Wake Interaction Research Articles

Wind farm power maximization through wake steering with a new multiple wake model for prediction of turbulence intensity

A new multiple wake model is developed for wind farm power prediction and wind farm control. First, numerical simulations are conducted for two wind turbines under different layout sets, and the characteristics of mean velocity and turbulence intensity in multiple wakes are systematically investigated. A new multiple wake model considering the local effective turbulence on the rotor and the wake interaction effects is proposed. The proposed model can favorably predict the mean velocity and turbulence intensity distributions in multiple wake regions, as well as the power production in wind farm comparing with numerical simulations and field measurements. Finally, the new proposed multiple wake model is applied to wind farm modelling and optimization framework, which enables the maximization of wind farm power production by wake steering control. The wind sector width of 2° with the wind speed bin of 0.5 m/s is proposed for the lookup-table-based wake steering optimization. The proposed values reduce the prediction error of annual energy production gain from 34.5% to 3.2% comparing with the conventional values of 5° and 1 m/s. In addition, the yaw offset limit of ±15° is recommended to satisfy both the maximization of power production and the safety requirement of International Electrotechnical Commission (IEC) standard.

Data driven learning model predictive control of offshore wind farms

This paper presents a data-driven control approach for maximizing the total power generation of the offshore wind farm by using a recently developed learning model predictive control (LMPC) algorithm. The control is designed by coordinating yaw angle control actions of wind turbines to mitigate the wake interactions among the turbines for increasing the total farm power production, which is termed as wake redirection. This paper mainly focuses on designing the architecture and methodology of the LMPC for wind farm, including a unified wind turbine wake interaction model, the LMPC for minimizing an iteration cost function, the recursive feasibility, stability and convergence analysis. Extensive comparative studies are conducted to verify the performance of the LMPC in comparison with the existing model predictive control (MPC) method under the same wind speed conditions. The results show that the wind farm yields up to 15% more power production by using the LMPC than the conventional MPC.

Underlying mechanisms of propeller wake interaction with a wing

Abstract

On the Huse-Muren model for wake interaction with vertical offshore cylinders

There are many offshore applications that incorporate vertical cylindrical-shaped structural elements such as marine risers, TLP tendons and near shore pier designs. A better understanding of complex wake flows on the drag dominated force predictions is very much of interest for offshore design problems. This study investigates the Huse-Muren analytical wake model, whose mathematical formulation was founded on the earlier definitive studies of Prandtl and Schlichting. Developing a deeper understanding of their key findings that were based upon central assumptions and key approximations is central to this research. Analysis of their model leads to the development of dimensionless expressions for the drag coefficient correction ratio. Both the DNV-RP recommended drag force coefficients and some of Sarpkaya's U-tube measurements are used to investigate whether their analytical model could be used to bound the experimental data. The Keulegan-Carpenter number, A0/D ratios, Sarpkaya's Beta parameter and Reynold's number are used to interpret the range of model applicability. For the current cross flow case, variables identified in Huse and Muren's formulation lead to a dimensionless expression involving the period of in-line oscillation and the velocity of the cross-current resulting in a parameter similar to the Keulegan-Carpenter number. The ratio of this parameter to the Keulegan-Carpenter number allows one to address the drag coefficient correction ratio where a cylinder is simultaneously subjected to in-line oscillatory flow and a steady cross current coming from the transverse direction. A numerical simulation is performed to investigate the behavior of the correction ratio in this flow condition, and the results are presented and discussed.

The impact of turbulence and turbine operating condition on the wakes of tidal turbines

Before initiating a study on the interaction of multiple wakes, it is imperative that turbine wake hydrodynamics are studied in isolation. In this paper CFD computer simulations of downstream turbine wakes have been run using a scale-resolving hybrid turbulence model known as a detached eddy simulation. To allow validation of the CFD simulations the computer models were supported by flume measurements with a lab scale tidal stream turbine run at three tip-speed ratios and three turbulence conditions, varying both turbulence intensity and length-scale.From the study it was demonstrated that turbulence intensity has a significant impact on the wake development for both recovery and width. The turbulence length scales of between 0.25 and 1.0 rotor diameter did not have a significant impact on the wake.The turbine operating condition also had an impact on the resulting wakes. In the near wake, centreline velocity recovery was found to increase with increasing turbine thrust due to flow being diverted towards the turbine nacelle. For a volumetric averaged wake, greater power extraction was found to cause the greatest near-wake deficit. Wake width was found to increase with increasing tip-speed ratio (and therefore turbine thrust).

Integrated wind farm layout and control optimization

Abstract. The objective of this paper is to investigate the joint optimization of wind farm layout and wind farm control in terms of power production. A successful fulfilment of this goal requires the following: (1) an accurate and fast flow model, (2) selection of the minimum set of design parameters that rules or governs the problem, and (3) selection of an optimization algorithm with good scaling properties. For control of the individual wind farm turbines with the aim of wind farm production optimization, the two most obvious strategies are wake steering based on active wind turbine yaw control and wind turbine derating. The present investigation is limited to wind turbine derating. A high-speed linearized computational fluid dynamics (CFD) Reynolds-averaged Navier–Stokes (RANS) solver models the flow field and the crucial wind turbine wake interactions inside the wind farm. The actuator disc method is used to model the wind turbines, and utilizing an aerodynamic model, the design space of the optimization problem is reduced to only three variables per turbine – two geometric and one carefully selected variable specifying the individual wind turbine derating setting for each mean wind speed and direction. The full design space is spanned by these (2N+NdNsN) parameters, where N is the number of wind farm turbines, Nd is the number of direction bins, and Ns is the number of mean wind speed bins. This design space is decomposed into two subsets, which in turn define a nested set of optimization problems to achieve a significantly faster optimization procedure compared to a direct optimization based on the full design space. Following a simplistic sanity check of the platform functionality regarding wind farm layout and control optimization, the capability of the developed optimization platform is demonstrated on a Swedish offshore wind farm. For this particular wind farm, the analysis demonstrates that the expected annual energy production can be increased by 4 % by integrating the wind farm control into the design of the wind farm layout, which is 1.2 % higher than what is achieved by optimizing the layout only.

Fluid forces acting on three and four long side-by-side flexible cylinders undergoing flow-induced vibration(FIV)

Multiple side-by-side cylinders are widely employed in many engineering applications. Consequently, flow-induced vibrations (FIV), which have a significant influence on structural reliability, have drawn considerable concern from many investigators. Due to the complicated wake interactions behind the cylinders, the hydrodynamic characteristics of multiple side-by-side cylinders subject to FIV are obviously different from those of a single cylinder. Hydrodynamic force coefficients play an important role in evaluating structural reliability and predicting fatigue damage. However, few relevant investigations have been performed on hydrodynamic force coefficients of multiple side-by-side flexible cylinders, which require further research. In this paper, the hydrodynamic force coefficients of three and four side-by-side flexible cylinders with a spacing ratio of 6.0 are obtained by an inverse analysis method based on the oscillation displacements. In the resonance regions, the hydrodynamic force coefficients of multiple cylinders exhibit variation trends similar to those of a single cylinder, especially at higher reduced velocities. In the mode switch regions, relatively large deviations exist between the coefficients of multiple cylinders and single cylinder. The inner cylinder in the three-cylinder system shows prominently distinctive behaviour compared with the outer two cylinders. For the four-cylinder system in a side-by-side arrangement, the outer two cylinders show relatively slight differences between each other compared with the inner two cylinders.

Aerodynamic Performance of an Octorotor SUAV with Different Rotor Spacing in Hover

To study the aerodynamic performance of hovering octorotor small unmanned aerial vehicles (SUAV) with different rotor spacing, the computational fluid dynamics (CFD) method is applied to analyze the flow field of an octorotor SUAV in detail. In addition, an experimental platform is built to measure the thrust and power of the rotors with rotor spacing ratios L/D of 1.0, 1.2, 1.4, 1.6, and 1.8, sequentially. According to the theory of momentum, rotor aerodynamic performance is obtained with qualitative analysis. Further analysis with numerical simulation is presented with the flow field of the octorotor SUAV, the vorticity distribution, velocity distribution, pressure distribution, and streamline. The results show that the aerodynamic performance varies with the rotor spacing. Specifically, the aerodynamic performance is poor at L/D = 1.0, which is accompanied with strong interaction of wake and tip vortexes and interaction with each other. However, the aerodynamic efficiency is much improved with a larger rotor spacing, especially achieving the highest at L/D = 1.8, which is considered to be the best rotor spacing ratio for this kind of octorotor SUAV.

Aerodynamic Characteristics of Helicopter with Ducted Fan Tail Rotor in Hover under Low-Speed Crosswind

The tail rotor of a helicopter operating under low-speed crosswind undergoes highly complex flow due to the interaction between the main rotor, fuselage, and tail rotor system. In this study, numerical simulations have been conducted on the complete configuration of a helicopter with a ducted fan tail rotor system (comprising a main rotor, ducted fan tail rotor, fuselage, and empennage) to analyze the wake interaction in hovering flight under various crosswind directions. The flow characteristics around the tail rotor, the tail rotor thrust, and the yawing moment of the helicopter are investigated and evaluated. The aerodynamic forces are compared with those of a helicopter with an open-type tail rotor. The results indicate that the aerodynamic performance of the ducted fan tail rotor is highly affected by the wakes of both the main rotor and port wing. Nevertheless, the helicopter with a ducted fan tail rotor is observed to be much more directionally stable under various crosswind directions, than that with an open-type tail rotor. This is because the rotor is protected by the fixed part of the tail rotor system in the former case.

FAST.Farm development and validation of structural load prediction against large eddy simulations

AbstractFAST.Farm is a mid‐fidelity engineering tool developed by the National Renewable Energy Laboratory targeted at accurately and efficiently predicting wind turbine power production and structural loading in wind farm settings, including wake interactions between turbines. FAST.Farm is based on several principles of the dynamic wake meandering (DWM) model, but also addresses limitations of previous DWM implementations. Previous FAST.Farm studies have shown the similarities and differences between FAST.Farm and large eddy simulations for rigid turbine cases. The objective of this work is to quantify the ability of FAST.Farm to accurately predict turbine structural response in a small wind farm. This is done by comparing FAST.Farm structural response to SOWFA‐OpenFAST results for three laterally‐aligned turbines. The purpose of this study is to characterize the similarities and differences between FAST.Farm and a higher fidelity model for predicting turbine structural response in a wind farm for differing atmospheric inflows. Strong statistical agreement was found between FAST.Farm and SOWFA‐OpenFAST structural response for the non‐waked upstream turbine, and good agreement was found for the downstream turbines for most structural quantities. Higher differences were seen for downstream turbines with low ambient turbulence intensity or yawed turbines, suggesting areas for FAST.Farm wake dynamics modeling improvements. For all cases and turbines, small statistical differences were seen between blade deflections and bending moments, with larger differences for tower‐top and tower‐base bending moments. Overall, the results establish confidence for applying FAST.Farm to wind farm power and loads analyses and identify areas where further model validations and model improvements should be targeted.

Wind tunnel testing of wake steering with dynamic wind direction changes

Abstract. The performance of an open-loop wake-steering controller is investigated with a new unique set of wind tunnel experiments. A cluster of three scaled wind turbines, placed on a large turntable, is exposed to a turbulent inflow and dynamically changing wind directions, resulting in dynamically varying wake interactions. The changes in wind direction were sourced and scaled from a field-measured time history and mirrored onto the movement of the turntable. Exploiting the known, repeatable, and controllable conditions of the wind tunnel, this study investigates the following effects: fidelity of the model used for synthesizing the controller, assumption of steady-state vs. dynamic plant behavior, wind direction uncertainty, the robustness of the formulation in regard to this uncertainty, and a finite yaw rate. The results were analyzed for power production of the cluster, fatigue loads, and yaw actuator duty cycle. The study highlights the importance of using a robust formulation and plant flow models of appropriate fidelity and the existence of possible margins for improvement by the use of dynamic controllers.

Behaviors of two semi-passive oscillating hydrofoils with a tandem configuration

Two hydrofoils with a tandem spatial configuration can share the same flow window. The wake vortex interaction and blockage effect have noticeable impacts on the response of two hydrofoils for a semi-passive system. In this study, the responses of two oscillating hydrofoils with a tandem arrangement are examined using a numerical approach. The results indicate that the downstream hydrofoil has an inferior energy harvesting performance because it operates in the upstream hydrofoil wake. Several special cases were observed. As a result of a negative interaction in the wake vortices, (i) two hydrofoils were shown to have different heaving equilibrium positions and thus no longer share the flow window; and (ii) the downstream hydrofoil was demonstrated to have a larger heaving amplitude but a lower energy harvesting performance. At a very high oscillation frequency, the downstream hydrofoil tends to incur an irregular response owing to the intense vortex formation and shedding, as well as a constant interaction of the wake vortices. The downstream hydrofoil achieves an unexpectedly better energy harvesting performance due to a favorable interaction of the wake vortices. However, the entire system can’t achieve high efficiency due to the poor energy harvesting performance of the upstream hydrofoil. When two semi-passive oscillating hydrofoils have a tandem configuration, negative wake vortex interactions should be avoided instead of seeking a favorable interaction.

Effect of In-Line Tandem Configuration on Performance and Scaling of Pitching Hydrofoils

Simulations are reported on the behavior of two tandem symmetric foils in an in-line configuration undergoing pitching motion. The Reynolds number was varied between 1000 and 4000, whereas the pitching Strouhal number ranged from 0.2 to 0.5 and the amplitude of pitching varied from 4 to 8 deg. The streamwise separation distance between the foils was fixed at one chord length. The performance of the downstream foil was significantly worsened (up to 80% in efficiency) by the tandem effects. The Strouhal number, Reynolds number, and pitching amplitude had substantial impact on the performance of the downstream foil. For the upstream foil, however, the performance improved by up to 147% in efficiency at all ranges of Reynolds number, Strouhal number, and pitching amplitude. The scaling of the individual foil performance metric and the system collective performance metric were investigated for tandem foils. The scaling developed for isolated foils does hold for tandem foils in the in-line configuration with modified empirical coefficients that depend on reduced frequency and Reynolds number in a range of and . However, the leading foil behavior at lower is more scattered compared to higher , which suggests more significant wake interactions that influence the scaling and performance of the leading foil.

Performance and Wake Characterization of a Model Hydrokinetic Turbine: The Reference Model 1 (RM1) Dual Rotor Tidal Energy Converter

The mechanical power and wake flow field of a 1:40 scale model of the US Department of Energy’s Reference Model 1 (RM1) dual rotor tidal energy converter are characterized in an open-channel flume to evaluate power performance and wake flow recovery. The NACA-63(4)-24 hydrofoil profile in the original RM1 design is replaced with a NACA-4415 profile to minimize the Reynolds dependency of lift and drag characteristics at the test chord Reynolds number. Precise blade angular position and torque measurements were synchronized with three acoustic Doppler velocimeters (ADV) aligned with each rotor centerline and the midpoint between the rotor axes. Flow conditions for each case were controlled to maintain a hub height velocity, uhub= 1.04 ms−1, a flow Reynolds number, ReD= 4.4 × 105, and a blade chord length Reynolds number, Rec= 3.1 × 105. Performance was measured for a range of tip-speed ratios by varying rotor angular velocity. Peak power coefficients, CP= 0.48 (right rotor) and CP= 0.43 (left rotor), were observed at a tip speed ratio, λ= 5.1. Vertical velocity profiles collected in the wake of each rotor between 1 and 10 rotor diameters are used to estimate the turbulent flow recovery in the wake, as well as the interaction of the counter-rotating rotor wakes. The observed performance characteristics of the dual rotor configuration in the present study are found to be similar to those for single rotor investigations in other studies. Similarities between dual and single rotor far-wake characteristics are also observed.

Analytical three‐dimensional wind flow model for real‐time wind farm simulation

Wind energy conversion systems have gained an essential share of the electricity supply and are increasingly competitive with conventional power plants based on fossil fuel. Wind turbines are expected to continue their increase in rated power and size, exceeding 10 MW and more, imposing major challenges concerning not only the operation of future power grids but also mechanical stress on individual wind turbines, their aerodynamic interaction and holistic wind farm control strategies as well as simulation models being able to cover all these aspects. This study deals with requirements of a real-time capable three-dimensional wind flow model for wind farm simulation, covering key issues from wind modelling through wake interactions to prospects of advanced control strategies. Building upon this, a modular framework is proposed and implemented by integrating existing validated analytical models. Simulation results demonstrating the model capabilities are presented, discussed and evaluated concerning future research focus.

Deflected wake interaction of tandem flapping foils

Abstract

Concurrent Optimal Re/Active Power Control for Wind Farms Under Low-Voltage-Ride-Through Operation

A novel concurrent control scheme of optimal re/active power regulation in a wind farm is proposed in this letter, of which the objective is to minimize the detrimental shortfall of active power during low-voltage-fault events. Distinguished from the existing works, the proposed control scheme incorporates an efficient optimization module into the wind turbine controllers. Specifically, it features the merit that the practical feasibility of tracking the resultant references can be ensured via considering multiple constraints in presence of complex wake interactions. Case studies on a permanent-magnet-synchronous-generator based wind farm built in a joint DIgSILENT-MATLAB platform are carried out to demonstrate the effectiveness of the proposed control scheme.

Real-time identification of clusters of turbines

The implementation of wind farm control strategies requires knowledge of the turbines that are aerodynamically coupled through wake effects. As the wind changes direction, the clusters of coupled turbines may vary within the array. The present study proposes a simple method to identify clusters of turbines coupled by the wake interaction. The method uses the correlation of the power production fluctuations of the turbines in the wind farm. Large Eddy Simulations (LES) results of the flow in a 4×4 turbine array have been used to validate the proposed approach. This study introduces a tool which can be integrated into a control system to optimize wind plant power production.

On the multi-scale turbulent structure interactions within wind farms

The quantification of the scale-by-scale interaction of multi-wakes and background flow is required to estimate turbines behaviour within wind farms. The paper provides insight on the evolution of multi-scale turbulence of a model wind farm operating in turbulent boundary layer. The wind farm configured with aligned turbines set at streamwise spacing between units of 5 diameters by spanwise spacing of 2.5 diameters, corresponding to an array of 8x3 turbines. Howtwire anemometry was used to acquire high-resolution measurements of streamwise velocity fluctuations at various locations. Experimental results suggest that the contribution of multi-scale turbulence structures to the kinetic energy of the wake is highly dependent on the location inside the wind farm, the evolution is significantly modulated by turbine rotation, wake interactions and outer flow, large scale motions mainly dominate outer and far wakes, while small scales are popular in inner and near wakes.

Yaw induced wake deflection-a full-scale validation study

Aerodynamic wake interactions between turbines located in wind power plants cause both a loss in power production and an increase in fatigue loading of the wind farm turbines. Yaw induced active wake deflection is one possible wind farm control strategy, which can be applied to mitigate wake effects on nearby downstream located wind turbines. In the present study three flow models of different fidelities are applied to mimic a full-scale study of wake deflection recorded by an advanced synchronised setup of two long-range pulsed scanning lidars. The investigated case studies encompass a base case with (approximately) zero yaw setting supplemented by two non-zero yaw cases of 17.5° and -14.5°, respectively. The model results are compared mutually as well as with the result of the full-scale measurement campaign.