Verification of computational simulations of the NREL 5 MW rotor with a focus on inboard flow separation

The aerodynamic characteristics of the NREL 5 MW rotor have been examined using a Reynolds‐averaged Navier–Stokes method, OVERFLOW2. A comprehensive off‐body grid independence study has been performed. A strong dependence on the size of the near‐body wake grid has been found. Rapid diffusion of the wake appears to generate an overprediction of power and thrust. A large, continuous near‐wake grid at a minimum of two rotor diameters downstream of the rotor appears to be necessary for accurate predictions of near‐body forces. The NREL 5 MW rotor demonstrates significant inboard flow separation up to 30% of span. This separation appears to be highly three dimensional, with a significant amount of radial flow increasing the size of the separated region outboard. A simple, continuous full‐chord fence was applied at the maximum chord location of the blade, within the region of separation. This non‐optimized device reduced the boundary‐layer cross‐flow and resulting separation. The fence increased energy capture by nearly 1% at a wind speed of 8 m s^?1 and slightly increased blade loading over the length of the span. Copyright © 2011 John Wiley & Sons, Ltd.     

Modelling and analysis of the flow field around a coned rotor

The influence of coning a wind turbine rotor is analysed numerically using the blade element momentum (BEM) method and an actuator disc model combined with the Navier–Stokes equations. The two models are compared and shortcomings of the BEM model are discussed. As a first case, an actuator disc with a constant normal loading of C_T = 0·89 is considered. In accordance with theoretical predictions and investigations by Madsen and Rasmussen (European Wind Energy Conference, Nice, 1999; 138–141), the computations demonstrate that the power coefficient based on the projected area of the actuator disc is invariant to coning. The induced velocities, however, are no longer constant, but vary as a function of spanwise position. Next, the flow past the 2 MW Tjæreborg wind turbine is computed with and without coning. The most important findings from this study are that, although the power is reduced when the rotor is coned, the power coefficient based on the projected area is only slightly changed. The computations show that upstream coning results in a 2%–3% point higher power production than the corresponding downstream coning of the rotor. The Navier–Stokes computations show that the integrated loading, i.e. the root shear force, is higher than the one predicted by the BEM method, which is reduced approximately in proportion to the projected area. Copyright © 2001 John Wiley & Sons, Ltd.     

Analysis of a tip correction factor for horizontal axis turbines

It is imperative to include three‐dimensional tip flow corrections when using low‐order rotor models that rely on the flow independence principle to compute the blade forces. These corrections aim to account for the effect of pressure equalization at the tips and the accompanying spanwise pressure gradients on the outboard sections, by reducing the computed axial and tangential forces as the blade tips are approached. While Glauert‐type corrections are conventionally employed for actuator disc‐type computations, alternative corrections are required for actuator line computations as they use a finite blade representation. We present actuator line computations of the Model Rotor Experiments in Controlled Conditions (MEXICO) rotor to investigate tip corrections. Using the tip correction factor proposed by Shen et al. (Wind Energy 2005; 8:457–475), the actuator line computations show an improvement in accuracy over similar computations undertaken without a tip correction factor included. Further improvement to the blade loading is achieved by recalibrating the tip correction factor using data extracted from blade resolved computations of the model rotor experiments in controlled conditions rotor. From the rotor resolved computations, the tip loss (reduction in the blade loading on the outboard sections) is found to be more aggressive in the tangential direction than the axial direction. To account for this, we recalibrate the tip correction factor separately in the axial and tangential directions to develop new directionally dependent tip corrections. The resulting actuator line computations show a further improvement in accuracy of the tangential blade loading, resulting in better prediction of the rotor power. Copyright © 2017 John Wiley & Sons, Ltd.     

Measurements and modelling of the power performance of a model floating wind turbine under controlled conditions

This paper describes power performance measurements undertaken on a model floating wind turbine installed on a tension‐leg platform (TLP) in a wind/wave generator facility. Initially, the surge of the platform was measured under different rotor and wave conditions. The surge behaviour depended considerably on the rotor tip speed ratio and the wave frequency and amplitude. High‐frequency data sampling techniques were then used to derive the instantaneous power coefficient and tip speed ratio directly from the measurements, together with the surge velocity of the floating system. The power measurements were compared with those predicted by three independent numerical models, two of which are based on the blade element momentum approach and the third involving a lifting‐line free‐wake vortex model. The fluctuations of the power coefficient with time predicted by the three models were in close agreement; however, these were all significantly larger than those derived from the rotor shaft torque measurements. This was found to be due to the limitations of the torque measurement technique. Although being accurate in measuring the time‐averaged torque, the sensor was incapable of measuring the high‐frequency low‐amplitude fluctuations in the rotor shaft torque induced by the TLP surge. This was confirmed using an alternative experimental technique involving hot‐wire near‐wake measurements. The study also investigated the influence of the platform surge motion on the time‐averaged power coefficients. Both the measurements and the free‐wake vortex model revealed marginal deviations in the time‐averaged power coefficients when compared with those obtained for a fixed, non‐surging rotor. Copyright © 2014 John Wiley & Sons, Ltd.     

Numerical study of turbulent flow around a wind turbine nacelle

This article presents a numerical method for the simulation of turbulent flow around the nacelle of a horizontal axis wind turbine. The flow field around the turbine and nacelle is described by the Reynolds‐averaged Navier–Stokes equations. The k–? model has been chosen for closure of the time‐averaged turbulent flow equations. The rotor disc is modelled using the actuator disc concept. The main purpose of this article is to assess the impacts of the variation of some operational parameters (e.g. blade pitch angle changes) and atmospheric turbulence upon the relationship between wind speed measured near the nacelle and freestream wind speed established for an isolated turbine. Simulation results were compared with experimental data (from a typical stall‐controlled, commercially available wind turbine rated higher than 600 kW). In general, good qualitative agreements have been found that validate the proposed method. It has been shown that a level of accuracy sufficient for use in power performance testing can be obtained only when a proper aerodynamic analysis of the inboard non‐lifting cylindrical sections of the blade is included. Furthermore, the numerical method has proven to be a useful tool for locating nacelle anemometers. Copyright © 2005 John Wiley & Sons, Ltd.     

Noise simulations of flap devices for wind turbine rotors

The rotor of a large diameter wind turbine experiences more substantial and more dynamic loads due to the fluctuating and heterogeneous wind field. The project SmartBlades 2.0 investigated rotor blade design concepts that alleviate aerodynamic loading using active and passive mechanisms. The present work evaluates the acoustics of the two load alleviating concepts separately, an inboard slat and an outboard flap, using the Fast Random Particle Mesh/Fast Multipole Code for Acoustic Shielding (FRPM/FMCAS) numerical prediction toolchain developed at DLR with input from the averaged flow field from RANS. The numerical tools produce a comparable flap side-edge noise spectrum with that of the measurement conducted in the Acoustic Wind Tunnel Braunschweig (AWB). The validated FRPM/FMCAS was then used to analyze the self-noise from a slat at the inboard section of a rotor blade with a 44.45 m radius and compared with that from the outboard trailing edge. Furthermore, the rotational effect of the rotor was included in the post-processing to emulate the noise observed at ground level. The findings show an increase in the slat's overall sound pressure level and a maximum radiation upwind of the wind turbine for the case with the largest wind speed that represents the off-design condition. In operational conditions, the slat adds at most 2 dB to the overall sound pressure level. The toolchain evaluates wind turbine noise with conventional or unconventional blade design, and the problem can be scaled up for a full-scale analysis. As such, the tools presented can be used to design low-noise wind turbines efficiently.     

Potential flow solutions for energy extracting actuator disc flows

The actuator disc is the oldest representation of a rotor, screw or propeller. Performance prediction is possible by applying momentum theory, giving integrated values for power and velocity. Computational fluid dynamics has provided much more flow details, but a full potential flow solution zooming in on these flow details was still absent. With the wake boundary discretized by vortex rings, flow states for energy extracting discs have been obtained for thrust coefficients up to 0.998. Boundary conditions are met with an accuracy of a few ‰. Results from momentum theory are confirmed. Most rotor design codes use momentum theory in annulus or differential form, assuming that the axial velocity v_x at the disc is uniform. However, the absolute velocity | v | is found to be uniform, and arguments for this are presented. The non‐uniformity of v_x is an inherent part of the flow solution caused by, in terms of momentum theory, the pressure acting at the annuli. This makes the annuli not independent from each other as assumed in current design codes. Although this was already known, it is now confirmed up to the highest thrust coefficients. Optimizing a rotor design should be carried out for the non‐uniform distribution of v_x. To enable this, an equation for the non‐uniformity as function of thrust and radial position is presented, being a surface‐fit to the calculated data. Qualitatively, the non‐uniform distribution does the same as the Prandtl–Glauert–Shen tip correction applied to a uniform distribution. Copyright © 2015 John Wiley & Sons, Ltd.     

Computational investigations of blunt trailing‐edge and twist modifications to the inboard region of the NREL 5 MW rotor

The effects of twist and section shape modifications in the inboard region on the aerodynamic characteristics of the NREL 5 MW rotor have been examined using a Reynolds‐averaged Navier–Stokes method OVERFLOW2. The baseline rotor blade was modified by increasing the trailing‐edge thickness over the inboard region by modifying the sections’ thickness distribution aft of the maximum thickness location. Results when compared with the baseline rotor show that a modest increase of trailing‐edge thickness to 10–20%c increased power capture by 1%. Further increases in trailing‐edge thickness decrease in effectiveness to the point of reducing power capture when thicknesses reach 40%c. Increasing trailing‐edge thicknesses also leads to an increase in thrust, but this load is concentrated in the inboard region, resulting in a small increase in root bending moments. The blunt trailing‐edge concept greatly reduces the spanwise extent of inboard flow separation evident in the baseline NREL 5 MW rotor. The low‐pressure region aft of the trailing edge, created by the geometry, acts to reduce the spanwise spreading of the inboard separation. Rotors with modified twist distributions over the inboard 35%R of span are also compared. Inboard twist angles are varied from + 6° to ? 6° from the baseline twist schedule. Increasing inboard blade twist reduces overall rotor power capture but reduces thrust at a faster rate. Power capture remains constant with decreasing inboard geometry twist, whereas thrust increases approximately linearly by 0.75% for a decrease in thrust of 6°. Copyright © 2012 John Wiley & Sons, Ltd.     

Quantifying wave and yaw effects on a scale tidal stream turbine

The behaviour of Tidal Stream Turbines (TST) in the dynamic flow field caused by waves and rotor misalignment to the incoming flow (yaw) is currently unclear. The dynamic loading applied to the turbine could drive the structural design of the power capture and support subsystems, device size and its proximity to the water surface and sea bed. In addition, the strongly bi-directional nature of the flow encountered at many tidal energy sites may lead to devices omitting yaw drives; accepting the additional dynamic loading associated with rotor misalignment and reduced power production in return for a reduction in device capital cost. Therefore it is imperative to quantify potential unsteady rotor loads so that the TST device design accommodates the inflow conditions and avoids an unacceptable increase in maintenance action or, more seriously, suffers sudden structural failure.The experiments presented in this paper were conducted using a 1:20th scale 3-bladed horizontal axis TST at a large towing tank facility. The turbine had the capability to measure rotor thrust and torque whilst one blade was instrumented to acquire blade root strain, azimuthal position and rotational speed all at high frequency. The maximum out-of-plane bending moment was found to be as much as 9.5 times the in-plane bending moment. A maximum loading range of 175% of the median out-of-plane bending moment and 100% of the median in-plane bending moment was observed for a turbine test case with zero rotor yaw, scaled wave height of 2 m and intrinsic wave period of 12.8 s.A new tidal turbine-specific Blade-Element Momentum (BEM) numerical model has been developed to account for wave motion and yawed flow effects. This model includes a new dynamic inflow correction which is shown to be in close agreement with the measured experimental loads. The gravitational component was significant to the experimental in-plane blade bending moment and was also included in the BEM model. Steady loading on an individual blade at positive yaw angles was found to be negligible in comparison to wave loading (for the range of experiments conducted), but becomes important for the turbine rotor as a whole, reducing power capture and rotor thrust. The inclusion of steady yaw effects (using the often-applied skewed axial inflow correction) in a BEM model should be neglected when waves are present or will result in poor load prediction reflected by increased loading amplitude in the 1P (once per revolution) phase.     

Smart control of a horizontal axis wind turbine using dielectric barrier discharge plasma actuators

Rotating stall around a small-scale horizontal axis wind turbine was experimentally studied to characterize and assess smart rotor control by plasma actuators. Phase-locked Particle Image Velocimetry was used to map the flow over the rotor blade suction surface at numerous radial stations at a range of tip-speed-ratios. Flow separation occurred from the inboard of the blade and spread radially outwards as the tip-speed-ratio reduced. Plasma actuators placed along the span that produced a chord-wise body force had very little effect on the flow separation, even when operated in pulsed forcing mode. In contrast, plasma actuators along the blade chord that produced a body force into the radial directions (plasma vortex generators) successfully mitigated rotating stall. Torque due to aerodynamic drag was reduced by up to 22% at the lowest tip-speed-ratio of 3.7, suppressing stall over the outboard 50% of the blade. This was due to quasi-two-dimensional flow reattachment in the outboard region, and shifting of a fully stalled zone towards the hub in the inboard region because the plasma-induced body force counteracted the Coriolis-induced radial flow. This can significantly increase the turbine power output in unfavourable wind conditions and during start-up.     

Extracting lift and drag polars from blade‐resolved computational fluid dynamics for use in actuator line modelling of horizontal axis turbines

Low order rotor models such as the actuator line method are desirable as an efficient method of computing the large range of operating and environmental conditions, required to design wind and tidal rotors and arrays. However, the integrated thrust and torque predictions for each rotor are dominated by the blade loading on the outboard sections, where three‐dimensional (3D) effects become increasingly significant, and the accuracy of the reduced order methods remains uncertain. To investigate the accuracy of the spanwise blade loading on an individual rotor, actuator line and blade boundary layer resolved computations of the Model Rotor Experiments in Controlled Conditions (MEXICO) rotor are presented. The high fidelity blade‐resolved simulations give good agreement with measured pressure coefficient and particle image velocimetry data. Alternative lift and drag polars are extracted from the 3D simulated flow fields as a function of radial position. These are then used as replacement inputs for the actuator line method. Significant improvement in the accuracy of the actuator line predictions is found when using these 3D extracted polars, compared with using simulated two‐dimensional lift and drag polars with empirical correction applied to the spanwise loading distribution. Additionally, the 3D flow field data is used to derive different axial and tangential spanwise loading corrections for use with the two‐dimensional blade polars. Copyright © 2016 John Wiley & Sons, Ltd.     

Superposition of vortex cylinders for steady and unsteady simulation of rotors of finite tip‐speed ratio

Joukowski introduced in 1912 a helical vortex model to represent the vorticity of a rotor and its wake. For an infinite number of blades but finite tip‐speed ratio, the model consists of a vortex cylinder of longitudinal and tangential vorticity, a root vortex and a bound vortex disk. A superposition of cylinders is used in this paper to model rotors of radially varying circulation. The relations required to form a consistent system of cylinders are derived. The model contains a term which is not accounted for in the standard blade element momentum (BEM) algorithm. This term is identified as the contribution from the pressure drop due to the wake rotation. The BEM algorithm can be corrected to account for this effect. Unlike previous work on the topic, the contribution is derived for a radially varying circulation. A high‐thrust correction is also presented to extend the model. The optimal power coefficient obtained with this model for the constant circulation rotor is assessed and compared with that of existing solutions. Results from prescribed thrust distributions are compared with that of actuator disk simulations. Steady simulations are performed to compare with the BEM algorithm. The model is also applied to compute the velocity field in the entire domain and perform unsteady simulations. Results for an unsteady simulation corresponding to a pitch change of the rotor is used to compare the model with measurements and a BEM code with a dynamic inflow model. Copyright © 2015 John Wiley & Sons, Ltd.     

Plasma‐based feed‐forward dynamic stall control on a vertical axis wind turbine

Dynamic stall was controlled on a double‐bladed H‐Rotor vertical axis wind turbine model using pulsed dielectric barrier discharge plasma actuators in a feed‐forward control configuration. The azimuthal angles of plasma actuation initiation and termination, that produced the largest increases in power, were determined parametrically on the upstream half of the turbine azimuth in a low‐speed blow‐down wind tunnel at wind speeds of 7 m/s. A mathematical model, together with instantaneous turbine speed, was used to estimate transient torque and power developed by the turbine under the influence of plasma actuation. Overall performance improvements were based on changes between the final actuated and initial baseline results. A remarkable result of this investigation was that a net turbine power increase of 10% was measured. This was achieved by systematically reducing plasma pulsation duty cycles as well as the plasma initiation and termination angles. Nevertheless, it was determined that further performance increases could be achieved by changing the actuator's dielectric material, increasing the turbine radius and developing a method for control of dynamic stall on both the upwind (inboard of the blades) and downwind (outboard of the blades) halves of the turbine azimuth. Copyright © 2014 John Wiley & Sons, Ltd.     

A refined tip correction based on decambering

A new tip correction for use in performance codes based on the blade element momentum (BEM) or the lifting‐line technique is presented. The correction modifies the circulation by taking into account the additional influence of the induction of the vortices in the wake, using the so‐called decambering effect and thin‐airfoil theory. A limitation of the standard Prandtl tip correction is that it represents the surface loading by a line distribution that does not take into account the actual shape of the rotor blade. Thus, the chord distribution does not appear as a parameter in the model, and the loading in the proximity of the tip is generally found to be overestimated. The new tip correction is implemented as an additional correction in order to represent the surface loading by a line distribution. Comparing computations using the new model with standard BEM results and computations using a 3D panel code show that the inclusion of the correction greatly improves the results. The new model also explains some of the discrepancies that earlier on have been observed when using a BEM technique based alone upon standard tip corrections. Copyright © 2015 John Wiley & Sons, Ltd.     

Non‐uniform velocity distribution effect on the Betz–Joukowsky limit

A simple analytic correction is derived for the maximum efficiency of an ideal wind turbine rotor, the Betz–Joukowsky limit. The analytic correction accounts for the effect that the non‐uniform atmospheric boundary layer velocity distribution has on the Betz–Joukowsky derivation. The maximum power coefficient predicted by using the atmospheric boundary layer velocity profile is slightly higher than that predicted by using a uniform velocity distribution. The application of the correction to a 100 m rotor diameter at 80 m hub height in a neutrally stratified boundary layer flow predicts a maximum power increase of 1–2%, depending on the approach terrain. Copyright © 2012 John Wiley & Sons, Ltd.     

The prediction of the hydrodynamic performance of marine current turbines

The development of a blade element momentum (BEM) model for the hydrodynamic design of marine current turbines is presented. The model includes routines for interpolation of 2D section data and extrapolation for stall delay. The numerical model is compared with experimental data obtained from tests of an 800 mm diameter model rotor carried out in a cavitation tunnel. The theoretical predictions are in good agreement with the experiments. Using this validated model, a typical 3D rotor is used to demonstrate parametric variations of the design parameters. The effect of tip immersion on possible cavitation is assessed for this rotor. The model is then used to solve the dynamic effects of a tidal profile. The effect of an increase in blade roughness is presented, indicating a relatively small reduction in power. This work demonstrates that the numerical model developed can provide a useful tool for the investigation of the hydrodynamic design and operation of marine current turbines.     

A new correlation on the MEXICO experiment using a 3D enhanced blade element momentum technique

The blade element momentum (BEM) theory is based on the actuator disc (AD) model, which is probably the oldest analytical tool for analysing rotor performance. The BEM codes have very short processing times and high reliability. The problems of the analytical codes are well known to the researchers: the impossibility of describing inside the one-dimensional code the three-dimensional (3D) radial flows along the span-wise direction. In this work, the authors show how the 3D centrifugal pumping affects the BEM calculations of a wind turbine rotor. Actually to ascertain the accuracy of the analytical codes, the results are compared with rotor performance, blade loads and particle image velocimetry measurements of the model experiment in controlled conditions. A reliable agreement with the measurement is obtained. A good improvement is gained when the blade stall state modified aerofoil data instead of the original aerofoil data are used in the calculations.     

Design of a wind turbine rotor for maximum aerodynamic efficiency

The design of a three‐bladed wind turbine rotor is described, where the main focus has been highest possible mechanical power coefficient, CP, at a single operational condition. Structural, as well as off‐design, issues are not considered, leading to a purely theoretical design for investigating maximum aerodynamic efficiency. The rotor is designed assuming constant induction for most of the blade span, but near the tip region, a constant load is assumed instead. The rotor design is obtained using an actuator disc model, and is subsequently verified using both a free‐wake lifting line method and a full three‐dimensional Navier–Stokes solver. Excellent agreement is obtained using the three models. Global CP reaches a value of slightly above 0.51, while global thrust coefficient CT is 0.87. The local power coefficient C_p increases to slightly above the Betz limit on the inner part of the rotor; the local thrust coefficient C_t increases to a value above 1.1. This agrees well with the theory of de Vries, which states that including the effect of the low pressure behind the centre of the rotor stemming from the increased rotation, both C_p and C_t will increase towards the root. Towards the tip, both C_p and C_t decrease due to tip corrections as well as drag. Copyright © 2008 John Wiley & Sons, Ltd.     

Re‐examining the precepts of the blade element momentum theory for coning rotors

Blade element momentum (BEM) theory remains the core analytic approach behind most wind turbine analysis codes. The development of the theory extends over 100 years and thus bears re‐examination for applications involving coned rotors, for which it was never designed. This article reviews the underlying assumptions of BEM with a view to properly formulating a modified BEM theory for coned rotors. The postulate is that the critical limitation of BEM is the usual assumed relation between disc and far wake induction. Proper consideration of the relative placement of the wake and radially induced velocity, using potential flow theory, yields corrections to the BEM method. Using the modified BEM method, results are presented and compared with computational fluid dynamics (CFD) studies for ideal and real rotors to validate the new approach. It is found that the main limitation of BEM in application to coned rotors is in the assumed wake position, not the planar disc or the independence of stream‐tube assumptions. The latter is found only to have a strong effect at high induction factors and thrust coefficients, which are manifest in deficient local thrust coefficient models that seek to model transient recirculating flow conditions within the confines of a steady stream‐tube method. Copyright © 2006 John Wiley &Sons, Ltd.