called overtaking phenomenon. Hereby, the tip vortices of the upstream turbine overtake those of the downstream turbine repeatedly. By applying targeted harmonic excitations at the upstream turbine’s blade tips a variety of modes are excited and grow with downstream distance. Dynamic mode decomposition of this perturbed flow field showed that the unstable out-of-phase mode is dominant, both with and without the presence of the second turbine. The perturbations of the upstream turbine’s helical vortex system led to the destabilization of the tip vortices shed by the downstream turbine. Two distinct mechanisms were observed: for certain frequencies the downstream turbine’s vortices oscillate in phase with the vortex system of the upstream turbine while for other frequencies a clear out-of-phase behaviour is observed. Further, short-wave instabilities were shown to grow in the numerical simulations, similar to existing experimental studies [1].

The effect of chordae tendineae on systolic flow

When using Computational Fluid Dynamics to simulate ventricular blood flow in the heart, it has been common practice to neglect the effect of the sub-valvular apparatus and the trabeculae on the flow conditions. In this study, we analyze the effect of neglecting the chordae tendineae on the fluid flow and pressure drop. To test the assumption we use a previously developed dynamic 3D model of the left ventricle, aorta and valves that is based on 3D echocardiographic recordings. To this model we add the chordae tendineae as a sub-grid model.The previously developed 3D model for the left ventricle during systole is based on real-time three-dimensional echocardiography (RT3DE) recordings of a 30 years old female volunteer. The segmented ventricular wall does not include details of the aorta and the mitral valve, so these were reconstructed. The subgrid model for the flow across the chordae tendineae is based on the Actuator Line Method, which means that they are represented by drag coefficients. The analysis shows that the effect of the chordae tendineae on the pressure drop and work efficiency of the normal heart during systole is minor, and it seems that for simulating ventricular fluid flow and pressure drop during systole, one can follow the current practice and ignore the chordae. However, there can be local effects such as small vortices behind the chordae. Whether such effects are important for a particular application must be evaluated for the given case.

Comparison of Actuator Line Method and Full Rotor Geometry Simulations of the Wake Field of a Tidal Stream Turbine

This study aims to investigate the wake characteristics of a horizontal axis tidal stream turbine supported by a monopile using a numerical approach. Computational fluid dynamics (CFD) simulations based on the open source software OpenFOAM have been performed to enhance understanding of a turbine’s wake. The numerical simulations adopt both the actuator line method and the full rotor geometry method. The numerical results are found to be consistent with experimental data, although some discrepancies are observed at a distance of one rotor diameter downstream. Comparison of numerical results from both methods is performed. The results show that both methods can obtain important flow features and provide similar simulation in the wake of the turbine model. The actuator line method is able to give a better prediction in stream-wise velocity distribution, although it underestimates the turbulence intensity, circumferential velocity and vorticity magnitude slightly, compared with the full rotor geometry method. It is also found that the wake of the monopile and the rotor interact strongly in the downstream field, especially in the region immediately behind the structure. A strong interaction occurs within approximately two rotor diameters downstream.

Numerical investigations on control strategies of wake deviation for large wind turbines in an offshore wind farm

The OpenFOAM package incorporated with actuator line method (ALM) is employed to calculate the aerodynamic characteristics of large wind turbines in an offshore wind farm. In order to mitigate the wake effect of upstream wind turbines and conduct the overall optimization of output power for the offshore wind farm, wake deviation simulations of nine yaw angles and fifteen tilt angles are carried out. Taking total output power of the offshore wind farm as the reference point and combining fluid field profiles of cross-sections in all simulations, we investigate the flow mechanism of wake's influence on the downstream turbines with different control strategies. The results show that, although the output power of the upstream wind turbine may decreases mildly, changing the wake direction with different control methods can significantly increase the total output power of the whole wind farm due to the substantial enhancement of the downstream wind turbine. Both control strategies of wake deviation can globally optimize the offshore wind farm and each has an optimal angle, while the yaw control strategy has better results and with regard to the tilt control strategy, the plus tilt angle is comparatively better than the minus angle.

Analysis of the coupled aeroelastic wake behavior of wind turbine

With the development of wind energy in recent years, the aeroelastic performances and the wake characteristics of a wind turbine and their effect on the downstream wind turbine have become research priorities. However, traditional aeroelastic analysis tools based on the Blade Element Momentum theory (BEM) cannot calculate the wake characteristics of a wind turbine. This study focuses on the coupled aeroelastic wake behavior of the wind turbine and developed a new tool called ALFBM, which combines the Actuator Line Method (ALM) and the nonlinear finite element Beam theory. The coupled aeroelastic wake behavior of a single NREL 5 MW wind turbine under the inlet velocity of 8 m/s and 11.4 m/s are studied. The aerodynamic and structural results such as power, thrust, natural frequencies, and tip deflections are compared with the results of former researches to verify this new analysis tool. Also, these results show that the stress stiffening effect and the nonlinear bending effect significantly decrease the frequency of flapwise mode by 12.7% and the tip deflection by 31%. It is found that the modal shape of every asymmetry flapwise mode varies periodically during the wind turbine rotation. The wake results demonstrate that the recovery of the velocity and vorticity in the far wake region (>5D) are underestimated when the deformation of the wind turbine blades are neglected.

Near-wake analysis of actuator line method immersed in turbulent flow using large-eddy simulations

Abstract. The interaction between wind turbines through their wakes is an important aspect of the conception and operation of a wind farm. Wakes are characterized by an elevated turbulence level and a noticeable velocity deficit, which causes a decrease in energy output and fatigue on downstream turbines. In order to gain a better understanding of this phenomenon this work uses large-eddy simulations together with an actuator line model and different ambient turbulence imposed as boundary conditions. This is achieved by using the Simulator fOr Wind Farm Applications (SOWFA) framework from the National Renewable Energy Laboratory (NREL) (USA), which is first validated against another popular Computational Fluid Dynamics (CFD) framework for wind energy, EllipSys3D, and then verified against the experimental results from the Model Experiment in Controlled Conditions (MEXICO) and New Model Experiment in Controlled Conditions (NEW MEXICO) wind tunnel experiments. By using the predicted torque as a global indicator, the optimal width of the distribution kernel for the actuator line is determined for different grid resolutions. Then, the rotor is immersed in homogeneous isotropic turbulence and a shear layer turbulence with different turbulence intensities, allowing us to determine how far downstream the effect of the distinct blades is discernible. This can be used as an indicator of the extents of the near wake for different flow conditions.

Stochastic characteristics for the vortical structure of a 5-MW wind turbine wake

In this study, we analyze the near-wake characteristics of a 5-MW single wind turbine using a large-eddy simulation with the actuator line method. It is observed that stable helical structures of wake had breaking process due to flow instability with high-frequency turbulence in the analysis of turbulence energy spectra and auto-covariance. For vortical structure detection, the swirling strength criterion showed good performance when describing the tip vortices and their various streaklines. In the stable region, the stable helical structure of the tip vortices was observed with little wake recovery and flow instability. However, in the unstable region, the tip vortices had various streakline advection patterns such as horizontal, upward, and downward. In the statistical analysis of the flow acceleration, the distribution and magnitude of the acceleration in the stable and unstable regions were compared in terms of the probability density function, root mean square, and kurtosis. At the hub height and at the opposite blade tips in the stable region, the kurtosis of the acceleration was approximately 20 and 40, respectively. At the height wherein the wake recovery was dominant in the unstable region, the value of the kurtosis was 67.

Actuator Line Method Simulations for the Analysis of Wind Turbine Wakes Acting on Helicopters

In the present study the actuator line method is used to simulate the wake of a wind turbine and ways to characterize its impact on the flight of a helicopter model through the wake are evaluated. The helicopter within the simulation setup is represented by a dynamic actuator line which was implemented into the flow solver FLOWer. In order to evaluate the loads on the helicopter the thrust force distribution is compared in different areas of the wake. For further improvement of the actuator line implementation different methods to evaluate the angle of attack are compared. This was examined as the angle of attack is a highly important parameter for the actuator line and helps to optimise the accuracy and performance of the actuator line method. As there is no experimental data available and the model of the helicopter comes with distinct simplifications the loads are compared to values without the wake of the wind turbine, thus in free flight.

Actuator-Line Model in a Lattice Boltzmann Framework for Wind Turbine Simulations

This paper presents the development of a solver based on a Lattice Boltzmann Method (LBM) to perform reliable wind turbine simulations: LaBoheMe. LBM offers an interesting framework with low dissipation and high performance computing compared to usual Computational Fluid Dynamics (CFD) solvers based on Navier-Stokes equations. In order to take wind turbines into account in the LBM solver, LaBoheMe is enhanced by the addition of local force terms representing the influence of the wind turbines on the flow. These forces are computed using an Actuator-Line Model (ALM). The present solver is a 2D LBM/ALM solver allowing the simulation of “slices” of a Vertical Axis Wind Turbine (VAWT).The Lattice Boltzmann and the Actuator Line methods are described and the coupling adopted in this work is detailed. Validation cases for pure LBM (Backward Facing Step, BFS) and coupled LBM/ALM are performed. The LBM/ALM coupling is validated against experimental data and compared with a vortex method and a large eddy simulation of the wake. Both blade forces and wake velocities are considered.

Optimal distribution width for ALM in LES for (NEW) MEXICO experiment

The Actuator Line Method exists for more than a decade and has become a well established choice for simulating wind rotors in computational fluid dynamics. Numerous implementations exist and are used in the wind energy research community. A crucial parameter of this method is the distribution width used to distribute the blade forces in the domain in order to attain numerical stability but also to apply the force term over a length which makes physically sense. In this work the implementation is verified with results from the (NEW) MEXICO experiment and the optimal distribution width is found by setting the global torque as target variable.

Numerical investigations of wake interactions of two wind turbines in tandem

Aerodynamic performance and wake interactions between two wind turbine models under different layouts are investigated numerically using large eddy simulation in conjunction with actuator line method based on the “Blind Test” series wind tunnel experiments from Norwegian University of Science and Technology. Numerical results of the power and thrust coefficients of the two rotors and wake characteristics are in good agreement with the experimental measurements. Extended investigations emphasizing the influence of different layout arrangements on the downstream rotor performance and wake development are conducted. Results show that layout arrangements have great influence on the power and thrust prediction of the downstream turbine.

Large-Eddy Simulation of wind turbines wakes including geometrical effects

Accurate simulation of wind turbine wakes is critical for the optimization of turbine efficiency and prediction of fatigue loads. These wakes are three-dimensional, complex, unsteady and can evolve in geometrically complex environments. Modeling these flows calls thus for high-quality numerical methods that are able to capture and transport thin vortical structures on an unstructured grid. It is proposed here to assess the performances of a fourth-order finite-volume LES solver to perform massively parallel scale-resolving simulations of wind turbines wakes. In this framework, the actuator line method that takes the effect of the wind turbine blades on the flow into account is implemented. It is demonstrated that both near and far parts of the turbine wakes are accurately modeled as well as geometrical details. The methodology is assessed on two different test cases and validated with experimental results. It is demonstrated that the flow predictions are of equivalent quality on both structured and unstructured grids. The influence of the geometrical details (e.g. nacelle and tower) on the wake development as well as the influence of the discretization scheme are also investigated.

A large-eddy simulation study of horizontal axis tidal turbine in different inflow conditions

In this paper, we present our recent work of numerical simulations of the horizontal axis tidal turbine, which combines the large eddy simulation with the actuator line method. The towing tank test is conducted to acquire the performance of an originally designed tidal turbine. We first validated our numerical model with the experimental data; in this case, a good agreement was obtained with the simple uniform flow. In addition, we used different inflow conditions in the simulations to investigate the impacts on the turbine wake. The results show that the structure and mixing rates of turbine wake were different for various inflow conditions. For turbulent inflow conditions, the turbine wake was asymmetric and mixes rapidly downstream of the turbine. The tip and root vortices can maintain stability for only a 1–2 blade radii distance. Furthermore, the preliminary results of the different turbulent intensities and the effect of the incident angle are presented.

Comparison of wind farm large eddy simulations using actuator disk and actuator line models with wind tunnel experiments

We compare wind farm large eddy simulations with the EPFL wind tunnel measurement by Chamorro and Porté-Agel (Bound-Lay. Meteorol. 136, 515 (2010) and Energies 4, 1916 (2011)). We find that the near turbine wake, up to 3 turbine diameters downstream, of a single turbine is captured better with the actuator line method than using the actuator disk method. Further downstream the results obtained with both models agrees very well with the experimental data, confirming findings from previous studies. For large aligned wind farms we find that the actuator disk model predicts the wake profiles behind turbines on the second and subsequent rows more accurately than the wake profile behind the first turbine row. The reason is that the wake layer profile that is created at hub height in very large wind farms is closer to the assumptions made in the actuator disk model than the logarithmic profile found in the inflow conditions. In addition, we show that, even in relatively coarse resolution simulations, adding the effect of the turbine nacelle and tower leads to a significant improvement in the prediction of the near wake features at 1 and 2 diameters downstream.

An Investigation of Wind Farm Power Production for Various Atmospheric Boundary Layer Heights

The dependency of the atmospheric boundary layer (ABL) characteristics on the ABL’s height is investigated by using large eddy simulations (LES). The impacts of ABL’s height on the wind turbine (WT) power production are also investigated by simulating two subsequent wind turbines using the actuator line method (ALM). The results show that, for the same driving pressure forces and aerodynamic roughness height, the wind velocity is higher at deeper ABL, while the wind shear and the wind veer are not affected by the depth. Moreover, the turbulence intensity, kinetic energy, and kinematic shear stress increase with the ABL’s height. Higher power production and power coefficient are obtained from turbines operating at deeper ABL.

Actuator line simulation of a tidal turbine in straight and yawed flows

In this numerical study a tidal turbine in straight and yawed flows is simulated using the actuator line (AL) method coupled with Large Eddy Simulation (LES) of turbulence for the turbine previously studied experimentally by Bahaj et al. (2007). Importantly, the AL model is fully coupled to an existing GPU based computational fluid dynamic solver, enabling high resolution simulations in reasonable time frames using desktop size server systems. Simulation results using the blade element actuator disk (BEAD) method are also presented to support the results from the AL method and highlight its advantages over the BEAD method. Results obtained from this study show that the AL method is capable of capturing wake unsteadiness and the tip and root vortices resulting from the turbine blades. Predicted power and thrust coefficients agree well with experimental data, being within 0.77% and 1.91%, respectively, at the design tip speed ratio. However, the absence of hub geometry in this method affects the downstream wake pattern along its centerline.

Numerical simulations of wake characteristics of a horizontal axis tidal stream turbine using actuator line model

The wake of a laboratory scale tidal stream turbine in a shallow water channel with a turbulent inflow is simulated using the hybrid LES/ALM technique, which combines large eddy simulation with the actuator line method. The turbulent inlet conditions are generated using the mapping method to avoid a precursor running and large space for saving data. The numerical results demonstrated the usefulness of the mapping technique as well as some shortcomings that still remain to be addressed. Good agreement between numerical predictions and experimental data is achieved for both the mean and turbulent characteristics of the flow behind the turbine. The examination of changes in turbulence intensity and turbulent kinetic energy in the streamwise direction confirms the existence of a peak and transition to a highly turbulent flow about three diameters downstream of the turbine, which means that the distinct characteristics of the streamwise changes of turbulence intensity or turbulent kinetic energy may serve as an effective indicator for the flow regime transition and wake behaviour.

Validation of the actuator line method for simulating flow through a horizontal axis tidal stream turbine by comparison with measurements

The purpose of the present work is to evaluate the capability of the Actuator Line Method (ALM) to simulate flow through a horizontal axis tidal stream turbine. A numerical model combining the ALM with large eddy simulation technique is developed and applied to compute the flow past a laboratory-scale tidal stream turbine. The flow field is analysed in terms of streamwise mean velocity, turbulence intensity, turbulent kinetic energy and the decay rate of the maximum turbulent kinetic energy behind the turbine. It is found that the ALM performs well in predicting the mean flow and turbulence characteristics behind the turbine. The flow field predicted show a clear transition from an organised vorticity region near the turbine to a highly turbulent flow downstream. The location of this transition and the controlling parameters are discussed but further investigation, both numerical and experimental is required in order to clarify its effects on the flow structure and the performance of downstream turbines in tidal turbine arrays.

Wind–Wave Interaction Effects on a Wind Farm Power Production

In the current study, the effects of the nonlocally generated long sea surface waves (swells) on the power production of a 2 × 2 wind farm are investigated by using large-eddy simulations (LES) and actuator-line method (ALM). The short sea waves are modeled as a roughness height, while the wave-induced stress accounting for swell effects is added as an external source term to the momentum equations. The results show that the marine atmospheric boundary layers (MABLs) obtained in this study have similar characteristics as the MABLs observed during the swell conditions by many other studies. The current results indicate also that swells have significant impacts on the MABL. As a consequence of these changes in the MABL, swells moving faster than the wind and aligned with the local wind direction increase the power extraction rate.

Comparison of OpenFOAM and EllipSys3D actuator line methods with (NEW) MEXICO results

The Actuator Line Method exists for more than a decade and has become a well established choice for simulating wind rotors in computational fluid dynamics. Numerous implementations exist and are used in the wind energy research community. These codes were verified by experimental data such as the MEXICO experiment. Often the verification against other codes were made on a very broad scale. Therefore this study attempts first a validation by comparing two different implementations, namely an adapted version of SOWFA/OpenFOAM and EllipSys3D and also a verification by comparing against experimental results from the MEXICO and NEW MEXICO experiments.