Rotor Aerodynamics Research Articles

Prediction of the Broadband Noise Radiated from a Fan Rotor of a High-Bypass Ratio Turbofan Engine: Take-off Condition

The purpose of this study is to predict the broadband noise generated by the fan rotor of a high bypass ratio turbofan engine at take-off condition. A numerical investigation of the unsteady aerodynamics of a fan rotor was described, with emphasis on acoustics. Three case studies were examined; namely, design point, a choked point and a near surge point. It was found that the dominant broadband noise mechanisms are due to interaction of turbulence of the incoming flow with the engine casing and nose, interaction of the turbulent boundary layers on the rotor blades with their trailing edges, and interaction of the rotating blade with the turbulence in the incoming flow. The most important noise source was the interaction of the turbulent boundary layers on the rotor blades with their trailing edges. Choked case scored the highest noise level as it has the maximum turbulent kinetic energy levels, maximum mass flow rate and thus maximum relative Mach number. Near-surge case has the lowest noise level except at the lower part of the suction side of the rotor where the reverse flow and vortices caused higher levels of noise.

Development of a Fast Fluid-Structure Coupling Technique for Wind Turbine Computations

Fluid-structure interaction simulations are routinely used in the wind energy industry to evaluate the aerodynamic and structural dynamic performance of wind turbines. Most aero-elastic codes in modern times implement a blade element momentum technique to model the rotor aerodynamics and a modal, multi-body, or finite-element approach to model the turbine structural dynamics. The present paper describes a novel fluid-structure coupling technique which combines a three- dimensional viscous-inviscid solver for horizontal-axis wind-turbine aerodynamics, called MIRAS, and the structural dynamics model used in the aero-elastic code FLEX5. The new code, MIRAS- FLEX, in general shows good agreement with the standard aero-elastic codes FLEX5 and FAST for various test cases. The structural model in MIRAS-FLEX acts to reduce the aerodynamic load computed by MIRAS, particularly near the tip and at high wind speeds.

Unconventional Rotor Power Response to Yaw Error Variations

Continued inquiry into rotor and blade aerodynamics remains crucial for achieving accurate, reliable prediction of wind turbine power performance under yawed conditions. To exploit key advantages conferred by controlled inflow conditions, EU-JOULE DATA Project and UAE Phase VI experimental data were used to characterize rotor power production under yawed conditions. Anomalies in rotor power variation with yaw error were observed, and the underlying fluid dynamic interactions were isolated. Unlike currently recognized influences caused by angled inflow and skewed wake, which may be considered potential flow interactions, these anomalies were linked to pronounced viscous and unsteady effects.

LDV measurement of boundary layer on rotating blade surface in wind tunnel

Wind turbines generate electricity due to extracting energy from the wind. The rotor aerodynamics strongly depends on the flow around blade. The surface flow on the rotating blade affects the sectional performance. The wind turbine surface flow has span-wise component due to span-wise change of airfoil section, chord length, twisted angle of blade and centrifugal force on the flow. These span-wise flow changes the boundary layer on the rotating blade and the sectional performance. Hence, the thorough understanding of blade surface flow is important to improve the rotor performance. For the purpose of clarification of the flow behaviour around the rotor blade, the velocity in the boundary layer on rotating blade surface of an experimental HAWT was measured in a wind tunnel. The velocity measurement on the blade surface was carried out by a laser Doppler velocimeter (LDV). As the results of the measurement, characteristics of surface flow are clarified. In optimum tip speed operation, the surface flow on leading edge and r/R=0.3 have large span-wise velocity which reaches 20% of sectional inflow velocity. The surface flow inboard have three dimensional flow patterns. On the other hand, the flow outboard is almost two dimensional in cross sectional plane.

Aerodynamic Separability in Tip Speed Ratio and Separability in Wind Speed- a Comparison

From extensive application over a number of years, it has been established that the nonlinear rotor aerodynamics of typical medium and large wind turbines exhibit an effectively global separability property, in other words the aerodynamic torque of the machine can be defined by two independent additive functions. Two versions of the separability of aerodynamic torque for variable speed wind turbines are investigated here; the separated function, related to wind speed, in the first version is only dependent on that variable and not rotor speed and in the second version is only dependent on tip speed ratio. Both provide very good approximations to the aerodynamic torque over extensive neighbourhoods of T0, at least from 0 to 2T0.

Comparison of the lifting-line free vortex wake method and the blade-element-momentum theory regarding the simulated loads of multi-MW wind turbines

Design load simulations for wind turbines are traditionally based on the blade- element-momentum theory (BEM). The BEM approach is derived from a simplified representation of the rotor aerodynamics and several semi-empirical correction models. A more sophisticated approach to account for the complex flow phenomena on wind turbine rotors can be found in the lifting-line free vortex wake method. This approach is based on a more physics based representation, especially for global flow effects. This theory relies on empirical correction models only for the local flow effects, which are associated with the boundary layer of the rotor blades. In this paper the lifting-line free vortex wake method is compared to a state- of-the-art BEM formulation with regard to aerodynamic and aeroelastic load simulations of the 5MW UpWind reference wind turbine. Different aerodynamic load situations as well as standardised design load cases that are sensitive to the aeroelastic modelling are evaluated in detail. This benchmark makes use of the AeroModule developed by ECN, which has been coupled to the multibody simulation code SIMPACK.

Evaluation of a wind speed estimator for effective hub-height and shear components

Estimates of the effective wind speed disturbances acting on a wind turbine are useful in a variety of control applications. With some simplifications, it is shown that for zero yaw error, any wind field interacting with a turbine can be equivalently described using a hub-height (uniform) component as well as linear horizontal and vertical shear components. A Kalman filter-based wind speed estimator is presented for estimation of these effective hub-height and shear components. The wind speed estimator is evaluated in the frequency domain using the FAST aeroelastic simulator with the National Renewable Energy Laboratory's 5 MW reference wind turbine model and realistic hub-height and shear disturbances. In addition, the impact of the inflow model, used to simulate the rotor aerodynamics, on the Kalman filter performance is investigated. It is found that the estimator accuracy strongly depends on the inflow model used. In general, the estimator performs well up to a bandwidth of 1 Hz when the inflow model used for simulation matches the model used to create the linear Kalman filter model and blade pitch angle remains close to the linearization operating point. However, inaccuracies in the linear model of the turbine when dynamic inflow is used for simulation as well as nonlinearities in the turbine dynamics due to blade pitch actuation cause performance to degrade. Finally, the improvement gained by employing a non-causal wind speed estimator is assessed, showing a minor increase in performance. Copyright © 2014 John Wiley & Sons, Ltd.

Effects of Surge on Rotor Aerodynamics of Offshore Floating Wind Turbine

This paper, using the blade momentum theory combined with dynamic inflow correction and stall delay correction, analyses how periodic surge affect rotor aerodynamics of the NREL 5MW turbine operating at three different regions of its power curve. Results show that surge has the largest effects on rotor aerodynamics in region under rated wind speed while the smallest in region above that. Besides, oscillation amplitudes of rotor aerodynamic loads are in linear correlation with surge frequency and amplitude in most cases, except that rotor power and torque in region above rated wind speed is in linear correlation with the square of surge frequency. Results of this analysis would provide reference data for designs of floating wind turbine systems.

Numerical Simulation and Investigation of Transonic & Symmetrical Airfoil for Helicopter Main Rotor Blade Application

Helicopter rotor aerodynamics is prognosticated to be one of the most perplexing and enigmatic affliction encountered by both researchers and aviators throughout the ages. The bewilderment of the flow field around the main rotor blade ceaselessly remains unrequited in tangible flight environments. Appalling calamities repeatedly befall owing to these unforeseen and equivocal instances. In order to extricate these impediments, one must go back to the brass tacks and apprehend the cause. However, it is every so often exceedingly disconcerting to obtain experimental data due to intricacy, perplexity and substantial price tag. Subsequently, computational simulation is progressively becoming more of a preferred choice in recent times. Bearing this in mind, this study intended to simulate and visualize the air flow configuration of the main rotor blade using symmetrical and transonic airfoils to demarcate their physiognomies and behavior. Results have revealed that the transonic airfoil has a higher lift coefficient (Cl) than the symmetrical airfoil. Contrariwise, if used in an actual rotorcraft, the transonic airfoil can cause stern apprehension in terms of stability and control.

The cup anemometer, a fundamental meteorological instrument for the wind energy industry. Research at the IDR/UPM Institute.

The results of several research campaigns investigating cup anemometer performance carried out since 2008 at the IDR/UPM Institute are included in the present paper. Several analysis of large series of calibrations were done by studying the effect of the rotor's geometry, climatic conditions during calibration, and anemometers' ageing. More specific testing campaigns were done regarding the cup anemometer rotor aerodynamics, and the anemometer signals. The effect of the rotor's geometry on the cup anemometer transfer function has been investigated experimentally and analytically. The analysis of the anemometer's output signal as a way of monitoring the anemometer status is revealed as a promising procedure for detecting anomalies.

Unsteady aerodynamics and aeroacoustics of a fan rotor of a high-bypass ratio turbofan engine

The purpose of this study is to predict the broadband noise generated by the fan rotor of a high bypass ratio turbofan engine. A numerical investigation of the unsteady aerodynamics of a fan rotor is described, with emphasis on acoustics. Two flight conditions are discussed: cruise and takeoff. For each case, three case studies are examined: namely, operating point, a choked point and a near surge point. It is found that the dominant broadband noise mechanisms are due to interaction of turbulence of the incoming flow with the engine casing and nose, interaction of the turbulent boundary layers on the rotor blades with their trailing edges and interaction of the rotating blade with the turbulence in the incoming flow. The most important noise source is the interaction of the turbulent boundary layers on the rotor blades with their trailing edges. Choked case exhibits the highest noise level as it has the maximum turbulent kinetic energy levels, maximum mass flow rate and thus maximum relative Mach number. Near-surge case has the lowest noise level except at the lower part of the suction side of the rotor where the reverse flow and vortices causes higher levels of noise.

Modification of Leishman–Beddoes model incorporating with a new trailing-edge vortex model

The Leishman–Beddoes (L-B) model for airfoil dynamic stall is widely used in both helicopter rotor and wind turbine aerodynamics because it has fewer parameters and specific physical conceptions. However, in some cases of airfoil dynamic stall, the differences between the calculated results and experimental data are obvious, especially for the reattachment of airfoil dynamic stall. In order to observe the characteristics of dynamic stall about rotor airfoil, some dynamic stall experiments by PIV technology and CFD simulations based on RANS equations are accomplished. As a result, a trailing-edge vortex is observed which is not considered in the present L-B models. To consider the effect of this trailing-edge vortex on the unsteady aerodynamic characteristic, a new model of trailing-edge vortex is presented in this paper. In this method, the normal force induced by trailing-edge vortex is calculated by a similar methodology as leading-edge vortex, and the moment is modified due to the trailing-edge vortex. Consequently, the aerodynamic loads calculated by the modified L-B model at different reduced frequencies, Mach numbers, and airfoils are closer to the experimental data in the reattachment of dynamic stall. Therefore, it is demonstrated that the new trailing-edge vortex model further reflects the essence of trailing-edge vortex during the process of dynamic stall.

Adjoint Method for Shape Optimization in Real-Gas Flow Applications

An adjoint-based shape optimization approach for supersonic turbine cascades is proposed for application to organic Rankine cycle (ORC) turbines. The algorithm is based on an inviscid discrete adjoint method and encompasses a fast look-up table (LuT) approach to accurately deal with real-gas flows. The turbine geometry is defined by adopting state-of-the-art parameterization techniques (NURBS), enabling to handle both global and local control of the shape of interest. A preconditioned steepest descent method has been chosen as gradient-based optimization algorithm to efficiently search for the nearest minimum. The potential of the optimization approach is first verified by application on the redesign of an existing converging–diverging turbine nozzle operating in thermodynamic regions characterized by relevant real-gas effects. A significant efficiency improvement and a more uniform flow at the blade outlet section are achieved, with expected beneficial effects on the aerodynamics of the downstream rotor. The optimized configuration is also assessed by means of high-fidelity turbulent simulations, which point out the capability of the present inviscid approach in optimizing supersonic turbine cascades with very limited computational burdens. Finally, the newly developed real-gas adjoint method is compared against adjoints based on ideal equations of state on the same design problem. Results show that the performance gain obtained by a fully real-gas optimization strategy is by far higher than that achieved with simplified approaches in case of ORC turbines. This proves the relevance of including accurate thermodynamic models in all steps of ORC turbine design.

How to improve open rotor aerodynamics at cruise and take-off

AbstractA key challenge in open rotor design is getting the optimum aerodynamics at both the cruise and take-off conditions. This is particularly difficult because the operation and the requirements of an open rotor are very different at cruise compared to takeoff. This paper uses CFD results to explore the impact of various design changes on the cruise and take-off flow-fields. The paper then considers how a given open rotor design is best operated at take-off to minimise noise whilst maintaining high thrust. The main findings are that various design modifications can be applied to control the flow features that lead to lost efficiency at cruise and increased noise emission at take-off. A breakdown of the lost power terms from CFD solutions demonstrates how developments in open rotor design have led to reduced aerodynamic losses. At take-off, the operating point of the open rotor should be set such that the non-dimensional lift is as high as possible, without causing significant flow separation. This can be achieved through suitable amounts of re-pitch and speed up applied to a design. Comparisons with fully three-dimensional CFD show that the amount of re-pitch required can be determined using simplified methods such as two-dimensional CFD and a Blade Element Method.

Research on Wind Turbine Blade Load Based on the Analysis of Structural Dynamics

Wind energy technology is an integrated technology, which involves aerodynamics, structural dynamics, meteorology, mechanical engineering, electrical engineering, control multiple disciplines technology, materials science, environmental science and other areas. This paper studies the structural dynamics of the wind turbine. On one hand, modern wind turbine is composed of various interacting components and subsystems, and its aerodynamic rotor design technique involves controlling a wide range of areas systems, mechanical systems, electrical systems. On the other hand, the wind turbine has characteristics different from the usual mechanical systems. Wind turbine power source is natural randomness of strong wind, The leaves are often run in a stall condition. The system has a strong stochastic dynamic process. The transmission system irregular power input is abnormal . The main structural components exposed to several times higher than normal rotating mechanical fatigue loading. Thus the unique characteristic of dynamics of the wind turbine is formed . Analysis of wind turbine blade load dynamics of basic research carried out in this article by means of an appropriate structure coordinates.

Design trade-off study between efficiency and rotor forcing attenuation in a transonic turbine stage

A multi-objective optimisation procedure was applied to the 3D design of a transonic turbine vane row, considering efficiency and stator outlet pressure distortion, which is directly related to the forcing induced in the rotor. The characteristic features that define different individuals along the Pareto Front were described, analysing the differences between high efficiency airfoils and low interaction. Pressure distortion was assessed by means of a model that requires only of the computation the steady flow field in the domain of the stator. The reduction of aerodynamic rotor forcing was validated via unsteady multistage aerodynamic computations carried out with NUMECA FINETM/Turbo. A well known loss prediction method was used to perform total loss decomposition to quantify the influence on efficiency of reducing rotor forcing. Results show that when striving for efficiency, the rotor was affected by few, but intense shocks. On the other hand, when the objective was the minimisation of distortion, multiple shocks appeared.

Development and application of a dynamic stall model for rotating wind turbine blades

In unsteady conditions of wind turbines, both the dynamic stall phenomenon and the three-dimensional (3D) rotational effect affect the rotor aerodynamics. The dynamic stall mechanism for rotating wind turbine blades is first investigated. Through the comparison of the aerodynamic data between the rotating blade and the two-dimensional (2D) airfoil, the normal force slope in the attached flow and the separation point expression in the separated flow are modified in the Beddoes-Leishman (B-L) dynamic stall model for rotating NREL wind turbine blades. The modified model is validated by the comparison between the calculation results and the experimental results of the lift and drag coefficients at different radial positions. Both the hysteresis loop shapes and the calculation values are closer to the experiment than the 2D dynamic stall model. The present dynamic stall model is then coupled to a free vortex wake model. The coupled model is used to calculate the unsteady blade aerodynamic loads and the low speed shaft torque of the NREL wind turbine in a yawed condition. The accuracy is greatly improved by the corrections presented in the paper.

The Effect of Mounting Vortex Generators on the DTU 10MW Reference Wind Turbine Blade

The aim of the current work is to analyze possible advantages of mounting Vortex Generators (VG's) on a wind turbine blade. Specifically, the project aims at investigating at which radial sections of the DTU 10 MW Reference Wind Turbine blade it is most beneficial to mount the VG's in order to increase the Annual Energy Production (AEP) under realistic conditions. The present analysis was carried out in several steps: (1) The clean two dimensional airfoil characteristics were first modified to emulate the effect of all possible combinations of VG's (1% high at suction side x/c=0.2-0.25) and two Leading Edge Roughness (LER) values along the whole blade span. (2) The combinations from Step 1, including the clean case were subsequently modified to take into account three dimensional effects. (3) BEM computations were carried out to determine the aerodynamic rotor performance using each of the datasets from Step 2 along the whole blade span for all wind speeds in the turbine control scheme. (4) Employing the assumption of radial independence between sections of the blades, and using the results of the BEM computations described in Step 3, it is possible to determine for each radial position independently whether it is beneficial to install VG's in the smooth and LER cases, respectively. The results indicated that surface roughness that corresponds to degradation of the power curve may to some extent be mitigated by installation of VG's. The present results also indicated that the optimal VG configuration in terms of maximizing AEP depends on the degree of severity of the LER. This is because, depending on the condition of blade surface, installation of VG's on an incorrect blade span or installation of VG's too far out on the blade may cause loss in AEP. The results also indicated that the worse condition of the blade surface, the more gain may be obtained from the installation of VG's.

The role of conservative forces in rotor aerodynamics

AbstractThe theory to predict the performance and loads on rotors (propellers, screws, windmills) has a history of more than a century. Apart from modern computational fluid dynamics and vortex panel models taking the true blade geometry into account, most other models proceed from an infinitely thin actuator disc or line. These models assume an externally defined force field distributed at the disc or line, representing the loads on the real rotor. Given this force field, the flow is solved by momentum balances or by the equations of motion. The use of external force fields was discussed in textbooks of the first decades of the 20th century, but has received little attention since then. This paper investigates the higher-order effect of adding thickness to the actuator disc or changing the actuator line to a blade with cross-sectional dimensions. For the generation of a Rankine vortex by a force field acting on an actuator disc with thickness, an exact solution has been found in which not only the thrust and torque determine the flow, but also a radial force. This force is conservative, in contrast to the other force components. For rotor blades, a conservative normal and radial force acting on the chordwise bound vorticity is present. This explains the experimentally observed inboard motion of the tip vortex of model wind turbine rotors before the wake induction field drives it outboard. Simulations by computational fluid mechanics and a vortex panel code reproduce the inboard motion, but an actuator line analysis, in which the chordwise vorticity is absent, does not. The conservative load is only $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}1\mbox{--}2\, \%$ of the thrust on the entire blade but ${\approx }10\, \%$ of the thrust at the tip ($r/R>0.9$). Conservative forces at the disc and rotor blade vanish for vanishing disc thickness or blade cross-section, so play no role in any of the infinitely thin actuator disc or line methods. However, if higher-order effects of non-zero dimensions are to be modelled, the conservative force field has to be included.

Modeling of Wind Energy on Isolated Area

In this paper, a model of the wind turbine (WT) with permanent magnet generator (PMSG) and its associated controllers is presented, The increase of wind power penetration in power systems has meant that conventional power plants are gradually being replaced by wind farms. In fact, today wind farms are required to actively participate in power system operation in the same way as conventional power plants. In fact, power system operators have revised the grid connection requirements for wind turbines and wind farms, and now demand that these installations be able to carry out more or less the same control tasks as conventional power plants. For dynamic power system simulations, the PMSG wind turbine model includes an aerodynamic rotor model, a lumped mass representation of the drive train system and generator model. In this paper we propose a model with an implementation in MATLAB / Simulink, each of the system components off-grid small wind turbines. DOI: http://dx.doi.org/10.11591/ijpeds.v4i2.4859