Interaction Of Rotor Wake Research Articles

Field-data-based validation of an aero-servo-elastic solver for high-fidelity large-eddy simulations of industrial wind turbines

Abstract. To design the next generations of wind turbines, engineers from the wind energy industry must now have access to new numerical tools, allowing the high-fidelity simulation of complex physical phenomena and thus a further calibration of lower-order models. For instance, the rotors of offshore wind turbines, whose diameters can now exceed 200 m, are highly flexible and fluid–structure interactions cannot be neglected any longer. Accordingly, this paper presents a new aero-servo-elastic solver designed to perform high-fidelity large-eddy simulation (LES) of wind turbines, as well as of rotor–wake interactions classically occurring in wind farms. In this framework, the turbine blades are modeled as flexible actuator lines. In terms of operating parameters (rotation speed and pitch angles) and power output, the solver is first validated against field data from the Westermost Rough offshore wind farm, for three different operation points. A very good agreement between the numerical results and field data is obtained. To push the validation further, additional results are compared to those given by a certified aero-servo-elastic solver used in the industry, which relies on a blade element momentum (BEM) method. The internal loads throughout the first blade and the deflections at the tip are studied in detail, and some discrepancies are observed. Of a reasonable amplitude overall, those are legitimately related to intrinsic modeling differences between the two solvers.

Aerodynamic analysis for different operating states of floating offshore wind turbine induced by pitching movement

The floating offshore wind turbine (FOWT) may experience dynamic stall and complex rotor-wake interaction, such as the vortex ring state (VRS) or propeller working state (PWS), under large platform movements, which can lead to intricate alterations in both aerodynamic parameters and flow states. In this paper, we propose a reliable criterion based on the velocity field of FOWT to distinguish its operating states (VRS or PWS). The aerodynamic characteristics and flow mechanism of the FOWT are considered in details as different operating states induced by a large pitch motion are experienced. Excessive angle of attack (AOA) will cause a dynamic stall, generating a large number of stall vortices and inducing violent fluctuations in the normal and tangential forces of the blade. When the wind turbine experiences VRS or PWS, the continuity of the tip vortex is broken by the blade–vortex or vortex–vortex interaction, complicating the flow state and even resulting in a negative thrust and AOA. In comparison with the remainder of the blade, the blade tip is more likely to experience VRS or PWS owing to its higher movement velocity.

Experimental study of the wake interaction between two vertical axis wind turbines

AbstractWakes and wake interactions in wind turbine arrays diminish energy output and raise the risk of structural fatigue; hence, comprehending the features of rotor–wake interactions is of practical relevance. Previous studies suggest that vertical axis wind turbines (VAWTs) can facilitate a quicker wake recovery. This study experimentally investigates the rotor–wake and wake–wake interaction of VAWTs; different pitch angles of the blades of the upwind VAWT are considered to assess the interactions for different wake deflections. With stereoscopic particle image velocimetry, the wake interactions of two VAWTs are analysed in nine distinct wake deflection and rotor location configurations. The time‐average velocity fields at several planes upwind and downwind from the rotors are measured. Additionally, time‐average loads on the VAWTs are measured via force balances. The results validate the rapid wake recovery and the efficacy of wake deflection, which increases the available power in the second rotor.

Wake interactions behind individual-tower multi-rotor wind turbine configurations

Multiple rotors on single structures have long been proposed to increase wind turbine energy capture with no increase in rotor size, but at the cost of additional mechanical complexity in the yaw and tower designs. Standard turbines on their own very-closely-spaced towers avoid these disadvantages but create a significant disadvantage; for some wind directions the wake turbulence of a rotor enters the swept area of a very close downwind rotor causing low output, fatigue stress, and changes in wake recovery. Knowing how the performance of pairs of closely spaced rotors varies with wind direction is essential to design a layout that maximizes the useful directions and minimizes the losses and stress at other directions. In the current work, the high-fidelity large-eddy simulation (LES) code Exa-Wind/Nalu-Wind is used to simulate the wake interactions from paired-rotor configurations in a neutrally stratified atmospheric boundary layer to investigate performance and feasibility. Each rotor pair consists of two Vestas V27 turbines with hub-to-hub separation distances of 1.5 rotor diameters. The on-design wind direction results are consistent with previous literature. For an off-design wind direction of 26.6°, results indicate little change in power and far-wake recovery relative to the on-design case. At a direction of 45.0°, significant rotor-wake interactions produce an increase in power but also in far-wake velocity deficit and turbulence intensity. A severely off-design case is also considered.

Rotor-wake interactions in a wind turbine row: a multi-physics investigation with large eddy simulation

Large Eddy Simulation (LES) is growing in importance to support the development, validation and tuning of efficient and reliable engineering models. Such models are essential for the wind energy industry to predict the performance of wind turbines, especially when rotor-wake interactions are involved. In this paper, we present high-fidelity numerical results obtained with a new aero-servo-elastic solver relying on LES and the actuator line method. Two sub-configurations of the Westermost Rough wind farm are investigated: an isolated wind turbine and a seven-turbine row. For validation purposes, field data are compared to time-averaged numerical results for the rotation speed, blade pitch angle, output power, and blade loads. A very good agreement between the results and field data is obtained for the isolated turbine. For the seven-turbine row, some discrepancies are observed. Those can primarily be explained by the inherent difficulty of translating both the actual wind conditions and the influence of the neighboring row into boundary conditions.

Dynamic inflow model for a floating horizontal axis wind turbine in surge motion

Abstract. Floating offshore wind turbines may experience large surge motions, which can cause blade–vortex interaction if they are similar to or faster than the local wind speed. Previous research hypothesized that this blade–vortex interaction phenomenon represented a turbulent wake state or even a vortex ring state, rendering the actuator disc momentum theory and the blade element momentum theory invalid. This hypothesis is challenged, and we show that the actuator disc momentum theory is valid and accurate in predicting the induction at the actuator in surge, even for large and fast motions. To accomplish this, we develop a dynamic inflow model that simulates the vorticity–velocity system and the effect of motion. The model's predictions are compared to other authors' results, a semi-free-wake vortex ring model, other dynamic inflow models, and CFD simulations of an actuator disc in surge. The results show that surge motion and rotor–wake interaction do not result in a turbulent wake or vortex ring state and that the application of actuator disc momentum theory and blade element momentum theory is valid and accurate when applied correctly in an inertial reference frame. In all cases, the results show excellent agreement with the higher-fidelity simulations. The proposed dynamic inflow model includes a modified Glauert correction for highly loaded streamtubes and is accurate and simple enough to be easily implemented in most blade element momentum models.

Aeromechanics and Aeroelastic Stability of Coaxial Rotors

Coaxial rotor aeroelasticity is complex due to the counter-rotating wake system, rotor lift offset, periodic blade passage loads, unsteady rotor wake interactions, reduced rotor speed, and stiff hingeless blades. In this study, the aeroelastic stability of a coaxial rotor is examined in hover and forward flight. The rotor wake is modeled with the viscous vortex particle method, a grid-free approach for calculating vortex interactions over long distances. The spanwise blade aerodynamic loading is calculated using a computational-fluid-dynamics-based reduced-order model in attached flow, and the ONERA dynamic stall model in separated flow. Two propulsive trim procedures are developed: one with the propulsor not operating, and the other with the vehicle at level attitude. An aeroelastic stability analysis based on Floquet theory is applied to the periodic system. A novel graphical method is developed to identify coupling between blade modes of the two rotors. The effects of lift offset and advance ratio on the hub loads, inflow distribution, and aeroelastic stability are examined to provide an improved physical understanding of the aeroelastic interactions. Results indicate that the blade passage effect is caused by the bound-circulation-induced inflow. The first and second lag modes are the least stable modes in hover and forward flight.

Validation of a free vortex filament wake module for the integrated simulation of multi-rotor wind turbines

Multi wind tUrbine Simulation Tool (MUST) is a new CENER's in-house code, based on NREL's OpenFAST code, designed to allow the aero-hydro-servo-elastic modelling of novel multi-rotor wind turbine concepts. MUST also includes a new aerodynamic module, called AeroVIEW (Aerodynamic Vortex fIlamEnt Wake) based on an implementation of the Free Vortex filament Method (FVM) combined with an unsteady Lifting Line (LL). This model allows to represent the influence existing between all the rotors, in each one of them. In this article, a validation of AeroVIEW performance for single and multi-rotor configurations is presented. First, a study of the wake properties (wake geometry, core radius and circulation of tip vortex filament) obtained with AeroVIEW has been successfully compared with several MexNext III cases. Then, an analysis of the rotor wakes interaction, in different arrays of wind turbines, has been made, obtaining power and thrust increases, due to this rotor-wake interaction, for different conditions. The increases obtained in power are very similar to results of high fidelity tools found in the literature. Moreover, this beneficial effect on the power generated, has a counterpart in the average thrust, whose increase is around half of the power production increase.

Effect of surge motion on rotor aerodynamics and wake characteristics of a floating horizontal-axis wind turbine

Aerodynamics of a floating horizontal-axis wind turbine (FHAWT) is subject to its platform’s six degrees of freedom (DOFs) motion, especially the surge motion, and may become much more complicated than that of a fixed-base wind turbine. This paper aims at investigating the aerodynamics of the FHAWT’s rotor and the characteristics of its wake under surge motion. To explore this, a CFD method with the improved delayed detached eddy simulation (IDDES) is applied to a 1:50 model FHAWT. First, it is demonstrated that the rotor’s aerodynamics including the thrust, torque and rotor power may be greatly affected by surge motion even though it is small. During surge motion, the light dynamic stall and rotor-wake interaction phenomena are observed. In addition, the near and far wake may be obviously influenced by surge motion. More importantly, the wake-recovery process under surge motion is slower than the one without surge motion. In general, the surge motion impacts the aerodynamics of the FHAWTs’ rotor and the characteristics of its wake greatly, therefore should receive due attention in the design procedure and the arrangement of wind farms.

Strongly-coupled aeroelastic free-vortex wake framework for floating offshore wind turbine rotors. Part 2: Application

This two-part paper presents the integration of the free-vortex wake method (FVM) with an aeroelastic framework suitable to model the rotor-wake interactions engendered by floating offshore wind turbine (FOWT) rotors in operation. Part 1 of this paper introduces the numerical development and validation of an aeroelastic framework. Due to a lack of experimental aeroelastic benchmarks for FOWTs, a series of validation studies are conducted against the rotor aerodynamic and structural performance of the National Renewable Energy Laboratory (NREL) 5-MW reference wind turbine. Part 2 of this paper focuses on the modeling and simulating different aeroelastic operational conditions of FOWTs. Numerical results of the current framework capture consistently the aerodynamic rotor performance, such as power, thrust, and torque of wave-induced pitching FOWTs. In addition, the presented aeroelastic framework yields additional information about the power, thrust, and torque fluctuations due to the out-of-phase blade passing frequency and corresponding blade deflections. The fidelity of the presented framework demonstrates, for the first time, an FVM-based aeroelastic method capable of carrying out investigations on rotor-wake interactions and relevant aeroelastic phenomena of FOWTs.

Strongly-coupled aeroelastic free-vortex wake framework for floating offshore wind turbine rotors. Part 1: Numerical framework

This two-part paper presents the integration of the free-vortex wake method (FVM) into an aeroelastic framework suitable to model the rotor-wake interactions engendered by floating offshore wind turbine (FOWT) rotors in operation. Part 1 of this paper introduces the numerical development and validation of an aeroelastic framework. Because no experimental aeroelastic benchmarks exist for FOWTs, a series of validation studies are conducted against the rotor aerodynamic and structural performance of the National Renewable Energy Laboratory (NREL) 5-MW reference wind turbine. Part 2 of this paper focuses on the modeling of different aeroelastic operational conditions of FOWTs. Numerical results of the current framework captures consistently the aerodynamic rotor performance, such as power, thrust, and torque of wave-induced pitching FOWTs. In addition, the presented aeroelastic framework captures additional information about the power, thrust, and torque fluctuations due to the out-of-phase blade passing frequency and corresponding blade deflections. The fidelity of the presented framework showcases, for the first time, an FVM-based aeroelastic method capable of carrying out investigations on rotor-wake interactions and relevant aeroelastic phenomena of FOWTs.

Optimization of tonal noise control with flow obstruction

The tonal noise control of an axial low-speed fan system with flow obstruction has been achieved both numerically and experimentally. Its primary noise source caused by rotor-wake interaction has been directly and accurately simulated with a Lattice Boltzmann Method. The latter has then allowed deciphering the noise control mechanism of an upstream sinusoidal obstruction: the vortex rings shed by the obstruction yield a second noise source at the rotor-blade leading edge. The obstruction itself does not create any significant noise and is acoustically transparent. An industrially-applicable numerical methodology has then been proposed to obtain the optimal obstruction design for a given fan geometry and operating condition, with a maximum of six simulations of the fan system without and with the obstruction being static and slowly rotating. Simulations with rotating obstructions provide the optimal lobe amplitude and an optimal obstruction angular position, which are found to be 20 mm and about 16° respectively for the present fan system both numerically and experimentally. The frequency selectivity of the obstruction and the linear variation of the secondary source level with the lobe amplitude have also been confirmed.

Development and interaction of rotor wakes

The present work shows the results of a series of experimental investigations of the wake development behind a model rotor subject to upstream disturbances created either by another rotor or by a disk. The experiments are carried out in a water flume in order to control the flow and to carry out visualizations and to perform optical diagnostics. The aim of the work is to clarify similarities and differences in the wake of a wind turbine subject to different inflow disturbances, and in particular to see if there is any difference in the rotor wake resulting from a upstream disturbance created by a rotor and one created by an immobile disk. The background for the study is an on-going discussion if disks can replace rotors in laboratory experiments. In the paper, we will also show new experimental data that support our main conclusion, which is that strong differences exist between the near wakes characteristics of a rotor and a disk.

Rotor wake interactions with an obstacle on the ground

ABSTRACTAn investigation of the flow around an obstacle positioned within the wake of a rotor is described. A flow visualisation survey was performed using a smoke wand and particle image velocimetry, and surface pressure measurements on the obstacle were taken. The flow patterns were strongly dependent upon the rotor height above the ground and obstacle, and the relative position of the obstacle and rotor axis. High positive and suction pressures were measured on the obstacle surfaces, and these were unsteady in response to the passage of the vortex driven rotor wake over the surfaces. Integrated surface forces are of the order of the rotor thrust, and unsteady pressure information shows local unsteady loading of the same order as the mean loading. Rotor blade-tip vortex trajectories are responsible for the generation of these forces.

Effect of Weak Outlet-Guide-Vane Heterogeneity on Rotor–Stator Tonal Noise

Four numerical simulations of the NASA Active Noise Control Fan rig with some modifications have been performed to investigate the influence of the heterogeneity of the stator row on the tonal noise radiation for a realistic turbofan with a high hub-to-tip ratio. These simulations achieved with the Powerflow solver based on the lattice Boltzmann method provide a direct acoustic prediction for both tonal and broadband noise. The numerical simulations are used to evaluate the noise contributions of the rotor-wake interaction with the stator vanes, and the rotor interaction with both the inlet distortion and the potential field generated by the stator row. Analytical models are evaluated on this configuration using the flow description provided by the simulations. Rotor-wake and inlet-distortion interaction noise generation with in-duct propagation uses the classical Amiet’s response, whereas Parry’s model is used for the rotor response to the potential field. The simulation is used to evaluate simple excitation models for the potential field and the rotor-wake evolutions. The analytical results for homogeneous and heterogeneous configurations compare well with the detailed acoustic modal powers extracted from the direct acoustic field simulated with Powerflow. The wake interaction remains the dominant source in the present heterogeneous configurations.

Influence of Tip Speed Ratio on Wake Flow Characteristics Utilizing Fully Resolved CFD Methodology

Dominant flow structures in the wake region behind the turbine employed in the Blind Test campaign [1], [2] is investigated numerically. The effect on the wake configuration at variable operating conditions are studied. The importance of the introduction of turbine tower inside the numerical framework is highlighted. High-fidelity simulations are performed with Multiple Reference Frame (MRF) numerical methodology. A thorough comparison among the cases is presented, and the wake evolution is analyzed at variable stations downstream of the turbine. Streamlines of flow field traveled towards ground adjacent to turbine tower and strongly dependent on the operating tip speed ratio. Wake is composed of tower shadow superimposed by rotor wake. Shadow of the tower varies from x/R=2 until x/R=4 and breaks down into small vortices with the interaction of rotor wake. This study also shows that the wake distribution consists of two zones; inner zone composed of disturbances generated by blade root, nacelle and the tower, and an outer zone consisting of tip vortices.

Converged statistics for time-resolved measurements in low-speed axial fans using high-frequency response probes

Fast-response probes in multistage turbomachinery are typically used to measure unsteady flows and turbulence in a number of traverse locations throughout the machine (rotor–stator inter-regions, inlet and outlet sections, tip clearance gaps…). When used intensively, they provide complete information of time-resolved flow structures, including wake patterns, wake mixing, wake–wake and rotor-wake interactions or turbulence transport in 2D planes and even 3D pictures if the raw signals are post-processed accurately.The segregation between deterministic, unsteady features and turbulent scales is essential to understand the unsteady mechanisms responsible for the energy transfer and requires an accurate selection of the sampling frequencies and the total length of the measured traces to assure a valid statistical reduction. Similar considerations must be made if they are filtered in a frequency basis (for example, filtering low-scale turbulence or extracting only BPF components), employing well-designed periodograms or power spectra with minimum scatter and large periods of time integration.This work presents the effect of number of periods (ensembles), resolution in which the averaged periods are reconstructed and turbulence intensity on the experimental accuracy of ensemble-averaged measurements in low-speed axial fans using fast-response probes. In particular, the statistical analysis is established in terms of convergence (residuals) between time-resolved traces retrieved using different sampling frequencies and number of total samples. The possible effects of three-dimensionality, the measured regions (hub, tip, midspan) or the sensibility to turbulence levels is also explored.A technique to quantify the convergence of the phase-locked averaging (PLA) processes is applied to a low-speed axial fan, with twin configurations of rotor–stator and stator–rotor arrangements. As a starting point, a concise survey of usual practices employed by other authors in the literature for axial fans and compressors is firstly reviewed, in order to identify fundamental parameters and values typically adopted to guarantee convergence. Finally, typical requirements are given as a function of the variable analyzed, the wake pattern to be described or the global disorder of the flow structures inside axial flow fans.

A Comparison of Coaxial and Conventional Rotor Performance

The performance of a coaxial rotor in hover, in steady forward flight, and in level, coordinated turns is contrasted with that of an equivalent, conventional rotor with the same overall solidity, number of blades, and blade aerodynamic properties. Brown's vorticity transport model is used to calculate the profile, induced, and parasite contributions to the overall power consumed by the two systems, and the highly resolved representation of the rotor wake that is produced by the model is used to relate the observed differences in the performance of the two systems to the structures of their respective wakes. In all flight conditions, all else being equal, the coaxial system requires less induced power than the conventional system. In hover, the conventional rotor consumes increasingly more induced power than the coaxial rotor as thrust is increased. In forward flight, the relative advantage of the coaxial configuration is particularly evident at pretransitional advance ratios. In turning flight, the benefits of the coaxial rotor are seen at all load factors. The beneficial properties of the coaxial rotor in forward flight and maneuver, as far as induced power is concerned, are a subtle effect of rotor-wake interaction and result principally from differences between the two types of rotor in the character and strength of the localized interaction between the developing supervortices and the highly loaded blade-tips at the lateral extremities of the rotor. In hover, the increased axial convection rate of the tip vortices appears to result in a favorable redistribution of the loading slightly inboard of the tip of the upper rotor of the coaxial system.

Lynx Main Rotor/Tail Rotor Interactions: Mechanisms and Modelling

A flight trial has been conducted by the Defence Research Agency (DRA) Bedford using a Lynx AH Mk5 helicopter fitted with an instrumented tail rotor to collect data on tail rotor aerodynamic performance. The analysis carried out to date has concentrated on the effects of main rotor wake interactions on the tail rotor. This paper describes the trial and the analysis in broad outline and discusses in detail mechanisms to explain three out of the six main rotor/tail rotor interactions found. The modelling of these effects to improve the fidelity of ground-based flight simulation is also covered.

Rotor wake interaction with separated flow

Aerodynamic interactions represent one of the toughest challenges to computational methods for the prediction of rotorcraft performance, loads, and noise. Efforts over the past ten years using a simple hemisphere/cylinder airframe under a teetering two-bladed rotor have shown that most of the dominant effects can be predicted using potential-flow concepts, in the absence of massive flow separation and strong vortex-surface collision. Here, the prospects for modeling the separated flow interaction are studied. The rotor wake interacts with the separated flow over a pressure-instrumented boom, downstream of a backstep cut into the original cylinder. The undisturbed backstep flow is characterized using spectral analysis of the velocity and surface pressure fields. The interaction between the tip vortex and the backstep free shear layer is visualized using laser sheet videography, and measured in detail using laser velocimetry and pressure sensing. The tip vortex dominates the interaction, causing periodic destruction and reconstruction of the shear layer, and large-amplitude longitudinal motion of the reattachment zone. However, the characteristics of the undisturbed shear layer are still detected in the surface pressure spectra. The tip vortex and its collision with the boom surface appear to be unaffected by the shear layer interaction. It is argued that this experiment is a conservative representation of the interaction, so that occurrences on real rotorcraft should be less complex. Thus, while the separated flow interaction appears to be extremely complex at first sight, the dominant effects are surprisingly simple, and may be easily modeled.