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Abstract
The numerical simulations for cycloidal propellers based on five airfoils with different thickness are presented in this paper. The CFD simulation is based on sliding mesh and URANS. The result of CFD simulation indicates that all test cases share similar flow pattern. There are leading edge vortex and trailing edge vortex due to blade dynamic stall. Interaction between the vortices shed from upstream blade and the downstream blade is observed. There is also variation of the flow velocity encountered by the blade due to downwash in the cycloidal rotor cage. These factors result in large fluctuations of the aerodynamics forces on the blade. The comparison of the forces and flow pattern indicates that the thickness of the airfoil is very important parameter that influents the flow pattern and hence the performance of the cycloidal propeller.
... From this study they noticed that curved profiles resulted in reduced performance as compared to symmetrical ones. In turn, Hu and Zhang [10,11] conducted a combined experimental and numerical study for cycloidal rotors with five airfoil profiles. The results showed that the NACA 0015 airfoil achieved the highest figure of merit (FM) and the maximum thickness of the airfoil significantly affected the flow field and the aerodynamic performance of the cycloidal propeller at macro scale. ...
This study analyzes the effect of DBD plasma actuators on the unsteady flow of a cycloidal rotor with six blades of NACA 0016 type using computational fluid dynamics (CFD). The flow field modeling is performed through a sliding mesh technique that was used with k-ω SST turbulence model. The plasma body force generated by a dielectric barrier discharge DBD actuator is modeled with the phenomenological Shyy's model. The impact of plasma actuators on the flow is studied using two actuators located on the upper and bottom surfaces of the airfoil in each of the cyclorotor blades. Three cases are studied: actuation off; actuation over the upper surface of the blades; actuation over the bottom surface of the blades. The vorticity field and the evolution of the lift, thrust and power consumption are analyzed for when a control law is designed that combines both actuations based on the two last cases. The results show that the use of the proposed control law for plasma actuation improves the cyclorotor lift peak value per blade in 11% and the overall thrust by 1.5%. It is also demonstrated that the plasma actuator mitigates the virtual camber on the cyclorotor, this effect is named counter-virtual camber. We show that DBD actuators produce a reduction on the recirculation bubble at the blades suction surface. This effect is important to reduce the thickness of the blades, bringing improvements in terms of mitigating unsteady periodic mass acceleration in cyclorotors.
... The experiments and numerical analysis for cycloidal propellers were also performed by the authors. [17][18][19][20][21] The cyclogyro that can fly with good stability were developed. 21 Our previous researches indicated that two-dimensional simulations can produce time-averaged aerodynamic forces with good accuracy, although the blade tip vortices (BTV) were neglected. ...
Cycloidal rotor is a rotor whose blades pitch around the pitching axis and revolve around the rotor shaft that is parallel to blade. It can generate omni-directional thrust with high efficiency. In this paper, the numerical simulation models were validated by the wind tunnel experiments. Then physics of the cycloidal rotor in forward flight was studied. The effects of blade number and advance ratio were qualitatively discussed based on the numerical simulation results. Results of the analysis indicate that the rotor with more blades will result in smoother force curve, so that there will be lower vibration. However, the rotor with three or four blades will be the most efficient. The maximum forward flight efficiency is obtained from medium-to-high advance ratio. The efficiency is comparable with that of a screw propeller at the same Reynolds number. At low advance ratio, the peak lift on the blade can be observed when the blade is located at the lower left part of its trajectory. This is caused by positive blade pitch angle and relatively large inflow speed, which is similar to the downwash in the rotor cage under hovering status. From medium-to-high advance ratio, the thrust and lift primarily originated from the plunging motion of the blade. If the advance ratio is high enough, there will be negative horizontal force and positive torque, which means that the blade is taking energy from the inflow. Resultant force of the horizontal and vertical force does not vary too much with the advance ratio. But with the advance ratio approaching 1.0, the direction of the resultant force will point upwards and no horizontal force can be observed.
In the operation of cycloidal rotors the flow structure deformation, and associated curvature, limits the possibility of achieving an optimum working point. In this study, first, we started by performing a detailed analysis of the start-up characteristics of a cycloidal rotor in order to identify the evolution of kinematic and dynamic characteristics of its operation. Afterwards, we identified the effects of applying multiple plasma boundary layer control, of dielectric barrier discharge type. This application is defined in both sides of the blades, since in cyclorotor movement they change from pressure to suction side over one blade rotation. A control law is defined to regulate the operation of the plasma actuators as the rotor blade move in azimuthal direction. The coupled multiphysics simulation of the cyclorotor and plasma flow is performed by adding to Fluent two user define functions to model the complex cyclorotor movement and the plasma momentum effect. The k-\(\omega \) SST turbulence model is used in the computations. The study is performed for a cyclorotor comprised of six NACA 0016 blade profiles that rotate at 200 rpm. The results demonstrate the advantage of using multiple plasma actuators in order to control the blade flow field. In particular, the configuration with six actuators, three in each side, was the most effective, by improving the thrust by 2.3%, as compared to the base case, and achieving a reduction in power requirements of 0.9%.
A study was conducted to demonstrate the development of a micro twin-rotor cyclocopter capable of autonomous hover. The blades used on the twin-cyclocopter were fabricated mostly out of foam with carbon-fiber reinforcement inside and a single-layer 0/90 degree carbon composite prepreg skin wrapped around the foam core at the blade tips. Foam helped in maintaining the required airfoil shape for the blades, while most of the bending and torsion stiffness was provided by the carbon-fiber structure embedded inside the foam. One of the key requirements for the success of a cyclocopter was a simplified lightweight blade-pitching mechanism. A feedback control system was required to provide sufficient attitude damping and stiffness to achieve stable hover.
This paper presents a 2D computational investigation on the dynamic stall phenomenon associated with unsteady flow around the NACA0012 airfoil at low Reynolds number (Rec≈105). Two sets of oscillating patterns with different frequencies, mean oscillating angles and amplitudes are numerically simulated using Computational Fluid Dynamics (CFD), and the results obtained are validated against the corresponding published experimental data. It is concluded that the CFD prediction captures well the vortex-shedding predominated flow structure which is experimentally obtained and the results quantitatively agree well with the experimental data, except when the blade is at a very high angle of attack.
Performance and flowfield measurements were conducted on a small-scale cyclorotor for application to a micro air vehicle. Detailed parametric studies were conducted to determine the effects of the number of blades, rotational speed, and blade pitching amplitude. The results showed that power loading and rotor efficiency increased when using more blades; this observation was found over a wide range of blade pitching amplitudes. The results also showed that operating the cyclorotor at higher pitching amplitudes resulted in improved performance, independently of the number of blades. A momentum balance performed using the flowfield measurements helped to quantify the vertical and sideward forces produced by the cyclorotor; these results correlated well with the force measurements made using load balance. Increasing the number of blades increased the inclination of the resultant thrust vector with respect to the vertical because of the increasing contribution of the sideward force. The profile drag coefficient of the blade sections computed using a momentum deficit approach correlated well with typical values at these low chord Reynolds numbers. Particle image velocimetry measurements made inside the cage of the cyclorotor showed that there are rotational flows that, when combined with the influence of the upper wake on the lower half of the rotor, explain the relatively low efficiency of the cyclorotor.
This paper investigates numerically the unsteady separated turbulent flows around an oscillating airfoil pitching in a sinusoidal pattern that induces deep dynamic stalls. The flow is in the regime of relatively low Reynolds number of the order of 105 based on the chord length of the airfoil. Both the URANS and the more advanced DES approaches are employed. The URANS is coupled with two advanced turbulence models, namely the RNG k−ε model and the Transition SST model (γ−Reθ model) and the DES is coupled with the SST k−ω model. A comparison with experimental data shows that the SST k−ω based DES approach is superior to the URANS approach and presents generally good agreement with the experimental data, although the prediction of experimentally observed peek stall angle of attack may not be warranted. The details of the complex flow development of the dynamic stall and the boundary layer transition have been discussed.
The viability of cycloidal rotor (cyclocopter) concept for developing hover-capable micro-air vehicles was analyzed through performance and flow field measurements. Detailed parametric studies were conducted to identify the dependence of the rotor performance on the amplitude of the blade pitch, blade airfoil profile, blade flexibility, and rotational speed. All experiments were conducted using a three-bladed, model scale cyclocopter that was tested up to the design rotational speed of 2,000 RPM. While higher pitch angles were found to increase the power loading (thrust/power) of the cyclocopter, the bending and torsional flexibility of the blades deteriorated the performance. Similarly, fixed camber proved to be detrimental to the rotor performance. Thrust measurements suggested the presence of a sidewise force (along with the vertical lift), similar to those found with lifting cylinders. Digital particle image velocimetry (DPIV) measurements made in the wake of the cyclocopter provided evidence of wake skewness, resulting in sideward force. The thrust produced by the cyclocopter was found to increase with a geometric pitch of 45° without showing any signs of stall. This behavior was explained through DPIV measurements that showed the presence of high induced velocities, resulting from the trailing tip vortices. These velocities are comparable to the rotor blade velocities, reducing substantially the aerodynamic angles of attack experienced by the rotor blades, thereby, preventing the occurrence of stall. DPIV measurements also identified several interesting flow features that include the presence of a leading edge vortex, similar to a dynamic stall vortex. The slipstream boundary obtained by following the path of the tip vortices was found to contract, as expected for a lifting rotor.
This paper describes the systematic performance measurements conducted to understand the role of rotor geometry and blade pitching kinematics on the performance of a microscale cycloidal rotor. Key geometric parameters that were investigated include rotor radius, blade span, chord, and blade planform. Because of the flow curvature effects, the cycloidal-rotor performance was a strong function of the chord/radius ratio. The optimum chord/radius ratios were extremely high, around 0.5-0.8, depending on the blade pitching amplitude. Cycloidal rotors with shorter blade spans had higher power loading (thrust/power), especially at lower pitching amplitudes. Increasing the solidity of the rotor by increasing the blade chord, while keeping the number of blades constant, produced large improvements in power loading. Blade planform shape did not have a significant impact, even though trapezoidal blades with a moderate taper ratio were slightly better than rectangular blades. On the blade kinematics side, higher blade pitching amplitudes were found to improve the power loading of the cycloidal rotor. Asymmetric pitching with a higher pitch angle at the top than at the bottom produced better power loading. The chordwise optimum pitching axis location was observed to be around 25-35% of the blade chord. The power loading of the optimized cycloidal rotor was higher than that of a conventional microrotor.
This paper describes the systematic experimental and computational studies performed to obtain a fundamental understanding of the physics behind the lift and thrust production of a cycloidal rotor (cyclorotor) in forward flight fora unique blade pitching kinematics. The flow curvature effect (virtual camber and incidence due to the curvilinear flow) was identified to be a key factor affecting the lift, thrust, and power of the cyclorotor in forward flight. The experimental study involved systematic testingof amicro air vehicle-scale cyclorotorinan open-jet wind tunnel using a custom built three-component balance. The key parameters varied include rotor chord/radius ratio and blade pitching axis location because these two parameters have a strong impact on flow curvature effects. Because of the virtual camber/incidence effects and the differences in the aerodynamic velocities around the azimuth, the blades producea small downward lift when they operate in the upper half ofthe circular trajectory and a large upward lift in the lower half, producing a net lift in the upward direction. The magnitude of this lift depends on the chord/radius ratio and the blade pitching axis location, and the direction of lift depends on the sense of rotation. The positive thrust on the cyclorotor is produced when the blades operateinthe rear halfof the rotor, while they produce a small negative thrustasthey operateinthe frontal half. The lift per unit powerofthe rotorisincreased with chord/radius ratio until a c/R of 0.67. Moving the pitching axis location closer to the leading edge also increased the lift producing efficiency of the cyclorotor. It was observed that the optimum chord/radius ratio for maximum thrust per unit power decreased with forward speed. A key conclusion was that the lift producing efficiency (lift per unit power) of the rotor (for a constant thrust) increased with forward speed while the thrust producing efficiency (fora constant lift) decreased with forward speed. This study also disproves the conventional argument that a cyclorotor needs two completely different pitching schedules for efficient hover and forward flight because it is shown that a simple phase shifting of the hover kinematics could result in efficient forward flight kinematics provided the cyclorotor has a high chord/radius ratio. Copyright © 2013 by the American Institute of Aeronautics and Astronautics, Inc.
The last few years have proved that Vertical Axis Wind Turbines (VAWTs) are more suitable for urban areas than Horizontal Axis Wind Turbines (HAWTs). To date, very little has been published in this area to assess good performance and lifetime of VAWTs either in open or urban areas. At low tip speed ratios (TSRs<5), VAWTs are subjected to a phenomenon called 'dynamic stall'. This can really affect the fatigue life of a VAWT if it is not well understood. The purpose of this paper is to investigate how CFD is able to simulate the dynamic stall for 2-D flow around VAWT blades. During the numerical simulations different turbulence models were used and compared with the data available on the subject. In this numerical analysis the Shear Stress Transport (SST) turbulence model seems to predict the dynamic stall better than the other turbulence models available. The limitations of the study are that the simulations are based on a 2-D case with constant wind and rotational speeds instead of considering a 3-D case with variable wind speeds. This approach was necessary for having a numerical analysis at low computational cost and time. Consequently, in the future it is strongly suggested to develop a more sophisticated model that is a more realistic simulation of a dynamic stall in a three-dimensional VAWT.
This paper describes a series of systematic experimental studies conducted to optimize the performance of a MAV scale cycloidal rotor. Parametric studies were performed to study the effects of blade airfoil section, blade pitching amplitude, asymmetric pitching, location of blade pitching axis and number of blades (at constant solidity) on the performance of the cycloidal rotor. Higher blade pitch angles were found to improve thrust and increase the power loading (thrust per unit power) of the cycloidal rotor. Asymmetric pitching, with a higher pitch angle at the top than at the bottom of the circular blade trajectory, produced better power loading. The optimum pitching axis location was observed to be around 25-35% of the blade chord. For a constant solidity, the rotor with lesser number of blades produced higher thrust and a 2-bladed rotor had the best power loading. Using the results from these parametric studies, an optimized cycloidal rotor with a significantly improved power loading over a conventional MAV rotor was developed.
This paper describes the aeroelastic model to predict the blade loads and the average thrust of a micro-air-vehicle-scale cycloidal rotor. The analysis was performed using two approaches: one using a second-order nonlinear beam finite element method analysis for moderately flexible blades and a second using a multibody-based large-deformation analysis (especially applicable for extremely flexible blades) incorporating a geometrically exact beam model. An unsteady aerodynamic model is included in the analysis with two different inflow models: single streamtube and double-multiple streamtube inflow models. For the cycloidal rotors using moderately flexible blades, the aeroelastic analysis was able to predict the average thrust with sufficient accuracy over a wide range of rotational speeds, pitching amplitudes, and number of blades. However, for the extremely flexible blades, the thrust was underpredicted at higher rotational speeds, and this may be because of the overprediction of blade deformations. The analysis clearly showed that the reason for the reduction in the thrust-producing capability of the cycloidal rotor with blade flexibility may be attributed to the large nosedown elastic twisting of the blades in the upper half cylindrical section, which is not compensated by a noseup pitching in the lower half-section. The inclusion of the actual blade pitch kinematics, unsteady aerodynamics, and flow curvature effects was found crucial in the accurate lateral force prediction.
Studies of a MAV-scale cycloidal rotor (cyclocopter) were performed using performance and flow field measurements. Detailed parametric studies were conducted to determine the effects of rotational speed, the amplitude of the blade pitch angle, and the number of blades on cycloidal rotor performance. It was found that a 5-bladed cyclocopter system operating with 40° pitch produced the best power loading (thrust per unit power), with a figure of merit (based on the definition given in the paper) of 0.4. Thrust measurements suggested the presence of a sidewise force along with the vertical lift, similar to those found on rotating circular cylinders. This was confirmed using particle image velocimetry (PIV) measurements, which showed evidence of a wake skewness. The magnitudes of the measured vertical and sidewise forces were confirmed from the flow field measurements using a momentum balance technique. The thrust produced by the cyclocopter was found to increase until a geometric pitch of 40° was reached without showing any signs of blade stall. This behavior was explained using the PIV measurements, which showed the presence of high induced velocities in the rotor wake, which reduced angles of attack and delayed the occurrence of stall. Blade lift and drag were estimated from the flow field measurements, and were found to correlate well with the balance measurements. The PIV measurements clearly showed the rotational nature of the flow inside the cycloidal rotor, which is a significant source of energy loss, as well as the fact that the lower half of the rotor operates in the wake of the upper half. The induced power loss manifested as a large induced power factor in the modified momentum theory when compared to measurements.
A cyclocopter propelled by the cycloidal blade system, which can be described as a horizontal rotary wing, is a new concept of VTOL vehicle. In this paper, structural design and analysis are carried out for the aerodynamically optimized cyclocopter rotor system. Design variables such as stacking sequence (ply angles), number of plies and locations of spars of composite blade are determined through MSC/NASTRAN, RSM and some other optimizing processes. The maximum stress obtained by static analysis of each parts of the rotor is within the failure criteria. The rotor system is designed for the purpose of avoiding possible dynamic instabilities by inconsistency between frequencies of rotor rotation and some low natural frequencies of rotor. Also dynamic analysis for the airframe which supports the rotor, engine, etc. is carried out.
Abstract Acyclocopter,propelled by the ,cycloidal blade system, which can be described as a horizontal rotary wing, is a new concept of VTOL vehicle. In this paper, cyclocopter rotor system is optimized for design variables such as radius of rotor, rotating speed, maximum pitch angle, chord and span of blade, number of blades and airfoil. To apply response surface methodology, datum acquired by STAR-CD for aerodynamic ,design ,variables and MSC/NASTRAN for ,structural ,ones. Genetic algorithm is used to find the optimal values from the constructed,response ,surface ,while ,objective functions are thrust, required power and stress of blade.
A combined experimental and numerical study of a cyclogiro, rotor operating at Reynolds numbers,about 40,000 has been conducted. The study reveals complex flow field with complex (unsteady) interactions between the blades and the wakes of other blades. In spite of this complexity, time-averaged integral forces acting on the rotor can be predicted by a simple momentum theory, properly corrected for thrust-producing area of the rotor and for large Magnus effect. The effective thrust producing area was estimated to be about half of its projected area, suggesting that the effectiveness of a cyclogiro rotor may be comparable with that of a heavy-loaded helicopter rotor.
A cyclocopter propelled by a cycloidal blade system is a new concept of vertical takeoff and landing aircraft. The cycloidal blade system, which can he described as a horizontal rotary wing, offers powerful thrust levels and a unique ability to change the direction of the thrust almost instantly. This paper investigates the development of the cyclocopter with four rotors, file aircraft was designed through computational fluid dynamics and finite element structural analyses. Elliptic blades and a swash plate were applied to the rotor system to improve the rotor performance and control mechanism. Efficient dc brushless motors and lithium-polymer batteries were used for power transmissions. Almost all parts of the rotor blades and fuselage were manufactured out of composite material. Thrust and required power were measured experimentally on the test bed. The experimental result shows that the cyclocopter on produce sufficient thrust for both hovering and low-speed forward flight.
The cycloidal propeller, which can be described as a horizontal rotary wing, offers powerful thrust levels, and a unique ability to change the direction of the thrust almost instantly. In this work, cycloidal propulsion system was designed and implemented with sinusoidal low pitch system to investigate the fundamental characteristics of hovering states. Before experimental study, computational analysis was made in commercial CFD tool, CD Adopco/Star- CD. This work gives schematic view of flow field around rotor. And it predicts the load variation on a blade with respect to revolution and the total thrust from cycloidal rotor, at specific condition. CFD analysis also makes it possible to check the camber effect. The propulsion system was built up with sinusoidal low pitch control mechanism. Thrust, power and efficiency were measured for various conditions of parameter setting. It was accomplished by designing the equipment which can change the rotational radius, number of blade, phase angle of eccentricity and amplitude of eccentricity.
Aerodynamics of the Cyclogiro
- Gil
- L Yuvaval
Gil, I and YuVAVAl, L. Aerodynamics of the Cyclogiro, AIAA 2003-3473.
Development and Flight Testing of a Twin-Rotor Cyclocopter Micro Air Vehicle
- M Benedict
- R Gupta
- C Inderjit
- Design
BeneDicT, M., GupTA, r. and inDerJiT, c. Design, Development and Flight Testing of a Twin-Rotor
Cyclocopter Micro Air Vehicle, Proceedings of the 67th Annual National Forum of the American
Helicopter Society, Virginia Beach, VA, USA, 3-5 May, 2011.