Airfoil Section Research Articles

Criticism study of J-Shaped darrieus wind turbine: Performance evaluation and noise generation assessment

Vertical Axis Wind Turbines (VAWTs) are suitable for use in populated areas. The main parameter of the design of this type of the wind turbine is the cross sectional profile of the turbine blade. Additionally, the turbine performance and the generated aerodynamic noise of the VAWTs are critical issues that could limit the spreading of these turbines especially in the residential zones. In the last years, some publications investigated the J-shape Darrieus turbine and praised the performance of this design. Therefore, the present investigation introduces a criticism of the J-shape Darrieus Vertical Axis Wind Turbine (J-SD-VAWT) with three-bladed rotor. An accurate comparison has been introduced for three different turbines with different airfoils sections (NACA 0015, NACA 0021 and S1046). The comparison included the standard airfoil design and the J-shape of the same airfoils from two points of views of turbine performance and noise generation. Aero-acoustics and performance of the proposed design was thoroughly investigated using the Computational Fluid Dynamics (CFD). Both the Unsteady Reynolds-averaged Navier-Stokes (URANS) equations and the impermeable Ffowcs Williams and Hawkings (FW-H) equation were simultaneously solved. The effects of J-shape cut ratio, speed ratio, and turbine solidity were investigated. For all configurations, the results indicated that J-shape reduced the performance and did not reduce the turbine noise.

Experimental and numerical study of wing boundary layer behavior in propeller flowfield

In this research, the effects of propeller slipstream on wing boundary layer and transition front were studied through wind tunnel tests and numerical analysis. In this respect, the flow around a NACA 6-series airfoil section was simulated in a computational fluid dynamics solver without and with propeller flowfield. The numerical study, presented in this paper, was concerned with the effect of propeller slipstream on both wing aerodynamics and boundary layer treatment. For experimental tests, oil flow visualization technique was used to determine laminar to turbulent transition location, laminar separation bubble with turbulent reattachment and turbulent separation line over the wing surfaces. Existence of propeller slipstream changed pressure and skin friction distribution over the wing surfaces, in both chordwise and spanwise directions and it hence affected on the wing loading distribution. Also upstream propeller affected on boundary layer characteristics including laminar/turbulent transition onset and separation over the wing. The results showed that the transition location moved toward the leading edge, and the separation bubble was washed out due to propeller slipstream.

Wind Turbine Aerodynamic Modeling in Icing Condition: Three-Dimensional RANS-CFD Versus Blade Element Momentum Method

Icing limits the performance of wind turbines in cold climates. The prediction of the aerodynamic performance losses and their distribution due to ice accretion is essential. Blade element momentum (BEM) is the basis of blade structural studies. The accuracy and limitations of this method in icing condition are assessed in the present study. To this purpose, a computational study on the aerodynamic performance of the full-scale NREL 5 MW rotor is performed. Three-dimensional (3D) steady Reynolds-averaged Navier–Stokes (RANS) simulations are performed for both clean and iced blade, as well as BEM calculations using two-dimensional (2D) computational fluid dynamics (CFD) sectional airfoil data. The total power calculated by the BEM method is in close agreement with the 3D CFD results for the clean blade. There is a 4% deviation, while it is underestimated by 28% for the iced one. The load distribution along the clean blade span differs between both methods. Load loss due to the ice, predicted by 3D CFD, is 32% in extracted power and the main loss occurs at the regions where the ice horn height exceeds 8% of the chord length.

Free vibration analysis of composite aircraft wings modeled as thin-walled beams with NACA airfoil sections

This study reports the formulation of a realistic aircraft wing model based on thin-walled composite beam theory. In this model, NACA 4-digit-series airfoils were selected for the wing-section. The circumferentially asymmetric stiffness lay-up was implemented and two different sets of coupling systems corresponding to the proposed model were utilized. The eigenvalue problem was solved by the extended Galerkin method and the natural frequencies were computed as a function of the ply angle. The results of the free vibration analysis along with the governing system of the proposed model contributes to the theory of anisotropic thin-walled beams, emphasizing the importance of thin-walled structures in the design of advanced aeronautical structures.

Weakly-nonlinear interactions of broadband Tollmien–Schlichting waves in a non-self-similar boundary layer on an airfoil

Abstract The present investigation is devoted to initial nonlinear stages of laminar–turbulent transition initiated by broadband disturbances in an essentially non-self-similar boundary layer, developing on a two-dimensional airfoil. The measurements are performed at fully controlled disturbance conditions with respect to the distribution of the initial spectra of excited 3D (in general) Tollmien–Schlichting (TS) waves. The main attention is devoted to the investigation of resonant (and non-resonant) interactions and amplification of broadband low-amplitude 3D instability waves in the presence (and in absence) of one or two primary harmonic 2D TS-waves. The broadband, “noise-like” TS-waves are composed of controlled perturbations, pseudo-random in time and space, with a continuous frequency–wavenumber spectrum. These perturbations simulate typical background disturbances usually observed at so-called “natural” disturbance conditions. The harmonic primary TS-wave simulates a primary instability mode, which has been formed by the linear instability mechanism. In fact, this paper represents the third step in a set of detailed investigations (Wurz et al., 2012 [1] ) and Wurz et al., 2012 [2] ) of weakly-nonlinear stages of transition performed in the same base flow (on an airfoil) but at increased complexity of initial disturbance conditions. In contrast to the studies of periodic resonant regimes described in refs. Wurz et al. (2012) [1] and Wurz et al. (2012) [2] , no specific (preinstalled) disturbance frequencies or/and spanwise wavenumbers were excited in the boundary layer (except for the primary wave). So, the flow was free to amplify almost any mode of the frequency–wavenumber spectrum by means of any available mechanisms of TS-wave interaction. Such disturbance conditions are the closest to the so-called “natural” ones, i.e. to those observed typically for uncontrolled external perturbations. Four main groups of regimes of disturbance excitation were studied: (i) two harmonic fundamental waves and a pair of frequency detuned subharmonics, (ii) the low-amplitude broadband disturbances only and (iii) the low-amplitude broadband disturbances together with harmonic 2D primary TS-waves, and (iv) the high-amplitude broadband disturbances only. It is found that in case (i) the superposition of the resonances takes place in a nonlinear way and joint efforts of the two fundamental waves are similar to a fundamental-mode amplitude enhancement, which always leads to the increase of the subharmonic amplification rates. In case (ii) the perturbations develop downstream in agreement with the linear stability mechanism. In case (iii) strong resonant interactions between the broadband perturbations and the harmonic primary TS-wave is observed in a wide frequency range. In case (iv) very similar strong resonant interactions are observed between 2D and 3D spectral modes starting already at significantly lower amplitudes of the 2D modes in the frequency range of the primary fundamental modes. In cases (i), (iii) and (iv) the amplification is double-exponential (exponent in exponent), similar to that found in isolated tuned and detuned resonances of deterministic modes in papers Wurz et al. (2012) [1] and Wurz et al. (2012) [2] . For all resonantly interacting disturbances the phase synchronism conditions are found to be satisfied. Based on the cumulative results obtained in the framework of this intensive investigation, the most important observation is the predominant role of resonant TS-wave interactions in the non-self-similar boundary layer developing on a natural laminar flow airfoil section.

Vortex Shedding Lock-In due to Pitching Oscillation of a Wind Turbine Blade Section at High Angles of Attack

The unsteady flow around a pitching two-dimensional airfoil section (NREL S809) has been simulated using unsteady RANS with the transition SST turbulence model. This geometry is chosen to represent a wind turbine blade in a standstill configuration. The Reynolds number is Re=106 based on a chord length of 1 m. A prescribed sinusoidal pitching motion has been applied at a fixed amplitude of 7° for a range of high angles of attack 30°<α<150°. At these incidences, the airfoil will behave more like a bluff body and may experience periodic vortex shedding. It is well known that, in bluff body flows, oscillations can lead to a lock-in (lock-in) of the vortex shedding frequency, fv, with the body’s motion frequency, fp. In order to investigate the susceptibility of airfoil to lock-in, the frequency ratio r (r=fp/fv0) has been varied around r=1. The lock-in region boundaries have been proposed, and an analysis of the effect of the oscillation amplitude has been conducted. The lock-in map obtained suggests that, for the vibration amplitude considered, the risk of vortex-induced vibration is more significant in the regions of α≈40° and α≈140°, i.e., for shallower characteristic lengths. Finally, a lumped parameter wake oscillator model has been proposed for pitching airfoils. This simple model is in qualitative agreement with the CFD results.

Rotor aerodynamic shape design for improving performance of an unmanned helicopter

In order to improve the effective load of an unmanned helicopter, a new blade shape with a new airfoil section, non-linear negative twist, varied chord length and double-swept blade tip was designed by employing a CFD method coupled with an optimization method. Firstly, a new airfoil is designed with improved aerodynamic characteristics. By comparing the numerical data, the new airfoil has better lift-drag ratio, i.e., the maximum lift-drag ratio increases from 63.03 to 66.29 at Mach number of 0.3 and from 70.17 to 72.13 at Mach number of 0.4, compared with the original NACA8H12 airfoil. In addition, a new blade shape is designed based on the new airfoil. By comparing the numerical data, the CT of the design rotor increases from 4.79×10−3 to 5.07×10−3, and the maximum FM increases from 0.67 to 0.72 at the design state. After that, a verification test is performed in hovering flight. The test data indicated the a maxumu thrust increase of 3.18% at the design state. Meanwhile, the FM of design rotor is improved about 3.41% compared with the original rotor.

Modification of turbulent airfoil section flow using a combined control action

Both numerically and experimentally, the possibility of using a combined method to control the incompressible turbulent boundary layer on a symmetric airfoil section of 12-% relative thickness implemented via the blowing/suction of air through a finely perforated section provided on the wing surface part was examined. The study was performed at Reynolds number Rec = 0.7·106 in the range of angles of attack α = −6 ÷ 6°. The mechanism of action of this flow control method on the aerodynamic characteristics of the wing was identified. An ambiguous pattern of the effect due to blowing/suction from the viewpoint of ensuring a maximum lift, a gain in wing lift-to-drag ratio, and a reduction of wing aerodynamic drag was revealed.

SIMULATION OF 2D FLOW AROUND OF AIRFOILS AT LOW-SPEED WIND TUNNEL WITH OPEN JET TEST-SECTION

At present, there is a great interest in the development of new airfoils for wind turbines and high-lift wings of unmanned aerial vehicles (UAV). The requirements for such airfoils differ from conventional aircraft airfoils, because of structural reasons and extreme operating conditions. So, wind turbine airfoils operate frequently under fully separated flow when stall is used for power regulation at high wind speeds. At the same time design of airfoils for wings UAV poses the problem of availability of high-lift at low Reynolds number. Modern airfoils are to a large extent developed from numerical methods. However, the complex flow conditions such as separation at high angles of attack, laminar separation bubbles and the transition from laminar to turbulent flow are difficult to predict accurately. Hence, testing of airfoils at a two-dimensional condition is an important phase in airfoil design. The development and validation of a 2D testing facility for investigation of single and multi-element airfoils in the wind tunnel Т-102 with open test section are considered in this article. T-102 is a continuous-operation, closed-layout wind tunnel with two reverse channels. The test section has an elliptical cross-section of 4 ×2,33 m and a length of 4 m. Two big flat panels of the L × H=3 ×3,9 m size installed upright on balance frame aligned with the free stream are used for simulating two-dimensional flow in the tunnel test section. The airfoil section in the layout of a rectangular wing is mounted horizontally between flat panels with minimum gaps to ensure 2D flow conditions. The aerodynamic forces and pitch moment acting on the model were measured by wind tunnel balance. To determine boundary corrections for a new test section of wind tunnel, the experimental investigation of three geometrically similar models has been executed. The use of boundary corrections has provided good correlation of the test data of airfoil NACA 6712 with the results obtained from the wind tunnel except for lift and drag coefficient values at high angles of attack.

Effects of the airfoil section, the chord and pitch distributions on the aerodynamic performance of the propeller

The main objectives of this study are to investigate parametrically the possible use of alternative airfoils (Joukowski and Gottingen) for propellers and to assess the effects of varying the chord and pitch angle distributions as well as the use of multiple airfoils along the blade on the performance parameters of the propeller. In this study, a validated home-built FORTRAN code based on the BEM method with incorporated tip and compressibility losses is used. The detailed investigation of the blade geometry is done to help in selecting a configuration that is efficient and easy to manufacture. The linear pitch distribution is found to reduce the coefficients of thrust and power as well as higher blade loading at the intermediate region and lower loading at the tip region in comparison with the Gottingen 796-based propeller. The results show that the power coefficient and efficiency of the generalized Joukowski-based propeller are greater than the respective coefficients of Gottingen 796-based propeller for advanced ratio J = 0.85 and higher. The predicted results indicate that the use of the elliptical chord distribution provokes reduction in the blade loading at the tip region and increases at the intermediate region of the blade. It is found also that it reduces the coefficient of thrust, torque and power in comparison with the blade having the reference chord distribution.

Aero-structural Investigation of Sesquiplane Wind Turbine Blades

Abstract The flexibility of large-scale wind turbines requires new structural blade designs to guarantee rigidity and avoid mechanical interference between the blades and tower. Segmented and partial reinforcement are effective approaches to improve the structural performance of large blades. This paper presents a novel sesquiplane blade design inspired by the biplane and sesquiplane aircraft, which have a similar development process. Wind tunnel testing and finite element simulation were conducted to investigate the aerodynamics and structural performance of the new design. The results indicate the sesquiplane blade, with airfoil sections in both the main and reinforcing spars, can notably enhance the integral rigidity and bearing capacity without a substantial loss in aerodynamic performance.

Hard sail optimization and energy efficiency enhancement for sail-assisted vessel

The shipping industry has valued wind energy in the last decades due to the global increasingly severe environmental situation. The sail-assisted vessels have employed generous types of sails to exploit wind energy and reduce the energy consumption. To meet the ever-increasing energy saving requirements of sail-assisted vessels, this paper investigates the hard sail design optimization and energy efficiency enhancement problems for sail-assisted vessel. By using parametric section airfoil parametrization (PARSEC) method and particle swarm optimization algorithm, we fulfill the optimization of the wingsail aerodynamic design. The performance of our optimized airfoil has been verified by multi-point theory, computational fluid dynamics (CFD) examination, and energy efficiency design index (EEDI) calculation. The maximum lift-to-drag ratio of the coupled wingsails has increased by 10% than that of the arc-shaped sail. And the vessel equipped with our coupled wingsails can remarkably reduce EEDI by 18.2% and 4% when compared to non-sails vessel, and the vessel equipped with variable-camber sail (VCS), respectively.

Active load alleviation potential of adaptive wind turbine blades using shape memory alloy actuators

AbstractUpscaling of wind turbine blades calls for implementation of innovative active load control concepts that will facilitate the flawless operation of the machine and reduce the fatigue and ultimate loads that hinder its service life. Based on aeroelastic simulations that prove the enhanced capabilities of combined individual pitch and individual flap control at global wind turbine scale level, a shape adaptive concept that encompasses an articulated mechanism consisting of two subparts is presented. Shape memory alloy (SMA) actuators are investigated and assessed as means to control the shape adaptive mechanism at airfoil section level in order to alleviate the developed structural loads. The concept is embedded in the trailing edge region of the blade of a 10‐MW horizontal axis wind turbine and acts as a flap mechanism. Numerical simulations are performed considering various wind velocities and morphing target shapes and trajectories for both normal and extreme turbulence conditions. The results prove the potential of the concept, since the SMA controlled actuators can accurately follow the target trajectories. Power requirements are estimated at 0.22% of the AEP of the machine, while fatigue and ultimate load reduction of the flap‐wise bending moment at the blade root is 27.6% and 7.4%, respectively.

A piezoelectric energy harvesting scheme based on stall flutter of airfoil section

Abstract The stall flutter of the airfoil section has the characteristics of self-excitation and violent vibration. As a new type of energy source, it could provide power for sensors and other devices. In this paper, a new energy harvesting scheme is proposed. A beam is fixed to the trailing edge of the airfoil section along the chord line and a piezoelectric transduction is stick to the appropriate position of the cantilever beam. When the airfoil section flies in stall flutter, the piezoelectric cantilever beam is excited to vibration and then drives the piezoelectric transduction to produce energy output. The system energy harvesting model consists of the dynamic model exported by the Lagrange Equation and the electromechanical coupling equation. The numerical simulation of the model shows that output voltage of a piezoelectric patch has stable amplitude of 1134.2 V and the output power has stable amplitude of 0.3233 W under the condition of resistance load R = 4 M Ω . The output voltage and power are greatly increased. The influence laws of load resistance, wind velocity, airfoil section torsion stiffness, sticking position of the piezoelectric patch, and wind gust are also analyzed by simulation. These analyses are conducive to the subsequent design of similar programs.

Static and Dynamic Analysis of a NACA 0021 Airfoil Section at Low Reynolds Numbers Based on Experiments and Computational Fluid Dynamics

The wind industry needs airfoil data for ranges of angle of attack (AoA) much wider than those of aviation applications, since large portions of the blades may operate in stalled conditions for a significant part of their lives. Vertical axis wind turbines (VAWTs) are even more affected by this need, since data sets across the full incidence range of 180 deg are necessary for a correct performance prediction at different tip-speed ratios. However, the relevant technical literature lacks data in deep and poststall regions for nearly every airfoil. Within this context, the present study shows experimental and numerical results for the well-known NACA 0021 airfoil, which is often used for Darrieus VAWT design. Experimental data were obtained through dedicated wind tunnel measurements of a NACA 0021 airfoil with surface pressure taps, which provided further insight into the pressure coefficient distribution across a wide range of AoAs. The measurements were conducted at two different Reynolds numbers (Re = 140 k and Re = 180 k): each experiment was performed multiple times to ensure repeatability. Dynamic AoA changes were also investigated at multiple reduced frequencies. Moreover, dedicated unsteady numerical simulations were carried out on the same airfoil shape to reproduce both the static polars of the airfoil and some relevant dynamic AoA variation cycles tested in the experiments. The solved flow field was then exploited both to get further insight into the flow mechanisms highlighted by the wind tunnel tests and to provide correction factors to discard the influence of the experimental apparatus, making experiments representative of open-field behavior. The present study is then thought to provide the scientific community with high quality, low-Reynolds airfoil data, which may enable in the near future a more effective design of Darrieus VAWTs.

Numerical Investigation on Evolution of Tip Vortices Generated by Low-Aspect Ratio Rectangular Wings at High Angle of Attack

The onset and evolution of tip vortices generated by the vertical tip of a straight half wing, with a NACA 0012 airfoil section, were studied at a Reynolds number of 1 × 105 by a steady-state calculation at high angles of attack, namely, 10°, 18°, and 42°, where the maximum lift, deep stall, and second peak of the lift coefficient occur, respectively, with three different aspect ratios: 2, 3, and 4. It was found that the counter-rotating vortex wrap initiated within the short wake period at higher angles than the deep stall and affected the contraction of the core trail, as well as resulting in the calculated swirl velocity component exhibiting a significant deviation from the conventional model fit. It was noted that the swirl strength of tip vortices increased continuously, even up to the deep stall phase, where the lift started to decrease immediately after the first peak. Wake contraction was found to follow an exponential decay in the earlier wake region. Compared to both the infinite wing and the elliptical wing, the finite square wing seems to have an offset in the lift coefficient, which could represent the tip loss.

Управление ламинарно-турбулентным переходом на крыловом профиле путем распределенного отсоса через мелкоперфорированную поверхность

The results of experimental investigations of the influence of distributed suction through a finely perforated section of a symmetric airfoil on the spatial development of disturbances in the boundary layer are presented. It was found that distributed suction reduces by 10 times the intensity of natural disturbances of the boundary layer and by 20 times the intensity of artificial disturbances generated by an external acoustic field. A spectral analysis of disturbances showed that suction reduces the intensity of high-frequency fluctuations for both natural and forced disturbances. It was found that the distributed suction affects the average flow – when the suction is on, the separation of the boundary layer near the trailing edge of the wing is eliminated. It was found that distributed suction significantly affects the mean flow, up to eliminating the boundary-layer separation near the trailing edge of the wing.

Effect of Flap on the Wind Turbine-Concentrator Combination

Wind energy is a renewable energy resource that can be used to ensure energy sustainability without causing environmental pollution. In this content, many studies are going on to acquire more energy from the wind. One of the methods to increase the energy production rate of the wind turbine is increase the mass flow rate which moves through the turbine working section. This can be achieved by equipping the turbine with a concentrator. The purpose of this study is to optimize the concentrator with flap geometry leading maximum flow speed increase at the turbine zone and compare it single airfoil shape concentrator. The actuator porous disc model is used to represent wind turbine in the concentrator. Response Surface Method is used as optimization methods with 2D CFD analyses. Concentrator with flap increases the flow speed by a factor 1.2 while single airfoil section increases by a factor 1.1.

Novel Cyclic Blade Pitching Mechanism for Wind and Tidal Energy Turbine Applications

A vertical axis drag-based turbine is proposed that allows for an improved performance by feathering its blades during recovery strokes to eliminate adverse blade forces. The turbine blades resemble flat plates and pitch by 90 ∘ between the two turbine strokes using a novel dual-cam mechanism. This passive mechanism orients the blades vertically during the drive stroke for maximum effective area and horizontally for minimum effective area during the recovery stroke. This allows maximizing the positive drive stroke force and minimizing the recovery stroke losses, in turn maximizing the net energy capture and the turbine performance. It is called the cyclic pitch turbine, and a mathematical model is developed that predicts the turbine performance. It shows that the turbine is self-starting for all orientations and has a higher and more uniform static torque coefficient than the popular Savonius turbine. The dynamic analysis also indicates a higher performance, and the predicted values for torque and power coefficients match very closely with those from water channel and wind tunnel experiments on a prototype. Results of testing several blade shapes indicate that airfoil section blades with long and narrow continuous shapes that have less area towards the blade’s tip result in higher performance.

Experimental Analysis of a NACA 0021 Airfoil Section Through 180-Deg Angle of Attack at Low Reynolds Numbers for Use in Wind Turbine Analysis

Wind turbine industry has a special need for accurate post stall airfoil data. While literature often covers incidence ranges [−10 deg, +25 deg], smaller machines experience a range of up to 90 deg for horizontal axis and up to 360 deg for vertical axis wind turbines (VAWTs). The post stall data of airfoils is crucial to improve the prediction of the start-up behavior as well as the performance at low tip speed ratios. The present paper analyzes and discusses the performance of the symmetrical NACA 0021 airfoil at three Reynolds numbers (Re = 100 k, 140 k, and 180 k) through 180 deg incidence. The typical problem of blockage within a wind tunnel was avoided using an open test section. The experiments were conducted in terms of surface pressure distribution over the airfoil for a tripped and a baseline configuration. The pressure was used to gain lift, pressure drag, moment data. Further investigations with positive and negative pitching revealed a second hysteresis loop in the deep post stall region resulting in a difference of 0.2 in moment coefficient and 0.5 in lift.