Rotor Configuration Research Articles

Analytical and Computational Study of Hover Performance of Novel Dissimilar Coaxial Rotor

This paper studies a novel dissimilar coaxial rotor concept with significantly reduced rotor–rotor aerodynamic interaction during hovering flight. The proposed concept’s performance is systematically compared with regular coaxial and conventional single rotor configurations using 1) analytical and 2) blade element momentum theory (BEMT)-based analysis for hover flight condition. The hover performance among different rotor configurations is compared using a baseline main rotor combined with various antitorque mechanisms. Through this study it is established that the dissimilar coaxial rotor configuration shows improved performance for torque equilibrium cases when compared to both conventional and regular coaxial configurations for moderate- to high-thrust conditions with power reduction of 16–37% when compared to a conventional single main rotor–tail rotor configuration and 14–17% of power saving when compared to a regular coaxial rotor during hovering flight with rotor thrust in the range of . The analytical method from momentum theory and BEMT results correlate well for the induced and profile torque balanced conditions. The proposed concept appears to be an efficient alternative to the conventional single main rotor with tail rotor and regular coaxial rotor for hovering flight conditions within the scope of the analysis.

Generation of a wingtip vortex pair using a pressure-side fin

We present results from a combined experimental and numerical investigation of the flow around a rectangular NACA 0012 wing, whose tip geometry is modified by the addition of a perpendicular fin. The purpose of this modification is to generate a system of two closely-spaced concentrated tip vortices, instead of the familiar single trailing vortex. Experiments were carried out in a water tunnel, where velocity measurements were made in the near wake of the wing using Stereoscopic Particle Image Velocimetry, in order to determine the characteristics of the tip vortex pair. Various configurations were tested by changing the position, dimensions and angle of attack of the fin. It was found that the fin could generate a strong and concentrated secondary vortex only when placed on the pressure side of the wing. Visualisations of surface streamlines using oil painting in a wind tunnel, as well as numerical simulations of the flow in the wingtip region, show that separation occurs with the fin on the suction side, even at low angles of attack. Both experiments and simulations were used to determine the influence of fin parameter variations on the tip vortex pair, with respect to its circulation ratio and separation distance. Whereas co-rotating vortex pairs of similar circulation could be generated easily, the fin's ability to produce counter-rotating secondary vortices is rather limited. The current results have relevance in the context of reducing the impact of concentrated tip vortices, which for rotor configurations can lead to strong blade-vortex interactions, causing fatigue and aerodynamic noise.

Aerodynamics of Sweep in a Tandem-Bladed Subsonic Axial Compressor Rotor

Abstract The present numerical investigation aims to understand the effect of the chordwise sweep on the performance of a tandem rotor. In a tandem compressor rotor, diffusion is achieved through two smaller airfoils, which are positioned in a particular fashion to nullify the effect of the boundary layer separation. The forward and aft rotor blades have different blade setting angles from the hub to the tip. Executing a chordwise sweep turns out to be an invalid tandem configuration due to the intersection of the forward and the aft rotor. To overcome this, a certain amount of lean is provided to the aft rotor. Therefore, the configurations selected for the current investigation are blades with a combination of sweep and lean. Two different forward sweep configurations and one backward sweep configuration are investigated. A significant improvement in the stall margin is observed for the forward-swept rotor configuration. However, the stage loading and efficiency of the unswept rotor are found to be higher than the forward-swept rotor case. Forward sweep reduces the tip loading of the forward rotor, which delays the tip stall. Backward sweep appears to be more detrimental in terms of the operating range of the tandem rotor, causing a substantial drop in the stall margin. The performance of the swept and the unswept rotor is compared at the design mass flow and near the stall mass flow rate. The pay-off derived from the chordwise sweep is elucidated with the help of blade loading, tip leakage vortices, blockage, skin friction lines, and various vortex interactions.

Comparative study of unsteady flow mechanisms for swept and radially stacked rotors in a dual-stage counter-rotating compressor

Counter-rotating (CR) compressor is a potential configuration to increase the thrust-to-weight ratio of aero-engines. The present study numerically investigated a dual-stage CR compressor by solving unsteady Reynolds-averaged Naiver–Stokes equations. The unsteady flow mechanisms were analyzed and compared for swept and baseline (radially stacked) CR rotors. Both of these rotor configurations were numerically simulated for the entire design speed line. The unsteady flow field near the mid-span and blade tip characterized by tip leakage vortex (TLV) was analyzed in detail for both near peak efficiency (NPE) and near stall (NS) conditions. According to the results, blade loading of the second rotor (R2) was much higher than that of the first rotor (R1). Moreover, the blade loading was higher in the swept rotors in comparison with the baseline CR rotors. In both baseline and swept CR compressors, the potential effect of R2 on the static pressure distribution of R1 was more significant compared to the opposite effect. Under the NS condition, the potential effect was confined in the rear zone of R1 where the potential flow of R2 passed. Despite the obvious impact of wake-induced negative jet on the flow field of R2 blade passage, the static pressure of R2 blade exhibited no evident variation except at 90% spanwise position. Although the blade loading of the swept CR compressor was increased, the potential effect of R2 on R1 was considerably reduced due to growth of axial gap between R1 and R2. Since the blade sweep improved the flow field, the wake passing effect on R2 was more moderate compared to that of the baseline CR compressor. The perturbation velocity magnitude of the swept CR compressor was approximately half of that of the baseline CR compressor. The TLV of R2 was periodically disturbed, and therefore, low axial velocity patches were observed on the TLV trajectory of R2. In addition, the swept CR compressor exhibited smaller low axial velocity patches of TLV due to weaker aerodynamic interaction between R1 and R2.

A Modified Permanent Magnet-Assisted Synchronous Reluctance Motor Design for Torque Characteristics Improvement

A modified configuration for Permanent Magnet Assisted Synchronous Reluctance Motors (PMaSyRM) is studied in this paper. Conventionally, the PM materials are inserted in the flux barriers region of the rotor. Consequently, the PMs sizing parameters will be restricted to the available space that may limit the efficient use of PMs benefits. In this study, a portion of the rotor structure is replaced with an Interior PM (IPM) rotor so that an axially segmented integration of IPM and SyRM is obtained. Thus, the PMs design is performed specifically in the IPM segment independent of the SyRM scheme, while the SyRM segment remains PM-less. The main motivation of this concept is to improve the contribution of the PM material in the total torque as well as utilizing the special property of the proposed rotor configuration for pulsating torque reduction. A computationally cost-effective algorithm is proposed for the optimal design of IPM-SyRM motor by which a low-power design is carried out for a lightweight electric vehicle. The results for torque profile characteristics, PM volume, power-factor, and efficiency are compared with a conventional PMaSyRM that has been optimally designed as the reference case. Finally, a prototype is constructed to verify the theoretical analysis.

Swiveling Magnetization for Anisotropic Magnets for Variable Flux Spoke-Type Permanent Magnet Motor Applied to Electric Vehicles

This paper investigates the application of anisotropic low-coercive force (LCF) magnets to a novel variable-flux spoke-type permanent magnet synchronous motor (VFS-PMSM) for electrical vehicles with a wide speed range. In the VFS-PMSM, flux is regulated by swiveling the magnetization of the anisotropic LCF magnets instead of directly magnetizing or demagnetizing them. The previously proposed VFS-PMSM uses only isotropic LCF magnets for easily swiveling the magnetic pole direction, resulting in lower torque density. The challenge thus lies in the feasibility to swivel the magnetic pole direction of the anisotropic LCF magnet, and the impact of the different magnetization strengths of the anisotropic magnets on the motor performance. This paper first studies the feasibility to swivel the magnetization direction of anisotropic LCF magnets through experiments. It is confirmed that the magnetization direction can be successfully swiveled by 90 degrees with a reduced external magnetizing field. Then, two VFS-PMSM topologies and various rotor configurations are compared in terms of key performance indices to determine critical sizing factors for performance enhancement. Finite element analysis is used for simulations. In comparison with the VFS-PMSM equipped with isotropic LCF magnets, the maximum torque of the proposed topology can be improved for the same flux adjustment ability. Alternatively, the flux adjustment ability can also be enhanced by 37.43% for the same maximum torque.

A new wake detection methodology to capture wind turbine wakes using adaptive mesh refinement and Large Eddy Simulation

The development of turbulent vortical wakes released downstream of wind turbines is crucial as it presents many technological implications for wind farm design and exploitation. The numerical prediction of these wakes constitutes a challenging problem as they involve the shedding of fine vortical structures, their instabilities, and interactions with a turbulent ambient flow. A Large Eddy Simulation (LES) approach allows capturing such flow phenomena, which implies a suitable mesh. Adaptive Mesh Refinement (AMR) is used to refine the mesh in the wind turbine wake to limit the computational cost. A methodology is developed to define and capture the wake envelope adequately. Three main parts of this methodology can be identified: The wind turbine wake detection, the target cell size required and adaptation frequency. The target cell size needed to properly capture the wind turbine wake is investigated in previous work [1], while this paper focuses on wind turbine wake detection. A strategy based on a progress variable with a source term in the rotor region is used to capture the wake. This variable is transported by the flow and thus defines the wake envelope. AMR is used to refine the mesh within this region. To validate the method, a comparison between an adaptive mesh case and a reference mesh case has been performed on a single rotor and a two aligned rotor configuration. For both, the wind turbine wake tracking method is effective. The progress variable is transported correctly and leads to a well-defined wake area. The mesh is refined adequately within it. The physical comparison between cases showed similar results, while the performance comparison showed a computational cost reduction of 30% in the single turbine configuration and 50% in the two turbines configuration. Therefore, our methodology coupled with adaptive mesh refinement can adequately capture wind turbine wake, define an accurate wake envelope and decrease the computational cost for the same physical precision.

Influence of Supersonic Nozzle Design Parameters on the Unsteady Stator–Rotor Interaction in Radial-Inflow Turbines for Organic Rankine Cycles

Abstract The use of radial-inflow turbine (RIT) for organic Rankine cycle (ORC) plant sizes below 100 kW is promising, although the application remains challenging. In fact, the single-stage arrangement imposed by economic constraints and hence the large expansion ratio, together with the large molecular weight, which characterizes organic fluids, usually result in highly supersonic flows, so making the use of transonic stators often mandatory. Particularly, the influence of RIT stator design parameters on losses and the level of unsteadiness seen by the subsequent rotor is still scarcely addressed in published literature. The present work investigates the effect of the convergent–divergent stators design parameters and the resulting downstream flow field non-uniformity on the unsteady stator–rotor interaction and loss generation in ORC RIT. To this end, two stator and rotor configurations which differ by the stator design parameters (i.e., discharge metal angle and number of vanes) have been tested by means of three-dimensional (3D) unsteady CFD calculations accounting for real-gas properties. The results show that larger stator–rotor interaction is present for the case featuring higher vane count and lower outlet metal, which also features the largest fluctuations of power output and pressure force on blade, together with a substantially lower average total-to-static efficiency, as a result of a larger stator downstream pitchwise non-uniformity. Ultimately, the results indicate that only aiming for optimal stator efficiency during the stator design phase might lead to highly suboptimal configurations from the whole turbine stage point of view.

Experimental and numerical investigation of the unbalance behavior of rigid rotors supported by spiral-grooved gas journal bearings

The forced response of rigid rotors, supported by herringbone-grooved gas journal bearings are investigated numerically and experimentally. The rotor configurations are varied by using end cylinders, attached at both sides of the shaft. Three rotors with different center of mass positions relative to the bearing locations are investigated for a variation of unbalance conditions. The numerical predictions by a quasi-linear unbalance approach and a full transient analysis are compared with experimentally obtained data. Different unbalance configurations are tested for validating the numerical models at orbital motions larger than 50% of the clearance and for bearing compressibility numbers of up to Λ=42. The investigation shows a maximum difference of 19.5% between nominal model predictions and experimental data at maximum orbital motions. The influence of the center of mass position on the rotor motion is proven to be significant and agrees with the numerical predictions. Further, the test unit is operated at different tilt angles, allowing to investigate the influence of static loading due to gravitational force on the forced response. It is found, that under different static loading, the rotor orbital motion remains unaltered and its influence can be neglected in the unbalance analysis.

A Benchmarking Platform and a Control Allocation Method for Improving the Efficiency of Coaxial Rotor Systems

Coaxial rotors used on multirotor Micro Aerial Vehicles (MAVs) are complex aerodynamic systems that are typically treated in a simplified manner, operating in sub-optimal conditions. In this letter, we propose: i) an open-source benchmarking platform for coaxial rotor systems that allows us to analyse and improve their efficiency and ii) a map of the whole actuation domain of coaxial systems based on extensive experimentation. In particular, we test three systems built using off-the-shelf components and different rotor configurations. Results demonstrate the existence of a maximum efficiency boundary, which covers the whole thrust range of each system. We also analyze how this boundary changes with respect to the rotor configuration. We compare it with the performance of coaxial rotors controlled with the current standard method (i.e., equal commands for both rotors). Finally, we propose a control allocation strategy that improves the efficiency of coaxial rotors by up to 11% over the current industry standard. Implementation on an octocopter with four sets of coaxial rotors validates the proposed methods across two different rotor separation designs and two different payload scenarios.

An automated method for the identification of interaction tone noise sources on the beamforming maps of counter-rotating rotors

Counter-rotating rotor configurations are considered more efficient than their single rotor counterparts. Consequently, the coaxially aligned rotors have appeared in the fixed-wing aircraft sector and are appearing in the quickly developing unmanned aerial vehicle sector, where they are expected to play a significant role, especially for long haul and heavy load configurations. As their noise levels have proven to be rather significant, the localization and reduction of the noise of such counter-rotating blade sets is a relevant topic of interest. One of the dominant contributors to counter-rotating rotor noise is interaction tones. Interaction tones appear at combinations of the harmonics of the blade passing frequencies of the two rotors and are significant throughout the spectra. In this paper, an automated method is presented that analyzes an entire series of beamforming noise source maps using principal component analysis-based methods in order to identify the dominant noise generation mechanisms in frequency bins that are associated with interaction tones. The processing technique is presented herein through the investigation of counter-rotating open rotor datasets developed for a fixed-wing aircraft configuration. With the proposed method, an objective mean has been provided for separating apart contributions from various noise sources, which can be automated, making the processing and investigation of large sets of measurement data rather quick and easy. The method has been developed such that the results of the analysis are easy to comprehend even without specialized prior knowledge in the area of counter-rotating rotor noise.

Parametric Analysis for Performance Optimization of Line-Start Synchronous Motor with Interior Asymmetric Permanent Magnet Array Rotor Topology

Line-start synchronous motors have attracted researchers’ interest as suitable replacements of asynchronous motors due to their high efficiency, which has been provoked by strict regulations regarding applicable efficiency classes of motors in the EU market. The research becomes even more challenging as it takes into consideration the diverse rotor topologies with different magnet locations for this type of motor. The rotor configuration with an interior asymmetric permanent magnet (PM) array rotor was chosen for analysis and optimization in this paper as this specific configuration is particularly challenging in terms of placing the magnets with adequate dimensions into the existing rotor of the asynchronous motor with a squirrel cage winding, in order simultaneously to obtain good operational characteristics such as high efficiency and power factor, good overloading capability and low material consumption. Therefore, an optometric analysis is performed in order to find the best configuration of the air gap length, magnet thickness, magnet width and number of conductors per slot, along with modifications of the rotor slot. The motor outer dimensions remained unchanged compared with the starting model of the line-start motor derived from the asynchronous motor, which is a product of the company Končar. The optimized model obtained higher efficiency, power factor and overloading capability than the starting model, along with good starting and synchronization capabilities.

Real-time prediction framework for frequency-modulated multirotor noise

A comprehensive multirotor noise assessment framework is developed to predict the noise of rotational-speed-controlled rotor configurations in real-time. The key objectives are to synthesize the frequency-modulated multirotor noise and analyze the frequency modulation (FM) characteristics. The framework includes modules associated with the flight control, aerodynamics, time reconstruction, noise prediction, and time-frequency analysis (TFA). In addition to the hybrid blade element momentum model, the aerodynamics module contains a linear inflow model, a Beddoes wake model, and an unsteady aerodynamic correction model. The convective form and source-time dominant algorithms are used in the acoustic analogy for tonal noise prediction. The FM characteristics are identified using the synchrosqueezing-based high-resolution TFA for strongly non-stationary signals. The framework is verified through validation and verification studies for diverse rotor configurations and flight conditions. During the cruise flight of the multirotor, the tonal noise exhibits simultaneous frequency and amplitude modulations. In wind gust conditions, these modulations result from rotational speed variations, acoustic wave interference, and Doppler shifting. By clarifying the non-stationary noise signal in diverse flight environments, the proposed framework can facilitate noise assessment in the perception-influenced design stage of multirotor configurations.

A New Design Procedure for Rotor Laminations of Synchronous Reluctance Machines with Fluid Shaped Barriers

A new procedure for the design and optimization of the rotor laminations of a synchronous reluctance machine is presented in this paper. The configuration of the laminations is symmetrical and contains fluid-shaped barriers. The parametrization principle is used, which executes variations in the lamination geometry by changing the position, thickness and shape of the flux barriers. Hence, the optimization procedure analyzes the various configurations through finite element simulations, by means of the communication between MATLAB and Flux 2D. In the post processing stage, the best geometry which optimizes mean torque, torque ripple, efficiency and power factor is selected. Once the best rotor configuration is defined, further investigations allow improving its performance by modifying the current angle, the stator winding and the thickness of the radial ribs.

A novel bearingless interior permanent magnet slice motor for pump*

A 2 pole bearingless interior permanent magnet (IPM) motor with slice rotor configuration is presented in this paper. A novel IPM rotor is designed considering various specifications such as force constant, torque constant and cogging torque. Cogging torque and resulting vibrations affects the motor and levitation operation significantly. Since the cogging torque is a result of the motor geometry, finite element (FE) simulation is used to simulate various rotor geometries to find the desired rotor configuration. FE simulations are also used to obtain other parameters like force and torque constant, and magnetic stiffness for designing control. The final design is fabricated and tested for closed loop levitation control and speed control. The simulation results and experimental control system performance is shown and explained in the paper.

Computational Study on Noise of Urban Air Mobility Quadrotor Aircraft

This paper investigates the acoustics of a one-passenger and a six-passenger quadrotor urban air mobility (UAM) aircraft in level flight based on a high-fidelity computational fluid dynamics (CFD) approach. The CFD simulations are carried out using the HPCMP CREATETM-AV multidisciplinary rotorcraft analysis and simulation tool Helios. The acoustic simulations are performed using the acoustic prediction tool PSU-WOPWOP. A total of three CFD models are simulated: a onepassenger isolated rotor configuration, a one-passenger full configuration with a fuselage, and a six-passenger isolated rotor configuration. The noise comparison between the one-passenger isolated rotor case and the full configuration case shows that the vehicle fuselage increases the A-weighted sound pressure level (SPL) up to 5 dB. The acoustic comparison between the one-passenger and the six-passenger isolated rotor configuration shows that the maximum overall SPL difference is up to 14 dB. Furthermore, it is shown that the noise of the six-passenger configuration is approximately 11 dB lower than that of a similar-sized conventional helicopter in an overhead scenario. The community noise impact of UAM aircraft is also assessed and compared to various background noise levels. The results show that the one-passenger quadrotor noise can be fully masked by freeway noise at an altitude greater than or equal to 1000 ft, while the six-passenger quadrotor noise can only be partially masked by freeway noise even at an altitude of 1000 ft.

Architecture Design and Flight Control of a Novel Octopus Shaped Multirotor Vehicle

This paper presents the design, modeling and control of an Over-actuated Tilt-Octocopter vehicle (OTO) which can fly in any direction in an inclined attitude. The structure of octopus is designed, considering the flight efficiency and maneuverability of the vehicle with a general optimization method of rotor configuration to meet the mission requirements. Kinematic and dynamic modeling of OTO is carried out, and a model-based trajectory controller is designed. This new structure can decouple the position and attitude of OTO and improve the ability of tracking and fault tolerance of OTO. Finally, the numerical simulation and prototype experimental results verify the advantages of vehicle design.

Forward Flight Performance Analysis of Supercritical Airfoil in Helicopter Main Rotor

In this research, the aerodynamic performance and flow characteristics of NASA SC (2)-0714 airfoil and HH02 airfoil in the helicopter main rotor are evidently analyzed. The supercritical airfoil is used in the aircraft for attaining better transonic and high-speed flow characteristics. Moreover, a specialized helicopter airfoil called HH02 is used in the Apache helicopter rotor for increasing the operational speed. As most of the high-speed helicopters are using four-bladed main rotor configuration, it is analyzed with prior attention. The lift and thrust act in different directions for the forward phase of the flight whereas the lift and thrust act in the same direction for the other phases of flight, which is analyzed in this work. The computational analysis is done by using ANSYS Fluent whereas the rotational analysis is done by using the Multiple Reference Frame (MRF) method. An analysis is carried out for different RPM of 400,600 and 800 with the combination of 0.3, 0.4 and 0.5 Mach numbers with the assistance of grid independence test. The result shows that the NASA SC (2)-0714 rotor increases the thrust of the rotor around 5% to 10% under various rotor and forward speeds. Thus, the results proves that the supercritical airfoil is highly capable of producing higher thrust and good aerodynamic characteristics.

Novel PM-Assisted Synchronous Reluctance Machines Using Asymmetrical Rotor Configuration

This paper proposes a novel asymmetrical permanent magnet assisted synchronous reluctance machine (APMA-SynRM) with high cost performance, namely it has the advantages of high torque performance and low cost. This machine has an asymmetrical rotor configuration featured by low-cost ferrite and expensive NdFeB, and two kinds of material are placed asymmetrically on both sides of the “U”-shaped barriers, which causes the magnet-axis-shift (MAS) effect. Consequently, the proposed APMA-SynRM improves the utilization of the reluctance torque and permanent magnet torque. The influences of permanent magnet volume ratio and hybrid permanent magnet on torque performance are further investigated, and the optimal topologies are designed through a multi-objective genetic algorithm. Moreover, the machine performance of APMA-SynRM is analyzed and compared with that of the conventional permanent magnet assisted synchronous reluctance machine (PMA-SynRM), which confirms that the proposed APMA-SynRM can reduce the cost of permanent magnets but still achieve similar torque and wider constant power speed range performance.

Computational Fluid Dynamics Transition Models Validation for Rotors in Unsteady Flow Conditions

Computational fluid dynamics transition models are evaluated for rotors in unsteady axial and in forward flight conditions. The study is carried out using CREATETM-AV Helios. Three transition models are considered: amplification factor transport (AFT), Langtry–Menter (LM), and the LM with a crossflow transition model. The LM model is modified to allow for Galilean invariance. The first rotor configuration is the German Aerospace Center (DLR) rotor tested in the Rotor Test Facility in Göttingen (RTG). It is a four-blade, 2.13-foot-radius rotor in axial flow with pitching blades, operating at Reynolds numbers of and at three quarter radius, for the two test cases studied. The second configuration is the pressure-sensitive-paint rotor. It is a model-scale, 5.58-foot-radius, three-blade rotor operating at a high-advance-ratio, high-thrust forward flight condition and a hover Reynolds number of at three quarter radius. For both rotors, good agreement is found between the computed and the measured transition locations, obtained using the differential infrared thermography technique. The AFT and LM models produced remarkably consistent predictions. The crossflow model predicted an early transition onset compared with the test data.