Blade Tip Speed Research Articles

A Model for Performance Estimation of Flapping Foil Operating as Biomimetic Stream Energy Device

During the recent period intensive research has focused on the advancement of engineering and technology aspects concerning the development and optimization of wave and current energy converters driven by the need to increase the percentage of marine renewable sources in the energy-production mix, particularly from offshore installations. Most stream energy-harvesting devices are based on hydro-turbines, and their performance is dependent on the ratio of the blade-tip speed to incident-flow speed. As the oncoming speed of natural-occurring currents varies randomly, there is a penalty for the latter device’s performance when operating at non-constant tip-speed ratio away from the design value. Unlike conventional turbines that are characterized by a single degree of freedom rotating around an axis, a novel concept is examined concerning hydrokinetic energy converters based on oscillating hydrofoils. The biomimetic device includes a rotating, vertically mounted, biomimetic wing, supported by an arm linked at a pivot point on the mid-chord. Activated by a controllable self-pitching motion the system performs angular oscillations around the vertical axis in incoming flow. In this work, the performance of the above flapping-foil, biomimetic flow energy harvester is investigated by application of a semi-3D model based on unsteady hydrofoil theory and the results are verified by comparison to experimental data and a 3D boundary element method based on vortex rings. By systematical application of the model the power extraction and efficiency of the system is presented for various cases including different geometric, mechanical, and kinematic parameters, and the optimal performance of the system is determined.

Uncertainty quantification on the effects of rain-induced erosion on annual energy production and performance of a Multi-MW wind turbine

Wind turbine blade erosion has risen to the attention of researchers and industry lately in an effort to keep ageing wind farms productive. Although not new, erosion-related blade damage seems to be more severe in recent, particularly off-shore, installations. With the high blade-tip speeds of modern wind turbines, installation in rainy locations can cause significant damage. While all the players in the industry agree that a reduction on Annual Energy Production (AEP) has to be expected, its magnitude remains uncertain, with wide range of variability forecasted in published research. This work proposes a probabilistic framework to assess AEP reductions, allowing for a better understanding of the key mechanism that cause turbine power loss and for a better quantification of AEP losses. The method is tested on the DTU10MW reference case. Erosion-related uncertainties are estimated based on available literature data. Lift and drag coefficients of the airfoils are derived using CFD, and the entire wind turbine is simulated aero-servo-elastically using a Blade Element Momentum code. An arbitrary Polynomial Chaos method is used to estimate the uncertainties associated to key turbine figures due to the erosion inputs. Results show how AEP reductions, while still significant, are lower than most published literature indicates.

팁제트 로터의 평형끝단속도를 결정하기 위한 예측 방법론 연구

In this study, we conducted a study on an optimal method to determine an equilibrium tip speed. The equilibrium tip speed is a blade tip velocity when the tip-jet rotor is in a torque-balanced state that a torque generated by the tip-jet and a torque generated by a rotation of the rotor are in a balanced state. The equilibrium tip speed is determined by using linear interpolation method and polynomial regression method to apply the tip-jet rotor torque obtained by 3D steady flow analysis considering JWI(Jet stream tip Wake Interference). Polynomial regression method is a faster and more accurate method than linear interpolation to figure out the speed. Also, a simple method to obtain the relation between the blade tip speed and the tip-jet rotor torque is presented.

Research on Effect of Icing Degree on Performance of NACA4412 Airfoil Wind Turbine

In order to study the effect of icing on the wind turbine blade tip speed ratio and wind energy utilization coefficient under working conditions, it is important to better understand the growth characteristics of wind turbine blade icing under natural conditions. In this paper, the icing test of the NACA4412 airfoil wind turbine was carried out using the natural low temperature wind turbine icing test system. An evaluation model of icing degree was established, and the influence of wind speed and icing degree on the performance parameters of wind turbines was compared and analyzed. It is shown that icing is mainly concentrated on the leading edge of the blade and the windward side. The icing on the leeward surface is concentrated near the leading edge of the blade. The amount of icing varies significantly along the direction of blade expansion, with the least amount of ice accumulation at 0.2R, followed by 0.6R, and most at 0.95R. At the beginning of the experiment, the icy thickness at the 0.2R, 0.6R and 0.95R locations increased at a rate of 12.03%, 8.87% and 6.00% respectively, and then the growth rate gradually decreased and stabilized. At the same wind speed, the blade tip speed ratio and wind energy utilization coefficient are inversely related to the change trend of the icing degree. When the blade icing degree is 0.82%, 1.49% and 2.73%, the maximum tip speed ratio is reduced by 7.1%, 14.1% and 21.2% and the maximum wind energy utilization coefficient is reduced by 36.3%, 51.2% and 61.6% respectively.

Effect of Boundary Layer and Rotor Speed on Broadband Noise from Wind Turbines

Trailing edge surface of aerofoil is an important source of broadband aerodynamic noise production. In this paper, three aerofoil self-noise mechanisms from turbulent boundary layer near trailing edge surface are studied. Numerical computations were performed for a three bladed 2 MW horizontal axis upwind turbine of blade length 37 m and source height of 80 m, for wind speeds of 8-15 m/s. A weighted 1/3rd octave band sound power levels (SPL) are evaluated for receiver located at distance of total turbine height and at 2 m above ground. The results obtained for sound power level using baseline models showed maximum values occurring between 300 Hz and 1 kHz region of spectrum. The trends for BPM model showed a reduction of ~2 dBA near 1 kHz region of spectrum at 10 m/s, but Grosveld’s and Lowson model were identical and agreed over the entire spectrum. The effect of rotational speed on sound power levels using three baseline models are illustrated at a wind speed of 8 m/s for 2 MW turbine. Results showed that for a change of ±10% rotor speed from the rated value, there is an increase of 2 to 6 dBA over the entire sound spectrum due to differences in blade tip speed.

Design of Integral Variable Structure Controller in Speed Tracking of Wind Driven Generator

In order to obtain the optimum power factor through maintaining the ratio of blade tip speed to wind speed, a new method for controller design of wind driven generator speed tracking is proposed basing on variable structure control theory. Integral variable of generator speed is introduced into the system equation. Switching surface of variable structure controller is founded with the error of generator speed. Adaptive estimation for wind disturbance and uncertain parameters of system dynamics model is adopted to derive control algorithm for rotation torque in the process of generator speed tracking. Simulation results show that speed tracking precision of wind driven generator is improved by integral variable structure controller. The method proposed in this article is effective and feasible.

Quadcopter propeller noise and its control

Simple scaling analysis is used to show that truly significant reductions in propeller noise are possibly by increasing the propeller diameter while reducing its rotational speed, thereby reducing its blade tip speed, while maintaining a given level of static thrust. The scaling analysis is also used to show that efficiency improvements accompany the reductions in radiated noise. The significant reductions in noise and the increase in efficiency predicted were verified experimentally by measuring the radiated noise and power requirements for both large, slowly rotating propellers and small, high speed propellers, at equivalent static thrusts, where the large propellers serve as possible replacements for the small propellers in typical quadcopter applications. The bulk of the noise reductions offered by large and slowly rotating propellers compared with small, high speed propellers is shown to be maintained at equivalent net force (thrust minus weight conditions), as required for practical implementation into a quadcopter, even using readily available hobby grade components. Further significant noise reductions are possible with lightweight and custom engineered components.

Prediction of aerodynamics performance of continuously variable-speed wind turbine by adaptive neuro-fuzzy methodology

Horizontal-axis wind turbines (HAWT) have the constant rotor speed, while the blade tip speed changes continuously. This could reduce power performance of the wind turbine. In this paper, the accuracy of soft-computing technique was employed for aerodynamics performance prediction based on continuously variable-speed horizontal-axis wind turbine with optimal blades. The process, which simulates the $$\varphi$$ (relative wind angle), BEP (blade element parameter), SP (solidity parameter), CPtot (total power coefficient), CPl (local power coefficient), and CT (local thrust coefficient), with adaptive neuro-fuzzy inference system (ANFIS) was constructed. The inputs were local speed ratios λr and different values of drag-to-lift ratio e. The performance of proposed system is confirmed by the simulation results. The ANFIS results are compared with the experimental results using root-mean-square error and coefficient of determination and Pearson’s coefficient. The experimental results show that an improvement in predictive accuracy and capability of generalization can be achieved by the ANFIS approach. The effectiveness of the proposed strategies is verified based on the simulation results.

Unbalance evaluation of a scaled wind turbine under different rotational regimes via detrended fluctuation analysis of vibration signals combined with pattern recognition techniques

This work aims to propose a different approach to evaluate the operating conditions of a scaled wind turbine through vibration analysis. The turbine blades were built based on the NREL S809 profile and a 40-cm diameter, while the design blade tip speed ratio (λ) is equal to 7. Masses weighing 0.5, 1.0, and 1.5 g were added to the tip of one or two blades in a varying sequence with the intent of simulating potential problems and producing several scenarios from simple imbalances to severe rotor vibration levels to be compared to the control condition where the three blades and the system were balanced. The signals were processed and classified by a combination of detrended fluctuation analysis with Karhunen-Loève Transform, Gaussian discriminator, and Artificial Neural Network, which are pattern recognition techniques with supervised learning. Good results were achieved by employing the above cited recognition techniques as more than 95% of normal and imbalanced cases were correctly classified. In a general way, it was also possible to identify different levels of blade imbalance, thus proving that the present approach may be an excellent predictive maintenance tool for vibration monitoring of wind turbines.

DEM analysis of the effect of particle shape, cohesion and strain rate on powder rheometry

Discrete Element Method (DEM) is used to simulate the flow of particles addressing the influence of shear strain rate, particle shape and cohesion on the flow characteristics. For this purpose, the dynamics of particle motion in the Freeman Technology FT4 rheometer is analysed. The simulations are first validated by comparison with experiments with cohesive particles, i.e. silanised glass beads, from the literature. Particles with faceted shapes, sharp corners and edges are then simulated and found to require significantly higher energy to flow compared to spherical particles. The presence of truncated vertices, typical of active pharmaceutical ingredients, influences the flow behaviour drastically. The results of this analysis therefore reveal the importance of considering the actual particle shape in DEM simulations when faceted particles are considered. Finally, a rheological model describing the relationship between the dimensionless shear stress and the inertial number for several particle shapes, cohesion values and blade tip speeds is proposed. The outcome of this study may lead to a unified rheological description of powder flow, which incorporates the effect of cohesion, shape and shear strain rate.

Effect of airfoil profile on aerodynamic performance and economic assessment of H-rotor vertical axis wind turbines

This research focuses on the effects of the asymmetric airfoil profiles on aerodynamic performance and economic evolution of a vertical axis wind turbine (VAWT) at different blade heights, solidities, and tip speed ratios (TSR or λ). The aerodynamic performance of six asymmetric airfoils, S809, S814, RISØ-A1-24, Du 93-W-210, FFA-W3-241, and FX66-S196-V1, was calculated using double multiple-stream tube (DMST) theory and blade element methods for determination of their performance for tip speed ratios from 1 to 12, and solidities of 0.2–0.6, were considered for this study. All calculations focused on the Khaf area (rural zone) in Iran and considered two heights: 10 m and 40 m. To verify the performance of the developed code, results were compared with experimental power coefficient data for NACA0012 airfoil. For FFA-W3-241 airfoil, maximum power coefficient was obtained at solidity of 0.5 and tip-speed ratio of 4. This aerodynamic excellence resulted in 22.4% and 21.9% increase in annual energy production at hub heights (h) of 10 m and 40 m, respectively, while keeping the total investment costs constant. Moreover, the ratio of wind-generated electric power sales to the total investment cost was found to be 4.33 (0.15/0.0346) for 15 years of operation.

Design Space Exploration of a Beam Flexible Hub Concept for an Inside-Out Ceramic Turbine Using a Simplified Rotordynamic Finite Element Model

This paper presents a new flexible hub design for the inside-out ceramic turbine (ICT) rotor configuration. This configuration is used in microturbines to integrate ceramic blades in order to increase turbine inlet temperature (TIT), which leads to higher cycle efficiency values. The ICT uses an outer composite rim to load the ceramic blades in compression by converting the centrifugal loads of the blades into hoop stresses in the composite rim. High stresses in the composite rim lead to high radial displacement of the blades. This displacement is compensated by using flexible hub in order to maintain the contact with the blades. However, hub flexibility can lead to rotordynamic problems as heavy hub deformation will induce high stresses in it. Thus, stresses in the hub are induced by both rotordynamics and centrifugation, requiring a multi-objective design process, which has yielded geometries that limited, until now, the blade tip speed to 358 m/s. In this paper, a simplified rotordynamics finite element model of a flexible hub is developed to allow quick design iterations. Using the model, a design space exploration of this hub concept is done while considering centrifugation and rotordynamics. Experimental validation is conducted on a simplified ICT prototype up to 129 krpm, i.e., an equivalent blade tip speed of 390 m/s. Finally, predictions from the experimentally calibrated model show that the tested prototype hub could reach a blade tip speed of 680 m/s.

A dynamic stall model for analysis of cross-flow turbines using discrete vortex methods

Abstract Cross-flow turbine blades operate in an unsteady environment over large ranges of angle of attack and Reynolds number, often enduring dynamic stall. The hydrodynamics are further complicated because the circular orbit of the blades changes their effective curvature. In order to address these technical challenges with modeling cross-flow turbines, this article presents a novel dynamic stall model with flow curvature correction. The dynamic stall model extends Beddoes-Leishman dynamic stall theory, including several effects such as proper asymptotic behavior at large (∼90°) angles of attack, influence of reduced-pitch rate on the stall angle, dynamics of the flow separation point, and changes in effective camber due to flow curvature. Further, the model enables rapid hydrodynamic analysis of cross-flow turbines using free vortex methods. Important design parameters such as blade thickness and camber are explicitly accounted for in the model, which enables parametric study of turbine configurations and therefore makes the model useful for system-level turbine optimization. Numerical predictions are validated by experimental data for several cases, including ranges of blade profile, toe angle, solidity, and tip speed ratio.

Full Annulus Simulations of a Transonic Axial Compressor Stage with Distorted Inflow at Transonic and Subsonic Blade Tip Speed

This article reports on systematic numerical studies on an axial compressor stage with distorted inflow. Four operating points at two speedlines were simulated with an inflow distortion generated by a 120 ∘ -sector segment with a wedge-type cross-section. With this setup, the interaction between the distorted inflow and the rotor flow was studied. The focus was put on the differences of the interaction between the distorted inflow and rotor flow as well as on the compressor behaviour at subsonic and transonic blade tip speeds, as the general mechanisms have been analysed in more detail in previous publications. The distorted flow itself is not influenced by the blade tip speed, but the interaction phenomena depend strongly on the spool speed and operating point. Additionally, the blade tip speed influences the circumferential sector of the compressor stage exit, which is affected by the distorted flow. The impact reaches from a small sector at 65% spool speed, with the peak efficiency operating point up to nearly the entire annulus at 100% spool speed, near the stall operating point.

Experimental Validation of a Wind Turbine Turbulent Inflow Noise Prediction Code

This paper describes the comparison of a semi-empirical wind turbine noise prediction code to a large volume of acoustic data gathered on a full-scale 2.3 MW wind turbine. Particular focus is placed on the turbulent inflow noise model due to the relatively small amount of validation that has been performed previously. Two novel techniques are developed in order to effectively validate the model. The first is that the turbulence is characterized in the acoustic model using a single quantity, the turbulence dissipation rate, instead of the combination of turbulence intensity and integral length scale as has been used in previous studies. The second development is a method of measuring the turbulence dissipation rate using blade-mounted accelerometers such that the turbulence dissipation rate in the vicinity of the blade is accurately determined. Using these methods, it is shown that the flat-plate airfoil analytical turbulence noise model underpredicts the turbulent inflow noise levels by 3–5 dB, whereas semi-empirical corrections for finite-thickness airfoils reduce prediction errors to less than 3 dB. Comparisons are also made on the basis of scaling with blade tip-speed, scaling with turbulence dissipation rate, and noise directivity.

Influence of Rotor Tip Blockage on Near Stall Blade Vibrations in an Axial Compressor Rig

Rotor blade vibrations observed in the Darmstadt transonic compressor rig are investigated in this paper. The vibrations are nonsynchronous and occur in the near stall (NS) operating region. Rotor tip flow fluctuations traveling near the leading edge (LE) against the direction of rotation (in the rotor relative frame of reference) with about 50% blade tip speed are found to be the reason for the occurrence of the vibrations. The investigations show that the blockage at the rotor tip is an important factor for the aeroelastic stability of the compressor in the NS region. It is found that by application of a recirculating tip injection (TI) casing treatment, the aeroelastic stability increases as a result of reduced blockage in the rotor tip region.

Boundary-layer transition measurements on Mach-scaled helicopter rotor blades in climb

In this work, laminar-turbulent boundary-layer transition is investigated on the suction side of Mach-scaled helicopter rotor blades in climb. The phenomenon is assessed by means of temperature-sensitive paint (TSP). Results are compared to a data sample acquired by infrared (IR) thermography and accompanied by integral thrust- and local surface pressure measurements at two radial blade sections. Spatially, high-resolved data allow for precise detection of boundary-layer transition along the outer 60% of the blade span. Results obtained via TSP and IR show remarkable agreement with minor deviations due to different surface qualities of the respective blades tested. TSP data are obtained at various collective pitch angles and three different rotating speeds corresponding to chord Reynolds and Mach numbers based on blade tip speed of $$Re_{\rm tip} = 4.8 - 9.3\times 10^5$$ and $$M_{\rm tip} = 0.29 - 0.57$$ , respectively. The transition position is detected with an accuracy of better than 1% chord and the findings show overall coherence as blade loading and tip chord Reynolds number are varied. Experimental findings are shown to be consistent with two-dimensional simulations using the $$e^N$$ -envelope method for transition prediction. Based on quantitative agreement between measured and calculated surface pressures, a comparison of the corresponding transition results suggests a critical amplification factor of $$N_{\rm cr.} = 5.5$$ best suited for transition prediction in the rotating test facility of the DLR Gottingen.

Retraction Notice: Design of Wind Turbine Blade with Thick Airfoils and Flatback and its Aerodynamic Characteristic

large-scaled blade has posed many problems related to design and production. After introducing the features of blade with thick airfoils and flatback, based on relevant parameters of Huaren 100 kW wind turbine, the paper designed blade with thick airfoils and flatback, introduced blade parameter design, and analyzed the aerodynamic performance of blades using GH bladed software, obtaining the relationship between power output of wind turbine with blade tip speed ratio Cp. Furthermore, it analyzed the aerodynamic performance of original design blades, modified blades and Huaren 100 kW blades, and assessed the aerodynamic performance of modified blade.

Investigation of high-speed rubbing behavior of labyrinth-honeycomb seal for turbine engine application

The labyrinth-honeycomb seal has been widely used in gas turbine engines as an abradable gas path seal to protect the rotor from wear and damage in rubbing interaction. It usually works with a stepped labyrinth because the knife-edged tips could produce a special dynamic sealing system, and then the minimum clearance is possible between the rotor and stationary component. To investigate the high-speed rubbing behavior between a Hastelloy-X honeycomb material and a GH4169 double stepped labyrinth, nine rubbing tests were conducted using a high-speed abrasion test rig while the blade tip speed varied from 150 to 450 m/s, and the incursion rate from 120 to 360 µm/s. The abradability of honeycomb made from Hastelloy-X was fully verified by analyzing the visual rubbing observations, rubbing forces, and impact acceleration. It is shown that compression deformation happens to the honeycomb material during the rubbing process with the labyrinth blade except for a simple cutting mechanism, which is mainly affected by the parameter of incursion rate. Thermal ablation and oxidation were the main damage occurring on the labyrinth tip and appeared more obviously at a higher blade tip speed. Rubbing forces and impact acceleration were obtained from a piezoelectric dynamometer and acceleration sensor during the rubbing process. At a blade tip speed of 300 m/s and incursion rate of 360 µm/s, radial and tangential forces show their maximum values of 716 N and 871 N, respectively. The peak value of acceleration presents 341g with the highest blade tip speed of 450 m/s and the highest incursion rate of 360 µm/s. All testing results provide a great deal of effective information on high-speed rubbing behavior for the abradablility evaluation of a honeycomb.

The physical modelling and aerodynamics of turbulent flows around horizontal axis wind turbines

This paper aims to assess the reliability of turbulence models in predicting the flow fields around the horizontal axis wind turbine (HAWT) rotor blades and also to improve our understanding of the aerodynamics of the flow field around the blades. The simulations are validated against data from the NREL/NASA Phase VI wind turbine experiments. The simulations encompass the use of twelve turbulence models. The numerical procedure is based on the finite-volume discretization of the 3D unsteady Reynolds-Averaged Navier-Stokes equations. The resulting simulations are compared with the full range of experimental data available for this case.The main contributions of this study are in establishing which RANS models can produce quantitatively reliable simulations of wind turbine flows and in presenting the flow evolution over a range of operating conditions. At low (relative to the blade tip speed) wind speeds the flow over the blade surfaces remains attached and all RANS models tested return the correct values of key performance coefficients. At higher wind speeds there is circumferential flow separation over the downwind surface of the blade, Moreover, within the separation bubble the centrifugal force pumps the flow outwards, which at the higher wind speeds suppresses the formation of the classical tip vortices. RANS models which do not rely on the linear effective viscosity approximation generally lead to more reliable predictions at higher wind speeds. By contrast some popular linear effective viscosity models perform the poorest over this complex flow range. Finally all RANS models are also able to predict the dominant (lowest) frequency of the pressure fluctuations.