Blade Lift Research Articles

INNOVATIVE DESIGN SOLUTION FOR QUICK-HEATING ACTIVE THERMAL TARGET FOR NIGHT SHOOTING

The paper presents a concept of a heated active thermal target designed for training and shooting with thermal sights at night conditions. The innovative solution contains a control system and a target with specially matched parameters providing several times greater power at the beginning of heating than the power needed to maintain a demanded difference of temperatures in the steady state. After reaching the required temperature, the control system does not give full voltage to the target. The times of voltage supply and its switching off are so selected that the temperature of the target corresponds to the required value. The time needed to obtain the required temperature in the proposed solution is several times shorter than in the currently used configurations. This allows the heater to be activated at the same time as the blade lift is activated. The solution eliminates disadvantages of heated targets used for shooting up to now.

Method for eliminating aerodynamic lift vibration of rigid rotor helicopters based on the novel sine-trim model

A novel methodology called sine-trim is proposed from the phasor superposition principle for AC voltages in electrical engineering to eliminate the aerodynamic lift vibration of rigid rotor helicopters. An advancing blade concept rotor aerodynamic analysis model is presented on the basis of free-wake method associated with the inner- and outer-trim modules. The inner-trim module transforms the blade lift distribution curve of a rigid lift-offset rotor into a similar sinusoidal one by adjusting the blade pitch in each azimuth. An acceleration algorithm that could greatly improve the inner-trim calculation efficiency is provided. The outer-trim module selects the amplitude of the lower rotor as the preset trim value, and it is reliable and effective in trimming the overall forces and moments of the helicopter by modifying the lift sinusoid curve parameters of the upper and lower rotors. In the sine-trim, the upper and lower rotors have similar sinusoidal lift distribution parameters, and their phases and amplitudes exhibit a direct inverse relationship. Calculation results show that after being sine-trimmed, the total aerodynamic lift vibration of the rigid rotors could be eliminated regardless of the change in flight and operating conditions, such as flight speed, number of blades, and rotor rotational speed. The optimal blade pitch control distribution, which is a comprehensive superposition of many higher harmonic inputs, is obtained in all azimuths. For the same lift-offset, after sine-trimming, the effective lift-to-drag ratio may have a small degree of loss compared with the conventional advancing blade concept rotor.

Active flow control for power enhancement of vertical axis wind turbines: Leading-edge slot suction

Vertical axis wind turbines (VAWTs) suffer from a poor power performance at low tip speed ratios, where their blade aerodynamics are dominated by unsteady separation and dynamic stall. Therefore, to enhance their aerodynamic performance, separation control is highly desired. The present study intends to suppress the flow separation on VAWTs using boundary layer suction through a slot located near the blade leading edge. High-fidelity computational fluid dynamics simulations extensively validated with experiments are employed. A characterization of the impact of the suction amplitude, 0.5% ≤ AS ≤ 10%, and the suction location, 8.5 ≤ XS/c ≤ 28.5, is performed. The dependency of the obtained power gain on operating conditions, i.e. tip speed ratio, 2.5 ≤ λ ≤ 3.5, Reynolds number, 0.51 × 105 ≤ Rec ≤ 2.78 × 105, and turbulence intensity, 1% ≤ TI ≤ 25%, is studied. The results show that applying suction along the chordwise extent of the laminar separation bubble (LSB) can prevent its bursting, eliminate/postpone its formation, avoid the formation of the dynamic stall vortex and trailing-edge roll-up vortex, and delay the incipient trailing-edge separation. This will significantly increase the blade lift force, decrease the drag force, delay the stall angle and suppress the aerodynamic load fluctuations. For the reference turbine and for AS = 0.5% and XS/c = 8.5%, the power coefficient at λ of 2.5, 3.0 and 3.5 is enhanced by 247%, 83% and 24%, respectively. The suction location is critical while a minimum amplitude, e.g. AS = 0.5%, suffices. The optimal suction location is insensitive to TI, weakly sensitive to λ while comparatively more sensitive to Rec.

Numerical analysis of VAWT wind turbines: Joukowski vs classical NACA rotor’s blades

Abstract As wind energy technology is growing rapidly in the energy production sector, Vertical Axis Wind Turbines (VAWTs) are coming to the fore to tackle areas with low wind speeds. The choice of optimum configuration of these VAWTs can not only maximize their performance but is a deterministic feasibility factor for their deployment. In this study, the Joukowski airfoil is reconsidered as potential rotor airfoil for VAWT. Its performance is compared against classical symmetrical NACA0012 and cambered NACA4312 blades. The investigation is carried out using high resolution CFD modelling. Results reveals the higher performance of the Joukowski airfoil over the classical symmetrical and cambered NACA. This dominance assessed by the attained higher blade lift as well as higher torque and power coefficient almost over the entire operable tip speed ratio. It was also found that cambered blades, improve the overall performance of the VAWT compared to the symmetrical baseline with a better power coefficient, at a lower tip speed ratio. In general, the rotor develops more consistent and homogenized torque at a higher tip speed ratio. The attained improvement in the torque using Joukowski however negatively affected the self-starting ability of the VAWT.

Symmetric Versus Asymmetric Pitching of a Cycloidal Rotor Blade at Ultra-Low Reynolds Numbers

This paper provides a fundamental understanding of the unsteady aerodynamic phenomena on a cycloidal rotor blade operating with symmetric as well as asymmetric pitching at ultralow Reynolds numbers () by using a combination of experimental (force and flowfield measurements) and computational fluid dynamics (CFD) studies. The instantaneous blade fluid dynamic forces on a rotating cycloidal rotor blade were measured, which, along with particle image velocimetry (PIV) based flowfield measurements revealed the key fluid dynamic mechanisms acting on the blade. A two-dimensional CFD analysis of the cycloidal rotor was developed and systematically validated using both force and flowfield measurements. The CFD flow solution and PIV-measured flowfield correlated well, especially in the upper half of the circular blade trajectory, and both showed the formation and shedding of strong dynamic stall or leading-edge vortices at high pitch amplitudes, which is the reason for the stall delay and force enhancement. The dynamic virtual camber induced by the flow curvature decreased the blade lift in the upper half due to negative induced camber; however, the lift increased in the lower half due to positive camber. Therefore, introducing an asymmetry in pitch kinematics with higher pitch in the upper half and lower pitch in the bottom half counteracted this inherent lift asymmetry and improved efficiency through a more uniform lift production.

Force and flowfield measurements to understand unsteady aerodynamics of cycloidal rotors in hover at ultra-low Reynolds numbers

This paper provides a fundamental understanding of the unsteady fluid-dynamic phenomena on a cycloidal rotor blade operating at ultra-low Reynolds numbers (Re ∼ 18,000) by utilizing a combination of instantaneous blade force and flowfield measurements. The dynamic blade force coefficients were almost double the static ones, indicating the role of dynamic stall. For the dynamic case, the blade lift monotonically increased up to ±45° pitch amplitude; however, for the static case, the flow separated from the leading edge after around 15° with a large laminar separation bubble. There was significant asymmetry in the lift and drag coefficients between the upper and lower halves of the trajectory due to the flow curvature effects (virtual camber). The particle image velocimetry measured flowfield showed the dynamic stall process during the upper half to be significantly different from the lower half because of the reversal of dynamic virtual camber. Even at such low Reynolds numbers, the pressure forces, as opposed to viscous forces, were found to be dominant on the cyclorotor blade. The power required for rotation (rather than pitching power) dominated the total blade power.

A Crane Overload Protection Controller for Blade Lifting Operation Based on Model Predictive Control

Lifting is a frequently used offshore operation. In this paper, a nonlinear model predictive control (NMPC) scheme is proposed to overcome the sudden peak tension and snap loads in the lifting wires caused by lifting speed changes in a wind turbine blade lifting operation. The objectives are to improve installation efficiency and ensure operational safety. A simplified three-dimensional crane-wire-blade model is adopted to design the optimal control algorithm. A crane winch servo motor is controlled by the NMPC controller. The direct multiple shooting approach is applied to solve the nonlinear programming problem. High-fidelity simulations of the lifting operations are implemented based on a turbulent wind field with the MarIn and CaSADi toolkit in MATLAB. By well-tuned weighting matrices, the NMPC controller is capable of preventing snap loads and axial peak tension, while ensuring efficient lifting operation. The performance is verified through a sensitivity study, compared with a typical PD controller.

Lift enhancement and drag reduction of lifting blades through the use of end-plates and detached end-plates

Three-dimensional steady-state RANS simulations of the flow around a stationary finite-span blade have been carried out. The analysis is directed toward the vorticity dynamics, the forces acting on the blade and their spanwise distributions when using end-plates or detached end-plates, namely end-plates that are not in contact with the blade tips. The objective is to understand the physics at play in the vicinity of the blade tips with an emphasis on the detached end-plates, as the use of such devices could be beneficial for optimizing the performance of vertical-axis turbines. This paper also aims at providing simple guidelines regarding the use of end plates and detached end-plates. The results show that the presence of detached end-plates leads to an important increase of the blade lift coefficient in addition to a significant reduction of its drag coefficient. This is explained by the fact that the detached end-plates modify the path of the vorticity lines shed by the blade, thus affecting the vorticity distribution and the resulting circulation along the blade. This work also shows that the forces on the blade are more sensitive to the dimensions of the detached end-plates in the transverse direction than to any other geometric parameter.

Integration of Optimum Power for Wind Turbine Blade at Different Cross Section

This research analysis and optimizes the main wind horizontal turbine blade parameters for high-performance altitude with variable pitch blade angle for different blade cross-section unsymmetrical airfoil NACA 4412 and unsymmetrical airfoil supercritical Eppler 417. For deep specification, some wind horizontal turbine parameters kept constant through the proses method to integrate the highest behavior of windmill turbine power coefficient. The procedure analysis with FORTRAN.90 code ,then compare with German code and then optimized using Schmitz and Betz method for blade chord and lift to drag for blade pitch angle. From theoretical results discussion, important conclusions figured; also a recommendation for further work was suggested. Best optimization methods were Schmitz chord optimization and Lift/Drag twist optimization which increases the Cp 10.3% for Eppler 4417 and 9.5% for NACA 4412.All results were tabulated and plotted for all optimization results

High blade lift generation through short rotor–stator axial spacing in a tiny pump

Short rotor–stator axial spacing has been demonstrated to be beneficial to the aerodynamic performance of subsonic compressors. In this paper, it is further verified that this benefit partly originates from the improvement of the rotor blade lift. A tiny axial pump is investigated both numerically and experimentally with three different rotor–stator axial spacings. The time-accurate simulation indicates that both the peak and average rotor blade lifts are enhanced as the axial spacing decreases. The potential field of the downstream stator has significant effects on the pressure distribution in the rotor when the blade rows get close. The two-dimensional (2D) PIV measurement demonstrated that short axial spacing generates high circulation around the rotor blade due to the flow tube compression, which contributes to the blade lift improvement. Besides, close blade-rows axial spacing generates enhanced unsteadiness for the inlet flows of the downstream stator, which should be taken into consideration in the design process.

Optimization and Validation of Cycloturbine Blade-Pitching Kinematics via Flux-Line Theory

The newly developed flux-line theory identifies the maximum power harvested by a cycloturbine and the associated fluid–turbine interaction pattern. This work designs blade-pitching functions that maximize turbine power by identifying the blade lift coefficient functions required to optimally decelerate the flow, relating these waveforms to blade angle of attack functions through a novel semi-empirical curvilinear flow aerodynamics model, and finally computing optimal blade-pitch motions with freestream flow data from flux-line theory. At low and moderate tip speed ratios, the blades stall before achieving the required deceleration force. In those cases, more wind power is extracted by braking the flow through both the upstream and downstream portions of the cycloturbine. In a truck-mounted cycloturbine test, the flux-line optimal pitching kinematics outperformed sinusoidal and fixed-pitching kinematics. The turbine achieved a mean gross aerodynamic power coefficient of 0.44 (95% confidence interval: [0.388, 0.490]) and 0.52 (95% confidence interval: [0.426, 0.614]) at design tip speed ratios of 1.5 and 2.25, respectively, which exceeds all other low tip speed ratio vertical-axis wind turbines. Two-dimensional Reynolds-averaged Navier–Stokes simulations show that the optimal blade-pitching functions achieve high power coefficients by evenly extracting energy from the flow without blade stall or detached turbine wakes.

COMPUTATIONAL FLUID DYNAMICS STUDY OF DUSTY AIR FLOW OVER NACA 63415 AIRFOIL FOR WIND TURBINE APPLICATIONS

Gulf and South African countries have enormous potential for wind energy. However, the emergence of sand storms in this region postulates performance and reliability challenges on wind turbines. This study investigates the effects of debris flow on wind turbine blade performance. In this paper, two-dimensional incompressible Navier-Stokes equations and the transition SST turbulence model are used to analyze the aerodynamic performance of NACA 63415 airfoil under clean and sandy conditions. The numerical simulation of the airfoil under clean surface condition is performed at Reynolds number 460×103, and the numerical results have a good consistency with the experimental data. The Discrete Phase Model has been used to investigate the role sand particles play in the aerodynamic performance degradation. The pressure and lift coefficients of the airfoil have been computed under different sand particles flow rates. The performance of the airfoil under different angle of attacks has been studied. Results showed that the blade lift coefficient can deteriorate by 28% in conditions relevant to the Gulf and South African countries sand storms. As a result, the numerical simulation method has been verified to be economically available for accurate estimation of the sand particles effect on the wind turbine blades.

Performances and Stall Delays of Three Dimensional Wind Turbine Blade Plate-Models with Helicopter-Like Propeller Blade Tips

The research on three dimensional (3-D) wind turbine blades has been introduced (Sutrisno, Prajitno, Purnomo, & B.W. Setyawan, 2016). In the current experiment, the 3-D wind turbine blades would be fitted with helicopter-like blade tips and additional fins to the blade hubs to demonstrate some laminarizing features. It was found that additional helicopter-like blade tip to the turbine blade creates strong laminar flows over the surface of the blade tips. Supplementary, finned hub, fitted to the blade body, creates rolled-up vortex flows, weakens the blade stall growth development, especially for blades at high-speed wind. A proposed mathematical form of modified lifting line model has been developed to pursue further 3-d blade development study of 3-d wind turbine blade. Rolled up vortex effects, developed by finned of the base hub, has been acknowledged could demolish the turbulent region, as well as laminarize the stall domain to intensify the induced wind turbine blade lift.

Observations of the Growth and Decay of Stall Cells during Stall and Surge in an Axial Compressor

This research investigated unsteady events such as stall inception, stall-cell development, and surge. Stall is characterized by a decrease in overall pressure rise and nonaxisymmetric throughflow. Compressor stall can lead to surge which is characterized by quasi-axisymmetric fluctuations in mass flow and pressure. Unsteady measurements of the flow field around the compressor rotor are examined. During the stall inception process, initial disturbances were found within the rotor passage near the tip region. As the stall cell develops, blade lift and pressure ratio decrease within the stall cell and increase ahead of the stall cell. The stall inception event, stall-cell development, and stall recovery event were found to be nearly identical for stable rotating stall and surge cases. As the stall cell grows, the leading edge of the cell will rotate at a higher rate than the trailing edge in the rotor frame. The opposite occurs during stall recovery. The trailing edge of the stall cell will rotate at the approximate speed as the fully developed stall cell, while the leading edge decreases in rotational speed in the rotor frame.

Effect of turbulent inflows on airfoil performance for a Horizontal Axis Wind Turbine at low Reynolds numbers (Part II: Dynamic pressure measurement)

The present study aimed at highlighting the influence of turbulence on the dynamic stall characteristics of the Horizontal Axis Wind Turbine (HAWT) with wind tunnel experiments. UMY02-T01-26 airfoil that could efficiently perform at a high Reynolds number had been designed. The turbulence flow was generated by turbulence grids. To experimentally characterize these flows, HAWT blade surface pressures which were used to investigate the dynamic stall phenomenon were acquired by multiport pressure devices. The pressure distribution acting on the blade surface, lift coefficient and drag coefficient against the angle of attack were examined based on different turbulence intensities. For the angle of attack of α = 13°, the results indicated that in the increasing direction of the angle of attack, the lift coefficients of dynamic state showed larger values increase than those in the static state. At dynamic stall, in the case of Re = 1.5 × 105, the flow was separated at the leading edge of x/c > 0.1 in the increasing direction of the angle of attack. In the case of Re = 2.0 × 105, the range of the pressure hysteresis loop became narrower than that in the turbulence intensity of Re = 1.5 × 105.

Influence of wake dynamics on the performance and aeroelasticity of wind turbines

Wind turbines are currently a rapidly expanding form of renewable energy. However, there are numerous technological challenges that must be overcome before wind energy provides a significant amount of power in the United States. One of the primary challenges in wind turbine design and analysis is accurately accounting for the aerodynamic environment. This study is focused on a comprehensive verification and validation of the NREL FAST code, which is enhanced to include a free vortex wake model. The verification and validation is carried out through a comparison of blade lift distribution, wind turbine power and force and moment coefficients using a combination of CFD and experimental data. The results are also compared against Blade Element Momentum theory, and results from a 2001 double-blind NREL study on the prediction capabilities of wind turbine modeling tools. Results indicate that the enhanced aeroelastic code generally provides improved predictions. However, in several notable cases the predictions are only marginally improved, or even worse, than those generated using Blade Element Momentum theory aerodynamics. It is concluded that modeling of the aerodynamic environment remains incomplete, even after inclusion of wake effects. One important aspect identified is modeling of the unsteady aerodynamic lift characteristics of the rotor. Finally, the aeroelastic response in the combined presence of wake effects and inflow turbulence is examined. Significant differences are observed in loads, power, and structural response between results computed using the free wake model or simpler models, such as Blade Element Momentum theory.

An Experimental and Numerical Assessment of Airfoil Polars for Use in Darrieus Wind Turbines—Part I: Flow Curvature Effects

A better comprehension of the aerodynamic behavior of rotating airfoils in Darrieus vertical-axis wind turbines (VAWTs) is crucial both for the further development of these machines and for improvement of conventional design tools based on zero- or one-dimensional models (e.g., blade element momentum (BEM) models). When smaller rotors are designed with high chord-to-radius (c/R) ratios so as not to limit the blade Reynolds number, the performance of turbine blades has been suggested to be heavily impacted by a virtual camber effect imparted on the blades by the curvilinear flow they experience. To assess the impact of this virtual camber effect on blade and turbine performance, a standard NACA 0018 airfoil and a NACA 0018 conformally transformed such that the airfoil's chord line follows the arc of a circle, where the ratio of the airfoil's chord to the circle's radius is 0.25 were considered. For both airfoils, wind tunnel tests were carried out to assess their aerodynamic lift and drag coefficients for Reynolds numbers of interest for Darrieus VAWTs. Unsteady computational fluid dynamics (CFD) calculations have been then carried out to obtain curvilinear flow performance data for the same airfoils mounted on a Darrieus rotor with a c/R of 0.25. The blade incidence and lift and drag forces were extracted from the CFD output using a novel incidence angle deduction technique. According to virtual camber theory, the transformed airfoil in this curvilinear flow should be equivalent to the NACA 0018 in rectilinear flow, while the NACA0018 should be equivalent to the inverted transformed airfoil in rectilinear flow. Comparisons were made between these airfoil pairings using the CFD output and the rectilinear performance data obtained from the wind tunnel tests and xfoil output in the form of pressure distributions and lift and drag polars. Blade torque coefficients and turbine power coefficient are also presented for the CFD VAWT using both blade profiles.

Innovative Solution of Vertical Axis Wind Turbine, Suitable for Naval Industry Implementation (Numerical Methods and Analythical Calculus)

In this paper it is continued an analysis of blade design solutions for small power Vertical Axis Wind Turbines (VAWTs), investigating the best solution for a new design that has as objective the minimisation of vibrations, loads, and augmentation of power. It is investigated a classical Darrieus rotor with a NACA0018 blade profile, and then, it is studied the influence of blade flaps on the performance of the aerodynamic profile, aiming to improve the blade lift curve and the C L /C D ratio. It is concluded that the flaps are proven to be an optimized solution in view of aerodynamic performance of the profile, but only up to a certain angle, and thus, it encourages the further studies regarding their feasibility and their effect on the turbine performance.

Influence of Propeller Blade Lift Distribution on Whirl Flutter Stability Characteristics

Influence of Propeller Blade Lift Distribution on Whirl Flutter Stability Characteristics

Measuring the surface tension of yield stress fluids

With the aim of studying the impact of capillary forces on the flow of yield stress fluids we investigate the properties of a film formed by withdrawing a blade from a bath of such a material. We show that before a progressive breakage of the film, the force amplitude reaches a maximum which is independent of the initial depth of penetration and the timing for blade lifting, but increases with the material yield stress and the blade thickness. This critical force is shown to reflect both capillary and viscous effects, even at vanishing blade velocity. We demonstrate that the ratio of this force to the blade perimeter provides the surface tension of the yield stress fluid in the limit of a low (≪1) capillary number (ratio of yield stress times the blade thickness to surface tension). Moreover we show that all our data for the force to perimeter ratio fall along a master curve which may be used to deduce the surface tension from measurements obtained at a capillary number up to 1, even if viscous effects are significant. Finally Carbopol gels appear to have almost the same value of surface tension whatever their yield stress, but this value is almost 10% smaller than that of pure water.