scholarly journals Non-Empirical BEM Corrections Relating to Angular and Axial Momentum Conservation

Energies ◽  
2019 ◽  
Vol 12 (2) ◽  
pp. 320
Author(s):  
Søren Hjort
Keyword(s):  
Power Efficiency ◽  
Correction Term ◽  
Outer Part ◽  
Momentum Conservation ◽  
Navier Stokes ◽  
Speed Ratio ◽  
Power Extraction ◽  
Higher Power ◽  
Horizontal Axis Wind Turbines ◽  
Order Of Magnitude

The Blade-Element Momentum (BEM) model for Horizontal-Axis Wind Turbines (HAWTs), although extremely useful, is known to be approximate due to model formulation insufficiencies, for which add-ons and corrections have been formulated over the past many decades. Scrutiny of the axial and azimuthal momentum conservation properties reveals momentum simplifications and absence of momentum sources not included in momentum theory underlying the standard BEM. One aspect relates to azimuthal momentum conservation, the wake swirl. This correction can be expressed analytically. Another aspect relates to axial momentum conservation, the wake expansion. This correction is not analytically quantifiable. The latter correction term is therefore quantified from postprocessing a large number of axisymmetric Actuator Disk (AD) Navier-Stokes computations with systematic variation of disk loading and tip-speed ratio. The new momentum correction terms are then included in the BEM model, and results benchmarked against references. The corrected BEM is derived by re-visiting the governing equations. For a disk represented by a constant-circulation set of blades, the corrected BEM contains no approximation to the underlying conservation laws. The study contributes by bridging the gap between BEM and the axisymmetric AD method for all disk load levels and tip speed ratios relevant for a wind turbine. The wake swirl correction leads to higher power efficiency at lower tip-speed ratios. The wake expansion correction causes a redistribution of the potential for power extraction, which increases on the inner part of the rotor and decreases on the outer part of the rotor. The overall rotor-averaged value of Betz limit is unaffected by the new corrections, but exceeding Betz locally on the inner- and mid-section of the rotor is shown to be possible. The two corrections significantly improve the axi-symmetric static BEM modelling accuracy for the radial distributions as well as for the rotor-integrated quantities, by reducing errors, approximately one order of magnitude. The relevance of these corrections for modern multi-MW rotors is quantified and discussed.

Wake and Performance Predictions of Two- and Three-Bladed Wind Turbines Based on the Actuator Line Model1

2021 ◽  
Vol 143 (5) ◽  
Author(s):  
Sebastian Henao Garcia ◽  
Aldo Benavides-Morán ◽  
Omar D. Lopez Mejia
Keyword(s):  
Wind Turbine ◽  
Three Dimensional ◽  
Operating Conditions ◽  
Navier Stokes ◽  
Speed Ratio ◽  
Power Extraction ◽  
Low Incidence ◽  
And Performance ◽  
Turbine Design ◽  
Simulation Parameters

Abstract This paper challenges the standard wind turbine design numerically assessing the wake and aerodynamic performance of two- and three-bladed wind turbine models implementing downwind and upwind rotor configurations, respectively. The simulations are conducted using the actuator line model (ALM) coupled with a three-dimensional Navier Stokes solver implementing the k−ω shear stress transport turbulence model. The sensitivity of the ALM to multiple simulation parameters is analyzed in detail and numerical results are compared against experimental data. These analyses highlight the most suitable Gaussian radius at the rotor to be equal to twice the chord length at 95% of the blade for a tip-speed ratio (TSR) of ten, while the Gaussian radius at the tower and the number of actuator points have a low incidence on the flow field computations overall. The numerical axial velocity profiles show better agreement upstream than downstream the rotor, while the discrepancies are not consistent through all the assessed operating conditions, thus highlighting that the ALM parameters are also dependent on the wind turbine's operating conditions rather than being merely geometric parameters. Particularly, for the upwind three-bladed wind turbine model, the accuracy of the total thrust computations improves as the TSR increases, while the least accurate wake predictions are found for its design TSR. Finally, when comparing both turbine models, an accurate representation of the downwind configuration is observed as well as realistic power extraction estimates. Indeed, the results confirm that rotors with fewer blades are more suitable to operate at high TSRs.

scholarly journals The problem of the second wind turbine – a note on a common but flawed wind power estimation method

2012 ◽  
Vol 3 (1) ◽  
pp. 79-86 ◽  
Author(s):  
F. Gans ◽  
L. M. Miller ◽  
A. Kleidon
Keyword(s):  
Boundary Layer ◽  
Simple Model ◽  
Kinetic Energy ◽  
Atmospheric Boundary Layer ◽  
Wind Turbine ◽  
Wind Power ◽  
Momentum Conservation ◽  
Height Velocity ◽  
Power Extraction ◽  
Order Of Magnitude

Abstract. Several recent wind power estimates suggest that this renewable energy resource can meet all of the current and future global energy demand with little impact on the atmosphere. These estimates are calculated using observed wind speeds in combination with specifications of wind turbine size and density to quantify the extractable wind power. However, this approach neglects the effects of momentum extraction by the turbines on the atmospheric flow that would have effects outside the turbine wake. Here we show with a simple momentum balance model of the atmospheric boundary layer that this common methodology to derive wind power potentials requires unrealistically high increases in the generation of kinetic energy by the atmosphere. This increase by an order of magnitude is needed to ensure momentum conservation in the atmospheric boundary layer. In the context of this simple model, we then compare the effect of three different assumptions regarding the boundary conditions at the top of the boundary layer, with prescribed hub height velocity, momentum transport, or kinetic energy transfer into the boundary layer. We then use simulations with an atmospheric general circulation model that explicitly simulate generation of kinetic energy with momentum conservation. These simulations show that the assumption of prescribed momentum import into the atmospheric boundary layer yields the most realistic behavior of the simple model, while the assumption of prescribed hub height velocity can clearly be disregarded. We also show that the assumptions yield similar estimates for extracted wind power when less than 10% of the kinetic energy flux in the boundary layer is extracted by the turbines. We conclude that the common method significantly overestimates wind power potentials by an order of magnitude in the limit of high wind power extraction. Ultimately, environmental constraints set the upper limit on wind power potential at larger scales rather than detailed engineering specifications of wind turbine design and placement.

scholarly journals Simulation of 2-Coil and 4-Coil Magnetic Resonance Wearable WPT Systems

Proceedings ◽  
2021 ◽  
Vol 68 (1) ◽  
pp. 13
Author(s):  
Yixuan Sun ◽  
Stephen Beeby
Keyword(s):  
Power Efficiency ◽  
Power Transmission ◽  
Rotation Angle ◽  
Wireless Power ◽  
Peak Efficiency ◽  
Frequency Splitting ◽  
At Resonance ◽  
Higher Power ◽  
The Impact ◽  
Proper Frequency

This paper presents the COMSOL simulations of magnetically coupled resonant wireless power transfer (WPT), using simplified coil models for embroidered planar two-coil and four-coil systems. The power transmission of both systems is studied and compared by varying the separation, rotation angle and misalignment distance at resonance (5 MHz). The frequency splitting occurs at short separations from both the two-coil and four-coil systems, resulting in lower power transmission. Therefore, the systems are driven from 4 MHz to 6 MHz to analyze the impact of frequency splitting at close separations. The results show that both systems had a peak efficiency over 90% after tuning to the proper frequency to overcome the frequency splitting phenomenon at close separations below 10 cm. The four-coil design achieved higher power efficiency at separations over 10 cm. The power efficiency of both systems decreased linearly when the axial misalignment was over 4 cm or the misalignment angle between receiver and transmitter was over 45 degrees.

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

2020 ◽  
Vol 9 (1) ◽  
pp. 21
Author(s):  
Iro E. Malefaki ◽  
Kostas A. Belibassakis
Keyword(s):  
Vertical Axis ◽  
Flow Speed ◽  
Performance Estimation ◽  
Speed Ratio ◽  
Power Extraction ◽  
Flapping Foil ◽  
Flow Energy ◽  
Novel Concept ◽  
Angular Oscillations ◽  
Energy Converters

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.

scholarly journals Statistical Uncertainty of DNS in Geometries without Homogeneous Directions

2021 ◽  
Vol 11 (4) ◽  
pp. 1399
Author(s):  
Jure Oder ◽  
Cédric Flageul ◽  
Iztok Tiselj
Keyword(s):  
Channel Flow ◽  
A Priori ◽  
Time Integration ◽  
Navier Stokes ◽  
Statistical Uncertainty ◽  
Time Step ◽  
Order Of Magnitude ◽  
Averaging Time ◽  
The Individual ◽  
Time Required

In this paper, we present uncertainties of statistical quantities of direct numerical simulations (DNS) with small numerical errors. The uncertainties are analysed for channel flow and a flow separation case in a confined backward facing step (BFS) geometry. The infinite channel flow case has two homogeneous directions and this is usually exploited to speed-up the convergence of the results. As we show, such a procedure reduces statistical uncertainties of the results by up to an order of magnitude. This effect is strongest in the near wall regions. In the case of flow over a confined BFS, there are no such directions and thus very long integration times are required. The individual statistical quantities converge with the square root of time integration so, in order to improve the uncertainty by a factor of two, the simulation has to be prolonged by a factor of four. We provide an estimator that can be used to evaluate a priori the DNS relative statistical uncertainties from results obtained with a Reynolds Averaged Navier Stokes simulation. In the DNS, the estimator can be used to predict the averaging time and with it the simulation time required to achieve a certain relative statistical uncertainty of results. For accurate evaluation of averages and their uncertainties, it is not required to use every time step of the DNS. We observe that statistical uncertainty of the results is uninfluenced by reducing the number of samples to the point where the period between two consecutive samples measured in Courant–Friedrichss–Levy (CFL) condition units is below one. Nevertheless, crossing this limit, the estimates of uncertainties start to exhibit significant growth.

Hydrodynamic Design and Analysis of Horizontal Axis Marine Current Turbines With Lifting Line and Panel Methods

2011 ◽  
Author(s):  
J. Baltazar ◽  
J. Machado ◽  
J. A. C. Falca˜o de Campos
Keyword(s):  
Computational Models ◽  
Design Procedure ◽  
Horizontal Axis ◽  
Complete Analysis ◽  
Speed Ratio ◽  
Integral Boundary ◽  
Power Extraction ◽  
Marine Current ◽  
Lifting Line ◽  
Blade Geometry

This paper presents the computational models used by the authors at MARETEC/IST for hydrodynamic design and analysis of horizontal axis marine current turbines. The models combine a lifting line method for the optimization of the turbine blade geometry and an Integral Boundary Element Method (IBEM) for the hydrodynamic analysis. The classical lifting line optimization is used to determine the optimum blade circulation distribution for maximum power extraction. Blade geometry is determined with simplified cavitation requirements and limitations due to mechanical strength. The application of the design procedure is illustrated for a two-bladed 300 kW marine current turbine with a diameter of 11 meters. The effects of design tip-speed-ratio and the influence of blade section foils on power and cavitation inception are discussed. A more complete analysis may be carried out with an IBEM in steady and unsteady flow conditions. The IBEM has been extended to include wake alignment. The results are compared with experimental performance data available in the literature.

Arriving at the Optimum Overlap Ratio for an Elliptical-Bladed Savonius Rotor

2017 ◽  
Author(s):  
Nur Alom ◽  
Ujjwal K. Saha
Keyword(s):  
Numerical Study ◽  
Superior Performance ◽  
Navier Stokes ◽  
Speed Ratio ◽  
Small Scale ◽  
Efficient Manner ◽  
Ansys Fluent ◽  
Savonius Rotor ◽  
Velocity Magnitude ◽  
Overlap Ratio

The Savonius rotor appears to be particularly promising for the small-scale applications because of its design simplicity, good starting ability, and insensitivity to wind directions. There has been a growing interest in recent times to harness wind energy in an efficient manner by developing newer blade profiles of Savonius rotor. The overlap ratio (OR), one of the important geometric parameters, plays a crucial role in the turbine performance. In a recent study, an elliptical blade profile with a sectional cut angle (θ) of 47.5° has demonstrated its superior performance when set at an OR = 0.20. However, this value of OR is ideal for a semicircular profile, and therefore, requires further investigation to arrive at the optimum overlap ratio for the elliptical profile. In view of this, the present study attempts to make a systemic numerical study to arrive at the optimum OR of the elliptical profile having sectional cut angle, θ = 47.5°. The 2D unsteady simulation is carried out around the elliptical profile considering various overlap ratios in the range of 0.0 to 0.30. The continuity, unsteady Reynolds Averaged Navier-Stokes (URANS) equations and two equation eddy viscosity SST (Shear Stress transport) k-ω model are solved by using the commercial finite volume method (FVM) based solver ANSYS Fluent. The torque and power coefficients are calculated as a function of tip speed ratio (TSR) and at rotating conditions. The total pressure, velocity magnitude and turbulence intensity contours are obtained and analyzed to arrive at the intended objective. The numerical simulation demonstrates an improved performance of the elliptical profile at an OR = 0.15.

A Conceptual Model of Flapping-Wing Mechanism Inspired from the Results of Numerical Study

2013 ◽  
Vol 397-400 ◽  
pp. 783-788
Author(s):  
Xing Wei Zhang ◽  
Chao Wang ◽  
Hang Liu
Keyword(s):  
Conceptual Model ◽  
Power Efficiency ◽  
Stokes Equations ◽  
Numerical Study ◽  
Flapping Wing ◽  
Numerical Results ◽  
Navier Stokes ◽  
Active Deformation ◽  
Navier Stokes Equations ◽  
Wing Model

This paper investigates the aerodynamic forces of several plunging wing models by means of computational fluid dynamics. A finite volume method was used to solve the two-dimensional unsteady incompressible Navier-Stokes equations. The forces and power efficiency have been calculated and compared between sets of different models. Current work found that the nonsymmetrical moving can enhance the lift and thrust forces. The numerical results also prove that the flexible wing model can be use to improve the efficiency and reduce the input. Additionally, a new conceptual model for flapping wing mechanism with active deformation and adaptive nonsymmetrical driving motion is proposed base on the numerical results.

scholarly journals A Flux Controlled Memristor using 90nm Technology

2021 ◽  
pp. 1-6
Author(s):  
B.T. Krishna ◽  
◽  
Shaik. mohaseena Salma ◽  
Keyword(s):  
Power Supply ◽  
Power Dissipation ◽  
Power Efficiency ◽  
Current Mode ◽  
Cmos Technology ◽  
Theoretical Simulation ◽  
Transconductance Amplifier ◽  
Higher Power ◽  
Static Power ◽  
A Current

A flux-controlled memristor using complementary metal–oxide–(CMOS) structure is presented in this study. The proposed circuit provides higher power efficiency, less static power dissipation, lesser area, and can also reduce the power supply by using CMOS 90nm technology. The circuit is implemented based on the use of a second-generation current conveyor circuit (CCII) and operational transconductance amplifier (OTA) with few passive elements. The proposed circuit uses a current-mode approach which improves the high frequency performance. The reduction of a power supply is a crucial aspect to decrease the power consumption in VLSI. An offered emulator in this proposed circuit is made to operate incremental and decremental configurations well up to 26.3 MHZ in cadence virtuoso platform gpdk using 90nm CMOS technology. proposed memristor circuit has very little static power dissipation when operating with ±1V supply. Transient analysis, memductance analysis, and dc analysis simulations are verified practically with the Experimental demonstration by using ideal memristor made up of ICs AD844AN and CA3080, using multisim which exhibits theoretical simulation are verified and discussed.

scholarly journals Design aspects of Bi2Sr2CaCu2O8+δ THz sources: optimization of thermal and radiative properties

2021 ◽  
Author(s):  
Mikhail M Krasnov ◽  
Natalia D Novikova ◽  
Roger Cattaneo ◽  
Alexey A Kalenyuk ◽  
Vladimir M Krasnov
Keyword(s):  
Experimental Data ◽  
Power Efficiency ◽  
Open Space ◽  
Radiation Power ◽  
Impedance Matching ◽  
Dipole Antenna ◽  
Radiative Properties ◽  
Heat Management ◽  
Order Of Magnitude ◽  
Thz Sources

Impedance matching and heat management are important factors influencing performance of THz sources. In this work we analyze thermal and radiative properties of such devices based on mesa structures of a layered high-temperature superconductor Bi2Sr2CaCu2O8+δ. Two types of devices are considered, containing either a conventional large single crystal, or a whisker. We perform numerical simulations for various geometrical configurations and parameters and make a comparison with experimental data for the two types of devices. It is demonstrated that the structure and the geometry of both the superconductor and the electrodes are playing important roles. In crystal-based devices an overlap between the crystal and the electrode leads to appearance of a large parasitic capacitance, which shunts THz emission and prevents impedance matching with open space. The overlap is avoided in whisker-based devices. Furthermore, the whisker and the electrodes form a turnstile (crossed-dipole) antenna facilitating good impedance matching. This leads to more than an order of magnitude enhancement of the radiation power efficiency in whisker-based, compared to crystal-based devices. These results are in good agreement with presented experimental data.

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