Wind Turbine Wake Simulations Research Articles

Secondary flows in the actuator-disk simulation of wind-turbine wakes

This study explores the generation of secondary flows of Prandtl's second kind in the actuator-disk simulation of wind-turbine wakes. Leveraging large-eddy simulation data and conducting an analysis of the mean streamwise vorticity budget, we uncover the physical mechanisms contributing to this phenomenon. Our investigations attribute the emergence of such flows to the spatial gradients of the Reynolds stresses in the wake downstream of the turbines, which are, in turn, influenced by ground effects. To further investigate the phenomenon, we specifically isolate the impact of secondary flows on the wake by employing a model recognized for its incapacity to capture such dynamics: a two-equation Reynolds-averaged Navier–Stokes (RANS) model founded on the linear eddy-viscosity hypothesis. By comparing the predictions of the RANS model with those of large-eddy simulations and wind-tunnel experiments, we highlight the effect of secondary flows on the wake structure and, in particular, the upward shift of the wake. Motivated by the obtained results, we then enhance the baseline RANS model by introducing a non-linear term within the Reynolds stress tensor. This modification leads to a more accurate representation of Reynolds stresses, enabling the RANS model to capture the secondary flows in the wake. Our analysis emphasizes the importance of employing advanced RANS models in the simulation of wind farms.

Study on boundary conditions of k-ε-fp turbulence model for wind turbine wake simulation

ABSTRACT Wind turbine wake has a great impact on the power output and fatigue damage of the whole wind farm. As a result, wind turbine wake simulation has attracted attention from both researchers and engineers for decades, in which turbulence modeling is the key point. Among all the turbulence modeling methods for wind turbine wake, k − ε − f P , a RANS model proposed by Laan in 2015, can balance well between efficiency and accuracy. In the current research, the boundary conditions of k − ε − f P turbulence model have been studied comprehensively. It was found that the boundary conditions of the model are affected by the wind conditions of the actual wind turbine, resulting in poor simulation accuracy under high wind shear conditions. Five simulation cases have been carried and compared with field measurement results. It was found that the roughness length z 0 is the most important factor for the determination of boundary conditions. Considering the wind shear conditions around wind turbine rotor, a new determination method for roughness length z 0 of the ground has been constructed, which corrects the wind profile prediction under high wind shear conditions. This research will contribute to the micro-sitting of future large wind farms.

Wind turbine wake simulation with explicit algebraic Reynolds stress modeling

Abstract. Reynolds-averaged Navier–Stokes (RANS) simulations of wind turbine wakes are usually conducted with two-equation turbulence models based on the Boussinesq hypothesis; these are simple and robust but lack the capability of predicting various turbulence phenomena. Using the explicit algebraic Reynolds stress model (EARSM) of Wallin and Johansson (2000) can alleviate some of these deficiencies while still being numerically robust and only slightly more computationally expensive than the traditional two-equation models. The model implementation is verified with the homogeneous shear flow, half-channel flow, and square duct flow cases, and subsequently full three-dimensional wake simulations are run and analyzed. The results are compared with reference large-eddy simulation (LES) data, which show that the EARSM especially improves the prediction of turbulence anisotropy and turbulence intensity but that it also predicts less Gaussian wake profile shapes.

Large-eddy simulation of wind-turbine wakes over two-dimensional hills

Wind-turbine wakes over two-dimensional (2D) hills with different slope gradients are systematically investigated using large-eddy simulation with wind turbine parameterized as actuator disk model and hilly terrain modeled by immersed boundary method. The chosen hill models represent typical hilly terrains with and without flow recirculation in the wake of the hills. The flow characteristics of wind-turbine wakes [including mean velocity, wake-center trajectory, turbulence statistics, and mean kinetic energy (MKE) budgets] and the power performance are analyzed, and the related flow mechanisms are elucidated in our study. It is found that the velocity deficit in turbine wakes cannot be acceptably represented by the Gaussian model in the wake of the steep hill until at a further distance. It is also found that the assumption that the wake-center trajectory maintains a nearly constant elevation downwind of the hilltop proposed by Shamsoddin and Porté-Agel [“Wind turbine wakes over hills,” J. Fluid Mech. 855, 671–702 (2018)] may not be applicable in particular for the steep hill cases. Furthermore, the hilltop is the optimal location for turbine placement because the turbine harvests more wind energy due to the speed-up effect and suffers less fatigue loading due to the lower turbulence levels. Both the turbulence levels and the magnitude of vertical turbulent flux are found to drop below those of the flat ground case on the windward side of the hills, and they also decrease within the hill wake region compared with the no-turbine cases. A detailed analysis of MKE budgets reveals that the budgets of pressure transport and mean convection are mainly responsible for balancing the MKE in turbine wakes over hilly terrain.

Comparison of a mid-fidelity free vortex wake method to a high-fidelity actuator line model large eddy simulation for wind turbine wake simulations

The need for mid-fidelity wind turbine simulations is increasing due to their balance of accuracy and computational efficiency. A common form of mid-fidelity code is the free wake vortex method (FVWM), but these are known to inaccurately represent the far-wake depending on their implementation. Herein, we compare CACTUS, a FVWM code, to Nalu-Wind, an actuator line model large eddy simulation (ALM-LES) code with particular focus on the far wake. Specifically, we compare one CACTUS simulation, which necessarily has steady and uniform inflow, to three Nalu-Wind simulations using a steady and uniform inflow, a low turbulence inflow, and a high turbulence inflow. The comparison reveals interesting parallels between CACTUS and all Nalu-Wind cases, but ultimately demonstrates that CACTUS is not an appropriate tool for the simulation of the far-wake of a wind turbine.

Quantifying structural uncertainties in Reynolds-averaged Navier–Stokes simulations of wind turbine wakes

Reynolds-averaged Navier Stokes (RANS) based modeling is considered the mainstream computational fluid dynamics (CFD) approach for wind energy applications. Considering the inherent shortcomings associated with RANS models, quantification of uncertainties is of obvious importance if these models are to be used for design and optimization of wind farms. In the present work, structural uncertainties of RANS closure are quantified for simulations of wake flow behind a stand-alone wind turbine. The uncertainty is modeled by introducing perturbations to the Reynolds stress tensor. We specifically focus on perturbations in the eigenvalues of the tensor. The k-ω SST model and large-eddy simulation (LES) data are used as the baseline RANS model and reference data, respectively. Initially we compare the unperturbed RANS simulation against LES data, and show that the k-ω SST model generally tends to predict higher levels of isotropy in the turbulent wake. A comparison between LES data and differently perturbed k-ω SST simulations is made for the evolution of velocity deficit and turbulence intensity behind the turbine. A satisfactory coverage of LES profiles is observed when the amount of introduced perturbation is adjusted based on a priori comparison of the baseline RANS and LES results with the exception of the turbulent intensity immediately behind the turbine.

Assessment of weak compressibility in actuator line simulations of wind turbine wakes

The trend of increasing rotor diameters and tip-speeds has brought about concerns of non-negligible compressibility effects in wind turbine aerodynamics. The investigation of such effects on wakes is particularly difficult when using actuator line models (ALM). This is because crucial regions of the flow, i.e. the direct vicinity of the blade, are not simulated but represented by body forces. To separately assess the impact of compressibility on the wake and the ALM itself, we conduct large-eddy simulations (LES) where the forces of the ALM are prescribed and based on the local sampled velocity (standard procedure), respectively. The LES are based on the weakly-compressible Lattice Boltzmann Method (LBM). Further to the comparison of (near-)incompressible to compressible simulations we investigate cases with artificially increased compressibility. This is commonly done in weakly-compressible approaches to reduce the computational demand. The investigation with prescribed forces shows that compressibility effects in the wake flow are negligible. Small differences in the wake velocity (of max. 1%) are found to be related to local compressibility effects in the direct vicinity of the ALM. Most significantly, compressibility is found to affect the sampled velocity and thereby accuracy of the ALM.

Experimental validation and improvement of actuator line model in the large-eddy simulation of wind-turbine wakes

Wind-turbine wakes significantly affect the power output of downstream wind turbines. In order to improve the calculation accuracy of wind-turbine wakes in the atmospheric boundary layer, a new improved actuator line - large-eddy simulation (AL-LES) method is proposed and verified by experiments. The traditional AL-LES method is improved in three aspects. Firstly, the atmospheric turbulence is generated by a dynamic k-equation LES with a wall shear stress model and a buoyancy effect model. Secondly, the nacelle and tower are modelled based on the static actuator line method. Finally, the distribution of blade body force is improved by using an anisotropic 3D Gaussian function. Based on the results of three wind tunnel experiments conducted by Norwegian University of Science and Technology (NTNU), China Aerodynamics Research and Development Centre (CARDC) and Von Karman Institute for Fluid Dynamics (VKI), the improved AL-LES method is validated from the aspects of wind-turbine power performance, the generation of tip vortex, and the distribution of wake velocity under typical offshore and onshore conditions. In addition, a full-scale wind turbine installed in Gansu is used for the experimental and numerical research. The main conclusion is that compared with the traditional method, the new AL-LES method improves the numerical accuracy by nearly 22%, and can more accurately simulate the interaction between atmospheric turbulence and wind-turbine wakes. The results can help the research of wind-turbine wakes and the micro-location selection of wind farm.

Investigation of the near-wake behaviour of a utility-scale wind turbine

Super-large-scale particle image velocimetry and flow visualization with natural snowfall is used to collect and analyse multiple datasets in the near wake of a 2.5 MW wind turbine. Each dataset captures the full vertical span of the wake from a different perspective. Together, these datasets compose a three-dimensional picture of the near-wake flow, including the effect of the tower and nacelle and the variation of instantaneous wake expansion in response to changes in turbine operation. A region of high-speed flow is observed directly behind the nacelle, and a region of low-speed flow appears behind the tower. Additionally, the nacelle produces a region of enhanced turbulence in its wake while the tower reduces turbulence near the ground as it breaks up turbulent structures in the boundary layer. Analysis of the instantaneous wake behavior reveals variations in wake expansion – and even periods of wake contraction – occurring in response to changes in angle of attack and blade pitch gradient. This behaviour is found to depend on the region of operation of the turbine. These findings can be incorporated into wake models and advanced control algorithms for wind farm optimization and can be used to validate wind turbine wake simulations.

Development of the supercomputer version RIAM-COMPACT and application to wind turbine wake simulation

Development of the supercomputer version RIAM-COMPACT and application to wind turbine wake simulation

AD/RANS Simulations of Wind Turbine Wake Flow Employing the RSM Turbulence Model: Impact of Isotropic and Anisotropic Inflow Conditions

The Reynolds-averaged Navier–Stokes (RANS)-based generalized actuator disc method along with the Reynolds stress model (AD/RANS_RSM) is assessed for wind turbine wake simulation. The evaluation is based on validations with four sets of experiments for four horizontal-axis wind turbines with different geometrical characteristics operating in a wide range of wind conditions. Additionally, sensitivity studies on inflow profiles (representing isotropic and anisotropic turbulence) for predicting wake effects are carried out. The focus is on the prediction of the evolution of wake flow in terms of wind velocity and turbulence intensity. Comparisons between the computational results and the measurements demonstrate that in the near and transition wake region with strong anisotropic turbulence, the AD/RANS_RSM methodology exhibits a reasonably good match with all the experimental data sets; however, in the far wake region that is characterized by isotropic turbulence, the AD/RANS_RSM predicts the wake velocity quite accurately but appears to over-estimate the wake turbulence level. While the introduction of the overall turbulence intensity is found to give an improved agreement with the experiments. The performed sensitivity study proves that the anisotropic inflow condition is recommended as the profile of choice to represent the incoming wind flow.

A statistical model for wake meandering behind wind turbines

A new wake model is proposed to account for wake meandering in simulations of wind-turbine wakes performed on steady solvers, through a wake-meandering description based on the dispersion theory of Taylor (1921, P. Lond. Math Soc., vol. 20, pp. 196–211). Single-turbine simulations were performed by means of the linearised solver ORFEUS. By analysing the steady wake behind a turbine, a set of parameters describing the wake was first obtained and synthesised into a look-up table. The proposed meandering model extended the simulation results by superimposing the lateral and vertical meandering motions to the steady wake. As a result, the time-averaged velocity distribution of the wake was increased in width and reduced in intensity. Through this combination, the model provides rationale for the wake-deficit decrease and for the power underestimation effects of several wake models. The new wake model is validated against the Lillgrund and Horns Rev data sets.

The Actuator Line Model in Lattice Boltzmann Frameworks: Numerical Sensitivity and Computational Performance

The growing use of large-eddy simulations for the modelling of wind farms makes the need for efficient numerical frameworks more essential than ever. GPU-accelerated implementations of the Lattice Boltzmann Method (LBM) have shown to provide significant performance gains over classical Navier-Stokes-based computational fluid dynamics. Yet, their use in the field of wind energy remains limited to date. In this fundamental study the cumulant LBM is scrutinised for actuator line simulations of wind turbines. The numerical sensitivity of the method in a simple uniform inflow is investigated with respect to spatial and temporal resolution as well as the width of the actuator line’s regularisation kernel. Comparable accuracy and slightly better stability properties are shown in relation to a standard Navier-Stokes implementation. The results indicate the overall suitability of the cumulant LBM for wind turbine wake simulations. The potential of the LBM for future wind energy applications is clarified by means of a brief comparison of computational performance.

Wake behavior and control: comparison of LES simulations and wind tunnel measurements

Abstract. This paper applies a large-eddy actuator line approach to the simulation of wind turbine wakes. In addition to normal operating conditions, a specific focus of the paper is on wake manipulation, which is performed here by derating, yaw misalignment and cyclic pitching of the blades. With the purpose of clarifying the ability of LES methods to represent conditions that are relevant for wind farm control, numerical simulations are compared to experimental observations obtained in a boundary layer wind tunnel with scaled wind turbine models. 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.

Turbulence-resolving simulations of wind turbine wakes

Turbulence-resolving simulations of wind turbine wakes are presented using a high-order flow solver combined with both a standard and a novel dynamic implicit spectral vanishing viscosity (iSVV and dynamic iSVV) model to account for subgrid-scale (SGS) stresses. The numerical solutions are compared against wind tunnel measurements, which include mean velocity and turbulent intensity profiles, as well as integral rotor quantities such as power and thrust coefficients. For the standard (also termed static) case the magnitude of the spectral vanishing viscosity is selected via a heuristic analysis of the wake statistics, while in the case of the dynamic model the magnitude is adjusted both in space and time at each time step. The study focuses on examining the ability of the two approaches, standard (static) and dynamic, to accurately capture the wake features, both qualitatively and quantitatively. The results suggest that the static method can become over-dissipative when the magnitude of the spectral viscosity is increased, while the dynamic approach which adjusts the magnitude of dissipation locally is shown to be more appropriate for a non-homogeneous flow such that of a wind turbine wake.

Assessment of inflow boundary conditions for RANS simulations of neutral ABL and wind turbine wake flow

It is known that wind turbines actually operate in the lower part of the atmospheric boundary layer (ABL), the modelling of this ABL flow is an important precondition for the simulations of wind turbine wakes. So, the capabilities of various inlet boundary conditions with related modelling methodologies in constructing equilibrium ABL are assessed firstly through cases of ABL flows over uniformly rough terrain, to ascertain that there are no substantial changes in the prescribed profiles throughout the whole computational domain. In this process, six popular turbulence inflow profiles, including four uniform and two non-uniform ones, are considered and investigated. Then, sensitivity studies on inflow profiles for predicting wind turbine wake development are carried out. Through comparing with the Sexbierum field experimental data, in terms of wake velocity and turbulence intensity along the cross-wind direction at several downstream positions, this study finds out that the shape and magnitude of wake velocity and wake turbulence profiles are significantly affected by different inflow profiles. Possible reasons for this sensibility are discussed, and accordingly, some suggestions are given to improve the accuracy of wind turbine wake simulation.

Sensitivity Analysis of Inflow Conditions and Turbulence Models on Wind Turbine Wake Simulation Using an AD/RANS Approach

Sensitivity Analysis of Inflow Conditions and Turbulence Models on Wind Turbine Wake Simulation Using an AD/RANS Approach

The turbulence scales of a wind turbine wake: A revisit of extended k-epsilon models

The turbulence time and length scales of a single wind turbine wake subjected to atmospheric turbulence are calculated from two large eddy simulations that differ in ambient turbulence intensity. The smallest turbulence length scale in the wake is about half the rotor radius and it increases for higher ambient turbulence levels. The large eddy simulations are compared with Reynolds-averaged Navier-Stokes simulations employing the standard and three extended k-ε models: the k-ε-fP model of van der Laan, the k-ε model of Shih and k-ε model of Durbin. It is shown that all three extended k-ε models can be written in a similar form. All Reynolds-averaged Navier-Stokes based turbulence models predict turbulence time scales that are comparable to the turbulence time scales of the large eddy simulations. The standard k-ε model underpredicts the velocity deficit because the turbulence length scale is overpredicted compared to the large eddy simulations. The performance of the k-ε model of Durbin shows to be very dependent on the ambient turbulence level and it is therefore less suited for wind turbine wake simulations. The k-ε model of Shih and the k-ε-fP model of van der Laan are recommended to be used for wind turbine wake simulations because their results are similar and compare well with results of large eddy simulations for both a low and high ambient turbulence intensity due to a limitation of the turbulence length scale in the near wake.

Study on Numerical Simulation of Wind Turbine

Wake effect may decrease the overall power output of the wind farm. It is necessary to study the wind turbine wake, which would benefit wind turbine micro-sitting selection optimization. In order to improve the accuracy of wind turbine wake simulation, dynamic mesh method based on CFD (computational fluid dynamics) is proposed. Simulation comparison tests between dynamic meshing method and actuator disk method have been done with a 1.5MW wind turbine. The result shows that with the effect of the rotating blades being considered, dynamic meshing method can capture the characteristics such as vortexes, velocity distribution, flow recovery rate, and the wake radius along the axis direction of wind turbine wake more accurately.

Analysis and Modifications of Turbulence Models for Wind Turbine Wake Simulations in Atmospheric Boundary Layers

Computational fluid dynamics (CFD) simulations of wind turbine wakes are strongly influenced by the choice of the turbulence model used to close the Reynolds-averaged Navier-Stokes (RANS) equations. A wrong choice can lead to incorrect predictions of the velocity field characterizing the wind turbine wake and, consequently, to an incorrect power estimation for wind turbines operating downstream. This study aims to investigate the influence of different turbulence models, namely the k–ε, k–ω, SSTk–ω, and Reynolds stress models (RSM), on the results of CFD wind turbine simulations. Their influence was evaluated by comparing the CFD results with the publicly available experimental measurements of the velocity field and turbulence quantities from the Sexbierum and Nibe wind farms. Consistent turbulence model constants were proposed for atmospheric boundary layer (ABL) and wake flows according to previous literature and appropriate experimental observations, and modifications of the derived turbulence model constants were also investigated in order to improve agreement with experimental data. The results showed that the simulations using the k–ε and k–ω turbulence models consistently overestimated the velocity and turbulence quantities in the wind turbine wakes, whereas the simulations using the shear-stress transport (SST) k–ω and RSMs could accurately match the experimental data. Results also showed that the predictions from the k–ε and k–ω turbulence models could be improved by using the modified set of turbulence coefficients.