Uniform Inflow Conditions Research Articles

Numerical investigations of coupled aeroelastic performance of wind turbines by elastic actuator line model

With the economic considerations of wind power generation, blades of wind turbines are gradually becoming longer and more flexible. Therefore, it is necessary to research the influence of coupling aerodynamic and elastic structure of blades of wind turbines. Moreover, in a wind farm, the turbulent wind is generated by the irregular movement of the atmosphere, which makes the elastic deformations of blades and the complex wake interference effect between wind turbines. Therefore, the coupled aeroelastic performance of the blade is applied to analyze the interaction of the wake flow field of two tandem wind turbines based on the elastic actuator line model and the Euler-Bernoulli beam theory. Firstly, the correctness of the coupled aeroelastic model is illustrated by comparing the aeroelastic response with OpenFAST software. Secondly, the influence of aeroelastic performances is researched on the structural deformations of blades. It is found that the aerodynamic power and thrust decrease after considering the deformations of blades. Furthermore, the interference effect on the thrust is more evident than aerodynamic power. Thirdly, the far wake region is affected apparently when the deformations of blades are taken into consideration, which causes the tip vortex interference phenomenon to delay. Finally, compared with the uniform inflow condition, the displacement and speed of the deformations of the blades are both increased under the turbulent inflow condition. Meanwhile, the disturbance of the wake vortex in the far wake region by the turbulent wind is stronger. Besides, by comparing the differences between the four vortex identification methods, it is significant that the ΩR method can better capture the vortex system behind the rotor plane without manual intervention threshold selection. The vortex structure also represents the vorticity of the wake vortex evolution.

The non-uniform inflow structure of reactor coolant pump based on Laser Doppler Velocimetry

The hydrodynamic fields generated by the steam generator (SG) head at the reactor coolant pump (RCP) inlet are experimentally and numerically investigated in this paper. The velocity profiles and turbulence intensity rate were obtained with a Laser Doppler Velocimetry (LDV) system. The velocity profiles under non-uniform inflow and uniform inflow conditions were obtained and compared. The numerical results on measurement sections were qualitatively and quantitatively validated against LDV data. The results indicated that the numerical method have a good agreement with the experiments. It can be known from the study that hydraulic performance of pump deteriorates with its inflow. In addition, the velocity profile of non-uniform inflow can be divided into three regions. These regions are constant high velocity region, large velocity gradient region and back flow region respectively. Furtherly, from the turbulence intensity rate analysis, it can be concluded that the non-uniform inflow is highly turbulent, which would bring significant performance changes to the pump.

Experimental Analysis of Wind Pressure Characteristics in a Reduced-Scale Model of a Slab-Shaped High-Rise Building at Different Inflow Conditions with Various Wind Flow Directions

Wind resistance is one of the most important safety targets for high-rise buildings, especially slab-shaped ones with relatively large length–width ratios. In this study, the characteristics of wind pressure on a reduced-scale model of a slab-shaped high-rise building were analyzed experimentally. The experiment was conducted using the DTC Initium electronic scanning pressure measurement system in the wind tunnel at the Xiamen University of Technology, China. The spatial distribution and time-frequency characteristics of the wind pressure signals were analyzed with various wind flow directions under uniform and boundary-layer inflow conditions. The results show that both of the inflow conditions and the wind directions have significant influences on the magnitude and distribution characteristics of the wind pressure on the building surfaces. The wavelet transform-based analysis shows that the wind pressure on the building surfaces presents obvious intermittent characteristics, with the instantaneous energies pulsating intensively in the time-frequency domain, illustrating the unsteady nature of the wind pressure loads on the building. The influence and risk of the unsteady pulsating pressure loads should be considered when evaluating the wind-resistant performances of this type of building.

Computational-fluid-dynamics analysis of a Darrieus vertical-axis wind turbine installation on the rooftop of buildings under turbulent-inflow conditions

Abstract. The behaviour of a rooftop-mounted generic H-rotor Darrieus vertical-axis wind turbine (H-VAWT) is investigated numerically in realistic urban terrain. The interaction of the atmospheric boundary layer with the different buildings, topography, and vegetation present in the urban environment leads to the highly turbulent-inflow conditions with continuously changing inclination and direction. Consequently, all these factors can influence the performance of a VAWT significantly. In order to simulate a small H-VAWT at rooftop locations in the urban terrain under turbulent-inflow conditions, a computational approach is developed. First, the flow field in the terrain is initialized and computed with inflow turbulence. Later, the mesh of wind turbine is superimposed on the mesh of the terrain at two distinct locations and different heights for further computation in the turbulent flow field. The behaviour of the H-VAWT is complex due to the 3D unsteady aerodynamics resulting from continuously changing the angle of attack, blade–wake interaction, and dynamic stall. To get more insight into the behaviour of a rooftop-mounted H-VAWT in turbulent flow, high-fidelity delayed detached-eddy simulations (DDESs) are performed at different rooftop positions and the results compared against the behaviour under uniform-inflow conditions in the absence of inflow turbulence. It is found that the performance of a wind turbine is significantly increased near the rooftop positions. The skewed flow at the rooftop location increases the complexity. However, this effect contributes positively to increasing the performance of H-VAWT wind turbines.

Investigation of a submerged fully passive energy-extracting flapping foil operating in sheared inflow

In this work a fully passive energy harvesting foil is studied computationally. An in-house 2nd order finite volume CFD solver, MaPFlow, is strongly coupled with a rigid body dynamics solver to investigate the foil operation under uniform and sheared inflow conditions with/without free surface. The mesh follows the airfoil motion using a radial basis function (RBF) mesh deformation approach. Initially, MaPFlow predictions are compared to experimental and numerical results available in the literature, where reasonable agreement is found. Next, one-phase simulations are considered for linearly sheared inflow for various shear rates. Results suggested that foil performance can be enhanced under sheared inflow conditions. Finally, two-phase simulations taking into account the free surface, for both uniform and sheared inflow, are considered. Predictions indicate a significant deterioration in performance of the system when the foil operates under the free surface due to the interaction of the shed vorticity with the free surface.

High-fidelity aeroelastic analyses of wind turbines in complex terrain: fluid–structure interaction and aerodynamic modeling

Abstract. This paper shows high-fidelity fluid–structure interaction (FSI) studies applied to the research wind turbine of the WINSENT (Wind Science and Engineering in Complex Terrain) project. In this project, two research wind turbines are going to be erected in the south of Germany in the WindForS complex-terrain test field. The FSI is obtained by coupling the CFD URANS–DES code FLOWer and the multiphysics FEM solver Kratos Multiphysics, in which both beam and shell structural elements can be chosen to model the turbine. The two codes are coupled in both an explicit and an implicit way. The different modeling approaches strongly differ with respect to computational resources, and therefore the advantages of their higher accuracy must be correlated with the respective additional computational costs. The presented FSI coupling method has been applied firstly to a single-blade model of the turbine under standard uniform inflow conditions. It could be concluded that for such a small turbine, in uniform conditions a beam model is sufficient to correctly build the blade deformations. Afterwards, the aerodynamic complexity has been increased considering the full turbine with turbulent inflow conditions generated from real field data, in both flat and complex terrains. It is shown that in these cases a higher structural fidelity is necessary. The effects of aeroelasticity are then shown on the phase-averaged blade loads, showing that using the same inflow turbulence, a flat terrain is mostly influenced by the shear, while the complex terrain is mostly affected by low-velocity structures generated by the forest. Finally, the impact of aeroelasticity and turbulence on the damage equivalent loading (DEL) is discussed, showing that flexibility reduces the DEL in the case of turbulent inflow, acting as a damper that breaks larger cycles into smaller ones.

Comparing wind turbine aeroelastic response predictions for turbines with increasingly flexible blades

Highly flexible blades are becoming more prevalent designs as a potential solution to the transportation challenges associated with large-scale wind turbine rotors. However, there is currently no quantitative definition of “highly flexible” blades. To further develop turbines with highly flexible blades, a precise definition of the term and accurate simulations of turbines with such blades are required. Assumptions made in the traditional aerodynamic model, Blade Element Momentum (BEM) theory, are violated in turbines with flexible blades. However, Free Vortex Wake (FVW) methods can more accurately model these turbine designs. Though more computationally expensive than BEM, FVW methods are still computationally tractable for use in iterative turbine design. The purpose of this work was to determine the blade flexibility at which BEM and FVW methods begin to produce diverging aeroelastic response results. This was accomplished by simulating the BAR-DRC reference turbine with increasingly flexible blades in a range of steady, uniform inflow conditions using OpenFAST, the National Renewable Energy Laboratory’s physics-based turbine engineering tool. Blade-tip deflections confirmed that BEM and FVW results diverge as blade flexibility increases. For the 212 m rotor diameter turbine used in this study, the two methods largely agreed for smaller blade deflections. But their results differed by an average of 5% when the out-of-plane blade-tip deflections exceeded 5% of the blade length and in-plane blade-tip deflections exceeded 1.25% of the blade length, with percent differences approaching 25% at the largest deflections.

Performance analysis of wind turbines with leading-edge erosion and erosion-safe mode operation

For offshore wind turbines, Leading-Edge Erosion (LEE) due to rain is posing a serious risk to structural integrity and can lead to a performance loss of the order of a few percent of the Annual Energy Production (AEP). A proposed mitigation strategy is the so-called Erosion-Safe Mode (ESM). In this work, the AEP losses caused by LEE or by operating in the ESM are compared for two reference turbines, i.e. the IEA 15MW and the NREL 5MW turbines. For both turbines, the performance is evaluated in uniform and sheared inflow conditions. The effects of erosion are modeled by creating clean and rough airfoil polars in XFOIL. It is assumed that erosion occurs once a critical blade element section speed is exceeded. Power curves for LEE and ESM are calculated by using the free-wake vortex method CACTUS. Results show that LEE negatively affects the power production below rated capacity, while operating in ESM predominantly sheds performance at rated power of the turbine. This study, therefore, shows that a break-even point for the ESM exists. The AEP loss due to erosion can be successfully mitigated with the ESM at sites with low mean wind speed, however, at sites with higher mean wind speed, operation with erosion leads to a lower AEP loss. The break-even point shows little sensitivity to the blade design and to mean shear variations, but strongly depends on the frequency ESM needs to be applied. The latter is driven by the predicted amount of damaging rain events. In conclusion, erosion-optimal operation is strongly governed by the site characteristics and much less by turbine design, and the viability of an ESM strategy can be significantly expanded by a better understanding of blade damage mechanisms and improved forecasting of the related weather events.

A New Loss Generation Body Force Model for Fan/Compressor Blade Rows: Application to Uniform and Non-Uniform Inflow in Rotor 67

Abstract Despite advances in computational power, the cost of time-accurate flows in axial compressor and fan stages with spatially non-uniform inflow is still too high for design-stage use in industry. Body force modeling reduces the computation time to practical levels, mainly by reducing the problem to a steady one. These computations are important to determine efficiency penalties associated with non-uniform inflows. Previous studies of body force methods have, in most cases, relied on computations with the presence of the blades to calibrate loss models. In some recent studies, uncalibrated models have been used, but such models can drop off in accuracy at conditions where separation would occur on the blade surfaces. In this paper, a neural-network-based loss model introduced in a recent paper by the authors is implemented for NASA rotor 67 for both uniform and non-uniform inflow conditions. For uniform inflow, the spanwise trend of entropy variation is generally captured with the new body force model. Although there are discrepancies at some span fractions, the present model generally predicts the compressor’s isentropic efficiency to within 3% compared to bladed Reynolds-averaged Navier–Stokes simulations. For non-uniform inflow, we consider a stagnation pressure profile representative of boundary layer ingestion. The results show that the region of maximum entropy generation is captured by the present model and the prediction of isentropic efficiency penalty due to the non-uniform inflow is only 0.2 points less than that determined from bladed computations.

The Effects of Turbulence Intensity and Tip Speed Ratio on the Coherent Structure of Horizontal-Axis Wind Turbine Wake: A Wind Tunnel Experiment

The evolution laws of the large-eddy coherent structure of the wind turbine wake have been evaluated via wind tunnel experiments under uniform and turbulent inflow conditions. The spatial correlation coefficients, the turbulence integral scales and power spectrum are obtained at different tip speed ratios (TSRs) based on the time-resolved particle image velocity (TR-PIV) technique. The results indicate that the large-eddy coherent structures are more likely to dissipate with an increase in turbulence intensity and TSR. Furthermore, the spatial correlation of the longitudinal pulsation velocity is greater than its axial counterpart, resulting into a wake turbulence dominated by the longitudinal pulsation. With an increase of turbulence intensity, the integral scale of the axial turbulence increases, meanwhile, its longitudinal counterpart decreases. Owing to an increase in TSR, the integral scale of axial turbulence decreases, whereas, that of the longitudinal turbulence increases. By analyzing the wake power spectrum, it is found that the turbulent pulsation kinetic energy of the wake structure is mainly concentrated in the low-frequency vortex region. The dissipation rate of turbulent kinetic energy increases with an increase of turbulence intensity and the turbulence is transported and dissipated on a smaller scale vortex, thus promoting the recovery of wake.

Large Eddy Simulations for Offshore Wind Farms Based on the Lattice Boltzmann Method

This study developed a large-eddy simulation code for offshore wind farms based on the lattice Boltzmann method. We present simulation results for a mesh convergence study for a single wind turbine performance and turbine wake and simulation results for a portion of the wind farm (8 wind turbines) under different inflow turbulence conditions. For Vestas V80 wind turbine with a rotor diameter of 80 m, the actuator line model shows the mesh convergence on a 2 m grid. The mesh dependence was significant for the uniform inflow condition for the wind turbine wake. In contrast, the resolution dependence on the mean velocity was significantly smaller for the turbulence inflow winds. The wind farm calculations captured trends in the observed wind turbine performance. Still, the following wind turbine performances were smaller than the observation data, suggesting the need to introduce rotational speed and pitch control into the simulations.

풍력터빈의 후류 우회를 위한 요각도 분석

The wake deficit from wind turbines is a major factor in reducing the performance of wind farms. Numerous studies have been performed on yaw angle control for the wind turbine rotor to mitigate the wake effect. It is necessary to establish a physical understanding of wake deflection and deformation at various yaw angles. In the present study, the turbine wake development at various yaw angles(-30° ~ 30°) and inflow wind speeds(7 ~ 9m/s) were analyzed via Large-Eddy Simulation coupled with Actuator Disk Model. Simulations were performed under uniform inflow conditions with the NREL 5MW wind turbine model placed at the center. The center, width, and shape of wake deficit profiles were calculated based on the velocity field and the vorticity of the wake. It shows that as the wake center deflection did not increase linearly with the yaw angle, and the slope of the wake deflection varied from the starting point to the end. In addition, the asymmetry of the wake distributions was analyzed based on the vorticity field which evolved into a kidney-shaped wake profile. Lastly, it was found that the wake deficit distribution held a self-similarity profile for the given range of uniform inflows.

Investigation on Thrust Characteristics of a Downstream Offshore Floating Wind Turbine under Yawed Inflow Conditions

In the natural marine environment, offshore floating wind turbines (OFWTs) inevitably experience yawed inflow conditions, which will make their aerodynamics more complicated than those experiencing uniform inflow conditions and difficult to understand. In the present study, the thrust characteristics of a wake-influenced OFWT under dynamic, static, and coupled yawed inflow conditions are investigated thoroughly. Analytical characterizations of yawed inflow and upstream wake are integrated into the blade element momentum (BEM) method to achieve the investigation. Based on this method, simulations by the FAST code have been conducted, and the results are analyzed. It is shown that the three inflow conditions have considerable influences on the thrust coefficient of the wind rotor or the normal force at the blade section, especially in the wake case where the downstream OFWT is located at a specific offset from the central line of a single upstream wake. In order to validate the analyses of simulation results, experimental tests by a set of dedicated apparatus are conducted. The comparison results are good, proving the reliability of simulation results. This work can provide some theoretical contributions to the aerodynamic design and control of OFWTs.

Influence of incoming flow velocity and mixture equivalence ratio on oblique detonation characteristics

In order to reveal the influence of incoming flow velocity and mixture equivalence ratio on the characteristics of oblique detonation, Euler equations coupled detailed chemical reaction model were used to numerically simulate the flow field of oblique detonation. We investigated the characteristic parameters and the morphology of oblique detonation with different wedge angles, velocities and mixture equivalence ratios of incoming flow. The results show that the variations of parameters for uniform inflow condition have obvious influence on the structure of the wave system and the internal stabilization of the oblique detonation flow field. With the increase of incoming flow velocity, the instability of oblique detonation flow field is suppressed. And the initiation position of the oblique detonation is advanced. Under the condition of high velocity inflow, the performance of oblique detonation propulsion is obviously improved. With the increase of the mixture equivalence ratio of inflow, the change law of oblique detonation initiation distance is U-shaped. The internal flow field of the oblique detonation becomes stabilized with the increase of excess oxygen. For the inflow equivalence ratio inhomogeneity, the deflagration front and oblique detonation front become distorted. In addition, the flow field properties of oblique detonation can be optimized by adjusting the inhomogeneity of equivalence ratio.

Aerodynamic modeling of wind turbine loads exposed to turbulent inflow and validation with experimental data

The present paper is intended to assess the ability of the state-of-the-art computational fluid dynamics (CFD) and blade element momentum (BEM) approaches for accurate load predictions on a 2.3 MW wind turbine rotor. Three different cases are considered, a steady uniform inflow condition, a turbulent uniform inflow condition and a turbulent inflow case in combination with shear and yaw. The CFD computations employ a delayed-detached eddy simulation (DDES) approach in combination with a high order (5th) WENO method for flux discretization. The BEM calculations apply several correction factors including recently developed dynamic stall and yaw models. Furthermore, a well established procedure at IAG to set-up BEM calculations consistent to CFD will be presented and verified. The results are compared with the field experimental data of the turbine for these three different flow conditions. The studies show that both CFD and BEM results are in a very good agreement with the experimental data not only on the mean load levels but also with regards to the load fluctuations. The differences between BEM, CFD and experimental data for most radial stations are less than 10%.

Sound Field Properties of Non-Cavitating Marine Propellers

The sound field properties of non-cavitating marine propellers are investigated using a hybrid method, in which the FWH (Ffowcs William-Hawkings) analogy is coupled with the BEM (Boundary Element Method) approach. The investigations include both the uniform and non-uniform inflow conditions. For both conditions, the dominant sound source terms and the decay rate of the noise with regard to the distance to propeller centre are investigated. The influence of the permeable surface dimensions in the permeable FWH approach on the hydroacoustic result is also investigated. To carry out the investigations, the formulation to calculate acoustic pressure generated by the propeller wake sheet is proposed for the first time. The issues associated to coupling permeable FWH approach and BEM are also discussed, including the fictitious volume flux problem and the consideration of the ship wake field. It was found that the influence of the permeable surface dimension is little for the 1st BPF (Blade Passage Frequency), but cannot be ignored for the 3rd BPF. In the uniform inflow situation the thickness terms are found to be dominant, while in the non-uniform inflow situation the loading terms are dominant.

Numerical and Experimental Analysis of a Singing Propeller Having Blunt Trailing Edges

This study considers a blunt trailing-edged propeller operating both in a uniform and a nominal wake fields. Experiments are performed in the cavitation tunnel of Hyundai Maritime Research Institute. The effects of propeller rotation speed, tunnel flow speed, and blade sheet cavitation growth on the generation mechanism of the singing propeller are investigated. The cavitation, sound, and vibration characteristics related to the singing phenomena are measured by a hydrophone, a microphone, an accelerometer, and a highspeed digital camera. The natural frequencies of propeller blades are predicted using a finite element method and verified by both contact- and noncontact-type impact hammer tests in air and underwater conditions. The inflow speed and angle of attack for each section of the propeller blades are calculated using the Reynolds-averaged Navier-Stokes equation-based flow analysis. Using a detached eddy simulation, the vortex shedding patterns and their frequencies are calculated. The predicted vortex shedding frequencies are compared with the measured singing frequency and blade natural frequency for determination of consistency. Under cavitation-free regime, the vortex shedding frequencies are predicted for normalized blade radial positions of .8R and .9R. The computed values are close to the two blade natural frequencies and also consistent with the double singing phenomena in the cavitation tunnel test. For fully developed blade sheet cavitation condition, the vortex formation in the wake region is observed to be strongly influenced by the cavitation growth on the pressure side surface. Propeller singing is diminished with the continuous growth of cavitation and is finally locked-off. The significant variation of the flow-induced sound and vibration levels are also observed for the locked-in and the locked-off conditions. The singing occurrence location and frequency under uniform inflow condition are analyzed to investigate the generation mechanism of propeller singing. This study can be applied to the analysis of singing location and its frequency of a propeller operating in the hull wake, which changes the angle of attack according to the propeller rotation angle.

The effect of dynamic near-wake modulation on utility-scale wind turbine wake development

High-resolution field-scale experiments using flow visualization with natural snowfall and high-fidelity large eddy simulations are combined to investigate the effect of dynamic turbine operation and atmospheric conditions on wind turbine wake mixing and recovery in the wake of a 2.5 MW wind turbine. Instantaneous near-wake expansion and deflection in response to changes in blade pitch and wind direction, termed dynamic wake modulation, is quantified using both techniques, demonstrating excellent agreement. The simulations are used to extend these results by calculating the energy flux into the wake 7 rotor diameters downstream, showing that dynamic turbine-atmospheric interactions enhance mixing in the far-wake. This finding is exhibited under both uniform and turbulent inflow conditions. Under turbulent flow, a synergistic relationship is also observed between dynamic wake modulation and wake meandering, as wake recovery can be further accelerated when the two phenomena occur together. The results of this study have implications for the development of more realistic far-wake models that include the significant impact of dynamic wake modulation on wake mixing and development. Additionally, the findings from the current study can be used to develop advanced control algorithms to speed up wake breakdown and recovery, further improving wind farm efficiency.

A moving-domain CFD solver in FEniCS with applications to tidal turbine simulations in turbulent flows

A moving-domain computational fluid dynamics (CFD) solver is designed by deploying the Arbitrary Lagrangian Eulerian Variational Multi-scale Formulation (ALE-VMS) enhanced with weak enforcement of essential boundary conditions (weak BC) into the open-source finite element automation software FEniCS. The mathematical formulation of ALE-VMS, which is working as a Large Eddy Simulation (LES) model for turbulent flows, and weak BC, which is acting as a wall model, are presented with the implementation details in FEniCS. To validate the CFD solver, simulations of flow past a stationary sphere are performed with moving meshes first. Refinement study shows the results quickly converge to the reference results with fixed grids. Then, the solver is utilized to simulate a tidal turbine rotor with uniform and turbulent inflow conditions. Good agreement is achieved between the computational results and experimental measurements in terms of thrust and power coefficients for the uniform inflow case. The effect of the inflow turbulence intensity on the tidal turbine performance is quantified.

Modified Power Curves for Prediction of Power Output of Wind Farms

Power curves are used to model power generation of wind turbines, which in turn is used for wind energy assessment and forecasting total wind farm power output of operating wind farms. Power curves are based on ideal uniform inflow conditions, however, as wind turbines are installed in regions of heterogeneous and complex terrain, the effect of non-ideal operating conditions resulting in variability of the inflow must be considered. We propose an approach to include turbulence, yaw error, air density, wind veer and shear in the prediction of turbine power by using high resolution wind measurements. In this study, two modified power curves using standard ten-minute wind speed and high resolution one-second data along with a derived power surface were tested and compared to the standard operating curve for a 2.5 MW horizontal axis wind turbine. Data from supervisory control and data acquisition (SCADA) system along with wind speed measurements from a nacelle-mounted sonic anemometer and wind speed measurements from a nearby meteorological tower are used in the models. The results show that all of the proposed models perform better than the standard power curve while the power surface results in the most accurate power prediction.