Wake Interaction Research Articles

Lagrangian Vortex Computations of a Four Tidal Turbine Array: An Example Based on the NEPTHYD Layout in the Alderney Race

This study investigates the wake interaction of four full-scale three-bladed tidal turbines with different ambient turbulence conditions, in straight and yawed flows. A three-dimensional unsteady Lagrangian Vortex Blob software is used for the numerical simulations of the turbines’ wakes. In order to model the ambient turbulence in the Lagrangian Vortex Method formalism, a Synthetic Eddy Method is used. With this method, turbulent structures are added in the computational domain to generate a velocity field which statistically reproduces any ambient turbulence intensity and integral length scale. The influence of the size of the structures and their density (within the study volume) on the wake of a single turbine is studied. Good agreement is obtained between numerical and experimental results for a high turbulence intensity but too many structures can increase the numerical dissipation and reduce the wake extension. Numerical simulations of the four turbine array with the layout initially proposed for the NEPTHYD pilot farm are then presented. Two ambient turbulence intensities encountered in the Alderney Race and two integral length scales are tested with a straight flow. Finally, the wakes obtained for yawed flows with different angles are presented, highlighting turbine interactions.

'No Man is an Island': Effects of social seclusion on social dream content and REM sleep.

Based on the Social Simulation Theory of dreaming (SST), we studied the effects of voluntary social seclusion on dream content and sleep structure. Specifically, we studied the Compensation Hypothesis, which predicts social dream contents to increase during social seclusion, the Sociality Bias - a ratio between dream and wake interactions - and the Strengthening Hypothesis, which predicts an increase in familiar dream characters during seclusion. Additionally, we assessed changes in the proportion of REM sleep. Sleep data and dream reports from 18 participants were collected preceding (n = 94), during (n = 90) and after (n = 119) a seclusion retreat. Data were analysed using linear mixed-effects models. We failed to support the Compensation Hypothesis, with dreams evidencing fewer social interactions during seclusion. The Strengthening Hypothesis was supported, with more familiar characters present in seclusion dreams. Dream social interactions maintained the Sociality Bias even under seclusion. Additionally, REM sleep increased during seclusion, coinciding with previous literature and tentatively supporting the proposed attachment function for social REM sleep.

Understanding the coherent structures for different fluctuation frequencies at partial load condition of a Francis turbine

More and more hydropower plants are operating as regulator of renewable energy such as solar and wind energy resources. Because these renewable energy resources are highly volatile, so hydraulic turbines have to switch working condition in a wide flow range to guarantee the steady output to electric grid. It’s known that there are many fluctuating frequency components in runner passage of hydraulic turbines, especially at part-load conditions, which are induced by unsteady flow phenomena like guide vane wake, flow separation or rotor-stator interaction (RSI) and so on. These fluctuation frequency components are varied with working condition due to different dominant inducing resources. Although we can estimate the frequency component resources according to rotating speed, numbers of blades and guide vanes, we can’t know exactly coherent structure pattern of every frequency component. In present study, firstly, we will compute the internal flow of a Francis turbine at lower discharge, and verify the validation of computational results by experimental data. Secondly, Dynamic Mode Decomposition (DMD) method will be employed to extract the three-dimensional coherent structures in runner passage and draft tube for every frequency component based on computational snapshots. Finally, we will demonstrate the patterns and locations of coherent structures for every frequency component and predict the coherent structure or fluctuation component generation resources.

Effects of uniform vertical inflow perturbations on the performance of flapping wings.

Flapping wings have attracted significant interest for use in miniature unmanned flying vehicles. Although numerous studies have investigated the performance of flapping wings under quiescent conditions, effects of freestream disturbances on their performance remain under-explored. In this study, we experimentally investigated the effects of uniform vertical inflows on flapping wings using a Reynolds-scaled apparatus operating in water at Reynolds number ≈ 3600. The overall lift and drag produced by a flapping wing were measured by varying the magnitude of inflow perturbation from JVert = −1 (downward inflow) to JVert = 1 (upward inflow), where JVert is the ratio of the inflow velocity to the wing's velocity. The interaction between flapping wing and downward-oriented inflows resulted in a steady linear reduction in mean lift and drag coefficients, and , with increasing inflow magnitude. While a steady linear increase in and was noted for upward-oriented inflows between 0 < JVert < 0.3 and JVert > 0.7, a significant unsteady wing–wake interaction occurred when 0.3 ≤ JVert < 0.7, which caused large variations in instantaneous forces over the wing and led to a reduction in mean performance. These findings highlight asymmetrical effects of vertically oriented perturbations on the performance of flapping wings and pave the way for development of suitable control strategies.

Numerical Analysis of Wind Wake Superposition and Boundary Layer Interaction

Understanding the wake impacts to a wind farm is critical for layout planning and turbine design. Wind Wake Models estimates wake development through a single turbine which is then aggregated with a wake variety of methods to calculate the combined wake profile for a wind farm. This study considered downstream wake interactions, and the energy content in the wake profile as the wake developed for three rows and five downstream turbines. This study also looks at the influence of different atmospheric stratifications to wind farm wake superposition. Our findings for the wake and boundary superimposition model determine that the streamwise velocity component range from 8.8 to 9.0 m s-1 (98 to 100 %) for the stable ABL ranges and 8.92 to 9.0 m s-1 (99 to 100 %) for the unstable ABL ranges. This study further determines that the minimum streamwise velocity for the stable and unstable ABL are 8.97 and 8.96 m s-1, and the mean normalized velocity deficit for the stable and unstable ABL are 0.972 and 0.975. The wake deficit for the superimposition of a coupled wake boundary model in the stable case surpasses that of the unstable case due to lower turbulence.

Evaluation of tilt control for wind-turbine arrays in the atmospheric boundary layer

Abstract. Wake redirection is a promising approach designed to mitigate turbine–wake interactions which have a negative impact on the performance and lifetime of wind farms. It has recently been found that substantial power gains can be obtained by tilting the rotors of spanwise-periodic wind-turbine arrays. Rotor tilt is associated with the generation of coherent streamwise vortices which deflect wakes towards the ground and, by exploiting the vertical wind shear, replace them with higher-momentum fluid (high-speed streaks). The objective of this work is to evaluate power gains that can be obtained by tilting rotors in spanwise-periodic wind-turbine arrays immersed in the atmospheric boundary layer and, in particular, to analyze the influence of the rotor size on power gains in the case where the turbines emerge from the atmospheric surface layer. We show that, for the case of wind-aligned arrays, large power gains can be obtained for positive tilt angles of the order of 30∘. Power gains are substantially enhanced by operating tilted-rotor turbines at thrust coefficients higher than in the reference configuration. These power gains initially increase with the rotor size reaching a maximum for rotor diameters of the order of 3.6 boundary layer momentum thicknesses (for the considered cases) and decrease for larger sizes. Maximum power gains are obtained for wind-turbine spanwise spacings which are very similar to those of large-scale and very-large-scale streaky motions which are naturally amplified in turbulent boundary layers. These results are all congruent with the findings of previous investigations of passive control of canonical boundary layers for drag-reduction applications where high-speed streaks replaced wakes of spanwise-periodic rows of wall-mounted roughness elements.

Near-Wall Flow in Turbomachinery Cascades—Results of a German Collaborative Project

This article provides a summarizing account of the results obtained in the current collaborative work of four research institutes concerning near-wall flow in turbomachinery. Specific questions regarding the influences of boundary layer development on blades and endwalls as well as loss mechanisms due to secondary flow are investigated. These address skewness, periodical distortion, wake interaction and heat transfer, among others. Several test rigs with modifiable configurations are used for the experimental investigations including an axial low speed compressor, an axial high-speed wind tunnel, and an axial low-speed turbine. Approved stationary and time resolving measurements techniques are applied in combination with custom hot-film sensor-arrays. The experiments are complemented by URANS simulations, and one group focusses on turbulence-resolving simulations to elucidate the specific impact of rotation. Juxtaposing and interlacing their results the four groups provide a broad picture of the underlying phenomena, ranging from compressors to turbines, from isothermal to non-adiabatic, and from incompressible to compressible flows.

Preface

The 23 papers in this volume constitute the proceedings of the 2021 Wake Conference. The Wake Conference series began in Visby, where it was held in 2009 and 2011. In 2013 the conference took place in Copenhagen where it was combined with the International Conference on Offshore Wind Energy and Ocean Energy. In 2015 and 2017 it went back to where it started, Visby. This year the conference will be held online due to the restrictions for Covid-19. Hosts for the conference are Professor Stefan Ivanell, Uppsala University and Professor Jens Nørkær Sørensen, Technical University of Denmark.Modern wind turbines are today clustered in large farms with a total production capacity reaching those of a nuclear power plant. When placed in a wind farm, the turbines will be fully or partially influenced by the wake of upstream turbines. This wake interaction results in a decreased power production, caused by the lower kinetic energy in the wind, and an increase in the turbulence intensity. Therefore, understanding the physical nature of vortices and their dynamics in the wake of a turbine is important for the optimal design of wind farms.The increased importance and interest in the field of wake and wind farm aerodynamics can be seen in the increased number of scientific articles on the subject. This citation growth essentially shows that the growth in the global production of electrical energy has become a scientific problem to be solved by scientists and engineers. In order to make a substantial impact on one of the most significant challenges of our time, global climate change, the wind industry’s growth must continue. A part of making this growth possible will require research into the physics of wind turbine wakes and wind farms.This conference is aimed at scientists and PhD students working in the field of wake dynamics. The conference covers the following subject areas: Wake and vortex dynamics, instabilities in trailing vortices and wakes, simulation and measurements of wakes, analytical approaches for modeling wakes, wake interaction and other wind farm investigations.We would like to thank all those that assisted in the review process for these proceedings and we are also thankful for the help we have received from the editorial team at IOP Publishing during the process.Visby, Sweden May 2021 Stefan Ivanell and Jens Nørkær Sørensen

Characteristics of fluid exciting force due to blade row interaction of a propeller turbine

As the use of renewable energy is promoted, much researches on propeller-type turbines are carried out. To improve reliability, it is important to evaluate the fluid exciting force due to blade row interaction that may cause vibration or fatigue fracture. In this paper, CFD(URANS) simulations and experimental studies were conducted to understand the characteristics of the fluid exciting force due to blade row interaction of a propeller turbine. To evaluate the fluid exciting force, strain gauges and pressure sensors were used in the closed-loop water channel. The fluid exciting force acting on the rotor due to the stator was measured by changing stator load type and the rotor-stator distance. Based on the difference in the attenuation tendency of potential interaction and wake interaction due to changes in the rotor-stator distance, the influences of these interactions could be distinguished. As results of experiments and computation, wake interaction affecting pressure fluctuation was hardly attenuated due to the swirling flow and changing the rotor-stator distance has little effect on the attenuation of blade row interaction.

Exploring the application of reinforcement learning to wind farm control

Optimal control of wind farms to maximize power is a challenging task since the wake interaction between the turbines is a highly nonlinear phenomenon. In recent years the field of Reinforcement Learning has made great contributions to nonlinear control problems and has been successfully applied to control and optimization in 2D laminar flows. In this work, Reinforcement Learning is applied to wind farm control for the first time to the authors’ best knowledge. To demonstrate the optimization abilities of the newly developed framework, parameters of an already existing control strategy, the helix approach, are tuned to optimize the total power production of a small wind farm. This also includes an extension of the helix approach to multiple turbines. Furthermore, it is attempted to develop novel control strategies based on the control of the generator torque. The results are analysed and difficulties in the setup in regards to Reinforcement Learning are discussed. The tuned helix approach yields a total power increase of 6.8% on average for the investigated case, while the generator torque controller does not yield an increase in total power. Finally, an alternative setup is proposed to improve the design of the problem.

Farm to farm wake interaction in WRF: impact on power production

This work focuses on the study of interactions between neighboring wind farms through a mesoscale numerical modelling approach. High resolution simulations with three wind farm parameterizations (WFP, EWP and IAWFP), implemented on the WRF model, over a set of “real world” neighboring onshore wind farms are performed. The sensitivity of the three parameterizations to wake development is analyzed. The older version of the WFP is also included to account for differences between versions in terms of wake length and intensity, and TKE production and advection. Finally, the impacts of upstream wind farms are studied through a series of simulations where they are “disabled” and the increases in power at downstream farms are quantified. These results and the adopted methodology have direct application and are highly relevant for the feasibility assessment of new wind farms in the shadow of other existing ones.

Experimental investigation of wind turbine wake and load dynamics during yaw maneuvers

Abstract. This article investigates the far wake response of a yawing upstream wind turbine and its impact on the global load variation in a downstream wind turbine. In order to represent misalignment and realignment scenarios, the upstream wind turbine was subjected to positive and negative yaw maneuvers. Yaw maneuvers could be used to voluntarily misalign wind turbines when wake steering control is targeted. The aim of this wind farm control strategy is to optimize the overall production of the wind farm and possibly its lifetime, by mitigating wake interactions. While wake flow and wind turbine load modifications during yaw maneuvers are usually described by quasi-static approaches, the present study aims at quantifying the main transient characteristics of these phenomena. Wind tunnel experiments were conducted in three different configurations, varying both scaling and flow conditions, in which the yaw maneuver was reproduced in a homogeneous turbulent flow at two different scales and in a more realistic flow such as a modeled atmospheric boundary layer. The effects of yaw control on the wake deviation were investigated by the use of stereo particle imaging velocimetry while the load variation on a downstream wind turbine was measured through an unsteady aerodynamic load balance. While overall results show a nondependence of the wake and load dynamics on the flow conditions and Reynolds scales, they highlight an influence of the yaw maneuver direction on their temporal dynamics.

The curled wake model: a three-dimensional and extremely fast steady-state wake solver for wind plant flows

Abstract. Wind turbine wake models typically require approximations, such as wake superposition and deflection models, to accurately describe wake physics. However, capturing the phenomena of interest, such as the curled wake and interaction of multiple wakes, in wind power plant flows comes with an increased computational cost. To address this, we propose a new hybrid method that uses analytical solutions with an approximate form of the Reynolds-averaged Navier–Stokes equations to solve the time-averaged flow over a wind plant. We compare results from the solver to supervisory control and data acquisition data from the Lillgrund wind plant obtaining wake model predictions which are generally within 1 standard deviation of the mean power data. We perform simulations of flow over the Columbia River Gorge to demonstrate the capabilities of the model in complex terrain. We also apply the solver to a case with wake steering, which agreed well with large-eddy simulations. This new solver reduces the time – and therefore the related cost – it takes to simulate a steady-state wind plant flow (on the order of seconds using one core). Because the model is computationally efficient, it can also be used for different applications including wake steering for wind power plants and layout optimization.

Evaluation of Finite-State Dynamic Inflow for Rotors

In an effort to evaluate the accuracy of the finite-state dynamic inflow of Peters and He (“Finite State Induced Flow Models Part II: Three-Dimensional Rotor Disk,” Journal of Aircraft, Vol. 32, No. 2, 1995, pp. 323–333) for use in rotor design, comparisons between dynamic inflow-based calculations and measured test data are presented for a wide variety of single rotor configurations and operating conditions. The quantities compared include rotor performance parameters, blade flap bending moments, blade airloads, and blade pitch angles. The dynamic inflow calculations are performed using the U.S. Army’s Rotorcraft Comprehensive Analysis System. Rotor performance calculations are mostly accurate prior to stall, except for rotor torque calculations from an individual blade pitch control phase sweep. Blade airloads calculations do exhibit the correct dominant features, including those from blade tip vortices, but the calculated amplitudes of the wake-induced loading are significantly less than the test data; this leads to significantly lower amplitudes in the flap bending moment response and is problematic for calculating half peak-to-peak values in conditions with significant blade–wake interactions. Blade collective and cyclic pitch angles are reasonably accurate in edgewise flight.

Bubble dynamics and deformation of free liquid surface in aerated liquid storage tanks

OpenFOAM was utilized for analyzing bubble behavior and deformation of free liquid surface due to bubble formation and bursting in a rectangular container. Influence of three system parameters, orifice diameter, number of orifices and spacing between orifices, on various bubble dynamics and deformation of gas-liquid interface is presented. The study also incorporates information on bubble formation, bubbling frequency, and orientation of bubbles. Considering different orifice spacing, bubbling synchronicity is also reported. Details regarding interaction of wakes during bubble coalescence for single, double and triple inlet orifices are presented. The deformation of free liquid surface due to bubble formation and bursting is quantified using a new parameter called deformation index (D L * ). The analyses indicate that the frequency of bubble detachment is augmented with increase in orifice diameter and number of orifices. However, bubble detachment frequency is reduced when orifice spacing increased. Orientation of detached bubbles keeps on changing for larger orifice spacing. Results show that variations of these geometric parameters have substantial influence on free liquid surface deformation due to bubble bursting and other bubble behavior. Using results of these studies, one can develop a bubble-generating device for optimal performance.

Simulations of stably stratified flow past two spheres at Re = 300

Flows past two spheres immersed in a horizontally moving, linearly stratified fluid are investigated at a moderate Reynolds number of 300. Characterization of flow patterns considers representative geometrical configurations defined by varying both the distance between the spheres and their relative orientation to the free stream direction. Simulations are performed on unstructured meshes chosen to accurately resolve the dynamics of fluids in regions close to the spheres for Froude numbers Fr∈[0.25,∞]. Results illustrate the evolution of boundary layers, separation, and the wakes interaction under the influence of a gravity induced buoyancy force. Computations utilize a limited area, nonhydrostatic model employing non-oscillatory forward-in-time integration based on the multidimensional positive definite advection transport algorithm. The model solves the Navier–Stokes equations in the incompressible Boussinnesq limit, suitable for describing a range of mesoscale atmospheric flows. Results demonstrate that stratification progressively dominates the flow patterns as the Froude number decreases and that the interactions between the two spheres' wakes bear a resemblance to atmospheric flows past hills.

Local force calculations by an improved stress tensor discontinuity-based immersed boundary–lattice Boltzmann method

In the immersed boundary method, the volume force that is applied to enforce the no-slip boundary condition is equivalent to a discontinuity in the stress tensor across the boundary. In the stress tensor discontinuity-based immersed boundary–lattice Boltzmann method, which was proposed in our previous study [Suzuki and Yoshino, “A stress tensor discontinuity-based immersed boundary–lattice Boltzmann method,” Comput. Fluids 172, 593–608 (2018)], the boundary is represented by Lagrangian points that are independent of the background grid, and the discontinuity in the stress tensor is calculated on these points from desired particle distribution functions that satisfy the no-slip boundary condition based on the bounce-back condition. Although this method allows computation of the force locally acting on the boundary, the local force has a spurious oscillation along the boundary. In the present study, we remedy this problem by relaxing the bounce-back condition. To confirm the improvement achieved by using the new method, we apply it to simulate typical benchmark problems involving two- and three-dimensional flows with stationary or moving boundaries. We find that the proposed approach can effectively eliminate the spurious oscillation of the local force, and the results obtained with the improved method show good agreement with other numerical and experimental results. In addition, as an application of the proposed method to local force calculation, we investigate the effect of lift enhancement due to wing–wake interaction on a two-dimensional butterfly-like flapping wing.

The control of quadcopter propeller radiated noise

Simple scaling analysis is used to show that significant reductions in quadcopter propeller radiated noise can be achieved, at the hover condition, by reducing the quadcopter and/or payload weight to, in turn, reduce the propeller tip speed. The scaling analysis shows that the propeller radiated noise, at the hover condition, scales as the cube of the combined quadcopter and payload weight. The reductions in radiated noise predicted from the scaling analysis are verified experimentally with quadcopter propellers in an anechoic chamber. Experiments also show that reducing the propeller tip speed by increasing the propeller blade pitch or blade number, while maintaining a given static thrust, is ineffective at reducing the radiated noise. In fact, it is shown that too high a blade pitch and/or blade number can significantly increase propeller radiated noise even though the blade tip speed is significantly reduced. This is attributed to increased propeller blade and blade wake interactions for the increased blade pitch and number.

Wake redirection at higher axial induction

Abstract. The energy produced by wind plants can be increased by mitigating the negative effects of turbine–wake interactions. In this context, axial-induction control and wake redirection control, obtained by intentionally yawing or tilting the rotor axis away from the mean wind direction, have been the subject of extensive research but only very few investigations have considered their combined effect. In this study we compute power gains that are obtained by operating tilted and yawed rotors at higher axial induction by means of large-eddy simulations using the realistic native National Renewable Energy Laboratory (NREL) 5 MW actuator disk model implemented in the Simulator for On/Offshore Wind Farm Applications (SOWFA). We show that, for the considered two-row wind-aligned array of wind turbines, the power gains of approximately 5 % obtained by standard wake redirection at optimal tilt or yaw angles and reference axial induction can be more than tripled, to above 15 %, by operating the tilted or yawed turbines at higher axial induction. It is also shown that significant enhancements in the power gains are obtained even for moderate overinduction. These findings confirm the potential of overinductive wake redirection highlighted by previous investigations based on more simplified turbine models that neglected wake rotation effects. The results also complement previous research on dynamic overinductive yaw control by showing that it leads to large power gain enhancements also in the case where both the yaw and the overinduction controls are static, hopefully easing the rapid testing and implementation of this combined-control approach.

Turbulence properties in jets with fractal grid turbulence

Abstract