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Numerical simulation of a downstream hydrokinetic turbine operating in the wake of an upstream turbine for feedback control design is presented. Wake effects from an upstream turbine are quantified in terms of wake velocity and amplified turbulence levels. These effects are integrated in an in-stream hydrokinetic turbine numerical simulation that utilizes a Blade Element Momentum approach with a dynamic wake inflow model. Simulations are carried out on a fixed turbine model to simulate operation in river or tidal channels with conventional foundations, as well as on a compliantly moored turbine model such as those designed to operate in open ocean currents. [ABSTRACT FROM AUTHOR]

The low-order modeling of wind turbine dynamic wake is a challenging problem, which requires approximating high-dimensional, nonlinear dynamic systems with a limited number of states and linear structures. The Koopman-linear flow estimator is regarded as a promoting method in this area to approximate the dynamic wake field from a few physical-measured states. This paper presents optimization methods for the Koopman-linear flow estimator to improve its generalization applicability and modeling accuracy in specific applications. Firstly, the flow estimator's wake field prediction process is organized using Koopman modes and amplitudes; both are identified initially. Then, a optimization method is proposed to optimize the Koopman amplitudes for a given application scenario, which maintains a consistent form for uncontrolled and controlled systems based on the error-source analysis. After this, a sequential particle swarm optimization algorithm is adopted, which improves the computationally-intensive problem during sensor configuration optimization. The proposed algorithm quickly solves a feasible and optimized sensor configuration plan instead of the unavailable global-optimal one. The verification results show two conclusions: On the one hand, the nonlinearity during the yaw-induced wake deflection process is evident, which poses a significant challenge to the linear low-order approximation of the dynamic wake field. On the other hand, the proposed optimization methods highly improve the dynamic wake modeling accuracy under free-wake and yaw-controlled scenarios. Sparse sampling is necessary for dynamic wake behavior study in industrial. This paper solves the incomplete measurements and wake deflection nonlinearity problem caused by sparse sampling and promotes the generalization applicability for specific applications. • The Koopman amplitudes is defined and optimized for the Koopman-linear flow estimator. • Organize the uncontrolled and controlled systems with similar form and solving method. • Sensor configuration optimization problem is improved with low computational cost. • Error-accumulation of free-developed and yaw-controlled wake prediction is reduced. [ABSTRACT FROM AUTHOR]

With the continuous development of wind power industry, the power record of single wind turbine is constantly expanded, and the scale of wind power generation is becoming larger and larger. For large wind turbines, the wake effect cannot be ignored. It has an important influence on the fatigue load of the wind turbine, and also has a significant impact on the output power characteristics of the wind turbine. In order to study the influence of wind turbine wake on the load of the downstream wind turbine, several important wake parameters such as upstream and downstream wind turbine spacing, upstream wind turbine thrust coefficient and nature wind speed are assumed according to the theoretical results of some researchers. The physical quantity of wake region can be obtained according to the momentum theory. The relation between the natural wind speed, the wind speed and the thrust coefficient can be obtained by calculating the axial thrust based on the Bates' theory. The wind speed increases linearly with the natural wind speed, but with the increase of the thrust coefficient, the wind speed of the wind rotor will gradually decrease. When the natural wind speed is fixed, with the increase of thrust coefficient, the wind speed of the wind turbine will be reduced, and if the thrust coefficient can be large enough, then the wind speed of the wind rotor will tend to 0. However, there will be no thrust coefficient greater than or equal to 1 in the actual operation of the wind turbine. Use GH Bladed software to build a model of FL1500/70 1.5 and 3.0 MW doubly-fed wind turbine, and calculate the load response of the wake model to the downstream wind turbine. The conclusions are obtained by analyzing the calculation results which are as follows: 1) When the natural wind speed is constant, the wind speed at the wind rotor will decrease nonlinearly with the increase of thrust coefficient. The wake wind speed increases with the distance between 2 wind turbines, but the trend is gradually stabilized. But for different wind rotor radius, the larger the value, the closer to linear variation the relationship curve of wake wind speed and distance. 2) When the wind turbine is in the influence range of the wake, under the same wind speed and thrust coefficient, the farther the distance, the less the load will be affected by wake. In the case of larger absolute value of turbulence intensity difference, the load difference increases with the decrease of turbulence intensity difference, which is due to that the decrease of turbulence intensity under the wake effects makes the load increase; sometimes the load difference decreases with the increase of the difference of turbulence intensity, which means under the wake effect the turbulence increases and at the same time the load is reduced. The reason is when the turbulence increases, the load increases, but at the same time the wind speed affected by the wake effect becomes smaller, so the thrust coefficient decreases, followed by the decrease of the load. Comparing the turbulence intensity with the thrust coefficient, the reduction of thrust coefficient has a more influence on the load. 3) When the thrust coefficient of the upstream wind turbine increases, the load will increase; when the wind speed is greater than the rated wind speed, the thrust coefficient should be reduced by adopting the pitch control so as to reduce the fatigue damage of the wind turbine. 4) Based on the wind turbine modeling and numerical simulation calculation with Bladed software, the influence of upstream wind turbine wake on the fatigue loading of the wind turbine is discussed. The accuracy of this calculation which is used to determine the load response characteristics of wind turbines is limited, but it is very difficult to carry out on-site verification of the fatigue load data of wind turbine. Therefore, the data provided by the chart in this paper are for reference only. [ABSTRACT FROM AUTHOR]

A method to design a diffuser augmented wind turbine (DAWT) is proposed, using as a guiding point the optimal pressure drop at the turbine. The use of concepts and expressions derived from a 1D analytic model helped to reduce the number of computational fluid dynamics simulations needed to find the optimal configuration. The proposed configuration can extract energy from the flow with the same efficiency as the state-of-the-art shrouded wind turbine (SWT) configurations but generating a significantly smaller wake, which makes this configuration a good candidate for wind farms or tidal applications. Furthermore, as a product of the 1D model, universal curves for the power coefficient have been obtained, as a function of the thrust coefficient, or disk loading, which have been compared with numerical and experimental results, showing a good agreement. Finally, the maximum ideal power coefficient has been found for a given configuration, which helps to estimate the margin for improvement of an actual design. [ABSTRACT FROM AUTHOR]

The dynamics of the velocity field resulting from the interaction between the atmospheric boundary layer and a wind turbine array can affect significantly the performance of a wind power plant and the durability of wind turbines. In this work, dynamics in wind turbine wakes and instabilities of helicoidal tip vortices are detected and characterized through modal decomposition techniques. The dataset under examination consists of snapshots of the velocity field obtained from large-eddy simulations (LES) of an isolated wind turbine, for which aerodynamic forcing exerted by the turbine blades on the atmospheric boundary layer is mimicked through the actuator line model. Particular attention is paid to the interaction between the downstream evolution of the helicoidal tip vortices and the alternate vortex shedding from the turbine tower. The LES dataset is interrogated through different modal decomposition techniques, such as proper orthogonal decomposition and dynamic mode decomposition. The dominant wake dynamics are selected for the formulation of a reduced order model, which consists in a linear time-marching algorithm where temporal evolution of flow dynamics is obtained from the previous temporal realization multiplied by a time-invariant operator. [ABSTRACT FROM AUTHOR]

The long wake of a wind turbine has a significant impact on the performance of downstream turbines. Under the inspiration of migrating geese flying in a V or I formation to save energy, a novel wake energy reuse method is proposed to optimize the layout for Savonius-type vertical axis wind turbines (S-VAWT). VAWT wakes include a series of high speed and energy zones. On both sides of the upstream turbine, 7 × 16 transient two-dimensional numerical simulations are performed with Fluent to investigate wake structure, interaction effect and power coefficients ( C p ) of downstream turbines. Based on Kriging Method, a response surface model (Surrogate model) is created to describe the relationship between the optimization objective C p and layout positions. Finally, particle swarm optimization algorithm is applied to find the optimal relative layout position (5.25 m, −2.18 m) of the downstream turbine. The optimal position is located in the periodic high speed zone of the wake on the advancing blade side. And the optimal position is suitable for multi-turbines in a large wind farm. The optimization results show that C p of downstream turbines at optimal layout position is significantly increased from 0.2477 to 0.3044 (22.89% higher). [ABSTRACT FROM AUTHOR]

The flow structure in the wake of a model wind turbine is explored under negligible and high turbulence in the freestream region of a wind tunnel at Re ~ 7 × 104. Attention is placed on the evolution of the integral scale and the contribution of the large-scale motions from the background flow. Hotwire anemometry was used to obtain the streamwise velocity at various streamwise and spanwise locations. The pre-multiplied spectral difference of the velocity fluctuations between the two cases shows a significant energy contribution from the background turbulence on scales larger than the rotor diameter. The integral scale along the rotor axis is found to grow linearly with distance, independent of the incoming turbulence levels. This scale appears to reach that of the incoming flow in the high turbulence case at x/d ~ 35-40. The energy contribution from the turbine to the large-scale flow structures in the low turbulence case increases monotonically with distance. Its growth rate is reduced past x/d ~ 6-7. There, motions larger than the rotor contribute ~50% of the total energy, suggesting that the population of large-scale motions is more intense in the intermediate field. In contrast, the wake in the high incoming turbulence is quickly populated with large-scale motions and plateau at x/d ~ 3. [ABSTRACT FROM AUTHOR]

The increased turbulence behind the turbine induces a fatigue load on the downstream turbine blades. An accurate estimate of turbulent intensity in turbine wake is paramount to optimizing the placement of turbines in tidal turbine parks. A simple empirical model is developed using data fitting from numerical simulation of a non-rotational actuator disc model to estimate the added turbulence in realistic tidal stream conditions similar to the Alderney Race, with a rotor diameter to depth ratio of 40%. The study shows a self-similar Gaussian shape streamwise turbulent intensity in a lateral direction similar to the velocity deficit profile. The turbulent wake radius expands according to a power-law depending on the ambient turbulent intensity. The added turbulence model justifies that the major turbulence source in the near wake is attributed to the rotor as it is weakly dependent on the ambient turbulence in the flow. As no known existing empirical model for tidal turbine wake added turbulence, the model is compared with existing models of unbounded wind turbines. The proposed model estimates the average turbulence in the far wake can assist turbine placement in a tidal farm. • Turbulent wake expands according to power law and depends on the turbulent intensity. • High turbulent intensity facilitates faster recovery to ambient condition. • Added turbulence is weakly affected by the ambient turbulence in the flow. • The rotor is the major source of turbulence in the near wake. • The proposed model estimates the average turbulence in the far wake. [ABSTRACT FROM AUTHOR]

Wind energy has become one of the most commercially prospective renewable energies. However, the wake effect of wind turbine can reduce the power generation efficiency and increase the fatigue loading of downstream turbines. Hence, the wake effect study has attracted increasing interests. Compared with the extensive study on the single turbine wake, that on the superposition effect of multiple turbine (multi-turbine) wakes is limited. In this study, the characteristics of the wake velocity and turbulence intensity are studied and the exponential superposition model is proposed for the aligned multi-turbine wakes. Firstly, Simulator for Offshore Wind Farm Applications (SOWFA), a high-fidelity simulator for the interaction between wind turbine dynamics and the flow in a wind farm, is used to analyze the distribution of aligned multi-turbine wakes. It is observed that wakes reach the steady state from second turbine in the aligned turbines. Then influences of different factors on the accuracy of existing superposition models are studied. It is found spacing has significant effect on the performance of superposition models. Furthermore, the exponential superposition model with higher applicability is proposed for the wake velocity and turbulence intensity. Finally, this model is validated by the benchmark data of real wind farms. • Characteristics of the wake velocity and turbulence intensity of aligned multiple turbines are addressed. • Influences of different factors on the accuracy of existing superposition models are addressed. • Exponential superposition model is proposed for the wake velocity and turbulence intensity. • Validations in real wind farms show exponential superposition model has good performance. [ABSTRACT FROM AUTHOR]

The characterization of wind turbine wakes produced for different loading conditions of the turbine and incoming wind is a crucial task for design of wind farm layout. Indeed, the optimization of the power harvested from a wind farm can only be achieved if detrimental wake interactions are carefully inhibited or mitigated. In this paper the importance of performing wind turbine wake measurements is emphasized through two experiments, which are complementary for the experimental measurement techniques used and scientific results achieved. First, wind tunnel tests of a down-scaled wind turbine are presented, for which fixed-point turbulence measurements are leveraged to predict the hub vortex instability encountered in the near wake. The second experiment consists in wind LiDAR measurements of the wake produced by a utility-scale wind turbine under different regimes of the atmospheric thermal stability. It is shown that wind turbine wakes recover faster under convective conditions than for neutral ones. [ABSTRACT FROM AUTHOR]

There is a demand of analyzing tilted Vertical Axis Wind Turbine (VAWT) for the emerging applications like VAWTs operating on high rise buildings, Floating Axis Wind Turbine (FAWT) functioning in deep sea, and so on. For this purpose numerical validation of an existing experimental work of Vertical Axis Wind Turbine (VAWT) in upright and tilted conditions has been carried out. The numerical validation is accomplished by means of Computational Fluid Dynamics (CFD) analysis by solving Unsteady Reynolds Averaged Navier–Stokes (URANS) equation. A rigorous parametric study of choosing a right turbulence model for solving URANS equation, mesh dependency analysis and optimum time step is comprehended. After choosing the right parameters, validation is performed. Following that the flow fields of upright and tilted conditions are visualized. The possible reasons of getting higher power coefficient in tilted condition have been discussed. While carrying out the study, it is observed that in case of tilted configuration the wake stream shifts downward. This feature of VAWT in tilted condition could lead into effective sea surface utilization in floating offshore wind farms. [ABSTRACT FROM AUTHOR]

The wake evolution measured downstream of multi-megawatt wind turbines located in flat and complex terrains are described here. These high-resolution measurements at full-scale Reynolds number conditions are made with an instrumented drone that is equipped with a suite of sensors and detail the characteristics of the mean flow and turbulent kinetic energy in the evolving wake. Reynolds decomposition yields the nature of turbulent fluctuations in surface layer, and this decomposition is used to detail the turbulence statistics, degree of anisotropy and friction velocity. These measurements are shown to be suited for the further development of three-dimensional wake models that are currently under intensive development. [ABSTRACT FROM AUTHOR]

Experiments were performed using two model wind turbines operated in tandem with a bottom-fixed configuration and a floating configuration with both turbines allowed to freely oscillate in the streamwise direction. Wakes of both turbines were measured using stereoscopic Particle Image Velocimetry. Turbulent characteristics of the far wake of the first turbine acting as the inflow for the downwind turbine were characterized calculating the eddy viscosity and mixing length profiles from the obtained data. The influence of the far wake on the statistical properties of the near wake of the second turbine are compared between the fixed and oscillating configurations. The incoming mixing length clearly influences the Reynolds stresses and turbulence production of the near wake in the shear layer. Below, the connection between incoming mixing length and the near wake is less evident, due to the impact of the nacelle and rotation of the rotor. For the oscillating turbine, the Reynolds stresses and turbulence production in the near wake of the downwind turbine are damped. Vertical fluctuations were found to decrease though an increase in the mean vertical component. New challenges arise in the design of a floating offshore wind farms, in terms of farm layout and load estimations. [ABSTRACT FROM AUTHOR]

The aerodynamic characteristics of wakes in complex terrain have a profound impact on the energy yield of wind farms and on the fatigue loads on wind turbines in the wind farm. In order to detail the spatial variations of the wind speed, wind direction, and turbulent kinetic energy (TKE) in the near-wake, comprehensive drone-based measurements at a multi-megawatt (MW) wind turbine that is located in complex terrain have been conducted. A short-time Fourier transform (STFT)-based analysis method is used to derive time-localized TKE along the drone's trajectory. In upstream and in the near-wake, the vertical profiles of wind speed, wind direction, and TKE are detailed. There is an increase in the TKE from upstream to downstream of the wind turbine, and whereas, the characteristic microscale length scales increase with increasing height above the ground upstream of the turbine, in the near-wake the microscale lengths are of constant, smaller magnitude. The first-ever measurements of the pressure field across a multi-MW wind turbines rotor plane and of the tip vortices in the near-wake are also reported. It is shown that the pitch between subsequent tip vortices, which are shed from the wind turbines blades, increases in the near-wake as the wake evolves. These details of the near-wake can have an important effect on the subsequent evolution of the wake and must be incorporated into the three-dimensional (3D) field wake models that are currently under intensive development. [ABSTRACT FROM AUTHOR]

A time-domain simulation tool to predict the dynamic power output of wind turbines in an offshore wind farm was developed in this study. A wind turbine model consisting of first or second order transfer functions of various wind turbine elements was combined with the Ainslie's eddy viscosity wake model to construct the simulation tool. The wind turbine model also includes an aerodynamic model that is a look up table of power and thrust coefficients with respect to the tip speed ratio and pitch angle of the wind turbine obtained by a commercial multi-body dynamics simulation tool. The wake model includes algorithms of superposition of multiple wakes and propagation based on Taylor's frozen turbulence assumption. Torque and pitch control algorithms were implemented in the simulation tool to perform max-Cp and power regulation control of the wind turbines. The simulation tool calculates wind speeds in the two-dimensional domain of the wind farm at the hub height of the wind turbines and yields power outputs from individual wind turbines. The NREL 5MW reference wind turbine was targeted as a wind turbine to obtain parameters for the simulation. To validate the simulation tool, a Danish offshore wind farm with 80 wind turbines was modelled and used to predict the power from the wind farm. A comparison of the prediction with the measured values available in literature showed that the results from the simulation program were fairly close to the measured results in literature except when the wind turbines are congruent with the wind direction. [ABSTRACT FROM AUTHOR]

Marine hydrokinetic turbine, when operating in a shallow channel is subjected to the boundary proximity effects from a deformable free surface on top and the channel bottom. A close proximity of turbine to these boundaries modifies the flow-field around the turbine and affects device performance. Significant flow acceleration occurs in and around the turbine rotation plane; the magnitude of which depends on size of the turbine relative to the channel cross-section and is commonly referred to as solid blockage. In addition, the wake behind the turbine creates a restriction to the flow called wake blockage. We focus on unraveling the influence of boundary proximity and blockage on the turbine performance through coupled experimental and computational studies. The experiments were carried out in an open surface water channel with a three bladed, constant chord, untwisted marine hydrokinetic turbine submerged at different depths and performance was evaluated under various operating conditions. The findings were complimented by a steady state computational fluid dynamics study that was carried out to understand the effect of flow Reynolds number and solid blockage on the turbine performance. A reduction in tip-depth of immersion was observed to improve the turbine performance until it reached an optimum depth beyond which a reduction in performance was observed due to free surface interaction with wake and bypass region. A transient CFD analysis with volume of fluid approach was performed to incorporate free-surface and buoyancy effects and augment flow-field characterization behind the turbine in the wake, upper bypass, and lower bypass regions. For low tip clearance ratios, a significant drop (up to 5–10% of channel depth) in free surface was observed behind turbine with complex three dimensional flow structures that lead to a skewed wake affecting its expansion and restoration process. [ABSTRACT FROM AUTHOR]

We apply a modified proper orthogonal decomposition (POD) to large eddy simulation data of a wind turbine wake in a turbulent atmospheric boundary layer. The turbine is modeled as an actuator disk. Our analysis mainly focuses on the pragmatic identification of spatial modes, which yields a low order description of the wake flow. This reduction to a few degrees of freedom is a crucial first step for the development of simplified dynamic wake models based on modal decompositions. It is shown that only a few modes are necessary to capture the basic dynamical aspects of quantities that are relevant to a turbine in the wake flow. Furthermore, we show that the importance of the individual modes depends on the relevant quantity chosen. Therefore, the optimal choice of modes for a possible model could in principle depend on the application of interest. We additionally present a possible interpretation of the extracted modes by relating them to the specific properties of the wake. For example, the first mode is related to the horizontal large-scale movement. [ABSTRACT FROM AUTHOR]

On a wind farm, the wake has an important impact on the performance of the wind turbines. For example, the wake of an upstream wind turbine affects the blade load and output power of the downstream wind turbine. In this paper, a modified actuator line model with blade tips, root loss, and an airfoil three-dimensional delayed stall was revised. This full-scale modified actuator line model with blades, nacelles, and towers, was combined with a Large Eddy Simulation, and then applied and validated based on an analysis of wind turbine wakes in wind farms. The modified actuator line model was verified using an experimental wind turbine. Subsequently, numerical simulations were conducted on two NREL 5 MW wind turbines with different staggered spacing to study the effect of the staggered spacing on the characteristics of wind turbines. The results show that the output power of the upstream turbine stabilized at 5.9 MW, and the output power of the downstream turbine increased. When the staggered spacing is R and 1.5R, both the power and thrust of the downstream turbine are severely reduced. However, the length of the peaks was significantly longer, which resulted in a long-term unstable power output. As the staggered spacing increased, the velocity in the central near wake of the downstream turbine also increased, and the recovery speed at the threshold of the wake slowed down. The modified actuator line model described herein can be used for the numerical simulation of wakes in wind farms. [ABSTRACT FROM AUTHOR]

Wind tunnel experiments were performed, where the development of the wake of a model wind turbine was measured using stereo Particle Image Velocimetry to observe the influence of platform pitch motion. The wakes of a classical bottom fixed turbine and a streamwise oscillating turbine are compared. Results indicate that platform pitch creates an upward shift in all components of the flow and their fluctuations. The vertical flow created by the pitch motion as well as the reduced entrainment of kinetic energy from undisturbed flows above the turbine result in potentially higher loads and less available kinetic energy for a downwind turbine. Experimental results are compared with four wake models. The wake models employed are consistent with experimental results in describing the shapes and magnitudes of the streamwise velocity component of the wake for a fixed turbine. Inconsistencies between the model predictions and experimental results arise in the floating case particularly regarding the vertical displacement of the velocity components of the flow. Furthermore, it is found that the additional degrees of freedom of a floating wind turbine add to the complexity of the wake aerodynamics and improved wake models are needed, considering vertical flows and displacements due to pitch motion. [ABSTRACT FROM AUTHOR]

An actuator line method combining the actuator line technique, in which the body forces representing the blades of the wind turbine with a three-dimensional Navier-Stokes solver, is proposed to get flow fields about wind turbines. It avoids numerous meshes and huge computational for the simulation of blade boundary layers, and in replacement, more meshes and computational resource can be used to simulate the wind turbine wake flow filed, so the actuator line method is very suitable for the research of the wind turbine wake. Computational results for NH1500 blade are compared among the actuator line method with the Gauss distribution, BEM theory, CFD and wind tunnel experiment in the load distribution on the blade and wind turbine power coefficient, validating that the actuator line method is effective and efficient for wind turbine aerodynamic calculation. For completeness, the wind turbine wake flow filed calculated by the actuator line method is analyzed. [ABSTRACT FROM AUTHOR]

Simulations of wind turbine wakes are presented in this paper using the three-dimensional Reynolds- Averaged Navier-Stokes (RANS) equations employing the k-ε turbulence model appropriately modified for the atmospheric flows. The actuator disk approach is used to model the action of the turbine rotor. Modified formulations of the inlet conditions and the wall functions are used to allow consistency between the fully developed atmospheric boundary layer (ABL) inlet profiles and the wall function formulation. Results are presented and compared with three wind turbines running under neutral atmospheric conditions. The results demonstrate that the accurate simulation of the atmospheric boundary layer applying enhanced inlet conditions and wall function formulation consistent with the k-ε model can give very useful information about the wakes, directly contributing to the accurate estimation of the power of the downstream turbines. [ABSTRACT FROM AUTHOR]

Abstract: Experimental results on the wake properties of a non-rotating simplified wind turbine model, based on the actuator disc concept, and a rotating model, a three-blade wind turbine, are presented. Tests were performed in two different test sections, one providing a nominally decaying isotropic turbulent inflow (turbulence intensity of 4% at rotor disc location) and one providing a neutral atmospheric boundary layer above a moderately rough terrain at a geometric scale of 1:300 (determined from the combination of several indicators), with 13% of turbulence intensity at hub height. The objective is to determine the limits of the simplified wind turbine model to reproduce a realistic wind turbine wake. Pressure and high-order velocity statistics are therefore compared in the wake of both rotor discs for two different inflow conditions in order to quantify the influence of the ambient turbulence. It has been shown that wakes of rotating model and porous disc developing in the modeled atmospheric boundary layer are indistinguishable after 3 rotor diameters downstream of the rotor discs, whereas few discrepancies are still visible at the same distance with the decaying isotropic turbulent inflow. [Copyright &y& Elsevier]

This study aimed to determine the optimal rotor spacing of two vertical-axis wind turbines, which are simulated by miniature models arranged side-by-side with a relatively low aspect ratio. Wind tunnel experiments with a pair of 3-D printed model rotors were conducted at a uniform velocity. A series of experiments were conducted involving both incremental adjustments to the rotor gaps, g, and the rotational direction of each rotor. Increases in the power and the related flow patterns were observed in all three arrangements: Co-Rotating (CO), Counter-Up (CU), and Counter-Down (CD). The maximum phase-synchronized rotational speed occurs at the narrowest gap in the CD arrangement. Meanwhile, local maxima arise in the CO and CU arrangements at g/D < 1, where D is the rotor diameter. From an engineering perspective, the optimal rotor spacing is g/D = 0.2 with the CO arrangement, using the same two rotors rotating in the same direction. Based on flow visualization using a smoke-wire method at a narrower gap opening of 0.2D, the wake width in the case of the CU arrangement was remarkably narrower than those obtained in the CO and CD arrangements. In the CU arrangement, a movement towards the center of the rotor pair of the nominal front-stagnation point of each rotor was confirmed via flow visualization. This finding explains a reduction tendency in the rotational speed of the rotors via a reduction in the lift in the CU arrangement. [ABSTRACT FROM AUTHOR]

Future wind power developments may be located in complex topographic and harsh environments; forests are one type of complex terrain that offers untapped potential for wind energy. A detailed analysis of the unsteady interaction between wind turbines and the distinct boundary layers from those terrains is necessary to ensure optimized design, operation, and life span of wind turbines and wind farms. Here, laboratory experiments were carried to explore the interaction between the wake of a horizontal-axis model wind turbine and the boundary layer flow over forest-like canopies and the modulation of forest density in the turbulent exchange. The case of the turbine in a canonical boundary layer is included for selected comparison. The experiments were performed in a wind tunnel fully covered with tree models of height H / z h u b ≈ 0.36 , where z h u b is the turbine hub height, which were placed in a staggered pattern sharing streamwise and transverse spacing of Δ x / d c = 1.3 and 2.7, where d c is the mean crown diameter of the trees. Particle image velocimetry is used to characterize the incoming flow and three fields of view in the turbine wake within x / d T ∈ (2 , 7) and covering the vertical extent of the wake. The results show a significant modulation of the forest-like canopies on the wake statistics relative to a case without forest canopies. Forest density did not induce dominant effects on the bulk features of the wake; however, a faster flow recovery, particularly in the intermediate wake, occurred with the case with less dense forest. Decomposition of the kinematic shear stress using a hyperbolic hole in the quadrant analysis reveals a substantial effect sufficiently away from the canopy top with sweep-dominated events that differentiate from ejection-dominated observed in canonical boundary layers. The comparatively high background turbulence induced by the forest reduced the modulation of the rotor in the wake; the quadrant fraction distribution in the intermediate wake exhibited similar features of the associated incoming flow. [ABSTRACT FROM AUTHOR]

Abstract: Experimental results on the unsteady behaviour of the wake of a modelled wind turbine in an atmospheric boundary layer (ABL) wind tunnel are presented. Tests were performed by modelling in the wind tunnel an ABL above a neutral moderately rough terrain at a geometric scale of 1:400, and a wind turbine with the help of the actuator disk concept. In order to characterise the meandering properties of its wake, velocity spectra and space–time correlations were measured through hot wire anemometry, both in the wake of the modelled wind turbine and in the wake of a solid disk. Comparing these two configurations allowed the examination of the differences between the random motion of the whole wind turbine wake characterising the meandering in the first case, and the periodic oscillations of the well-known vortex shedding, which appears behind a bluff-body, in the second case. Finally, the same experiments were performed in homogeneous and isotropic turbulent flows to exhibit the role of the large atmospheric turbulent scales in the meandering process. [Copyright &y& Elsevier]

Abstract: Understanding the flow field around horizontal axis marine current turbines is important if this new energy generation technology is to advance. The aim of this work is to identify and provide an understanding of the principal parameters that govern the downstream wake structure and its recovery to the free-stream velocity profile. This will allow large farms or arrays of devices to be installed whilst maximising device and array efficiency. Wake characteristics of small-scale mesh disk rotor simulators have been measured in a 21m tilting flume at the University of Southampton. The results indicate that wake velocities are reduced in the near wake region (close behind the rotor disk) for increasing levels of disk thrust. Further downstream all normalised wake velocity values converge, enforcing that, as for wind turbines, far wake recovery is a function of the ambient flow turbulence. Varying the disk proximity to the water surface/bed introduces differential mass flow rates above and below the rotor disk that can cause the wake to persist much further downstream. Finally, the introduction of increased sea bed roughness whilst increasing the depth-averaged ambient turbulence actually decreases downstream wake velocities. Results presented demonstrate that there are a number of interdependent variables that affect the rate of wake recovery and will have a significant impact on the spacing of marine current turbines within an array. [Copyright &y& Elsevier]

Several methods of defining and estimating the width of a turbulent wake are presented and tested on the experimental data obtained in the wake past an asymmetric prismatic airfoil NACA 64(3)-618, which is often used as tip profile of the wind turbines. Instantaneous velocities are measured by using the Particle Image Velocimetry (PIV) technique. All suggested methods of wake width estimation are based on the statistics of a stream-wise velocity component. First, the expansion of boundary layer (BL) thickness is tested, showing that both displacement BL thickness and momentum BL thickness do not represent the width of the wake. The equivalent of 99% BL thickness is used in the literature, but with different threshold value. It is shown that a lower threshold of 50% gives more stable results. The ensemble average velocity profile is fitted by Gauss function and its σ-parameter is used as another definition of wake width. The profiles of stream-wise velocity standard deviation display a two-peak shape; the distance of those peaks serves as wake width for Norberg, while another tested option is to include the widths of such peaks. Skewness (the third statistical moment) of stream-wise velocity displays a pair of sharp peaks in the wake boundary, but their position is heavily affected by the statistical quality of the data. Flatness (the fourth statistical moment) of the stream-wise velocity refers to the occurrence of rare events, and therefore the distance, where turbulent events ejected from the wake become rare and can be considered as another definition of wake width. The repeatability of the mentioned methods and their sensitivity to Reynolds' number and model quality are discussed as well. [ABSTRACT FROM AUTHOR]

Abstract: In order to develop applications for micro-wind turbines, an experimental analysis of the flow field around integrated micro-wind turbines was performed. The wake flows of a single turbine and 5×5 array unit were measured by using hot-wire and ultrasonic anemometers and particle image velocimetry (PIV). The present array of turbines follows a fundamental lattice layout; however, it has the flexibility to optimize its layout according to the environmental conditions. hot-wire and ultrasonic anemometers and PIV measurements were used for stand-alone turbines and their integrated systems. Comparisons between the mean velocity field and turbulent intensity were described for stand-alone full-scale and 1/10-scale wind turbine models. Thereafter, a typical array of the 1/10-scale model was assumed and its wake flow was investigated in a wind tunnel. The velocity profile and turbulence behind the array were measured and studied at different streamwise locations. The scale effect and model similarities were discussed. The experimental results show that a zone exists with constant and linear wake deficit ratios in the downstream regions. [Copyright &y& Elsevier]