Flow Wake Research Articles

State-space dynamic inflow modelling for hovering rotors in fixed- and moving-ground effect

This paper's objective is to apply a novel identification technique to obtain a dynamic state-space wake inflow model accounting for the effects of a stationary/moving ground beneath a hovering rotor. The identification is based on a dataset of computational simulations (training set) provided by an in-house free-wake mid-fidelity solver. The evaluation of the wake contribution is optimised through a generalised mirror-image model to reduce the computational cost without losing accuracy. Stationary and moving ground effects are analysed and discussed in the present work, considering both parallel and inclined ground with respect to the rotor disk and heaving and pitching ground motion. The proposed state-space wake inflow model relates the inflow coefficients to a set of inputs, including the aerodynamic hub loads (akin to the well-known Pitt-Peters approach) and the kinematic degrees of freedom of the ground. For a number of rotor/ground configurations, the identified model is successfully validated against the simulations directly provided by the nonlinear, free-wake aerodynamic solver.

Analysis of aerodynamic characteristics of car diffuser for dissimilar diffuser angles on Sedan’s using CFD

A vehicle's aerodynamic qualities have a major impact on its performance, handling, safety, and comfort. Variations in aerodynamic characteristics, such as lift and drag forces, have a noticeable impact on vehicle performance at high speeds. Researchers can lessen the frequency of accidents in which a motorist loses control of their car on our roads by lowering the pressure differential between the road and the tires. Manufacturers can save time and money on research and development by using Computational Fluid Dynamics (CFD) instead of wind tunnel testing, appreciations to advancements in computing power. The focus of this study is to learn the different aspects of a vehicle influence on its drag coefficient. Using CFD, the ideal diffuser angle was identified, which maximizes the down force produced by the device. Analysis of the drag and turbulent kinetic energy on the vehicle at varied diffuser angles (10, 11, and 12°) and vehicle speeds was performed using Ansys 16.2 Fluent. As shown by the numerical study, the drag force is reduced to 119.019 N at the 12° diffuser angle, which also provides the highest velocity and the lowest turbulent kinetic energy value. Researchers found that adjusting the diffuser's tilt significantly altered the under-body flow's wake.

Wake Interactions of Two Tandem Semisubmersible Floating Offshore Wind Turbines Based on FAST.Farm

Wake effects commonly exist in offshore wind farms, which will cause a 10–20% reduction of whole power production as well as a 5–15% increase of fatigue loading on the wind turbine main structures. Obviously wake interaction between floating offshore wind turbine (FOWT) is more complicated, and needs careful assessment which is a prerequisite for active wake control (AWC). The primary objective of the present research is to investigate in detail how the wake inflow condition, streamwise spacing, turbulence intensity, and wind shear influence the power performance, platform motion dynamic and structural loading of FOWT. FAST.Farm, developed by the National Renewable Energy Laboratory (NREL), was used for simulating two tandem FOWTs in different conditions. Comparisons were made between FOWTs in different conditions on power performance and platform motion dynamic, which were presented through both time and frequency domain analysis. Damage equivalent loads change in FOWTs interference under typical working conditions were discussed and summarized. Half wake inflow would pose many challenges to the downstream FOWT. These research studies can be incorporated into further offshore wind farm wake models, providing applicable AWC strategies to reduce wake interference effects for higher energy production and for the longer life of FOWT.

Analysis of the anisotropy aerodynamic characteristics of downstream wind turbine considering the 3D wake expansion based on coupling method

The inhomogeneous wake velocity distribution caused by the variation of relative position has a great impact on aerodynamic anisotropy characteristics of downstream wind turbine (WT). To accurately and efficiently evaluate the aerodynamic anisotropy of the downstream WT under wake inflow, an aero-elastic-servo model combined with the 3D Jensen-Gaussian (3DJG) wake model and OpenFAST was proposed in this paper. The accuracy of the model was verified by comparing with Jonkman's research. The effect of the relative installation position of the two 5 MW turbines on the aerodynamic anisotropy of the downstream WT was analyzed. Results indicate that the power loss of the downstream WT can be up to 17.47%–51.53% in the case investigated in this study. Due to the 3D wake expansion and the momentum mixing effect, relative axial distance and relative radial distance alternately become the dominant factor for rotor power recovery when y/R < 1 and y/R ≥ 1. The phenomenon mentioned in rotor power is also indicated in downstream rotor torque and thrust. Due to the inhomogeneous wind velocity distribution of the wake inflow, the standard deviation and average value of the blade root flapwise moment (BRFM) suffered by downstream WT are 10.84% and 24.30% larger than that suffered by upstream WT respectively. This paper provides better guidance for optimizing the WT layout and quantification of the wake effect on downstream WT. • A novel method to estimate the anisotropy aerodynamics of downstream WT at different relative positions is developed. • The aero-elastic-servo interactions of the downstream WT under the 3D wake effect have been considered. • The 3D wake expansion and momentum mixing effect alternately become the dominant factor for rotor power recovery. • The standard deviation and average value of the BRFM suffered by downstream WT can be increased by up to 10.84% and 24.30%.

Quantitative evaluation of yaw-misalignment and aerodynamic wake induced fatigue loads of offshore Wind turbines

Wind turbines in an offshore wind farm are usually allocated in a restricted area, which will result in wake interactions for downstream turbines. In order to mitigate wake effects on power and loads, wake redirection control (WRC) has been proposed to steer wake away from downstream turbines by intentionally yawing the upstream ones. However, wake interactions in combination with yaw-misalignment would produce complex structural loading behaviours, which should be carefully evaluated in control design. In this respect, priori knowledge on fatigue load contribution from wake and yaw-offset could be beneficial for multi-objective control optimisation and its real-time implementation. Therefore, we aim to quantitatively evaluate both yaw-offset and wake induced fatigue loads on offshore wind turbines in this work. More specifically, a numerical turbulent wind-field generator is established by including the wake deficit modelling feature, so that aero-elastic simulation with parametrically controlled wake inflow becomes possible. Then, aero-elastic simulations covering a wide range of waked inflow and yaw-offset conditions are performed, establishing a comprehensive database for tower and blade damage equivalent loads, where different load trends can be observed. In addition, a load assessment model is established by polynomial regression, and correlation analysis results indicate the dominant factors on fatigue loads are wind velocity and turbulence intensity. Besides, yaw-offset is more important than wakes for rated conditions, and vice versa for below and above-rated conditions. Moreover, wind tunnel results have also shown generally consistent trends with model predictions. The established load database and regression model could be used as the load indicator for future design optimisation of wind farm WRC.

The effect of the PSS configuration on the hydrodynamic performance of the KP505 propeller behind the KCS

The presented paper has numerically investigated a new pre-swirl stator (PSS) configuration on a ship propulsion performance. The KP505 propeller and the KRISO container ship (KCS) are used, and the PSS is installed before the propeller to adjust wake inflow and increase the performance of the propeller. To simulate PSS in front of the propeller, the STAR-CCM + solver (computational fluid dynamics) is used. First, the hydrodynamic characteristics of the model propeller KP505 results are compared with the available measurements and experimental data with relatively good agreement. Then, the flow over the KCS is simulated, and its wake flow is captured. This wake flow is entered into the propeller with and without PSS. The present paper introduces a new PSS configuration with eight blades, and the angle of attack of 15° is modelled and mounted in front of the propeller. The thrust and torque coefficients at four advance coefficients (between 0.24–0.91) are presented and discussed. The numerical results indicate that the PSS with these parameters works well in the low ship speed and increases efficiency and decreases delivered power. Also, the PSS's effect reduces the pressure fluctuation by 16.48% in the backside of the propeller blade. • Pre-swirl stator is arranged in front of the KP505 propeller behind the KCS. • Thrust and resistance of the KCS are calculated at various speeds. • Velocities in front of and behind the propeller are presented and discussed. • Performance of propeller behind the KCS are determined at different speeds with and without PSS. • Pressure distribution, thrust and torque coefficients are presented and discussed during one cycle

Wind turbine wake inflow over a heterogeneous forest - comparison between measurement and LES simulation

In this work a new step in understanding the wind turbine (WT) wake behavior on forested areas is made. For this analysis, a pair of real scale WTs located on a site with moderately complex terrain and heterogeneous forest is simulated using Large Eddy Simulation (LES). This simulation is compared with met mast and power output measurements of two WTs in Ryningsnas, Sweden, considering near neutral stratification in the atmospheric boundary layer (ABL). Three validation steps are followed; first, the undisturbed wind profile is compared with met mast data and another similar LES code. Then, the wake for each WT wake impacting on the met mast at different directions is addressed. A feature of this pair of WTs is that these have different hub heights, but the same rated power and rotor diameter, which helps provide insight into how the tip clearance over the forest affects the operation and wake characteristics. Finally, power output deficits when the WTs are operating in each others wakes are compared to observed power deficits. For these simulations SOWFA, the Open FOAM project for wind farms simulation in ABL, is used. In this code, three new additions are made; the forest model, the mesh modification for complex terrain and the representation of the WT using an actuator disc model with local force adaptation for wind farm flows. The simulation results show a good performance on quantitatively and qualitatively capturing the velocity in the wake, but for TKE the simulation underestimates the magnitude, and fails to match the measured structure of the wake for one of two WTs. The power deficit on the impacted WTs is well captured, despite the complexity related to turbines with different hub heights. This study makes one of the first steps on validating LES simulations for wind farms in forest.

Control of power generated by a floating offshore wind turbine perturbed by sea waves

Abstract Offshore wind energy is expected to provide a significant contribution to the achievement of the European Renewable Energy targets. One of the main technological issues affecting floating offshore wind turbines concerns generated power fluctuations and structural fatigue caused by sea-wave/platform interactions. This paper presents a fully-coupled aero/hydro/servo-mechanic model for response and control of floating offshore wind turbines in waves, suitable for preliminary design. The wind-turbine is described by a multibody model consisting of rigid bodies (blades and tower) connected by hinges equipped with springs and dampers (for realistic low-frequency simulation). The aerodynamic loads are evaluated through a sectional aerodynamic approach coupled with a wake inflow model. A spar buoy floating structure supports the wind turbine. The hydrodynamic forces are evaluated through a linear frequency-domain potential solver, with the free surface deformation effects included through a reduced-order, state-space model. An optimal controller is identified and applied for rejection of annoying fluctuations of extracted power and structural loads. The developed comprehensive model has been successfully applied to a floating version of the NREL 5 MW wind turbine for stability analysis, as well as for the analysis of uncontrolled and controlled responses to regular and irregular short-crested sea waves. The proposed controller, based on the combined use of blade pitch and generator torque as control variables and the application of an observer for non-measurable aerodynamic and hydrodynamic states estimation, has been demonstrated to be effective in a wide frequency range for alleviation of both generated power fluctuations and vibratory loads.

Analytical model for the power–yaw sensitivity of wind turbines operating in full wake

Abstract. Wind turbines are designed to align themselves with the incoming wind direction. However, turbines often experience unintentional yaw misalignment, which can significantly reduce the power production. The unintentional yaw misalignment increases for turbines operating in the wake of upstream turbines. Here, the combined effects of wakes and yaw misalignment are investigated, with a focus on the resulting reduction in power production. A model is developed, which considers the trajectory of each turbine blade element as it passes through the wake inflow in order to determine a power–yaw loss exponent. The simple model is verified using the HAWC2 aeroelastic code, where wake flow fields have been generated using both medium- and high-fidelity computational fluid dynamics simulations. It is demonstrated that the spatial variation in the incoming wind field, due to the presence of wakes, plays a significant role in the power loss due to yaw misalignment. Results show that disregarding these effects on the power–yaw loss exponent can yield a 3.5 % overestimation in the power production of a turbine misaligned by 30∘. The presented analysis and model is relevant to low-fidelity wind farm optimization tools, which aim to capture the combined effects of wakes and yaw misalignment as well as the uncertainty on power output.

Space-time accurate finite-state dynamic inflow modeling for aeromechanics of rotorcraft

Abstract Wake inflow modeling is a crucial issue in the development of efficient and high-fidelity simulation tools for rotorcraft flight dynamics and aeroelasticity. This paper proposes a space-time accurate, finite-state, dynamic wake inflow modeling suitable for conventional and innovative rotor configurations, based on simulations provided by high-fidelity aerodynamic solvers. It relates the coefficients of a rotor-disc, radial-azimuthal wake inflow distribution to the rotor kinematic variables, and is capable to take into account the intrinsic periodicity of aerodynamic responses of rotors in steady forward flight. The proposed inflow modeling consists of a three-step process: (i) numerical evaluation of wake inflow due to perturbations of rotor kinematic variables, (ii) determination of transfer functions of multi-harmonic components of a suitable set of inflow coordinates, followed by (iii) their rational approximation and transformation into time domain to derive the differential operators governing multi-harmonic dynamics. The numerical investigation concerns the derivation of finite-state inflow models for single and coaxial rotors, through application of an aerodynamic boundary-element-method solver for potential flows. These are successfully validated by comparison with inflows directly calculated by the aerodynamic tool for arbitrary rotor perturbations.

Influence of atmospheric stability on the load spectra of wind turbines at alpha ventus

Dependence between fatigue loads on a wind turbine located in an offshore wind farm and atmospheric stability was analysed by using measurement data of alpha ventus. An investigation of the fatigue load spectra of the wind turbines was performed enabling a detailed evaluation besides the damage equivalent load. Therefore, load spectra of a turbine in free-stream and a turbine experiencing wake inflow conditions were compared. Atmospheric stability was taken into account by means of the Bulk Richardson number. It was shown, how atmospheric stability affects load spectra of tower base and blade root bending moment. Hereby, it was found that few events of high fatigue loading increase the damage equivalent load (DEL) substantially, especially in unstable conditions.

State-space coaxial rotors inflow modelling derived from high-fidelity aerodynamic simulations

Wake inflow modelling is a crucial issue in the development of efficient and reliable computational tools for flight dynamics and aeroelastic analyses of rotorcraft. The aim of this work is the development of state-space, wake inflow modelling techniques for coaxial rotors perturbed from steady flight conditions, based on simulations provided by high-fidelity aerodynamic solvers. These provide relationships of the coefficients of an approximated linear distribution of wake inflow over upper and lower rotor discs either with rotor controls and helicopter kinematic variables or with thrust and in-plane moments generated by the rotors. A three-step identification procedure is proposed. It consists in: (i) evaluation of wake inflow due to harmonic perturbations of rotor kinematics; (ii) determination of the corresponding inflow coefficients (and rotors loads) transfer functions, followed by (iii) their rational approximation and transformation into time domain to obtain the final differential form. Aerodynamic responses are predicted through a boundary element method tool for potential flows, capable of capturing effects due to wake distortion, multi-body interference (like that in coaxial rotors) and severe blade–vortex interactions. The derived state-space models are validated by correlation with inflow distributions directly calculated by the aerodynamic solver, for a coaxial rotor subject to arbitrary perturbations.

Numerical simulation and dynamical response of a moored hydrokinetic turbine operating in the wake of an upstream turbine for control design

Abstract 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.

Identification of rotor wake inflow finite-state models for flight dynamics simulations

The aim of this work is the development of dynamic, finite-state modelling of wake inflow generated by kinematic perturbations of rotors in steady flight conditions. Extracted from responses of high-fidelity aerodynamic solvers, it is suited for flight dynamics applications. A three-step identification procedure is proposed: (1) evaluation by a high-fidelity solver of the wake inflow due to harmonic perturbations of rotor kinematics, (2) determination of the corresponding inflow coefficient transfer functions, and (3) rational approximation of the transfer functions. Wake inflow models related to rotor loads (like the well-known Pitt–Peters model) are obtained, as well, as by-products of that proposed. Considering aerodynamic simulations provided by a solver based on a boundary element method for potential flows, the numerical investigation presents the validation of the proposed finite-state wake inflow modelling, along with the examination of identified models related to rotor loads, for a rotor in steady flight conditions subject to arbitrary perturbations.

Assessment of a comprehensive aeroelastic tool for horizontal‐axis wind turbine rotor analysis

AbstractThis paper presents the development of a computational aeroelastic tool for the analysis of performance, response and stability of horizontal‐axis wind turbines. A nonlinear beam model for blades structural dynamics is coupled with a state‐space model for unsteady sectional aerodynamic loads, including dynamic stall effects. Several computational fluid dynamics structural dynamics coupling approaches are investigated to take into account rotor wake inflow influence on downwash, all based on a Boundary Element Method for the solution of incompressible, potential, attached flows. Sectional steady aerodynamic coefficients are extended to high angles of attack in order to characterize wind turbine operations in deep stall regimes. The Galerkin method is applied to the resulting aeroelastic differential system. In this context, a novel approach for the spatial integration of additional aerodynamic states, related to wake vorticity and dynamic stall, is introduced and assessed. Steady‐periodic blade responses are evaluated by a harmonic balance approach, whilst a standard eigenproblem is solved for aeroelastic stability analyses. Drawbacks and potentialities of the proposed model are investigated through numerical and experimental comparisons, with particular attention to rotor blades unsteady aerodynamic modelling issues. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Generalized Aerodynamic Modeling of Dynamic Wake Curvature for Open Rotors With Slender Blades

This paper elaborates on the theoretical development of an analytical approach, capable of modeling the effect of dynamic wake curvature on the aeroelastic response of open rotors with slender blades. The classical solution of incompressible, potential flow derived for a curved vortex tube of uniform vorticity strength is employed. The previously developed curved vortex tube analysis is mathematically generalized to account for arbitrary radial and circumferential variations of circulatory disk loading. An orthogonality analysis is carried out to obtain a finite set of inflow perturbation coefficients that describe the aerodynamic effect of wake curvature in a generalized manner. The end result is a set of integral expressions that provide the interharmonic coupling between the inflow perturbations on the rotor disk due to a curved trailing wake and the corresponding variations of disk loading. The obtained perturbation coefficients are subsequently superimposed upon an existing finite-state induced flow model that assumes a skewed, noncurved cylindrical wake. The developed mathematical approach for fluid mechanics is coupled with an unsteady blade element aerodynamics model, a rotor blade structural mechanics model, and a nonlinear rotor dynamics model. The combined formulation is implemented in an existing helicopter flight mechanics code. The overall method is initially employed to assess the effect of wake curvature on the dynamic response of a small-scale articulated rotor with a flap frequency ratio equal to unity. Subsequently, the integrated model is deployed to investigate the influence of wake curvature and inflow modeling fidelity on the predicted oscillatory blade loads and transient control response of a full-scale helicopter rotor. Comparisons are carried out with flight test measurements as well as with complex free-wake analysis methods. It is shown that including the effect of wake curvature is essential for predicting the transient control response of the investigated rotor. Good agreement is demonstrated between the proposed analytical model and nonlinear predictions carried out by resolving the complex wake geometry. The developed fluid mechanics formulation is a time-accurate method derived from first-principles and is applicable to both axial and nonaxial flow conditions.

Optimal Design and Acoustic Assessment of Low-Vibration Rotor Blades

An optimal procedure for the design of rotor blade that generates low vibratory hub loads in nonaxial flow conditions is presented and applied to a helicopter rotor in forward flight, a condition where vibrations and noise become severe. Blade shape and structural properties are the design parameters to be identified within a binary genetic optimization algorithm under aeroelastic stability constraint. The process exploits an aeroelastic solver that is based on a nonlinear, beam-like model, suited for the analysis of arbitrary curved-elastic-axis blades, with the introduction of a surrogate wake inflow model for the analysis of sectional aerodynamic loads. Numerical results are presented to demonstrate the capability of the proposed approach to identify low vibratory hub loads rotor blades as well as to assess the robustness of solution at off-design operating conditions. Further, the aeroacoustic assessment of the rotor configurations determined is carried out in order to examine the impact of low-vibration blade design on the emitted noise field.

Sound signature of propeller tip vortex cavitation

The design of an efficient propeller is limited by the harmful effects of cavitation. The insufficient understanding of the role of vortex cavitation in noise and vibration reduces the maximum efficiency by a necessary safety margin. The aim in the present study is to directly relate propeller cavitation sound to tip vortex cavity dynamics. This is achieved by a dedicated experiment in a cavitation tunnel on a specially designed two-bladed propeller using a high-speed video camera and a hydrophone. The sound signature of a tip vortex cavity is not evidently present in the sound spectrum above the tunnel background. The addition of a simulated wake inflow results in a high amplitude broadband sound. With a decrease in the free-stream pressure the centre frequency of this sound decreases as a result of a larger vortex cavity diameter. In the near future each blade passage in the high-speed video will be analyzed in detail. The frequency content of the cavity dynamics can then be directly related to the measured sound. An analytic model for vortex cavity dynamics resulting in a cavity eigenfrequency using a vortex velocity model can finally be evaluated as a design instrument for estimation of broadband sound from propeller cavitation.

Dependence of offshore wind turbine fatigue loads on atmospheric stratification

The stratification of the atmospheric boundary layer (ABL) is classified in terms of the M-O length and subsequently used to determine the relationship between ABL stability and the fatigue loads of a wind turbine located inside an offshore wind farm. Recorded equivalent fatigue loads, representing blade-bending and tower bottom bending, are combined with the operational statistics from the instrumented wind turbine as well as with meteorological statistics defining the inflow conditions. Only a part of all possible inflow conditions are covered through the approximately 8200 hours of combined measurements. The fatigue polar has been determined for an (almost) complete 360° inflow sector for both load sensors, representing mean wind speeds below and above rated wind speed, respectively, with the inflow conditions classified into three different stratification regimes: unstable, neutral and stable conditions. In general, impact of ABL stratification is clearly seen on wake affected inflow cases for both blade and tower fatigue loads. However, the character of this dependence varies significantly with the type of inflow conditions – e.g. single wake inflow or multiple wake inflow.

An unsteady aerodynamic formulation for efficient rotor tonal noise prediction

Abstract An aerodynamic/aeroacoustic solution methodology for predction of tonal noise emitted by helicopter rotors and propellers is presented. It is particularly suited for configurations dominated by localized, high-frequency inflow velocity fields as those generated by blade–vortex interactions. The unsteady pressure distributions are determined by the sectional, frequency-domain Kussner–Schwarz formulation, with downwash including the wake inflow velocity predicted by a three-dimensional, unsteady, panel-method formulation suited for the analysis of rotors operating in complex aerodynamic environments. The radiated noise is predicted through solution of the Ffowcs Williams–Hawkings equation. The proposed approach yields a computationally efficient solution procedure that may be particularly useful in preliminary design/multidisciplinary optimization applications. It is validated through comparisons with solutions that apply the airloads directly evaluated by the time-marching, panel-method formulation. The results are provided in terms of blade loads, noise signatures and sound pressure level contours. An estimation of the computational efficiency of the proposed solution process is also presented.