Airfoil Section Research Articles

Dynamic Stall Flow Structure and Forces on Symmetrical Airfoils at High Angles of Attack and Rotation Rates

This article describes a direct comparison between two symmetrical airfoils undergoing dynamic stall at high, unsteady reduced frequencies under otherwise identical conditions. Particle image velocimetry (PIV) was performed to distinguish the differences in flow structure between a NACA 0021 and a NACA 0012 airfoil undergoing dynamic stall. In addition, surface pressure measurements were performed to evaluate aerodynamic load and investigate the effect of laminar separation bubbles and vortex structures on the pressure fields surrounding the airfoils. Airfoil geometry is shown to have a significant effect on flow structure development and boundary layer separation, with separation occurring earlier for thinner airfoil sections undergoing constant pitch-rate motion. Inertial forces were identified to have a considerable impact on the overall force generation with increasing rotation rate. Force oscillation was observed to correlate with multiple vortex structures shedding at the trailing-edge during high rotation rates. The presence of laminar separation bubbles on the upper and lower surfaces was shown to dramatically influence the steady-state lift of both airfoils. Poststall characteristics are shown to be independent of airfoil geometry such that periodic vortex shedding was observed for all cases. However, the onset of deep stall is delayed with increased nondimensional pitch rate due to the delay in initial dynamic-stall vortex.

Quantitative study of localized mechanisms of excitation of cross-flow instability modes in a swept-wing boundary layer

An experimental study of two efficient receptivity mechanisms of excitation of cross-flow (CF) instability modes is carried out in a boundary layer of a real airfoil section of a swept wing due to: (i) action of localized surface vibrations, and (ii) scattering of 2D freestream vortices on them. It is found that the two mechanisms lead to rather efficient excitation of CF-modes both at surface vibration frequency and at combination ‘vortex-vibration’ frequencies. First estimations of the corresponding localized receptivity coefficients are obtained. Direct comparison of the experimental amplification curves of the excited CF-modes with those calculated based on the linear stability theory (LST) has shown that the experimental data obtained at vibration frequency are in excellent agreement with the LST. At the same time, growth rates of the CF-modes excited at combination frequencies are found to be completely inconsistent with the LST. A possible explanation of this phenomenon via action of a new efficient distributed receptivity mechanism is suggested. This mechanism is associated with scattering of freestream vortices on rather high-amplitude CF-modes excited by surface vibrations.

Temperature effect on icing distribution near blade tip of large-scale horizontal-axis wind turbine by numerical simulation

To investigate the temperature effect on characteristics of icing distribution near the tip part of rotating blade of large-scale horizontal-axis wind turbine, numerical simulations were carried out on a horizontal-axis wind turbine rotor with rated power of 1.5 MW based on a quasi-three-dimensional computational method developed in this study. The icing simulation was focused on the part between blade tip and 30% length of blade from tip along span wise to blade root where the most serious icing area is according to the past researches. Eight sections along blade were selected, and the ice accretion on each airfoil was calculated. Eight temperature values from −6°C to −20°C were decided to investigate the effects of temperature on icing under the certain liquid water content and medium volume droplet. Three icing times were selected to research the ice accretion on blade surface with the increase in the time. According to the results, the icing distribution has the overall characteristics that the icing shape changes from horn icing shape to streamline icing shape with decrease in the temperature. The closer the blade airfoil section to blade tip, the more obvious the ice accretion is. This study can be a reference for the research on anti-icing and de-icing technologies for large-scale horizontal-axis wind turbine.

Performance of the NREL S826 airfoil at low to moderate Reynolds numbers—A reference experiment for CFD models

Abstract Lift, drag and surface pressure measurements are performed on a wing section of the NREL S826 wind turbine airfoil at eight Reynolds numbers ranging from 0 . 5 × 1 0 5 to 6 . 0 × 1 0 5 . Alongside with the measurements two types of Reynolds averaged Navier–Stokes (RANS) simulations are performed, one of which includes a laminar to turbulent transition model. The lift and drag characteristics are observed to be dominated by low Reynolds number effects for R e 0 . 7 × 1 0 5 , related to the presence of laminar separation bubbles (LSBs) on the suction side of the profile. For R e ≥ 0 . 7 × 1 0 5 the airfoil’s performance is rather independent of the Re-number for the present free stream turbulence intensities, while significantly higher peak lift is measured than in earlier experiments on the same airfoil. At high angles of attack, strong three-dimensional spanwise surface flow distribution reminiscent of a single stall cell is observed. The RANS simulations in a two-dimensional domain including the Langtry–Menter γ − R e θ transition model accurately predict lift and drag coefficients as long as the flow is fairly attached. Further, the γ − R e θ model simulations are observed to predict the location and average size of the LSBs in this region.

Numerical Investigation on Effect of Multiple Winglets for Wind Turbine Applications

The present article is an effort to examine the potential of multiple winglets to reduce the induced drag of the aerodynamic surface. The advantages of using multiple winglets include reduction of induced drag, increased L/D and improved performance of the Wind turbine. Computational Fluid Dynamics is utilized as to approach the effects of multiple winglets with NACA 24012 airfoil section for untwisted, rectangular wing. The testing of configurations is done at Reynolds number 290,000. FLUENT solver incorporated in ANSYS used for the numerical investigation of the steady flow over the wing. A substantial improvement in lift curve slope occurs with dihedral spread of the winglets. The dihedral spread also distributes the tip vortices.

Design and optimization of airfoils and a 20 kW wind turbine using multi-objective genetic algorithm and HARP_Opt code

Small wind turbines (SWTs) are ideal for supplying electricity to small remote communities that do not have grid access. However, literature review and trends point out that SWTs are far from fully developed. While larger wind turbines have been researched extensively and perfected, SWTs lack improvements in efficiency and capacity factor. In the present work, airfoil sections for a 20 kW wind turbine were generated using Multi-Objective Genetic Algorithm. The USP07-45XX family of airfoils was designed to achieve maximum lift-to-drag ratio from 4 to 10° angles of attack and to be insensitive to leading edge roughness. The USP07-45XX airfoils showed only slight change during clean and soiled conditions both in experiments and in numerical studies. The optimized airfoils were used in the design of a 20 kW wind turbine. The turbine design and optimization code developed by National Renewable Energy Laboratory (NREL) – HARP_Opt – was used to design and optimize the 20 kW turbine. The turbine was designed using soiled airfoil characteristics of the USP07-45XX family of airfoils. Power curves show cut in speed of 2 m/s and a rated speed of 9 m/s. This gives an annual energy production (AEP) of 4.787 × 104 kWh while having leading edge soiling on blades. The high resistance of the airfoil to soiling means that the AEP will not vary from its design value due to the turbine blades getting dirty or soiled.

Experimental investigation of propeller slipstream effects on the wing aerodynamics and boundary layer treatment at low Reynolds number

In this research, an experimental investigation was conducted to predict the Laminar-turbulent transition over the wing surface. Furthermore, the effects of a tractor propeller slipstream on both wing aerodynamics and transition front were studied. For tests, a rectangular wing was used with a NACA 6-series airfoil section and with a total of 22 pressure orifices. Unsteady pressure measurements were performed over the upper and lower surfaces of the wing in different spanwise locations at different incidence angles. Existence of propeller slipstream changed pressure distribution over the wing surfaces, in both chordwise and spanwise directions and hence affected the wing loading distribution. Statistical analysis of pressure signals was used to predict the boundary layer transition over the wing by computing the root mean square and skewness of the pressure data. The results showed that the transition location moves toward the leading edge due to propeller slipstream. Increase in propeller rotational speed causes that the turbulent flow covers whole portion of the wing surface.

Flow modulation by a mushroom-like coating around the separation region of a wind-turbine airfoil section

The flow over a mushroom-shaped microscale coating was experimentally inspected over a diverging channel that followed the pressure side of a wind turbine blade (S835). High-resolution particle image velocimetry was used to obtain in-plane velocity measurements in a refractive-index-matching flume at Reynolds number Reθ ≈ 1200 based on the momentum thickness. The results show that the evolution of the boundary layer thickness, displacement thickness, and shape factor change with the coating, contrary to the expected behavior of an adverse pressure gradient boundary layer over a canonical rough surface. Comparison of the flow with that over a smooth wall revealed that the turbulence production exhibited similar levels in both cases, suggesting that the coating does not behave like a typical rough wall, which increases the Reynolds stresses. Proper orthogonal decomposition was used to decompose the velocity field to investigate the possible structural changes introduced by the wall region. It suggests that large-scale motions in the wall region lead to high-momentum flow over the coated case compared to the smooth counterpart. This unique behavior of this surface coating can be useful in wind-turbine applications, with great potential to increase the power production.

Sensitivity of wind turbine airfoil sections to geometry variations inherent in modular blades

In the ongoing work to increase the efficiency of large-scale, horizontal-axis wind turbines, the modular blade concept has been proposed. This article works toward quantifying the aerodynamic performance of modular blades, whose baseline profile remains unchanged from conventional blades but which are susceptible to a larger degree of variation in both manufacturing tolerances and fabrication materials, by examining the effects of offsets in the leading edge and the use of a tensioned membrane-fabric as a flow surface over the aft. Wind tunnel tests were performed on modified DU00-W-212 and DU91-W2-250 sections and included standard pressure measurements, as well as surface deflection measurements of the fabric. For the case of offsets near the nose of the profile, the offsets produce up to 17% decrease in [Formula: see text]. For the case of fabric in the aft of the model, the [Formula: see text] variations are strongly dependent on the spanwise location of the measurement.

In-blade Load Sensing on 3D Printed Wind Turbine Blades Using Trailing Edge Flaps

As wind turbines become larger the loading on the blades also increases. Controlling a section of the trailing edge of the turbine airfoil is found to reduce load fluctuations on wind turbine blades. Here a detailed experimental setup is described showing the development of a compact airfoil section capable of measuring the surface pressure, root bending moment, and controlling a TEF simultaneously and in time resolved fashion to quantity the influence of a TEF on a wind turbine. This experimental work includes a trailing edge flap that covers 20% of an S833 airfoil with a chord of 178 mm. Surface pressure and blade root strain are measured for varying angles of attack and flap angle. Coefficient of lift and moment are obtained from the 54 pressure taps. The lift and drag forces are also obtained from the strain gages at the root of the blade. A 3D printed blade section is designed and built to house the actuation and sensing on the airfoil. The trailing edge flap was tested inside a 0.61 m wind tunnel as a baseline case and the results showed how the lift, drag, and root moment on the airfoil can change for different flap angles. The coefficient of lift changed by 30% for flap angle of 20°. The entire blade with the flap will also be installed on a 3.4 diameter wind turbine to study the influence of a flap on load variation.

A simplified shape optimization strategy for blended-wing-body underwater gliders

This paper presents a simplified shape optimization (SSO) strategy for blended-wing-body underwater gliders (BWBUGs). In this strategy, the 3-D shape optimization of the BWBUG is approximately simplified into three 2-D airfoil design problems. The three crucial sectional airfoils are firstly selected from the BWBUG and the Class-Shape function Transformation method is used for parameterization of the airfoils. Then, the airfoils are optimized using a kriging-based constrained global optimization (KCGO) method and the optimized BWBUG shape is finally formed with the optimized airfoils. To verify the practicability of the proposed SSO strategy, the shape of the BWBUG is optimized using a direct shape optimization (DSO) strategy as well. Results show that the lift-to-drag ratio of the BWBUG is improved by 4.1728% and 4.4328% with SSO and DSO strategies respectively, which shows that the SSO is comparable with the DSO strategy in the optimization results. However, the CPU time involved in SSO is only about 16% of that in DSO, which illustrates that SSO is an applicable and more efficient strategy for shape design optimization of BWBUGs.

Structural optimization of wind turbine blades with ring shear webs

The aim of this paper is to present an optimized structure for the design of wind turbine blades. It sets up a forced blade model with ring shear webs and deduces the equations for the inertia moment Iy of airfoil section, the maximum stress σmax and the compression ΔD of airfoil relative thickness. Then, the model and the equations are applied to one 750 kw wind turbine. By MATLAB software, the changing trends of the inertia moment Iy, the maximum stress σmax, and the compression ΔD, respectively, along with the ratio e of the inner chord length to the outer and the blade span x are gained. It is concluded that the bending strength around the blade root increases greatly. It also indicates that the bending strength of blades with ring shear webs is almost the same as the blades with solid shear webs when the ratio e is set as 0.4, and also the use of ring shear webs in wind turbine blade structure leads to lighter blades.

Effect of corrugated skins on the aerodynamic performance of the cambered airfoils

PurposeThis paper aims to study the effect of corrugated skins on the aerodynamic performance of the cambered NACA 0012 airfoils at different corrugations parameters, maximum cambers, Reynolds numbers and maximum camber locations.Design/methodology/approachIn this work, numerical approach is concerned, and results are obtained based on the finite volume approach. To characterize the effect of corrugated skins, the NACA 0012-corrugated airfoil section is chosen as the base airfoil, and different cambered corrugated airfoil sections are obtained by inclusion the camber to the base airfoil. In this research, the corrugation shape is a sinusoidal wave and corrugated skins are in the aft 30 per cent of airfoil chord. To investigate the effect of corrugations on the cambered sections, the drag coefficient and averaged lift curve slope for the corrugated airfoils are compared to those of the corresponding smooth sections.FindingsResults indicate that the effect of increase in the maximum camber and also Reynolds number on the relative zero-incidence drag coefficient is of little importance at low corrugation amplitudes, whereas at high corrugation, amplitude results in different behaviors. It is found that as the maximum camber increases, the deterioration in the relative curve slope introduced by corrugated skins is reduced, and reduction in this deterioration is significant for high corrugation amplitudes airfoils. It is shown that an increase in the maximum camber location has nearly no effect on the relative zero-incidence drag coefficient and also relative lift curve slope.Originality/valueThe outcome of the present research provides the clues for better understanding of the effect of different corrugations parameters on the aerodynamic performance of the unmanned air vehicles to have as high aerodynamic performance as possible in different mission profiles of such vehicles.

Wind turbine power improvement utilizing passive flow control with microtab

In this study, the effect of deploying microtabs on performance improvement of a horizontal axis wind turbine blade is numerically investigated in three-dimensions. The NREL Phase VI, a stall-regulated upwind wind turbine, is used as the baseline case. Different cases are considered to investigate the effects of spanwise location as well as the height variation of tabs along the blade span, on the flow over the rotor blade. In all cases, the tab is located at 95% chord of the airfoil section on the lower surface of the blade. Results reveal that locating microtabs at the outboard part of the blade has a greater impact on the rotor performance than the inboard part. However, both cases improve the output power in comparison with the baseline blade. Furthermore, due to the variations in Reynolds number and local angle of attack along the blade span, the influence of microtab height profile on output power is explored in details. In the best case, 17% of the wasted wind energy in the baseline blade is saved for the sub-rated wind speeds.

Aerodynamics and structural analysis of wind turbine blade

The ultimate objective of the paper is to increase the reliability of wind turbine blades through the development of the airfoil structure, to calculate an optimum blade shape for the procedure begins with the choice of airfoils characteristics. Then an initial wind blade design is determined using blade element momentum. The blade plays a pivotal role, because it is the most important part of the energy absorption system. Practical horizontal axis wind turbine (HAWT) designs use airfoils to transform the kinetic energy in the wind into useful energy and it has to be designed carefully to enable to absorb energy with its greatest efficiency. There are many factors for selecting a profile. One significant factor is the chord length and twist angle which depend on various values throughout the blade. In this work, the airfoil sections used in horizontal axis wind turbine (HAWT) are S818; S825 and S826 airfoils used in NREL phase 2 and phase 3 wind turbines. They have several advantages in meeting the intrinsic requirements for wind turbines in terms of design point, off-design capabilities and structural properties. The lift and drag coefficients data for these airfoils sections are available and Matlab code were used to obtain the coordinates of a wind turbine blade. Aerodynamic and static structural analyses are presented. The commercially available software FLUENT is employed for calculation of the flow field using the Reynolds-averaged Navier Stokes (RANS) in conjunction with the k-omega shear stress transport (SST), based on finite-element method (FEM). Both grid and time step were optimized to reach independent solutions. The present work represents the basis to develop an accurate three dimensional Horizontal-Axis Wind Turbine (HAWT) model and may be used to support wind tunnel experiments.

A new 360° airfoil model for predicting airfoil thrust potential in vertical-axis wind turbine designs

Vertical-axis wind turbine (VAWT) configurations expose the airfoil sections of turbine blades to angles of attack between 0° and 360°. In designing new VAWT configurations or improving existing configurations, some knowledge of the aerodynamic forces at these angles must be known. This paper presents a model for predicting 360° of aerodynamic forces acting on the s1210 airfoil at two low Reynolds-number conditions. This model is based upon results from wind-tunnel experimentation and achieves a close approximation of the measured performance of the airfoil. Models such as this provide the means to predict airfoil lift and drag characteristics and use those results to predict the thrust capabilities of the airfoil as the turbine blades rotate. The model is presented and compared with other post-stall models within the literature. These comparisons show that this model is superior to previous models when applied to the s1210. The model shows that two angles of attack regions between 0 and 30° and then again those between 180° and 300° provide the highest thrust contributions of the airfoil. Using models such as these for system-level analysis provides the means to assess the feasibility of different VAWT configurations. Furthermore, it provides the means to assess the proper spacing and distance between turbine blades to generate the desired thrust. This enables the designer to determine the optimum turbine geometry in order to maximize performance.

Computational fluid dynamics investigation into flow behavior and acoustic mechanisms at the trailing edge of an airfoil

Airfoil self-noise or trailing edge noise and shear noise were investigated computationally for a NACA 0012 airfoil section, focusing on noise mechanisms at the trailing edge to identify and understand sources of noise production using ANSYS Fluent. A two-dimensional computational fluid dynamics simulation has been performed for 0°, 8°, and 16° airfoil angles of attack capturing surface pressure contours, contours of turbulent intensity, contours of surface acoustic power level, vorticity magnitude levels across the airfoil profile, and x- and y-directional self-noise and shear noise sources across the airfoil profile. The results indicate that pressure gradients at the upper surface do increase as the angle of attack increases, which is a measure of vortices near the surface of the trailing edge associated with turbulence cease as the boundary layer begins to separate. Comparison of the turbulent intensity contours with surface acoustic power level contours demonstrated direct correlation between the energy contributed by turbulent structures (i.e. vortices) and the level of noise measured at the surface and within the boundary layer of the airfoil. As angle of attack is increased, both x and y sources have the same trends; however, y sources (perpendicular to the free-stream flow) appear to have a bigger impact as angle of attack is increased. Furthermore, as the angle of attack increased, shear noise contributes less and less energy further downstream of the airfoil and becomes dominated by noise energy from vortical structures within turbulence. The two-dimensional computational fluid dynamics simulation revealed that pressure, turbulent intensity, and surface acoustic power contours further corroborated the previously tested noise observations phenomena at the trailing edge of the airfoil.

Comprehensive Analysis of a Coaxial Ultralight Rotorcraft and Validation with Full-Scale Flight-Test Data

The results of full-scale flight testing in combination with comprehensive analysis of an ultralight counterrotating coaxial rotorcraft are presented. The CoAX 2D was developed and manufactured by edm aerotec in Geisleden, Germany. The helicopter features two-bladed, 6.5-m-diameter upper and lower rotors in teeter configuration with uniform airfoil sections and linearly twisted blades. The focus was on the development of a comprehensive analysis model using CAMRAD II, its validation with measured flight-test data, and results evaluation from both measurements and analysis. The CoAX 2D's power consumption, tail loads, hub loads, teeter angles, and blade-tip clearance were evaluated for both hover and forward flight. Performance and thrust correlated well between the analysis and the measurements. Effects from rotor-fuselage interference dominated the measured 2/rev hub load and 1/rev teeter angle harmonics. The numerical model also matched well with the measured vibrations during forward flight in terms of hub loads, teeter angles, and resulting blade-tip clearance. Minimal rotor-rotor blade-tip clearance occurred at low flight speeds because of dominating effects from rotor-rotor interference.

Influence of lateral distance and phase difference on energy absorption performance of undulating airfoils in a side-by-side arrangement

It is recognized that traveling wave motion can extract energy from moving water or wind under certain motion parameters. The NACA0012 airfoil section is used as a two-dimensional simplified model of the fishlike body in this paper. We numerically investigated the influence of lateral distance and phase difference on energy absorption performance of two undulating airfoils in a side-by-side arrangement. The findings show that: within a certain range, the energy absorption efficiency of the side-by-side undulating airfoils is higher than the data of a single airfoil. When the phase difference is 0°, the energy absorption efficiency of undulating airfoils increases first and then decreases with the increasing lateral distance, existing a maximum efficiency. When the lateral distance is constant, as the phase difference varies, the curve of absorption efficiency almost takes on an antisymmetric form with the period of 360°. As the phase difference increases, one of the airfoil’s efficiency decreases first, then increases and finally decreases, while the other one’s efficiency increases first, then decreases and finally increases.

Rotor airfoil profile optimization for alleviating dynamic stall characteristics

In order to alleviate the dynamic stall characteristics of rotor airfoil, a high-efficient optimization method is established by coupling the SQP method with CFD method. Aiming at improving the optimized efficiency, a new strategy of linear search is established to avoid the disturbance of the penalty factors, and the optimal efficiency improves more than 30% compared with traditional line search method. In order to reduce the design variables, the CST method is employed to parameterize the airfoil section. All the flow solutions were computed by solving the URANS equations, coupled with the Spalart–Allmaras turbulence model. To reduce pitching moment and drag coefficient under dynamic stall condition, a new airfoil has been optimized based on the OA209 airfoil by employing the present optimal method. Due to the inhibition of leading edge vortex, the peaks of pitching moment and drag coefficient of the optimized airfoil are decreased about 75.8% and 44.2% at point 1, 44.0% and 41.4% at point 2 compared with the OA209 airfoil. Moreover, the optimized airfoil has better lift stall characteristics compared with the OA209 airfoil. In addition, by comparing the 3-D dynamic stall characteristics of rotor blade, it is demonstrated that the peaks of pitching moment coefficients and drag coefficients of optimized airfoil are smaller than those of OA209 airfoil at different blade radial stations.