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Operating a conventional propeller at negative thrust results in the operation of positively cambered blade sections at negative angles of attack, leading to flow separation. Consequently, accurately simulating the aerodynamics of propellers operating at negative thrust poses a greater challenge than at positive thrust. This study offers a comprehensive assessment of the aerodynamic modeling capabilities of numerical methods, spanning low to high fidelity, for computing propeller performance across both positive and negative thrust regimes. Low-fidelity methods, namely, blade-element momentum and lifting line theories, effectively predict propeller performance trends at positive thrust. However, they fail to capture trends at negative thrust beyond the maximum power output point due to the neglect of three-dimensional flow effects. Both steady and unsteady Reynolds-averaged Navier-Stokes (RANS) simulations with y+<1 perform well across both positive and negative thrust conditions, with errors below 2% for both thrust and power magnitudes near the maximum power output point. Lattice-Boltzmann very-large-eddy simulations (LB-VLESs) with y+≤10 exhibit excellent agreement with experimental data with less than 1% error near the maximum power output point but with significant computational costs. Conversely, LB-VLESs with y+≥15 offer a more economical approach to capture general trends with the computational cost of the same order as unsteady RANS. However, wall models introduce errors in modeling flow separation, leading to a 16% overestimation of power magnitude near the maximum power output point. The results highlight the necessity of using tools with increased fidelity levels when considering propeller operation at negative thrust compared to the conventional positive thrust regime. [ABSTRACT FROM AUTHOR]

Coaxial, corotating (stacked) rotors have shown improved performance compared to conventional rotors, but the optimum configuration is highly sensitive to the rotor parameters. This paper reports the development of a low-order model, blade interaction prediction (BLIP), to efficiently predict the thrust and power of closely spaced rotor blades and quantify the effects of rotor geometry on total and individual stacked rotor performance. BLIP couples a vortex panel method and blade element momentum theory to combine the effects of bound circulation and rotor inflow. A cascade effect in the two-dimensional vortex panel method was implemented to incorporate the effect of airfoils rotating in a circle. Validation was performed with measurements of a 1.108-m-radius fixed-pitch stacked rotor with variable axial and azimuthal spacing, and excellent correlation was observed. It was found that small azimuthal angles and axial spacings are most effective in varying total thrust. The variation of thrust and power with azimuthal spacing was found to be a result of bound circulation, while the variation with axial spacing is due to rotor inflow interactions. [ABSTRACT FROM AUTHOR]

Renewable energy sources are essential to address climate change, fossil fuel depletion, and stringent environmental regulations in the subsequent decades. Horizontal-axis wind turbines (HAWTs) are particularly suited to meet this demand. However, their efficiency is affected by environmental factors because they operate in open areas. Adverse weather conditions like rain reduce their aerodynamic performance. This study investigates wind turbine power prediction under rainy conditions by integrating Blade Element Momentum (BEM) theory with explainable artificial intelligence (XAI). The S809 airfoil's aerodynamic characteristics, used in NREL wind turbines, were analyzed using ANSYS FLUENT and symbolic regression under varying rain intensities. Simulations at a Reynolds number (Re) of 1 × 106 were performed using the Discrete Phase Model (DPM) and k– ω SST turbulence model, with liquid water content (LWC) values of 0 (dry), 10, 25, and 39 g/m3. The lift and drag coefficients were calculated at various angles of attack for all the conditions. The results indicated that rain led to reduced lift and increased drag. The innovative aspect of this research is the development of machine learning models predicting changes in the airfoil coefficients under rain with an R 2 value of 0.97. The proposed XAI framework models rain effects at a lower computational time, enabling efficient wind farm performance assessment in rainy conditions compared to conventional CFD simulations. It was found that a heavy rain LWC of 39 g/m3 could reduce power output by 5.7% to 7%. These findings highlight the impact of rain on aerodynamic performance and the importance of advanced predictive models for optimizing renewable energy generation. [ABSTRACT FROM AUTHOR]

To reduce the cost of small wind turbines, a prototype of a butterfly wind turbine (6.92 m in diameter), a small vertical-axis type, was developed with many parts made of extruded aluminum suitable for mass production. An overspeed control system with movable arms that operated using centrifugal and aerodynamic forces was installed for further cost reduction. Introducing this mechanism eliminates the need for large active brakes and expands the operating wind speed range of the wind turbine. However, although the mechanism involving the use of only bearings is simple, the violent movement of the movable arms can be a challenge. To address this in the present study, dampers were introduced on the movable arm rotation axes to improve the movement of the movable arms. To predict the behavior of a movable arm and the performance of the wind turbine with the mechanism, a simulation method was developed based on the blade element momentum theory and the equation of motion of the movable arm system. A comparison of experiments and predictions with and without dampers demonstrated qualitative agreement. In the case with dampers, measurements confirmed the predicted increase in the rotor rotational speed when the shorter ailerons installed perpendicularly to the movable arms were used to achieve the inclination. Field experiments of the generated power at a wind speed of 6 m/s (10 min average) showed relative performance improvements of 11.4% by installing dampers, 91.3% by shortening the aileron length, and 57.6% by changing the control target data. The movable arm system with dampers is expected to be a useful device for vertical-axis wind turbines that are difficult to control. [ABSTRACT FROM AUTHOR]

The ducted propeller designed for a single working condition usually cannot meet the high efficiency requirements of vertical take-off and landing aircraft under multi-working conditions. The key to the multi-working condition design of the ducted propeller based on the blade element momentum theory lies in the calculation of the propeller thrust ratio and the correction of the inflow velocity under different working conditions. In the absence of propeller geometry, the momentum source method was used to estimate the duct thrust, and a method for correcting the inflow velocity was proposed. Based on the principle of minimum energy loss, the propeller geometry was established and the multiple reference frames method was used for coupled CFD (computational fluid dynamics) correction to obtain the relationship between chord length and twist angle, propeller thrust ratio and corrected inflow velocity under a single working condition. By changing the design lift coefficient and rotational speed, the chord length and twist angle of the ducted propeller could be matched under different working conditions. The design results show that the momentum source method may calculate a large proportion of the duct thrust, and the method of correcting inflow velocity can reduce the computational cost of coupled CFD solutions under the same working conditions. Multi-working condition optimization can effectively improve the overall efficiency of the ducted propeller. [ABSTRACT FROM AUTHOR]

The present paper proposes a general approach capable of determining the set of trim commands, aerodynamic control surface actuation, and/or engine thrust regulation that allow specific trimmed steady-state flight conditions of nonconventional vertical takeoff and landing vehicles under given performance constraints. Exploiting the peculiarity of these vehicles to have redundant controls, namely more controls than those strictly required to define steady-state flight conditions, their trim problem is recast into a constrained minimization of a cost function. This approach is of particular interest in that it allows the identification of control settings for multirotor vehicles that, at the same time, guarantee trimmed flight conditions and optimization of selected target functions such as, for instance, performance or noise emission. The proposed approach is numerically applied to quadcopter and hexacopter configurations for three flight conditions (level forward flight, climb, and coordinate turn), and three strategies of trim by different trim variables (rotor angular velocity, blade collective pitch, and combination of angular velocity and blade collective pitch). The effect of the introduced cost function is investigated, comparing the outcomes of minimum control effort and minimum torque strategies. The results achieved demonstrate the capability of the proposed algorithm to exploit redundant controls of multirotor vehicles for identifying selected optimal trim operative conditions. [ABSTRACT FROM AUTHOR]

Aircraft propellers produce relatively large in-plane loads, called propeller 1P loads, during maneuvers such as turning, diving, and lifting, and these loads can negatively affect the flight and control of the aircraft. In order to study the change rule of 1P aerodynamic loads, in this paper, a mathematical model of the propeller 1P aerodynamic loads has been developed based on the blade element momentum theory. This mathematical model was then corrected using the Pitt–Peters incoming flow correction method, the Prandtl tip correction method, and the propeller root flow correction method. Based on this mathematical model, a calculation procedure for the propeller 1P aerodynamic loads was developed using MATLAB software, and the accuracy of the procedure was verified by comparing the results with CFD simulation results. Numerical simulations show that the results calculated based on the proposed mathematical model for the coefficients of thrust, power, bending moment, and the tangential force of the propeller have an error of less than ±6.00% compared to the CFD simulation results. For a small inflow angle, the coefficients of bending moment and tangential force of the whole propeller fluctuate in a small range. But, as the inflow angle increases, the fluctuation range of the aerodynamic characteristic parameters of the propeller increases and the fluctuation becomes more complicated. Through numerical calculations, it has been shown that the mathematical model presented herein is reliable and accurate. In addition, it greatly shortens the calculation time and improves the calculation efficiency. It is expected that the proposed model can provide a certain help for the strength design of the propeller structure and the study of the aerodynamic performance of the whole aircraft. [ABSTRACT FROM AUTHOR]

This work presents an actuator disk-like body force method designed for propeller modeling, which is based on a full coupling between computational fluid dynamics (CFD) and blade element theory (BET). An analysis is conducted on the model to identify best practices for source term distribution. It is found that the source term volume shape has no impact on propeller loads and flowfield and that the velocities used for the BET analysis at each radial section should be evaluated exactly where half the source terms have been distributed in the CFD domain. Four tip-loss corrections, including two from literature, are also analyzed and compared to lifting-line and Reynolds-averaged Navier-Stokes (RANS) blade-resolved computations. The best practices and the most effective tip-loss correction lead to a final model that is compared to lifting-line, RANS, and unsteady RANS computations for different pitch angles and incidence angles, on the ONERA HAD-1 three-bladed light propeller. The RANS/BET body force model predicts thrust within 3% for axial flow and 8% for cases with incidence. The same accuracy is obtained for wake prediction. [ABSTRACT FROM AUTHOR]

The use of a diffuser in hydrokinetic turbines can improve the power coefficient. However, the risk of cavitation in the rotor blades increases. Studies suggest that backward-curved blades can reduce the axial load on the rotor and therefore prevent cavitation. Therefore, this work develops an optimization procedure applied to backward-curved blades in hydrokinetic turbines with diffusers based on the Blade Element Momentum Theory. The main contribution is to consider both the sweep effect and the presence of a diffuser in the optimization in an innovative way. We use a radial transformation function that adjusts the radial position considering the curvature of the blade during optimization under the effect of the diffuser. The results showed that the increase in blade curvature resulted in greater chord distributions and twist angles, especially at the blade tips. The Prandtl's loss factor was not sensitive to sweep, but the linked circulation increased at the blade tips, suggesting an increased risk of cavitation. Depending on the sweep angle, the optimized blades were able to mitigate or avoid cavitation. In particular, a sweep angle of 30 ∘ eliminated cavitation. This study indicated that the proposed optimization can effectively prevent cavitation, showing satisfactory results. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Ordnance Equipment Engineering is the property of Chongqing University of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Energy from river currents is one of the available renewable energy sources to produce electricity. An improvement in the efficiency of the horizontal axis current turbine is required for the successful utilization of this energy. This paper compares the power production of a horizontal axis current turbine and a turbine with two passive flow control attachments, the vortex generator, and the inserted tube. Firstly, the NACA S1210 hydrofoil shape has been selected. Then the variation of chord length and twist angle along the blade span has been determined. Finally, a 5-meter-diameter, three-bladed, horizontalaxis current turbine has been designed using the blade element momentum theory. The rotor has a maximum power coefficient and a maximum power of 0.47 and 42.71 kW, respectively, at the current speed of 2.1 m/s and a tip speed ratio of 6 when the hydrofoil with tubes has been chosen to design the horizontal axis current turbine. It has been proven that including vortex generators and tubes on the turbine can help increase power production by 9.8% and 14.7%, respectively. [ABSTRACT FROM AUTHOR]

Renewable energy sources are essential to address climate change, fossil fuel depletion, and stringent environmental regulations in the subsequent decades. Horizontal-axis wind turbines (HAWTs) are particularly suited to meet this demand. However, their efficiency is affected by environmental factors because they operate in open areas. Adverse weather conditions like rain reduce their aerodynamic performance. This study investigates wind turbine power prediction under rainy conditions by integrating Blade Element Momentum (BEM) theory with explainable artificial intelligence (XAI). The S809 airfoil’s aerodynamic characteristics, used in NREL wind turbines, were analyzed using ANSYS FLUENT and symbolic regression under varying rain intensities. Simulations at a Reynolds number (Re) of 1 × 106 were performed using the Discrete Phase Model (DPM) and k–ω SST turbulence model, with liquid water content (LWC) values of 0 (dry), 10, 25, and 39 g/m3. The lift and drag coefficients were calculated at various angles of attack for all the conditions. The results indicated that rain led to reduced lift and increased drag. The innovative aspect of this research is the development of machine learning models predicting changes in the airfoil coefficients under rain with an R2 value of 0.97. The proposed XAI framework models rain effects at a lower computational time, enabling efficient wind farm performance assessment in rainy conditions compared to conventional CFD simulations. It was found that a heavy rain LWC of 39 g/m3 could reduce power output by 5.7% to 7%. These findings highlight the impact of rain on aerodynamic performance and the importance of advanced predictive models for optimizing renewable energy generation.

Copyright of Machine Tool & Hydraulics is the property of Guangzhou Mechanical Engineering Research Institute (GMERI) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The flowfield of a coaxial counter-rotating rotor in hover was measured using particle image velocimetry. The goal of the measurements was to investigate the flow features of coaxial rotors, including interaction between the rotors, and to measure inflow over the rotor radius and azimuth. The phase-resolved measurements were performed at several azimuthal index angles behind the rotor blades, with 500 instantaneous flow realizations per azimuth. Axial velocity profiles extracted from the phase-averaged flowfields exhibited the effect of bound circulation associated with blade passage, the trailed vortex sheet, and the effect of the blade tip vortex. Tip vortex characteristics were extracted from the measurements and were shown to compare well with semi-empirical models in most cases. The measured inflow was captured well by blade element momentum theory when the mutually induced velocities were accounted for. The lower rotor was found to behave more like an isolated single rotor, whereas tip vortices from the upper rotor convected faster radially inward and downstream than the lower rotor tip vortices. Upper and lower rotor tip vortices exhibited similar core radii ranging from 12 to 18% of the rotor blade chord, and they grew at a similar rate. However, this vortex growth rate was different from that of an isolated single rotor. [ABSTRACT FROM AUTHOR]

Effective and reliable slipstream numerical simulation methods are important for propeller-driven aircraft design. This paper presents a high-precision and efficient momentum source method (HPE-MSM) based on a novel actuator disk load prediction model established by the frozen rotor method and blade element momentum theory. The simulation results of two benchmark test cases of an isolated propeller and a typical turboprop airliner show that the accuracy of the HPE-MSM proposed in this paper is close to the time-averaged unsteady results of the sliding mesh method (SMM), with a maximum error of 5% in aircraft lift and drag coefficients compared with experimental values. Meanwhile, the calculation efficiency of the HPE-MSM is comparable to quasi-steady methods, with only about 3.4% of the computation resources of the SMM in the whole aircraft simulation. This novel approach achieves high-precision and efficient simulation of the slipstream, which has the potential to improve the design level of propeller-driven aircraft. [ABSTRACT FROM AUTHOR]

Aircraft propellers produce relatively large in-plane loads, called propeller 1P loads, during maneuvers such as turning, diving, and lifting, and these loads can negatively affect the flight and control of the aircraft. In order to study the change rule of 1P aerodynamic loads, in this paper, a mathematical model of the propeller 1P aerodynamic loads has been developed based on the blade element momentum theory. This mathematical model was then corrected using the Pitt–Peters incoming flow correction method, the Prandtl tip correction method, and the propeller root flow correction method. Based on this mathematical model, a calculation procedure for the propeller 1P aerodynamic loads was developed using MATLAB software, and the accuracy of the procedure was verified by comparing the results with CFD simulation results. Numerical simulations show that the results calculated based on the proposed mathematical model for the coefficients of thrust, power, bending moment, and the tangential force of the propeller have an error of less than ±6.00% compared to the CFD simulation results. For a small inflow angle, the coefficients of bending moment and tangential force of the whole propeller fluctuate in a small range. But, as the inflow angle increases, the fluctuation range of the aerodynamic characteristic parameters of the propeller increases and the fluctuation becomes more complicated. Through numerical calculations, it has been shown that the mathematical model presented herein is reliable and accurate. In addition, it greatly shortens the calculation time and improves the calculation efficiency. It is expected that the proposed model can provide a certain help for the strength design of the propeller structure and the study of the aerodynamic performance of the whole aircraft.

Blade Element Momentum Theory (BEMT) -based approaches provide 15%–20% inaccurate load predictions under uniform steady inflow. Inaccuracies were related to unclear aerodynamic mechanisms causing the Prandtl Tip-Root Loss (TRL) correction to fail at high Tip-Speed Ratio (TSR) and the ambiguous drag coefficient treatment under Stall Delay (SD). With this study, we provide an in-depth flow analysis to explain what physical mechanisms are poorly captured in TRL and SD corrections and offer a potential solution for the improvement of the Prandtl model. Corrections are assessed by comparison with RANS CFD simulations of the NREL 5MW rotor at its design TSR, TSR 4, and TSR 10. The Prandtl tip-loss correction provides 5%–15% higher loading than CFD at TSR 10. The reason is a lower (0.5°) downwash angle of attack, providing a 5%–10% lift coefficient overestimation. We recommend employing the tip-loss correction of Zhong, as it considers the lift coefficient reduction due to the tip-vortex downwash. The investigated Eggers SD correction predicts an incorrect drag coefficient at TSR 4, while a better agreement is found for the lift coefficient. We conclude that only the latter requires a correction for SD in the inboard blade. • CFD-based assessment of BEMT corrections for tip-root-losses and stall delay. • Prandtl tip-loss correction is missing tip-vortex-induced lift coefficient reduction. • Zhong tip-loss model improves the Prandtl one for high tip-speed ratio and blunt tip. • CFD inboard blade loading matched by only a stall-delay-corrected lift coefficient. • Capturing the root-vortex downwash requires a modified Prandtl root-loss correction. [ABSTRACT FROM AUTHOR]

Recent years have seen a renewed interest in rotary-wing aircraft, due to the emergence of urban air mobility. In the conceptual design of such aircraft, fast estimation of rotor performance is important. A challenge is that rotor performance models require a detailed description of the blade geometry, which is not always available at this stage. We have developed a new, robust method for rapidly computing optimal rotor designs, which is derived from blade element momentum theory. In this paper, we derive the rotor design method and prove its convergence for all reasonable model inputs. We verify the design method by comparing the optimal rotor designs it computes to those found by a nonlinear programming algorithm. Thousands of numerical experiments yield relative errors of less than 13 and 5% in terms of the chord and twist profiles, respectively, and less than 0.3% in terms of efficiency, thrust, torque, and power coefficients. In addition, the design method computes optimal designs 170 times faster compared to the nonlinear programming algorithm. Based on these findings, we expect this new rotor design method to be useful in the conceptual design of rotary-wing aircraft. [ABSTRACT FROM AUTHOR]

System identification methods have played an essential role in the research and industry projects at the Institute of Flight System Dynamics of the Technical University of Munich. Besides the application of the established system identification approaches to multiple aircraft, novel methods have been developed at the institute to deal with the challenges associated with fixed- and rotary-wing aircraft system identification in the time and frequency domains. This paper provides an overview of these new developments, as well as practical application examples from the past and ongoing projects at the institute. After a brief introduction to the work at the institute, the paper describes the new advancements in the fields of time domain system identification and optimal input design by introducing optimal control methods. It continues with an alternative problem formulation for applying frequency domain system identification and parameter estimation methods to a novel flight control design and tuning approach. Additional topics from the past and ongoing research at the institute such as novel methods and practical findings in rotorcraft system identification and flight path reconstruction for different applications will be discussed and referenced. [ABSTRACT FROM AUTHOR]

With the increasing size of wind turbines, the inflow conditions are also becoming more and more complex, and the rotor speed and blade-pitch angle are unknown under complex inflow conditions, so in order to avoid establishing an equivalent wind speed model, the control system was coupled to the blade element momentum theory (BEMT) to establish an aerodynamic model. In addition, due to the increasing flexibility of blades, a structural model of blades that can solve any section shape and any material properties was established based on the geometrically exact beam theory. Finally, the aerodynamic model and the structural model were coupled to establish the aeroelastic model and implemented by C++. The model was applied to engineering calculations, and the aerodynamic characteristics of wind rotors and the dynamic response of blades under different low-level jets (LLJ) were calculated and analyzed. The results show that when the control system is coupled to the BEMT, part of the power error is transferred to the rotor speed for below-rated wind speeds, and all the power error is transferred to blade-pitch angle for the above-rated wind speeds. The structural model can accurately calculate the static, dynamic displacement and natural frequency of the blades. When the LLJ height is different, the control system weakens the influence of strong shear wind on the average aerodynamic force on the sweeping surface of the wind rotor, but the amplitude of aerodynamic force is still greatly affected by the LLJ height. When the aerodynamic force on the blade is similar, the law of structure dynamic response is the same, which is mainly affected by the natural frequency of the blade. Our work has important reference significance for calculating the aerodynamic characteristics of wind rotors and the dynamic response of blades. [ABSTRACT FROM AUTHOR]

This is a new installment in the series of publications that describe the mathematical modeling of the Floating Hybrid Generator Systems Simulator (FHYGSYS) tool. This work presents an improved mathematical model of the turbines of the floating hybrid system—consisting of an "OC3-Hywind" wind turbine and two marine current turbines—presented by the authors in previous publications. In this third installment, the modeling of the three turbines of the floating hybrid system is described using the Blade Element Momentum (BEM) theory. This modeling allows one to replace the one based on the One-Dimensional theory used in previous installments. For the operation of modeling with BEM, it has been considered necessary to implement a continuous feedback control system. In this case, two PID (proportional–integral–derivative) controllers have been implemented in each of the turbines. The first controls the torque on the turbine generator and the second controls the collective pitch angle of the blades. The results obtained are presented and validated through a code-to-code comparison with simulations carried out with FASTv8 under the same conditions and with the operating results of marine current turbines that exist in the literature. This improvement in the mathematical model offers the possibility of implementing other types of controllers that allow for the testing of different strategies of the floating hybrid control system, with the aim of maximizing energy production while ensuring the structural stability of the floating hybrid system. [ABSTRACT FROM AUTHOR]

Abstract This article proposes an efficient correction model that enables the extension of the blade element momentum method (BEM) for swept blades. Standard BEM algorithms, assuming a straight blade in the rotor plane, cannot account for the changes in the induction system introduced by blade sweep. The proposed extension corrects the axial induction regarding two aspects: the azimuthal displacement of the trailed vorticity system and the induction of the curved bound vortex on itself. The extended algorithm requires little additional processing work and maintains BEM's streamtube independent approach. The proposed correction model is applied to simulations of swept blade geometries based on the IEA 15 MW reference wind turbine. Results show good agreement with lifting line simulations that inherently can account for the swept blade geometry. Blade sweep couples bending and torsion deformations by curving the blade axis in the inplane direction. As such, it can be used to passively alleviate loads and, thus, aeroelastically tailor wind turbine blades. The implementation of aeroelastic tailoring techniques, and the aeroelastic analysis in general, becomes increasingly significant with the size of wind turbine rotors continually rising. Due to its low computing complexity, BEM remains a crucial tool in the aerodynamic and aeroelastic analysis of wind turbine rotors. Thus, the proposed correction model contributes to a fast and accurate evaluation of swept blade designs.

Copyright of Engineering Mechanics / Gongcheng Lixue is the property of Engineering Mechanics Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper intends to establish a blade element momentum theory (BEMT) model for analyzing the rotor-and-rotor interference in the hovering intermeshing rotor system. The model uses the blade tip/hub losses to portray the intrinsic three-dimensional rotor aerodynamic effects and introduces the skew downwash flow to determine the rotor-and-rotor interference. Then, the model employs the interference locus considering the intersecting angle and the rotor hub spacing to distinguish the interference operating region on the rotor disk. Next, the BEMT model results are validated against experimental data of the German Autonomous Rotorcraft for Extreme Altitudes (AREA) helicopter and the computational fluid dynamics simulation results of the JZ series helicopters. Moreover, the effects of the rotor performance on the intersecting angle are also considered. It is found that the interference operating region resembles a semi-ellipse shape on the intermeshing rotor disk, which is not sensitive to the collective pitch angles under given structural parameters. Second, the emerging thrust recovery region created by raising the intersecting angle from 20 to 30 deg has a limited enhancement to the rotor thrust and the rotor performance. However, the incremental intersecting angle will produce a significant variation of the rotor thrust in the horizontal component in the body coordinate system, providing a large design numerical margin for the lateral stability in the hovering intermeshing rotor system. [ABSTRACT FROM AUTHOR]

Fixed-pitch coaxial, contrarotating rotors are often used in multirotor aircraft in applications ranging from payload delivery to urban air mobility. This paper examines minimizing net power at a given thrust for the coaxial pair using both an analytical approach based on blade element momentum theory and experimental results conducted in a hover test stand, as well as experimental results in hover for an X8 configuration multirotor. Appropriate distribution of thrust (realized via differential rotor speeds) to the upper and lower rotors results in approximately 5% savings in total power. Under this condition the thrust distribution is approximately 60% by the upper rotor and 40% by the lower rotor (which occurs when the lower rotor shaft speed is roughly 10% higher than the upper rotor shaft speed). Similar ratios were obtained in both the analytical solution and in experiment, at various total thrust levels. Hover flight tests of the X8 multirotor also showed approximately 5% reduction in hover power at this RPM setting. [ABSTRACT FROM AUTHOR]

This article presents the findings from a numerical study on hydrokinetic turbines for application in isolated and low populated areas. A numerical code for the determination of the performance of horizontal axis hydrokinetic turbine is developed based on the blade element momentum method, optimized and validated. It is intended to investigate hydrofoils which are easy to manufacture and to maintain. These hydrofoils include hydrofoil and the Joukowski J 0015, GO 410, flat plate and NACA 0024 (reference) hydrofoils. The effects of the chord and twist angle distributions are investigated, and their influence on the hydrodynamic performance is evaluated. The results showed that the linear twist angle distribution along the blade showed the best performance. The power coefficient for the Joukowski J 0015-, Gottingen GO 410- and NACA 0024-based rotor blades is, respectively, 0.512, 0.506 and 0.513 indicating the good performance of the Joukowski hydrofoil. The combination of the Joukowski J0015 hydrofoil with linear chord and twist distributions showed the best performance. The flat plate hydrofoil-based rotor with constant chord and linear twist angle distributions showed a reasonable power coefficient of 0.324 and can be a viable solution considering facility of manufacturing and maintenance. [ABSTRACT FROM AUTHOR]

In this paper, the control problem of a wind turbine in region 3 (where the wind velocity is between the rated wind velocity and cut out wind velocity) has been investigated by considering the aerodynamic nonlinear behavior of the wind-structure interaction. The model has been developed by using the blade element momentum (BEM) theory to obtain the aerodynamic torque and aerodynamic loads in edgewise and flapwise directions. For validation, the aerodynamic behavior of the onshore NREL 5 MW turbine has been compared with the Fatigue, Aerodynamics, Structures, and Turbulence (FAST) aeroelastic code in terms of the power coefficient. Wind speed is modelled as a three-dimensional profile with Kaimal spectrum distribution. The wind profile data is modeled in the frequency domain by the Kaimal spectrum distribution function. Then, an inverse Fourier transformation is needed to convert the data into the time domain. In the next, the sliding mode approach is applied to the three DOFs model of the drivetrain system which includes the rotor speed, low-speed shaft angle, and pitch angle. Finally, to consider the complete behavior of the onshore wind turbine, an eleven DOFs model is obtained. This model considers three DOFs for the flapwise vibrations of the blades, another three DOFs for the edgewise vibrations of the wind turbine, two DOFs for the side-side and fore-aft vibrations of the tower, and three DOFs for the drive-train system of the wind turbine. The performance of the proposed sliding mode control methods has been compared with the conventional PID controller for the above-rated wind velocity region. Results demonstrate that the wind turbine performance has been improved. [ABSTRACT FROM AUTHOR]

The rising interest in the evolvability of electric vertical takeoff and landing (eVTOL) promises substantial potential in the field of urban air mobility (UAM). Challenges in energy storage density and geometry restriction both emphasize the propeller efficiency for endurance and takeoff weight, whereas the contra-rotating propellers (CRP) advantage is balancing high thrust and efficiency over a single propeller. The aim of this paper is twofold: (i) to present a novel rapid CRP blade shape optimization framework and (ii) to study the impact of the dual propellers revolution speed allocations on the overall CRP power efficiency. The core of the framework is the blade element momentum theory (BEMT)-based blade shape optimization considering the wake effect of the upper propeller by the rotational CFD (computational fluid dynamics) actuator-disc simulation method. The results show that for the same thrust, the optimized CRP at the equal revolution speed is superior to the original (upper-lower-identical) one by 5.9% in thrust-to-power ratio. The overall efficiency can be additionally lifted by 5.3% when the dual propellers share similar torques. By excluding the integral propeller CFD simulation and empirical parameters estimation, the framework enables the swift obtaining of an optimized CRP scheme while maintaining robustness as well. [ABSTRACT FROM AUTHOR]

The optimized chord and twist angle of the preliminary blade design through Blade Element Momentum theory are non-linear distributions, which adds to the complexity of blade manufacture and does not always guarantee the best aerodynamic performance. In this paper, the effect of the linearization on aerodynamic performance using Prandtl-Glauert correction model was investigated through four cases: case 1 and case 2 and case 3, where the chord and the twist angle are linearized and case 4, where sole chord is linearized. The effect of the linearization using Shen correction model while making a comparison to the linearization using Prandtl-Glauert correction model was also studied. The simulation is conducted for S809 wind turbine blade profile. The results show that case 4 using Shen correction model represents the best technique of linearization in terms of higher aerodynamic performance and easy manufacturing process. [ABSTRACT FROM AUTHOR]

To accurately predict the ship's manoeuvring and powering performance in actual seaways, it is crucial to gain an enhanced comprehension of the hydrodynamic behaviour of vessels navigating through waves. A critical component is the accurate determination of forces exerted on the hull and its appendages when the ship is operating at an angle of drift in waves. This is also significant for wind-assisted ships, which often operate with non-zero drift and rudder angles. Therefore, a deeper understanding of how drift and rudder angles affect hull–propeller–rudder interaction is required for investigating energy efficiency in waves. In this paper, a thorough numerical study is conducted to investigate the hydrodynamic interaction among the hull, propeller and rudder of the benchmark KRISO Container Ship (KCS) in regular head waves. The KCS is simulated at drift angles of −10°, 0°and +10°, combined with a series of rudder angles (−20°to +20°), representing quasi-static phases of actual ship manoeuvring in waves. Blade Element Momentum theory (BEMt) is adopted for modelling propeller action in all cases. Good agreement is found between experimental and numerical predictions regarding hull forces. This study contributes to better ship design due to ship manoeuvring and operations of wind-assisted vessels. • Drift effect on hull–propeller–rudder interaction in waves is studied numerically. • The propeller thrust loading effect on sideforce and yaw moments is studied in waves. • Towing tank tests for CFD validation include hull drags for three wavelengths. • Drift and rudder effects on wake fraction and thrust deduction are studied in waves. [ABSTRACT FROM AUTHOR]

In the present study, helicopter rotor blades with trailing edge flaps are studied to analyze their aerodynamic and performance characteristics for hover conditions. The effect of different characteristics of trailing edge flaps, i.e. flap cord, flap span, the radial location of the flap along the blade and flap deflection angle are inspected. Preliminary calculations are conducted using the properties of BO105 rotor. In order to examine the parameters of the rotor blade, an in-house aerodynamic code is developed by applying the Blade Element Momentum Theory formulations with tip loss effects and XFoil program is used to find 2-D airfoil characteristics. Consequently, effects of several parameters of trailing edge flap concept are demonstrated in several figures where variation of the Flap authority (FA) and the figure of merit authority (FMA) are examined. Flap authority is more affected by the change of Sp, Cr, Md, δ parameters in cases where the thrust coefficient is low. It is directly proportional to the change of each of the flap parameters with the change of the FA value. The FMA increases in low-loaded blades when the flaps approach the root, while it decreases when the flaps are positioned towards the blade tip.

This article presents flutter prediction of composite wind turbine blades through time domain aeroelastic analysis using unsteady blade element momentum (BEM) theory based aerodynamic model. Comparison of the flutter speeds obtained in the present study with those previously obtained through Eigen solution based approach using Theodorsen's theory based aerodynamic model for several different lamination sequences and blade configuration has been presented. It has been observed that both the approaches predict highest flutter speed at the same blade configuration and lamination sequence. However, flutter limit obtained through the present approach is less as compared to that obtained through Eigen solution based approach. [ABSTRACT FROM AUTHOR]

This article proposes an efficient correction model that enables the extension of the blade element momentum method (BEM) for swept blades. Standard BEM algorithms, assuming a straight blade in the rotor plane, cannot account for the changes in the induction system introduced by blade sweep. The proposed extension corrects the axial induction regarding two aspects: the azimuthal displacement of the trailed vorticity system and the induction of the curved bound vortex on itself. The extended algorithm requires little additional processing work and maintains BEM's streamtube independent approach. The proposed correction model is applied to simulations of swept blade geometries based on the IEA 15 MW reference wind turbine. Results show good agreement with lifting line simulations that inherently can account for the swept blade geometry. Blade sweep couples bending and torsion deformations by curving the blade axis in the inplane direction. As such, it can be used to passively alleviate loads and, thus, aeroelastically tailor wind turbine blades. The implementation of aeroelastic tailoring techniques, and the aeroelastic analysis in general, becomes increasingly significant with the size of wind turbine rotors continually rising. Due to its low computing complexity, BEM remains a crucial tool in the aerodynamic and aeroelastic analysis of wind turbine rotors. Thus, the proposed correction model contributes to a fast and accurate evaluation of swept blade designs. [ABSTRACT FROM AUTHOR]

Understanding the impacts of unsteady flows on tidal turbine performance and loadings prior to device deployment is essential for mitigating the effects of large hydrodynamic forces and avoiding premature fatigue. High-frequency velocity measurements from an energetic tidal channel were fed into a model that couples Blade Element Momentum (BEM) theory with dynamic stall and rotational augmentation corrections. A model turbine operating in real-world conditions was used to investigate different hub submergence depths and also extreme wave conditions. Mean turbine coefficients were more affected by varying levels of shear than by waves while standard deviations were more sensitive to shear than to the proximity of surface waves. Under long waves, power and thrust coefficients reached more than twice the average values and standard deviations are enhanced by over 70%. Waves moved the separation point towards the leading edge in over half the blade span and thus impacting the stall behaviour. These outcomes contribute to the understanding of power extraction and hydrodynamic forces around turbine blades in real-world conditions. This analysis is important for turbine developers to design resistant structures and thus extend device durability, but also to improve turbine performance. [ABSTRACT FROM AUTHOR]

The hydrokinetic turbine is used worldwide for electrical generation purposes, as such a technology may strongly reduce environmental impact. Turbines designed using backward swept blades can significantly reduce the axial load, being relevant for hydro turbines. However, few works have been conducted in the literature in this regard. For the case of hydrokinetic rotors, backward swept blades are still a challenge, as the authors are unaware of any optimization procedures available, making this paper relevant for the current state of the art. Thus, the present work develops a new optimization procedure applied to hydrokinetic turbine swept blades, with the main objective being the design of blades with reduced axial load on the rotor and possibly a reduction in the cavitation. The proposed method consists of an extension of the blade element momentum theory (BEMT) to the case of backward swept blades through a radial transformation function. The method has low computational cost and easy implementation. Once it is based on the BEMT, it presents good agreement when compared to experimental data. As a result, the sweep heavily affects the chord and twist angle distributions along the blade, increasing the turbine torque and power coefficient. In the case of the torque, it can be increased by about 18%. Additionally, even though the bound circulation demonstrates a strong change for swept rotors, Prandtl's tip loss seems to be not sensitive to the sweep effect, and alternative models are needed. [ABSTRACT FROM AUTHOR]

The main objective of this work is to characterise the prop-rotor of V/STOL (Vertical/short-take-off and landing) aircraft in various operating conditions, especially in transition flight. To this end, a BET (Blade element theory) based analytic model/tool with a low computational cost has been built to predict the aerodynamic performance of the prop-rotor. This BET-based model can predict prop-rotor thrust, torque, normal force, and yaw moment as well as their corresponding 1P load (once-per-revolution load on the single blade) regarding oblique flow. This model is verified and shows good consistency with the wind tunnel experimental data and CFD simulation results. Furthermore, by using this BET model/tool, the effects of axial advance ratio, tilt angle and collective pitch angle on these component loads on the prop-rotor are explored. Moreover, the effects of these variables on the vibratory amplitudes of the 1P load on the single blade and 3P harmonic loads on the prop-rotor due to inclined flow are presented as well. [ABSTRACT FROM AUTHOR]

In recent times, the application of small-scale horizontal axis wind turbines (SHAWTs) has drawn interest in certain areas where the energy demand is minimal. These turbines, operating mostly at low Reynolds number (Re) and low tip speed ratio (λ) conditions, can be used as stand-alone systems. The present study aims at the design, development, and testing of a series of SHAWT models. On the basis of aerodynamic characteristics, four SHAWT models viz., M1, M2, M3, and M4 composed of E216, SG6043, NACA63415, and NACA0012 airfoils, respectively, have been developed. Initially, the rotors are designed through blade element momentum theory (BEMT), and their power coefficient has been evaluated. Thence, the developed rotors are tested in a low-speed wind tunnel to find their rotational frequency, power, and power coefficient at design and off-design conditions. From BEMT analysis, M1 shows a maximum power coefficient (Cpmax) of 0.37 at λ = 2.5. The subsequent wind tunnel tests on M1, M2, M3, and M4 at 9 m/s show the Cpmax values as 0.34, 0.30, 0.28, and 0.156, respectively. Thus, from the experiments, the M1 rotor is found to be favorable than the other three rotors, and its Cpmax value is found to be about 92% of BEMT prediction. Further, the effect of pitch angle (θp) on Cp of the model rotors is also examined, where M1 is found to produce a satisfactory performance within ±5 deg from the design pitch angle (θp,design). [ABSTRACT FROM AUTHOR]

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Due to the fact that the noise caused by axial fan blades of vehicles is large, which seriously affects ride comfort, and there is no effective mathematical model to quantitatively study the contribution of the various parameters of the blades to the noise, a new method for calculating the load force of the blades is proposed. This method obtains the constant load force of the blade according to the blade element momentum theory and the characteristics of the blade structure of the axial fan for a vehicle. At the same time, this method obtains the non-constant load force of the blade by combining the non-constant thin-wing theory and experimental data and then vectors the constant load force and the non-constant load force to obtain the total load force of the blade to build a mathematical model of the relationship between the noise of the fan and the parameters of the blade. According to the model, the total sound pressure level of a fan is calculated numerically and further compared with the FLUENT software simulation and experimental results. The results show that the error of the total sound pressure level calculated by the numerical value is within 3 dB(A). This method provides an important basis for the study of a high-accuracy noise mathematical model and the optimization of blade parameters of low Mach-number fans. [ABSTRACT FROM AUTHOR]

This paper presents a theoretical and computational study to determine the optimal positions of airfoils along the length of the horizontal axis wind turbine blade. We used four and five-digit NACA airfoils to model a 54-meter blade. The lift, drag coefficient, and lift-to-drag ratio of each airfoil are determined by using QBlade software. The aerodynamic performance of the airfoils is studied based on the blade element momentum theory, and Matlab software is used for numerical implementation. The velocity and pressure distributions on each airfoil are assessed using computational fluid dynamics. We implement the thickness distribution techniques to adjust the positions of the airfoils along the length of the blade. It is noted that stresses reach their maximum values at the root and minimum at the tip section. Thus, the thicker (NACA 4420) and thinner (NACA 23012) airfoils are set at 20% of the maximum chord and 91.11% at the tip sections of the blades. The remaining sections of the blade are configured using linear interpolation methods. Specifically, the maximum chord length of the new design is reduced by 18.06% compared to the NACA 23012 rotor blade. Finally, the recommended tip speed ratio for the designed rotor blade is estimated using the graphs of the normal and tangential forces, thereby producing a safe and efficient design. [ABSTRACT FROM AUTHOR]

We propose a reinforcement learning strategy to control wind turbine energy generation by actively changing the rotor speed, the rotor yaw angle and the blade pitch angle. A double deep Q-learning with a prioritized experience replay agent is coupled with a blade element momentum model and is trained to allow control for changing winds. The agent is trained to decide the best control (speed, yaw, pitch) for simple steady winds and is subsequently challenged with real dynamic turbulent winds, showing good performance. The double deep Q-learning is compared with a classic value iteration reinforcement learning control and both strategies outperform a classic PID control in all environments Furthermore, the reinforcement learning approach is well suited to changing environments including turbulent/gusty winds, showing great adaptability. Finally, we compare all control strategies with real winds and compute the annual energy production. In this case, the double deep Q-learning algorithm also outperforms classic methodologies. • Reinforcement learning (Double deep Q-learning) is developed to maximize wind energy energy in changing turbulent winds. • BEMT allows three control parameters (rotational speed, blade pitch and rotor yaw). • Description of states, actions, rewards, training and validation are included. • Comparisons with a classic PID control and Value Iteration reinforcement learning are included. • Double deep Q-learning outperforms the other control mechanisms tested. [ABSTRACT FROM AUTHOR]

Diffuser technology placed around hydrokinetic rotors may improve the conversion of the fluid's kinetic energy into shaft power. However, rotor blades are susceptible to the phenomenon of cavitation, which can impact the overall power efficiency. This paper presents the development of a new optimization model applied to hydrokinetic blades shrouded by a diffuser. The proposed geometry optimization takes into account the effect of cavitation inception. The main contribution of this work to the state of the art is the development of an optimization procedure that takes into account the effects of diffuser efficiency, η d , and thrust, C T d . The authors are unaware of any other work available in the literature considering the effect of η d and C T d on the cavitation of shrouded hydrokinetic blades. The model uses the Blade Element Momentum Theory to seek optimized blade geometry in order to minimize or even avoid the occurrence of cavitation. The minimum pressure coefficient is used as a criterion to avoid cavitation inception. Additionally, a Computational Fluid Dynamics investigation was carried out to validate the model based on the Reynolds-Averaged Navier–Stokes formulation, using the κ − ω Shear-Stress Transport turbulence and Rayleigh–Plesset models, to estimate cavitation by means of water vapor production. The methodology was applied to the design of a 10 m diameter hydrokinetic rotor, rated at 250 kW of output power at a flow velocity of 2.5 m/s. An analysis of the proposed model with and without a diffuser was carried out to evaluate the changes in the optimized geometry in terms of chord and twist angle distribution. It was found that the flow around a diffuser-augmented hydrokinetic blade doubles the cavitation inception relative to the unshrouded case. Additionally, the proposed optimization model can completely remove the cavitation occurrence, making it a good alternative for the design of diffuser-augmented hydrokinetic blades free of cavitation. [ABSTRACT FROM AUTHOR]

Several physics-based enhancements are embedded in a low-fidelity general unducted rotor design analysis tool developed, py_BEM, including the local Reynolds number effect, rotational corrections to airfoil polar, stall delay model, high induction factor correction, polar at large angle of attack, exergetic efficiency calculation and momentum-based loss. A wind turbine rotor is analyzed in high fidelity designed from py_BEM using a 3D blade generator. It is a design derived from the NREL Phase VI rotor. Three design variations are analyzed using steady 3D CFD solutions to demonstrate the effect of geometry on aerodynamics. S809 and NACA 2420 airfoil properties are used for calculating the aerodynamic loading. Momentum, vorticity and energy transport are explained in depth and connected to entropy production as a measure of performance loss. KE dissipation downstream of the rotor is shown to be a significant contributor of entropy rise. Wake analysis demonstrates mixing with the free stream flow, which begins after 3 diameters downstream of the rotor and extends to about 25 diameters until the decay is very small. Vorticity dynamics is investigated using a boundary vorticity flux technique to demonstrate the relationship between streamwise vorticity and lift generated in boundary layers. Drag components are accounted as well. It is demonstrated using rothalpy that shaft power is not only torque multiplied by rotational velocity but a viscous power loss term must also be included. A multifidelity analysis of wind turbine aerodynamics is demonstrated by capturing flow physics at several levels. [ABSTRACT FROM AUTHOR]