Wake Control Research Articles

Study of three wake control strategies for power maximization of offshore wind farms with different layouts

The wake regulation by cooperative yaw control, axis induction control, or their combination can significantly improve the total power output of wind farms. However, a comprehensive study of the three control strategies is still lacking, which hinders their engineering applications. To this end, the optimized yaw angles, induction factors, and total power promotion are systematically compared for the three control strategies on both a regularly arranged wind farm and an irregularly arranged realistic offshore wind farm. The results indicate that all the three strategies work similarly to achieve a net power gain by enhancing the power output of the downstream turbines while slightly downgrading the upstream turbines. For the regularly arranged wind farm, very notable power promotions are obtained in the aligned direction, and all the three control strategies show similar performance, but the promotions almost disappear with a small misalignment. In contrast, power promotion is less sensitive to the wind direction for the irregularly arranged wind farm. The promotion of the yaw control is much more predominate than that of the induction control, and the combined control only slightly outperforms the single yaw control in the realistic wind farm. In the prevailing wind direction, the total power of the realistic wind farm can be increased by 2.1% by the combined control strategy under the wind speed of 6 m/s.

A Machine Learning Method for Modeling Wind Farm Fatigue Load

Wake steering control can significantly improve the overall power production of wind farms. However, it also increases fatigue damage on downstream wind turbines. Therefore, optimizing fatigue loads in wake steering control has become a hot research topic. Accurately predicting farm fatigue loads has always been challenging. The current interpolation method for farm-level fatigue loads estimation is also known as the look-up table (LUT) method. However, the LUT method is less accurate because it is challenging to map the highly nonlinear characteristics of fatigue load. This paper proposes a machine-learning algorithm based on the Gaussian process (GP) to predict the farm-level fatigue load under yaw misalignment. Firstly, a series of simulations with yaw misalignment were designed to obtain the original load data, which considered the wake interaction between turbines. Secondly, the rainflow counting and Palmgren miner rules were introduced to transfer the original load to damage equivalent load. Finally, the GP model trained by inputs and outputs predicts the fatigue load. GP has more accurate predictions because it is suitable for mapping the nonlinear between fatigue load and yaw misalignment. The case study shows that compared to LUT, the accuracy of GP improves by 17% (RMSE) and 0.6% (MAE) at the blade root edgewise moment and 51.87% (RMSE) and 1.78% (MAE) at the blade root flapwise moment.

Lower Serum Calcium Levels Associated with Disrupted Sleep and Rest-Activity Rhythm in Shift Workers.

Vitamin D deficiency is prevalent in many developed countries, and several studies suggest that vitamin D plays an essential role in brain function. A recent study showed that vitamin D deficiency was closely associated with daytime sleepiness and shorter sleep time. The relationshipbetween vitamin D levels and calcium levels is well established, and calcium level regulates slow-wave sleep generation. It is conceivable that the sleep disturbance in vitamin D deficiency may be due to an altered calcium level. Nonetheless, calcium levels, sleep disturbances, and activity rhythms have not been investigated directly. Therefore, we hypothesized that calcium and vitamin D levels might be important in regulating sleep and activity rhythm, and we analyzed the correlation with calcium levels by actigraphy analysis. Interestingly, a negative correlation was found between calcium level and sleep latency, total sleep time, use of sleep medicine, and daytime dysfunction among shift workers. In contrast, non-shift workers showed a negative correlation between the calcium level and the circadian phase. These findings suggest that low serum calcium levels may disrupt sleep–wake control and rest–activity rhythm, even if they are within the normal range.

Enhanced control of the turbulent flow past a circular cylinder with rotating rods inspired by an inviscid solution

This paper examines an active mechanism of wake control for a turbulent flow regime of Reynolds number 1000 through finite volume, three-dimensional DES simulations. Eight peripheral rods equally spaced about a central body are forced to spin around their axes in two arrangements: case 0, with uniform rotation speed and case 1, with rotation rates inspired by the solution of the potential flow around the body. The two cases were constrained to the same input kinetic energy for every set of rotations. We have found case 1 to be more effective to control the wake, eliminate turbulent structures, and attenuate mean drag and fluctuating lift. The interference of the rods in the flow past the main body resulted in a steady wake in both cases, provided enough rotation was supplied to the system. This result was previously achieved only in laminar regime. Novel to such a system, rotations midway between a vortex wake and a steady wake led streamwise vortices to show a mode-B-like mechanism of vorticity transfer. Case 1 generally suppressed vortex shedding with lower input kinetic energy and required less power than case 0, thus resulting in a more efficient configuration to suppress vortices in the wake.

Design, steady performance and wake characterization of a scaled wind turbine with pitch, torque and yaw actuation

Abstract. This paper describes the design and characterization of a scaled wind turbine model, conceived to support wake and wind farm control experiments in a boundary layer wind tunnel. The turbine has a rotor diameter of 0.6 m and was designed to match the circulation distribution of a target conceptual full-scale turbine at its design tip speed ratio. In order to enable the testing of plant-level control strategies, the model is equipped with pitch, torque and yaw actuation and is sensorized with integrated load cells, rotor azimuth and blade pitch encoders. After describing the design of the turbine, its steady-state performance and wake characteristics are assessed by conducting experiments in two different wind tunnels, in laminar and turbulent conditions, collecting wake data with different measurement techniques. A large-eddy simulator coupled to an actuator-line model is used to develop a digital replica of the turbine and of the wind tunnel. For increased accuracy, the polars of the low-Reynolds-number airfoil used in the numerical model are tuned directly from measurements obtained from the rotor in operation in the wind tunnel. Results indicate that the scaled turbine performs as expected: measurements are repeatable and consistent, and the wake appears to have a realistic behavior in line with expectations and with a similar but slightly larger scaled model turbine. Furthermore, the predictions of the numerical model are well in line with experimental observations.

The effect of canard’s optimum geometric design on wake control behind the car using Artificial Neural Network and Genetic Algorithm

Canard is a cutting-edge aerodynamic attachment for lowering the vehicle’s drag coefficient by efficiently directing the airflow as well as reducing the lift coefficient by enhancing down-force. This paper aims to simulate the airflow crossing over the car to investigate the effect of canards’ geometric design on the rear-body wake using the application of CFD-based optimization. Hence, 7 design variables based on the geometry of the canard are considered, and the objective functions are set to be drag and lift coefficients that are aimed to be minimized. Firstly, ANSYS Fluent is utilized to generate CFD calculations for a series of Design of Experiment (DOE) points. Then, the GMDH-ANN processes the results to elicit the polynomials that demonstrate the relation of design variables and objective functions. A genetic algorithm is next implemented for multi-objective optimization using polynomials as its input, and consequently, Pareto optimal points are achieved. The numerical results show that an appropriate design for canards on the rear bumper causes a potential drag and lifts reduction of 9.62% and 9.6% in comparison to the car without canards. Moreover, the size of the wake behind the car decreases and the differences in the pressure distribution between car fore-body and rear-body is reduced. Finally, the fuel efficiency is potentially enhanced due the changes in car drag coefficient and frontal area.

A Probabilistic Learning Approach Applied to the Optimization of Wake Steering in Wind Farms

Abstract The wake steering control in wind farms has gained significant attention in the last years. This control strategy has shown promise to reduce energy losses due to wake effects and increase the energy production in a wind farm. However, wind conditions are variable in wind farms, and the measurements are uncertain what should be considered in the design of wake steering control strategies. This paper proposes using the probabilistic learning on manifold (PLoM), which can be viewed as a supervised machine learning method, to enable the wake steering optimization under uncertainty. The expected power generation is estimated considering uncertainties in wind speed and direction with good accuracy and reduced computational cost for two wind farm layouts, which expand the application of machine learning models in wake steering. Furthermore, the analysis shows the potential gain with the application of wake steering control.

Wake control of a bluff-body via distributed jets over its surface

The flow passing a circular cylinder can trigger flow-induced vibrations such as the vortex-induced vibration. In this paper, the authors investigated an active method to control the cylinder wake flow. The control scheme was achieved by active blowing through a structured porous surface that was manufactured by 3D printing precisely. The blowing momentum was changed by various mass flow rates so that it defined different values of a non-dimensional momentum coefficient Cμ. The experimental investigation was conducted in a wind tunnel. A 2D particle image velocimetry system was used to measure global flow fields. The Reynolds number based on D was 10 000 in the subcritical region, where D is the cylinder diameter. The proper orthogonal decomposition (POD) was utilized as a reduced-order model. Experimental results showed that transformations could be found in POD modal characteristics and vortex shedding frequencies. Fluctuations in the global wake were suppressed. Moreover, intensities of turbulence kinetic energy and elements of the Reynolds stress tensor T were decreased in the near wake region. It can be concluded that active blowing jets through the structured porous surface of the circular cylinder can be used to control the surrounding flow with effective Cμ values.

Design and testing of a model-scale yaw mechanism for an experimental wind turbine model

In this study a new yaw mechanism for a small existing wind turbine model is designed and tested. The special requirement for this yaw device is its compact design due to the small length scaling factor of the model turbine. Such a dense design is crucial in order for the wake of the model turbine not to be influenced by the yaw mechanism. Furthermore, it has to have a fast system response and high accuracy to take into account the time scaling. Different concepts are investigated and a design located at the tower base is realized. To limit its drag, the structure is being concealed in an aerodynamically shaped cover. Wind tunnel measurements confirmed that the wake of the turbine is not influenced by the structure. A system analysis, using a Simulink model of the turbine helps finding the right motor and shows that the device has a fast system response with high accuracy. Consequently, the designed yaw mechanism is suited for the small wind turbine model, which can be used for experimental wake and wind farm control studies in the future.

Dynamic wake tracking based on wind turbine rotor loads and Kalman filtering

In a wind farm setting, the location of the wake to which a downwind turbine is exposed is of high interest. It can be used for closed-loop active wake control, ultimately leading to fatigue load reduction and higher power generation. This work proposes a method for dynamic tracking of the meandering wake centre. The rotor takes the role of a sensor, with its blades sampling through the incoming wind field. The measurement of the flapwise blade root bending moment and its decomposition into the non-rotating yaw and tilt moments is used. The latter are linked to the lateral and vertical wake location via a parametric model, tuned with training data from aeroelastic simulations. The implementation of an Extended Kalman Filter (EKF) adds robustness to the tracking and allows to include physical knowledge of the wake meandering to the estimation. For this, the governing equations of the dynamic wake meandering model (DWM) are used to describe the meandering motion as a random walk process. The performance, possibilities and limitations of the tracking method in various inflow conditions are shown and discussed. Generally the wake tracking works satisfying, with estimation errors below 10% of the rotor diameter under moderate turbulence intensity. The Extended Kalman Filter formulation provides the confidence in the tracked wake position. The work shows how this can be effectively used for wake impingement detection.

Resolvent-based approach for pmb {H_2}-optimal estimation and control: an application to the cylinder flow

We consider estimation and control of the cylinder wake at low Reynolds numbers. A particular focus is on the development of efficient numerical algorithms to design optimal linear feedback controllers when there are many inputs (disturbances applied everywhere) and many outputs (perturbations measured everywhere). We propose a resolvent-based iterative algorithm to perform (i) optimal estimation of the flow using a limited number of sensors, and (ii) optimal control of the flow when the entire flow is known but only a limited number of actuators are available for control. The method takes advantage of the low-rank characteristics of the cylinder wake and provides full-dimensional solutions by implementing a terminal reduction technique based on resolvent analysis. Optimal feedback controllers are also obtained by combining the solutions of the estimation and control problems. We show that the performance of the estimators and controllers converges to the true global optima, indicating that the important physical mechanisms for estimation and control are of low rank.Graphic abstract

A novel hybrid control strategy of wind turbine wakes in tandem configuration to improve power production

The wind turbines that operate in the wake region of the upstream turbines produce less power and may suffer serious structural issues due to highly unsteady flows, which can reduce the life expectancy of the turbines. In this study, a novel hybrid wake control strategy for the wind farm power generation enhancement is proposed, which is based on highly accurate large eddy simulations coupled with the actuator line method. The combined effects of yaw angle and tilt angle control methods on the performance improvement of downstream wind turbines in wind farm layouts have not yet been investigated. This work would be the first attempt to evaluate the hybrid wake-control strategy of tandem wind turbines. It is found that the vortex generation is stronger at lower tilt angles because more parts of the rotor are affected by the wakes of the upstream wind turbine. An optimisation analysis is also provided to find the optimum wake deflection angles of the upstream turbine to maximize the electrical power generation. The results show that an accumulative power production increment of 17.1% is achieved by controlling both yaw angle (θ=30o), and tilt angle (φ=24o). The power obtained in the present study is approximately 6.1% higher than previous wake control techniques. By using the hybrid control strategy, an annual energy production enhancement of 3.7% is achieved, which is higher than the previous wake-controlled wind farm layouts.

The Sleep-Promoting Ventrolateral Preoptic Nucleus: What Have We Learned over the Past 25 Years?

For over a century, the role of the preoptic hypothalamus and adjacent basal forebrain in sleep–wake regulation has been recognized. However, for years, the identity and location of sleep- and wake-promoting neurons in this region remained largely unresolved. Twenty-five years ago, Saper and colleagues uncovered a small collection of sleep-active neurons in the ventrolateral preoptic nucleus (VLPO) of the preoptic hypothalamus, and since this seminal discovery the VLPO has been intensively investigated by labs around the world, including our own. Herein, we first review the history of the preoptic area, with an emphasis on the VLPO in sleep–wake control. We then attempt to synthesize our current understanding of the circuit, cellular and synaptic bases by which the VLPO both regulates and is itself regulated, in order to exert a powerful control over behavioral state, as well as examining data suggesting an involvement of the VLPO in other physiological processes.

Numerical study on aerodynamic drag and noise of circular cylinders with a porous plate

The wake control of the flow around a cylinder is a classic problem of fluid mechanics, which possesses a wide range of research foundations and applications. To attenuate the flow oscillation and resistance, this paper proposes a control strategy of cylinder flow based on an afterbody-attached porous material plate in the subcritical flow regime (Re = 1.4×105). Three-dimensional numerical simulations are carried out by combining large eddy simulation (LES) with Ffowcs Williams-Hawkings (FW-H) acoustic analogy, and the validation is achieved through the available experimental and numerical data. Two configurations, namely the solid plate attached cylinder (SPAC) and the porous plate attached cylinder (PPAC), are dedicatedly compared. The results show that the PPAC has a remarkable effect on vortex shedding suppression and drag reduction. Especially, the PPAC even achieves a lift oscillation reduction by 97%, well above that by the SPAC. Meanwhile, both the reduction rates of resistance and lift fluctuation exhibit non-monotonic variations concerning the porosity, and their maximum effects appear at the porosity of 30% and 40% respectively. In principle, the PPAC greatly changes the separation pattern that the wake pressure elevates, turbulent kinetic energy decreases, and vortex core moves downstream, though the separation zone greatly enlarges. More importantly, the wake flow oscillation is significantly weakened. Besides, the OASPL around the cylinder is reduced by above 24 dB averagely by the PPAC with a porosity of 40%.

Flow control of wake around a wall-mounted cube using a horizontal hole of different diameters

This paper describes an experimental investigation of flow control for the wake around a wall-mounted cube using horizontal control holes (HCHs) of different diameters drilled from the center of the front surface to the rear surface of the cube. The cube has side lengths of D = 50 mm, and HCHs with diameters of d = 10, 12, and 14 mm are considered. The instantaneous velocity fields are measured at a Reynolds number of 7800 based on time-resolved particle image velocimetry in a water tunnel. The HCHs suppress the recirculation zone, turbulence intensity, and Reynolds stress, and these control effects gradually increase with the hole diameter. The issuing flow from the large-diameter HCH completely obstructs the development of the downwash flow to the bottom wall and decomposes the near-wake arch-type vortex into a double arch-type structure. As the hole diameter increases, the dominant frequency of the spectrum in the HCH wake increases. Proper orthogonal decomposition analysis indicates that the large-scale spanwise vortices are suppressed by HCH. In the dynamic evolution process of the cube wake, the HCH issuing flow hinders the interaction of shear flow on both sides, and the issuing vortices attract the spanwise vortices and accelerate their shedding.

Altered electromyographic responses to emotional and pain information in awake bruxers: case-control study.

This study aims to investigate how emotional information and pain-related information affect the activity of the masticatory muscles in participants with awake bruxism and controls. Different videos and texts, with positive, negative, and neutral valence or related to pain, were presented to a sample of university students, while their electromyographic (EMG) activity around the masseter muscle and their skin conductance were recorded. Two groups were selected, with 24 subjects each: one group of subjects with definitive awake bruxism (confirmed by posterior EMG activity) who also suffered from moderate jaw discomfort, and another group of subjects without bruxism. The results demonstrated that the subjects with definitive awake bruxism displayed greater muscular activity when presented videos and texts with negative valence, especially when related to pain, than the non-bruxist group. This study supports the idea that persons with bruxism who also suffer moderate levels of jaw discomfort present greater bruxism activity when watching pain-related stimuli, and to a lesser extent when watching negative stimuli. The increased muscular activity induced by negative and pain-related information might contribute to pain exacerbation and perpetuation in persons with bruxism who suffer from discomfort.

Accelerated Wind-Turbine Wake Recovery Through Actuation of the Tip-Vortex Instability

Advances in wind-plant control have often focused on more effectively balancing power between neighboring turbines. Wake steering is one such method that provides control-based improvements in a quasi-static way, but this does little to fundamentally change the wake recovery process, and thus, it has limited potential. This study investigates use of another control paradigm known as dynamic wake control (DWC) to excite the mutual inductance instability between adjacent tip-vortex structures, thereby accelerating the breakdown of the structures. The current work carries this approach beyond the hypothetical by applying the excitation via turbine control vectors that already exist on all modern wind turbines: blade pitch and rotor speed control. The investigation leverages a free-vortex wake method (FVWM) that allows a thorough exploration of relevant frequencies and amplitudes of harmonic forcing for each control vector (as well as the phase difference between the vectors for a tandem configuration) while still capturing the essential tip-vortex dynamics. The FVWM output feeds into a Fourier stability analysis working to pinpoint candidate DWC strategies suggesting fastest wake recovery. Near-wake length reductions of are demonstrated, although without considering inflow turbulence. Analysis is provided to interpret these predictions considering the presence of turbulence in a real atmospheric inflow.

Optimal closed-loop wake steering – Part 2: Diurnal cycle atmospheric boundary layer conditions

Abstract. The magnitude of wake interactions between individual wind turbines depends on the atmospheric stability. We investigate strategies for wake loss mitigation through the use of closed-loop wake steering using large eddy simulations of the diurnal cycle, in which variations in the surface heat flux in time modify the atmospheric stability, wind speed and direction, shear, turbulence, and other atmospheric boundary layer (ABL) flow features. The closed-loop wake steering control methodology developed in Part 1 (Howland et al., 2020c, https://doi.org/10.5194/wes-5-1315-2020) is implemented in an example eight turbine wind farm in large eddy simulations of the diurnal cycle. The optimal yaw misalignment set points depend on the wind direction, which varies in time during the diurnal cycle. To improve the application of wake steering control in transient ABL conditions with an evolving mean flow state, we develop a regression-based wind direction forecast method. We compare the closed-loop wake steering control methodology to baseline yaw-aligned control and open-loop lookup table control for various selections of the yaw misalignment set-point update frequency, which dictates the balance between wind direction tracking and yaw activity. In our diurnal cycle simulations of a representative wind farm geometry, closed-loop wake steering with set-point optimization under uncertainty results in higher collective energy production than both baseline yaw-aligned control and open-loop lookup table control. The increase in energy production for the simulated wind farm design for closed- and open-loop wake steering control, compared to baseline yaw-aligned control, is 4.0 %–4.1 % and 3.4 %–3.8 %, respectively, with the range indicating variations in the energy increase results depending on the set-point update frequency. The primary energy increases through wake steering occur during stable ABL conditions in our present diurnal cycle simulations. Open-loop lookup table control decreases energy production in the example wind farm in the convective ABL conditions simulated, compared to baseline yaw-aligned control, while closed-loop control increases energy production in the convective conditions simulated.

Study on equivalent fatigue damage of two in-a-line wind turbines under yaw-based optimum control

ABSTRACT Offshore environment contains rich and high-quality wind resource with higher wind speed and lower turbulence intensity. In the offshore wind farm, the wake flows of different wind turbines will interact with each other, which will cause the energy loss and fatigue damage increase on wind turbines, namely wake effect. To alleviate the wake effect in the wind farm, several control methodologies have been proposed. The previous researchers have made a great progress on the power-augmentation effect. However, the fatigue damage from the wake control was not fully quantified in most current research. As a result, in this study, the fatigue damage of wind turbine blade from the wake control will be researched based on the coupled model of structural dynamics model and large-eddy-simulation-based CFD model. The flow field and dynamic loading for the wind turbine blades are simulated and analyzed. The equivalent fatigue damage (DEL) caused by the wake control for the upstream and downstream wind turbines has been calculated. It was found that the yaw-based wake control can cause 50% fatigue damage increase with increasing yaw angle. This research will provide guidelines for the design of both large wind turbine and wind farm.

Blade planform design optimization to enhance turbine wake control

AbstractThis study considers optimizing the planform of wind turbine blades to ultimately enhance wind plant controls, namely, wake steering strategies. Adjoint‐enabled unsteady actuator line simulations are carried out to obtain gradients for optimization of several different performance objectives with respect to blade chord length at 10 locations along blade span. We demonstrate different blade design optimizations that can maximize time‐averaged lateral wake deflection, entrainment of kinetic energy, or total power of multiple turbines. Our optimized designs can produce a 4+% increase in wake deflection, a 4× increase in vertical kinetic energy entrainment, or a 3.6% increase in power when compared with the baseline case. While lateral wake deflection is only modestly sensitive to chord changes, we find that increasing the outboard chord length can dramatically increase kinetic energy entrainment, resulting in faster wake recovery and gains in net power. While this work develops only a few case studies emphasizing relative performance improvements and general trends, these results show the promise of a framework that combines mid‐fidelity computation with adjoint‐based optimization for control and design problems.