Wake Generator Research Articles

Optimal control of wind farm power output with delay compensated nested-loop extreme seeking control

In this paper, we propose to enhance the nested-loop extremum seeking control (NLESC)-based wind farm control strategy with the predictor-based delay compensation in order to improve its convergence characteristics under fluctuating wind. Earlier work has shown the effectiveness of NLESC for region-2 wind farm operation, i.e., maximizing the total power output of cascaded wind turbine array, while its convergence speed is highly limited by the delay of power output for downstream turbines due to wake propagation along the wind direction. By utilizing the delay compensated ESC proposed by Oliveira and Krstic, the delay compensated NLESC (DCNLESC) wind farm control is proposed, allowing the dither frequencies to be of similar magnitude as that in the single-turbine ESC. This can significantly improve the convergence speed of optimum tracking for real-time wind farm control. The wake propagation delay is estimated from turbine power outputs using cross correlation and proper filtering. Using the SimWindFarm platform, the proposed DCNLESC strategy is simulated with both a single-column three-turbine array and a 2 × 3 turbine array, under different wind speeds. The results show that the convergence speed toward the calibrated optimum is significantly improved over the NLESC operation. The convergence time for the upstream turbines' torque gain is reduced by 55%–14% in terms of integral time-weighted absolute error, while the impact on turbine fatigue loads is as low as no more than 3.5% increase on turbine tower and shaft.

Yaw Optimisation for Wind Farm Production Maximisation Based on a Dynamic Wake Model

In recent years, a major focus on wind farm wake control is to maximise the production of wind farms. To improve the power generation efficiency of wind farms through wake regulation, this study investigates yaw optimisation for wind farm production maximisation from the perspective of time-varying wakes. To this end, we first deduce a simplified dynamic wake model according to the momentum conservation theory and backward difference method. The accuracy of the proposed model is verified by simulation comparisons. Then, the time lag of wake propagation and its impact on wind farm production maximisation through wake meandering is analysed. On this basis, a yaw optimisation method for increasing wind farm energy capture is presented. This optimisation method uses the proposed dynamic wake model for wind farm prediction. The results indicate that the optimisation period is critical to the effect of the optimisation method on wind farm energy capture.

An investigation of spatial wind direction variability and its consideration in engineering models

Abstract. We propose that considering mesoscale wind direction changes in the computation of wind farm cluster wakes could reduce the uncertainty of engineering wake modeling tools. The relevance of mesoscale wind direction changes is investigated using a wind climatology of the German Bight area covering 30 years, derived from the New European Wind Atlas (NEWA). Furthermore, we present a new solution for engineering modeling tools that accounts for the effect of such changes on the propagation of cluster wakes. The mesoscale wind direction changes relevant to the operation of wind farm clusters in the German Bight are found to exceed 11∘ in 50 % of all cases. Particularly in the lower partial load range, which is associated with strong wake formation, the wind direction changes are the most pronounced, with quartiles reaching up to 20∘. Especially on a horizontal scale of several tens of kilometers to 100 km, wind direction changes are relevant. Both the temporal and spatial scales at which large wind direction changes occur depend on the presence of synoptic pressure systems. Furthermore, atmospheric conditions which promote far-reaching wakes were found to align with a strong turning in 14.6 % of the cases. In order to capture these mesoscale wind direction changes in engineering model tools, a wake propagation model was implemented in the Fraunhofer IWES wind farm and wake modeling software flappy (Farm Layout Program in Python). The propagation model derives streamlines from the horizontal velocity field and forces the single turbine wakes along these streamlines. This model has been qualitatively evaluated by simulating the flow around wind farm clusters in the German Bight with data from the mesoscale atlas of the NEWA and comparing the results to synthetic aperture radar (SAR) measurements for selected situations. The comparison reveals that the flow patterns are in good agreement if the underlying mesoscale data capture the velocity field well. For such cases, the new model provides an improvement compared to the baseline approach of engineering models, which assumes a straight-line propagation of wakes. The streamline and the baseline models have been further compared in terms of their quantitative effect on the energy yield. Simulating two neighboring wind farm clusters over a time period of 10 years, it is found that there are no significant differences across the models when computing the total energy yield of both clusters. However, extracting the wake effect of one cluster on the other, the two models show a difference of about 1 %. Even greater differences are commonly observed when comparing single situations. Therefore, we claim that the model has the potential to reduce uncertainty in applications such as site assessment and short-term power forecasting.

Decentralized yaw optimization for maximizing wind farm production based on deep reinforcement learning

This study describes a deep reinforcement learning (DRL) based decentralized yaw optimization method to maximize the power production of wind farms. Specifically, we apply the Multi-Agent Deep Deterministic Policy Gradient (MADDPG) algorithm to design agents separately for each turbine in the wind farm to make yaw decisions independently. This design allows setting different yaw rewards for the agents to promote faster and better convergence of MADDPG. To test the control effect of the proposed method, a novel analytical dynamic wake model is derived first, which can dynamically reflect the wake propagation of the wind turbine after the inflow wind speed, axial induction factor, and yaw angle change. Then, the static and dynamic characteristics of the proposed wake model are verified. The wind farm simulation is realized through the dynamic wake model, which provides an interactive environment for the simulation experiments to test the effect of the proposed DRL-based decentralized yaw optimization method. The results show that the proposed method can significantly increase the power generation of the wind farm, and the set yaw reward helps to guide MADDPG to converge to the optimal strategy.

Analysis of middle-to-far wake behind floating offshore wind turbines in the presence of multiple platform motions

Understanding the unsteady characteristics of the mid-to-far wake and comprehension of the aerodynamic performance of floating offshore wind turbines is essential for the further development of offshore wind farms. In this perspective, a developed Actuator line model is utilized to analyze an offshore turbine on four different platforms. The model reliability was examined through three sets of validations involving turbine output in fixed and floating conditions and wake expansion in terms of size and rate. The affected relative velocity by the platform motion contributes to the wake deformation and temporal effects on induced velocity. Angular platform motions produce a non-axisymmetric helical wake that raises the chance of meandering wake patterns. It was founded although platform movement generally can boost the recovery of mean velocity value, it may amplify the amplitude of velocity deficit fluctuation at further downstream by encouraging interactions and merging vortex rings. Consequently, the wake propagates into a form of stronger circles whose period, strength, and center are functions of turbine movement and operation conditions. By providing a computationally efficient tool, the findings emphasize the importance of wake propagation in designing and assessing farm layouts that operate in the presence of significant multi-motions of floating offshore wind turbines.

The high-Reynolds-number stratified wake of a slender body and its comparison with a bluff-body wake

The high-Reynolds-number stratified wake of a slender body is studied using a high-resolution hybrid simulation. The wake generator is a 6 : 1 prolate spheroid with a tripped boundary layer, the diameter-based body Reynolds number is ${Re}= U_\infty D/\nu = 10^5$ , and the body Froude numbers are ${Fr}=U_\infty /ND=\{2,10,\infty \}$ . The wake defect velocity decays following three stages with different wake decay rates (Spedding, J. Fluid Mech., vol. 337, 1997, pp. 283–301) as for a bluff body. However, the transition points among stages do not follow the expected $Nt = Nx/U_\infty$ values. Comparison with the wake of a circular disk in similar conditions (Chongsiripinyo & Sarkar, J. Fluid Mech., vol. 885, 2020) quantifies the influence of the wake generator – bluff versus slender – in stratified flow. The strongly stratified ${Fr}=2$ wake is in a resonant state. The steady lee waves strongly modulate the mean flow, and relative to the disk, the 6 : 1 spheroid (a high-aspect-ratio shape) wake at ${Fr}=2$ shows an earlier transition from the non-equilibrium (NEQ) stage to the quasi-two-dimensional (Q2D) stage. The NEQ–Q2D transition is followed by a sharp increase in the turbulent kinetic energy and horizontal wake meanders. At ${Fr}=10$ , the start of the NEQ stage is delayed for the spheroid. Transfers between kinetic energy and potential energy reservoirs (both mean and turbulence) are analysed, and the flows are compared in phase space (with local Froude and Reynolds numbers as coordinates). Overall, the results of this study point to the difficulty of finding a universal framework for stratified wake evolution, independent of the features of the body, and provide insights into how buoyancy effects depend on the wake generator.

Magnetic Reconnection in the Wakes of Cosmic Strings

The motion of cosmic strings in the universe leads to the generation of wakes behind them. We study magnetized wakes of cosmic strings moving in the postrecombination plasma. We show that magnetic reconnection can occur in the postshock region. Since the width of the cosmic-string wake is very small, the reconnection occurs over a very short length scale. The reconnection leads to a large amount of kinetic energy being released in the postshock region of the cosmic-string wake. This enhances the kinetic energy released during the reconnection. We make a rudimentary estimate of the kinetic energy released by the magnetic reconnection in cosmic-string wakes and show that it can account for low-energy gamma-ray bursts in the postrecombination era.

Generation of intense magnetic wakes by relativistic laser pulses in plasma

The emergence of petawatt lasers focused to relativistic intensities enables all-optical laboratory generation of intense magnetic fields in plasmas, which are of great interest due to their ubiquity in astrophysical phenomena. In this work, we study generation of spatially extended and long-lived intense magnetic fields. We show that such magnetic fields, scaling up to the gigagauss range, can be generated by interaction of petawatt laser pulses with relativistically underdense plasma. With three-dimensional particle-in-cell simulations we investigate generation of magnetic fields with strengths up to 10^{10} G and perform a large multi-parametric study of magnetic field in dependence on dimensionless laser amplitude a_{0} and normalized plasma density n_{e}/n_{c}. The numerical results yield scaling laws that closely follow derived analytical result B propto sqrt{a_{0}n_{e}/n_{c}}, and further show a close match with previous experimental works. Furthermore, we show in three-dimensional geometry that the decay of the magnetic wake is governed by current filament bending instability, which develops similarly to von Kármán vortex street in its nonlinear stage.

Tail-shake risks assessment & mitigation by wind-tunnel tests on air-intake installation on a heavy-weight H/C configuration

AbstractIn this work, the key role of the upper-deck design including engine installation as a potential source of tail-shake is at focus. The work is based on a Wind-Tunnel Test (WTT) campaign performed at the Airbus Helicopters’ Marignane wind-tunnel facilities on a high-fidelity minibody fuselage at scale 1:3.5 representing a generic heavy-helicopter upper deck.Two different engine intake installations for a Power Unit (PU) have been investigated; in a first configuration, the air intake is implemented at the pylon-fairing trailing edge. The second configuration consists in positioning two air intakes on each side of the pylon fairing, close to the maximum cross-section location. Different measurement methods to evaluate aerodynamic interactions and wake sources are proposed: flow-separation assessments from surface oil flow visualisations, time-resolved Particle Image Velocimetry (PIV) measurements and unsteady skin-pressure measurements at the cowlings. Tail-shake-related indicators are then proposed. Basically, a configuration that produces strong vortices characterised by a broadband spectral signature is believed to gather all the conditions for tail-shake to emerge.The flow over the clean configuration is first analysed for various combinations of angle-of-attack and sideslip, highlighting four different areas of flow separation at the cowlings. The complex flow topology around the upper deck is then assessed, which includes a spectral analysis of the flow in the PIV planes. The influence of the air intakes (operating or not) is then evaluated. When located at the pylon-fairing trailing edge and operating, the air intake has a spectacular impact on the flow-field topology. It is responsible for the generation of an intense broadband wake interacting with the pylon-fairing lip vortices, which is believed to be a potential source of tail-shake. The second air-intake configuration is also not favourable, as it requires enlarging the pylon fairing by 100mm, which causes an intense wake similarly to a blunt body. At last, a mitigation mean is proposed for the first configuration. It demonstrates a significant reduction of the wake intensity and broadband signature at the source.

A stabilised finite element framework for viscoelastic multiphase flows using a conservative level-set method

The development of stable numerical schemes for viscoelastic multiphase flows has implications for many areas of engineering applications. The principal original contribution of this paper is the implementation of a conservative level-set method to define implicitly the interface between fluid phases, fully integrated into the mathematical framework of viscoelastic flow. The governing equations are discretized using the finite element method and stabilisation of the constitutive equation is achieved using either the discontinuous Galerkin (DG) or streamline upwinding (SU) method. The discrete elastic viscous stress splitting gradient (DEVSS-G) formulation is also employed in the Navier-Stokes equations to balance the hyperbolic characteristics of the polymeric stress tensor. The numerical scheme is validated with reference to several benchmark problems and excellent quantitative agreement with published data is found for Newtonian and viscoelastic fluids, for both single and multiphase flows. The motion of a gas bubble rising in a viscoelastic fluid is studied in detail. The influence of polymer concentration, surface tension, fluid elasticity and shear-thinning behaviour, on flow features such as the development of filaments and cusps and the generation of negative wakes is explored.

Numerical Analysis of Wake Interaction of Darrieus Tidal Turbine in Shallow Water Application

Malaysia is rich in natural resources including coal and fossil fuels. These natural resources, however, will diminish and pollute the ecosystem. Thus, researchers have proposed the use of renewable energy sources such as solar, wind, wave, and other energies as potential solutions to resolve the issue. This study focuses on tidal energy, specifically the vertical axis tidal turbine (VATT) for shallow water application. While a majority of the VATT research shows that this device can work effectively in deep water, its effectiveness in shallow water has yet to be explored comprehensively. To analyse the turbine's performance, Computational Fluid Dynamics (CFD) method was applied. The Darrieus turbine was used in this study since it is the ideal turbine design for shallow water in Malaysia with an average current speed of 1.0 𝑚𝑚𝑚𝑚−1. The Darrieus turbine model used in this project is 5 metres tall and 4.3 metres in diameter. The simulation was evaluated based on wake characteristics. The Darrieus turbine was designed using the NACA0018 airfoil. Following the results of single turbine analysis, the wake has recovered to its ambience velocity at 60 metres behind the device. The wake generation due to the multi-row configuration of the devices was also examined. The three-turbine configuration verifies the concept that installing two turbines side by side boosts the performance significantly. Notably, the improvement is sufficient to mitigate the adverse effects due to the turbulence wakes from upstream turbines. The findings demonstrate that by employing a staggered design (1 device upstream and 2 devices downstream), the wake can recover its initial velocity faster due to a shorter wake generation distance.

Effects of Blowing and Density Ratios on Unsteady Film Cooling Effectiveness Using Fast-Response Pressure-Sensitive Paint

Abstract The transient effect of unsteady wakes on film cooling effectiveness using a fast response pressure sensitive paint (PSP) was studied. Testing was performed in a five-passage, low-speed wind tunnel using a scaled-up high pressure, blade profile. The blowing ratios studied include 0.4, 0.8, and 1.2 with a density ratio of 1.0 and 1.5 at each blowing ratio. An upstream wake generator produced Strouhal numbers of 0.1 and 0.3 while the wind tunnel inlet mainstream Reynolds number was held at a constant value of 300,000 for every test case. The results in this paper confirm the overall time averaged reduction in film cooling effectiveness with the addition of an unsteady wake while providing more detailed information on the transient response of the film cooling effectiveness, presented in the form of film cooling effectiveness RMS values. Results show that film cooling effectiveness RMS values increase on the leading-edge region of the blade and show little change on the pressure side of the blade with an increase in the Strouhal number. On the suction side, the area-averaged effectiveness RMS values decrease for the 0.8 and 1.2 blowing ratios and remain unchanged for the 0.4 blowing ratio. A change in the density ratio of the coolant from 1.0 to 1.5 has a minimal effect on area-averaged effectiveness RMS values for all areas of the blade.

Application of high accuracy Penalized Vortex in Cell method for the high Reynolds number turbomachinery flows.

This paper presents the high-order Penalized Vortex in Cell method applied to flow past an airfoil with a cavity and a T-106A turbine cascade. The first part of the paper treats about the parallel environment and algorithm performance where times for each operation are measured. Then further validation on the flow past cylinder at Re=9500 was shown, and flow past an airfoil simulation results were extended. For unbounded flow with stream-function correction based on Biot-Savart law, velocity profiles at the boundary were shown. A volumetric imbalance was determined. In the second part, formulation of pVIC method for periodic flows was discussed, and an example of calculation with a turbine cascade with a moving bar wake generator was demonstrated.

Flow Analysis of Tidal Turbine Array Interaction Using LES

Geographical location of Malaysia is endowed with natural resources such as coal and fossil fuels. Unfortunately, these natural resources will run out shortly if the Malaysian government does not take appropriate measures to conserve them. To ensure that the area has enough energy and sources, a system of utilizing and introducing renewable energy such as solar, rainwater, wave and tidal current energy is established. Vertical axis tidal turbine (VATT) for shallow water applications is the main topic of this study where computational fluid dynamics (CFD) via Large-eddy simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) simulation was performed in investigating the influence of the array interaction of tidal turbines. The VATT is the most suitable tidal turbine in Malaysia because the depth of the water in Malaysia ranges between 20m and 30m. In addition, the hypothetical actuator disk as a representation of the tidal turbine devices, with different turbulence models for simulating tidal turbine wakes interaction was performed. The tested turbulence models include the k-epsilon, Large Eddy Simulation (LES) and Smagorinsky-Lilly. The evaluation was done by comparing the wake generation between LES and RANS simulations’ results against the set of published experimental data regarding various arrays for the tidal turbines. The discussion involves three main parts: (i) pre-processing which concentrates on creating the model and setup parameters, (ii) processing which concentrates on visualizing the simulation data and lastly (iii) post-processing which concentrates on defining the results obtained from the ANSYS software.

Comparative numerical investigation of wake effect on low-pressure turbine endwall flow

In low-pressure turbine linear cascade experiments, the upstream stator vanes are often modeled by moving bar wake generators. Different from the majority of the earlier wake effect research which is concerned with less aggressively loaded airfoils, here front-loaded high-lift airfoils are considered for both stator and rotor. Implicit large-eddy simulations of the flow through a 50% reaction stage with stator vanes and rotor blades as well as a linear cascade with bar wake generators and rotor blades were carried out and the wake effect on the flow through the rotor was investigated. The chord-based Reynolds number for the simulations was 100,000. For the chosen front-loaded high-lift airfoil the stator wake is dominated by pronounced endwall structures which are lacking for the bar wake generator. Instantaneous flow visualizations reveal a periodic suppression of the two-dimensional rotor blade suction-side separation for both configurations. With upstream stator vanes, the wake endwall structures appear to be chiefly responsible for the suppression of the rotor blade passage vortex and corner separation. For the bar wake generator, the wakes are wider and the wake effect is spread out over a larger portion of the wake passing period. The passage vortex and corner separation remain mostly intact.

A Novel Wake Control Approach for Power Generation Improvement of Three Wind Turbines in a Wind Farm

Abstract The total power generation of a wind farm is significantly affected by the adverse wake generation of upstream wind turbines on the aerodynamic performance of the downstream wind turbines. This paper aims to provide a novel hybrid wake control strategy to improve the power generation of horizontal-axis wind turbines in a wind-farm layout. A numerical simulation based on the actuator line method with large eddy simulation (ALM–LES) is performed to investigate the airflow around three-dimensional National Renewable Energy Laboratory (NREL) 5 MW turbines in a three-by-one layout. The yaw angle (θ) and tilt angle (ϕ) of the wind turbines are in the range of (−30 deg<θ<30 deg) and (0<ϕ<35 deg), respectively. Firstly, the combined effects of yaw-control and tilt-control methods on the velocity profile, vorticity generation and turbulent kinetic energy in the wake region of the multiscale wind farm are investigated. Afterwards, the total power generation of the wind farm is compared with previous wake control methods of wind turbines. It was observed that the proposed hybrid wake control method could improve the total power generation by 9.94% compared to the previous wind turbine wake control techniques. The hybrid control strategy can deviate the wake much better than typical single-control methods. An optimization analysis is also provided to find the most appropriate yaw angles and tilt angles of the wind turbines subject to varying wind speeds.

The impact of swirl and wake strength on turbulent axisymmetric wake evolution

An experimental investigation of swirl and wake strength influence on axisymmetric turbulent wake evolution was conducted. A novel wake generator design wire mounted in a wind tunnel test section with low free-stream turbulence produced wake Reynolds numbers based on momentum thickness and free-stream velocity in excess of 14 000 and swirl numbers up to 0.4 with minimal blockage. Steady-state blade element momentum simulations of reference wind turbine designs indicated that wind turbines operate in the flow regimes studied, indicating the practical aspects of this work. Stereoscopic particle image velocimetry was used to acquire three components of velocity in the swirling wake at locations up to approximately ten diameters downstream. Quantitative measures of wake growth and decay were deduced using available equilibrium similarity scaling for the swirling wake. The results show an increase above 50% in growth and axial velocity decay rate constants over the range of swirl strength studied compared to those of the non-swirling wake. Tangential velocity decay constants were shown to decrease with swirl strength over the range of conditions studied. Notably, changes in wake strength have little influence on growth and decay rates when compared to changes in swirl strength for the flow regimes studied in this work.

3D Printed Graphene Piezoresistive Microelectromechanical System Sensors to Explain the Ultrasensitive Wake Tracking of Wavy Seal Whiskers

AbstractMany marine animals perform fascinating survival hydrodynamics and perceive their surroundings through optimally evolved sensory systems. For instance, phocid seal whiskers have undulations that allow them to resist noisy self‐induced vortex‐induced vibrations (VIV) while locking their vibration frequencies to wakes generated by swimming fishes. In this study, fully 3D‐printed microelectromechanical systems (MEMS) sensors with high gauge factor graphene nanoplatelets piezoresistors are developed to explain the exquisite sensitivity of whisker‐inspired structures to upstream wakes. The sensors are also used to measure natural frequencies of excised harbor (Phoca vitulina) and grey (Halichoerus grypus) seal whiskers and determine the effect of whisker orientation on the VIV, which can explain the possible natural orientation of whiskers during active hunting. Experimental investigations conducted in a recirculating water flume show that whisker‐inspired sensors successfully sense an upstream wake located up to 10× the whisker diameter by locking to the frequency of the wake generator, thus mimicking the sensing mechanism of the seal whisker. The combination of VIV reduction and frequency‐locking with the upstream wake generator demonstrates the whisker‐inspired sensor's high signal‐to‐noise ratio, indicating its efficiency in long‐distance wake sensing as well as its potential as an alternative to visual and acoustic sensors in underwater robots.

The effects of the cold expansion degree on the fatigue crack growth rate in rail steel

This study investigates the effects of the cold-expansion degree on the fatigue strength of rail steels. Two degrees of cold-expansion, 2% and 4%, and two stress levels were investigated. Results indicate that cold-expansion postpones crack initiation and extends fatigue life, especially with higher degrees of cold-expansion and lower stress levels. Linear superposition principle coupled with the weight function method results in an overestimated fatigue life of cold-expanded specimens, suggesting that reliable predictions can be achieved if both the residual stresses redistribution due to propagating crack and the crack closure effects due to plastic wake generation are considered in the simulations.

Stochastic model predictive controller for wind farm frequency regulation in waked conditions

As renewable energy penetrations increase, wind farms may be required to provide ancillary services such as frequency regulation. For wind farms, two issues complicate frequency regulation: turbine-wake interactions alter the available wind power, and the future power reference signal is uncertain. This paper presents a stochastic model predictive controller (MPC) for wind farms to track an uncertain frequency regulation signal while accounting for wakes. The controller uses a simplified Park model with wake propagation delay to predict turbine-wake interactions. The controller operates without any knowledge of the future power regulation signal or wind speed. The controller solves a nonconvex stochastic optimization program using scenario optimization. Our stochastic MPC tracks with 7%–10% less error than our deterministic MPC controllers.