Counter-rotating Rotors Research Articles

Individual Blade Control Approach for Active Vibration Suppression of a Lift-Offset Coaxial Rotorcraft

This study explores the best vibration reduction using an individual blade control (IBC) actuation scheme for a lift-offset coaxial helicopter in high-speed flight. The rotorcraft dynamics analysis model consists of coaxial, three-bladed counter-rotating rotors and a finite element fuselage stick model constructed based on the measured data of the XH-59A helicopter. The XH-59A helicopter is well-known for its severe vibration level encountered during the flight (over 0.5g), leading to the cancellation of the development program. A special focus is given to assessing the accuracy and efficiency of the integrated rotor–body vibration predictions evaluated between the one-way and two-way coupling methods in reference to the flight data of the vehicle. The two-way rotor–body coupled results show excellent correlations with the test data for rotor blade loads and airframe vibrations. The best actuation scenarios are then sought for the minimum vibration at the rotor hub and the pilot seat. The IBC pitch actuation effectively reduces the vibrations at both locations of the rotorcraft. Specifically, a multiple-harmonic IBC actuation enables to suppress the pilot seat vibration by 93% compared to the uncontrolled case, achieving a significantly reduced vibration level (below 0.05g).

Model Predictive Control of Counter-Rotating Motors for Underwater Vehicles Considering Unbalanced Load Variation

The propulsion system for underwater vehicles, driven by a counter-rotating permanent magnet synchronous motor (CRPMSM), can enhance the operational stability and efficiency of the vehicle. Due to the influence of complex underwater flows, the load imbalance of CRPMSM’s dual counter-rotating rotors may lead to severe issues of dual-rotor desynchronization rotation. Combining traditional vector control (VC) with master-slave control strategies can address the desynchronization problem when CRPMSM’s load changes. However, it results in significant speed fluctuations and a long transition time during the transition from load disturbance to synchronous rotation. This paper introduces a model predictive control (MPC) strategy to effectively resolve this issue. The incremental MPC model is established based on the mathematical model of CRPMSM in the dq coordinate system. The predictive control system forecasts the d- and q-axis components of stator currents for the next four control cycles. It selects the optimal control increments to minimize the cost function based on current predictions and different inverter voltage states. The obtained optimal d- and q-axis components of stator voltage are used to control CRPMSM under unbalanced load disturbances. Simulation results demonstrate that, compared to the VC strategy, CRPMSM utilizing the MPC strategy exhibits better dynamic performance with faster speed response and reduced torque fluctuations during load and speed variations.

Investigation of the Flow Fields of Coaxial Co-Rotating and Counter-Rotating Rotors in Hover Using Measurements and Simulations

The flow fields of a 2-m diameter two-bladed single rotor, a 2× 2-bladed coaxial corotating (stacked) rotor, and a 2× 2-bladed coaxial counterrotating (CCR) rotor in hover were measured using particle image velocimetry and computed using a finitevolume unsteady Reynolds-averaged Navier–Stokes (URANS) CFD model. Phase-resolved measurements were performed on the stacked rotor at nine azimuthal locations, and time-resolved measurements were performed on the CCR rotor at 64/rev with at least 500 flow realizations per azimuth for each operating condition. The goal of this study was to compare the flow features of these rotor configurations and explore the interactions between the rotors. Overall, there was good correlation between the measurements and simulations. In particular, the effect of index angle on the upper and lower rotor thrust sharing for the stacked rotor was predicted well by the simulation. The slipstream boundary for the stacked rotor was found to vary with the index angle. The slipstream boundary and vortex trajectories for the CCR rotor were found to vary with azimuthal location, indicating the effect of blade passage on the wake geometry. Simulations indicated a stronger dependence of the tip vortex trajectory on the index angle and thrust for the stacked rotor compared to the CCR rotor. The radial thrust distribution along the upper blades was found to depend on the index angle for the stacked rotor and showed small variation due to blade passage for the CCR rotor. A larger azimuthal dependence was seen for the radial thrust distribution on the lower rotor blades, primarily due to the proximity of the upper rotor tip vortices. The lower rotor radial thrust distribution was biased towards the blade tip, outside the upper rotor slipstream.

A novel experimental control method to suppress instability in a centrifugal compressor with two counter and co-rotating rotors

The use of a counter-rotating rotor in turbomachines can be a solution to improve performances by allowing reduction in size and weight, especially compared to multistage configuration. The present study aims are to demonstrate another advantage of the counter-rotating rotors: their application to expand the operating range of centrifugal compressors toward extremely low flow rates based on an active control method. This method was performed on two different counter-rotating configurations. The principle of the active control method is to adjust independently the rotation speed of each rotor to push back the instability appearance. Experimental results show that reducing the rotation speed of the upstream-rotor while fixing the speed of the downstream rotor can push the instability phenomenon toward a lower mass flow rate while decreasing the pressure rise and efficiency. Co-rotating mode is applied to push further the stable region toward low mass flow rates allowing a significant extension of the operating range. The experimental results reveal that the map width is shifted by about 50% over the studied range toward a lower mass flow rate for both counter-rotating centrifugal compressors when using the control method. This can be achieved when the absolute value of the upstream-rotor speed is within the range of 48%–61% of the rotational speed of the downstream rotor.

Phase-Resolved Flowfield Measurements of a Coaxial Counter-Rotating Rotor in Hover

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.

드론용 동축 반전 로터와 단일 로터의 지면효과에 따른 추력특성에 대한 수치 비교 연구

In this study it is attempted to compare the difference of increase rate of rotor thrust between coaxial counter-rotating rotor and single rotor due to ground effect by using computational fluid analysis. Rotor system was modeled with a commercial propeller usually used in small drones in both cases, and thrust was calculated with unsteady-state transient analysis by using Fluent 19.1 software. As a result, it is found that the rate of change of rotor thrust in accordance with the distance to the ground of coaxial rotor with a relatively large inter-propeller distance is quite different from the case of a single rotor.

Comparative energy performance analysis of four wind turbines with counter-rotating rotors in steady-state regime

In the worldwide effort to decarbonize the electricity generation, increasing the energy performance of renewable energy conversion systems is a continuing global concern and challenge for stakeholders. In this context, the paper presents the results of a comparative study on energy performance of four counter-rotating wind turbines of different types. In this regard, the authors propose a new reconfigurable structure of a counter-rotating wind system, able of operating in four distinct cases: with traditional electric generator (with fixed stator) or counter-rotating generator, with monomobile (torque adding) or bimobile (speed adding) speed increaser. These functional cases are obtained by the appropriate command of two intermittent clutches. A generalized algorithm for kinematic and static analytical modeling of the reconfigurable wind system in steady-state regime is proposed in the first part. Also, a new approach for optimizing the main design parameters, such as the ratio of wind rotor diameters, is detailed. The numerical simulations of the obtained closed-form model, for optimized value set of design parameters and various wind speed values, showed a slight energy advantage of using counter-rotating generators and monomobile transmissions, but accompanied by drawback of increased complexity. The main results of the paper, i.e. the new reconfigurable wind system concept and its analytical steady-state model of speeds, torques and efficiency, are useful for researchers and practitioner interested in developing optimized counter-rotating wind turbines.

Experimental study on noise reduction of multi-rotor by phase synchronization

Phase synchronization implies that the rotors are synchronized to the same rotation speed and different phases to minimize rotor noise in specific directions. In this study, the influence of phase synchronization on noise directivity and the reducing effect of a multi-rotor were studied experimentally. Based on the multi-rotor noise test platform built in the anechoic chamber, to study the noise control effect of phase synchronization, tests were performed on a small-scale dual-rotor and quadrotor. The influence of phase synchronization on the noise directivity and frequency characteristics was analyzed. Furthermore, the effectiveness of the proposed method was verified using noise-reduction experiments for the target points and regions. The experimental results revealed that: (1) phase synchronization is more advantageous for co-rotating rotors than counter-rotating rotors. Phase synchronization reduces the noise level in all azimuths for the co-rotating rotors, and only a few directions for the counter-rotating rotors. (2) Phase synchronization effectively reduces the sound pressure level in the low-frequency band and multiple harmonic frequencies for a single observation point, especially at the blade-passing frequency. (3) The regional noise is reduced by 1–5 dB and 1–11 dB for the counter-rotating and co-rotating dual-rotor, respectively. Therefore, the experiments show that phase synchronization has a significant advantage in noise control for multi-rotor aircraft.

Development of a Dual-Input Magnetic Gear Train for the Transmission System of Small-Scale Wind Turbines

Magnetic gear mechanisms have advanced to have a promising future in transmission technology. Previous research indicates that magnetic gear mechanisms might replace mechanical gear mechanisms in some applications. Small-scale wind turbines (SWT) with counter-rotating rotors that were initially fitted by bevel gears are proposed to be replaced by a coaxial magnetic gear train (CMGT). The CMGT is intended for use as a speed multiplier in order to obtain maximum power at low wind speeds, due to its beneficial transmission of power without physical contact. The primary objective of this study is to build a dual-input CMGT that will be employed in the transmission system of small-scale counter-rotating wind turbines. A dual-input CMGT is built through the analytical modeling of an equivalent magnetic circuit (EMC), which aims to predict the magnetic flux density in the air-gaps of CMGT. Several models within design constraints were compared to obtain the optimum design parameters of the preliminary CMGT design resulting from an EMC analysis. The optimized critical design parameters were then selected and analyzed using finite-element analysis (FEA) to depict the performance of the proposed SWT design. According to the findings, the developed design can generate an inner air-gap flux density of 0.8314 T and an outer air-gap flux density of 1.0200 T. The model likewise produces promising simulation results with an output transmitted torque in the inner rotor (output link) of 8.7 Nm, 56.9 Nm in the outer rotor, and 48.0 Nm in the carrier with pole-pieces. Thus, this design can generate higher torque than a bevel-geared wind turbine. The speed characteristics are also compromised in order to raise the generator’s rotating speed to generate more power. Finally, this study demonstrates the performance and embodiment design of the proposed SWT using CMGT.

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.

Aerodynamic Interactions of Side-by-Side Rotors in Ground Proximity

An experimental investigation is conducted to study the aerodynamic behavior of a two-rotor system in ground proximity. The counter-rotating rotors are placed side-by-side in the hovering condition. The time-averaged and unsteady flow behavior is studied when the rotor-to-rotor lateral distance and the distance between the rotors and the ground are varied. The experiments are performed using three-dimensional large-scale volumetric velocimetry with helium-filled soap bubbles as tracers, tracked by the particle motion analysis technique “Shake-The-Box.” The mean velocity field reveals the wake deflection due to the ground plane and the formation of toroidal-shape regions of separated flow below each rotor. The interaction of the wall jets formed by slipstream deflection results in a separation line with the flow emerging from the wall in a fountain-like pattern. Regimes of flow re-ingestion occur when the rotors are sufficiently far apart. The flowfield exhibits the tendency toward asymmetric states, during which the fountain flow column and the domain of re-ingestion shift closer to one of the rotors. A generic classification of flow regimes is proposed in relation to the behavior of two rotors in ground effect.

Experimental Study of a Novel Centrifugal Compressor With Two Successive and Independent Rotors

Abstract This study deals with a low pressure-ratio centrifugal compressor consisting of two counter-rotating rotors called a counter-rotating centrifugal compressor (CRCC). The design method based on the loss model was presented to determine the geometric parameters of the two counter-rotating rotors. According to this method, the rotor of a selected single rotor centrifugal compressor (SRCC) has been redesigned into two counter-rotating rotors (upstream and downstream rotors) by choosing the value of meridional length ratio (LR). The meridional view, the volute shape, and the operating parameters of SRCC are preserved during the design process. In a first step, the counter-rotating mode at a constant rotor speed of 11k rpm has been carried out. The overall characteristics of CRCC are compared to those of SRCC. In a second step, the map characteristic of CRCC is established for seven speed ratios. The results show that CRCC increases up to 4.6% for the pressure ratio and 3.5% for the efficiency compared to SRCC at the same tip-speed. In addition, CRCC can operate at a lower tip-speed by about 2k rpm to produce the same characteristics as SRCC, with better efficiency over a wide range of flow rates. However, the surge margin of the CRCC is shifted to higher flow rates. This disadvantage of the CRCC was solved by choosing the adequate pair of the rotational speeds of the two rotors that will be presented in other publication.

Noise of small-scale contra rotating rotors

This article presents the results of an experimental study of the noise characteristics of small-scale coaxial counter-rotating rotors. The tests were carried out in an anechoic soundproof chamber for F7/A7 counter-rotating rotors with 11 and 9 blades on the front and rear rotors, respectively. The intensity of noise emission by rotors was compared with the constancy of the thrust of the power units. Acoustic studies have shown the presence of tonal and broadband noise components at low rotation speeds. In addition to the usual noise components, tonal components were also observed for the rotor blade running combinations. The observed noise levels at these frequencies are equal, and in some directions are significantly higher than the noise levels at the frequencies of the blades of the first and second rows. In experiments, it was found that, while maintaining the equality of the rods, with an increase in the diameter, the noise levels at the rotors decrease due to a decrease in peripheral speeds. A decrease in the noise levels of rotors with an increase in the number of blades was established due to a decrease in aerodynamic loads on the blades and peripheral speeds. An important parameter here is the filling factor of the space swept by the blades. With an increase in this coefficient, the rotation noise decreases, but the vortex noise increases significantly.

Study of a Bi-Vertical Axis Turbines Farm Using the Actuator Cylinder Method

Vertical axis turbines, which extract kinetic energy from currents, can produce electricity independently from a current’s direction. Hence, this type of turbines raises interest for harvesting energy from tidal currents, where flow changes direction during flood and ebb tides, and where currents present large variation of direction during tide. Methods for representing vertical axis turbines in tidal farms should be implemented in order to predict correctly power production with an acceptable time cost. The Actuator Cylinder (AC) is one of them. Numerical results in terms of wakes, with the study of velocity profiles, and efforts are compared to experiences, as well as showed that the method is sufficiently accurate and for a reasonable computing time, which is of prime importance for tidal turbines farms studies. The Actuator Cylinder method is implemented in ANSYS Fluent in a 2D stationary resolution. The method is then applied to a double levels of two counter-rotating rotors marine turbine designed by Hydroquest. Wake and power production of a single turbine and several farm configurations are studied under different current conditions (magnitude and direction).

Generation of Nonspherical Liquid Metal Microparticles with Tunable Shapes Exhibiting an Electrostatic-Responsive Performance.

Nonspherical liquid metal microparticles (NLMs) show extraordinary potential in various applications due to their multifunctional and structural advantages. To one-step-produce shaped NLMs with high efficiency, high controllability, and free of template, a facile microfluidic strategy named rotary flow shearing (RFS) is reported. A high-speed viscous shearing flow is provided by two counter-rotating rotors in the carrier fluid, inducing continuous pinch-off of liquid metal flowing from a capillary tube positioned in face of the slit between two rotors. The real-time oxidation realizes the rapid solidification of the pinching neck and the liquid metal surface during the RFS process, resulting in massive NLMs. Different from other microfluidic methods, the RFS enables tunable shapes of NLMs, especially for working materials at high viscosities. The collected NLMs exhibit special electrostatic-responsive performances including translation, rotation, reciprocation, and lining up under the manipulation of an external electric field. Such NLMs can be promisingly used for the construction of novel micromotors and soft electronics.

Comparative Study of a Conventional, Coaxial Counter-Rotating, and Co-Rotating Rotor Aerodynamics in Hover

Comparative Study of a Conventional, Coaxial Counter-Rotating, and Co-Rotating Rotor Aerodynamics in Hover

Flow-field and wake analysis of novel double-rotor open-center tidal current turbine by CFD simulations

Tidal turbines represent a new frontier for extracting energy from a huge potential of renewable source such as the tides. Despite the technology being mature enough, new solutions aimed at improving the machines efficiency with reduced environmental impact and installation costs are currently under investigation. A novel tidal turbine, characterized by being self-balanced and not requiring structures and foundations, was recently proposed. The machine geometry presents an open-center, which affects the flow field and improves the machine performance in terms of extracted power and power coefficient. At the best of authors’ knowledge, despite being open-center turbines produced and installed, an evaluation of the fluid-dynamics effects of the open-center on machine performance is still missing. The main objective of the present work is to fill this gap and to contribute to have a deeper insight in the flow-field in the wake region and to investigate its effects on machine performance. The analysis is carried out by means of a CFD tool, for three different machine geometries: the open-center machine, which is modeled in single-rotor and double-rotors configurations, and the traditional wind-like machine. A comparison with literature experimental data is also included for validation purpose. For each analyzed case, the flow analysis is carried out for the optimal tip-speed ratio. A free stream velocity of 3 m/s is selected for simulations and a scaled model of the turbine having the diameter of about 1m is considered. The results show that the open-center configurations allow the achievement of higher performance in terms of power coefficient and extracted power, in comparison to the traditional wind-like turbines. In particular, for the new double-rotor prototype, the estimated Cp value is about 0.43, with improvements of about 15% with respect to traditional configurations. Moreover, the corresponding increase in the extracted power, which for 1m diameter prototype increases from 5.2 kW to 6 kW, is achieved with a lighter machine. The weight/power ratio, in fact, reduces from about 2 kg/kW for the standard machine configuration to 1.6 kg/kW for the new double-rotor prototype. The improvements in machine performance are related to the fluid dynamics characteristics of the central opening. This, in fact, causes a suction effect, which contributes to reduce the energy losses due to radial and tangential kinetic energy displaced out from the rotor disk that for a standard machine amount to about 36% of the inlet kinetic energy. Only part of these energy losses is recovered as mechanical power to the turbine, while the remaining part increases the flow kinetic energy lost in the open-center region. The amount of energy recovered as mechanical power at the turbine increases considerably in the double-rotor configuration, where, besides the suction effect of the open center, the external counter-rotating rotor gives the major contribution to the recovery of the kinetic energy lost in tangential and radial components of the inner rotor, which is then reduced down to 12.5%.

Performance evaluation and modelling of the Atir marine current turbine

In this study, the behaviour of a full-size prototype of a platform for tidal energy was analysed. The turbine of this platform is formed by two counter-rotating rotors aligned with the tidal current, such that one rotor is always under the wake of the upstream rotor. The platform was tested under real working conditions; the tidal current was emulated by towing. The results of these tests were used to develop and parameterize a turbine model, which is presented in this paper. The turbine was modelled in terms of power, torque, and thrust coefficients. This model was used to estimate the power curve of the platform and design control strategies that, in addition to maximizing energy production, contribute to the stability of the platform.

Conceptual Design of the Axial Compressor Stage with Counter-Rotating Rotors

The article deals with the design solution of the unconventional concept of compressor with counter-rotating rotors, especially in order to create a model which, due to its parameters in simulated operation, would achieve the higher values of compression ratio with satisfactory compression efficiency in comparison to conventional axial compressor. The primary goal of this work is to create a design of a compressor stage with counter-rotating rotors from a theoretical point of view. The compressor model was created respecting the problems with lubrication of geared mechanism, pressurization of labyrinth seals for preventing oil leaks and the transfer of compressed air inside hollow compressor blades. Subsequent design evaluation and optimization is based on airflow analysis of the compressor stage. Obtained results shows improvement in higher values of absolute pressure behind stages in comparison to conventional compressor stage. The secondary goal is to point out the existence and application of innovative and progressive technology of rapid prototyping in the world of aircraft component production and at the same time to emphasize its benefits in comparison with conventional production techniques applied in the initial stages of aircraft component production.

Performance and Wake Characterization of a Model Hydrokinetic Turbine: The Reference Model 1 (RM1) Dual Rotor Tidal Energy Converter

The mechanical power and wake flow field of a 1:40 scale model of the US Department of Energy’s Reference Model 1 (RM1) dual rotor tidal energy converter are characterized in an open-channel flume to evaluate power performance and wake flow recovery. The NACA-63(4)-24 hydrofoil profile in the original RM1 design is replaced with a NACA-4415 profile to minimize the Reynolds dependency of lift and drag characteristics at the test chord Reynolds number. Precise blade angular position and torque measurements were synchronized with three acoustic Doppler velocimeters (ADV) aligned with each rotor centerline and the midpoint between the rotor axes. Flow conditions for each case were controlled to maintain a hub height velocity, uhub= 1.04 ms−1, a flow Reynolds number, ReD= 4.4 × 105, and a blade chord length Reynolds number, Rec= 3.1 × 105. Performance was measured for a range of tip-speed ratios by varying rotor angular velocity. Peak power coefficients, CP= 0.48 (right rotor) and CP= 0.43 (left rotor), were observed at a tip speed ratio, λ= 5.1. Vertical velocity profiles collected in the wake of each rotor between 1 and 10 rotor diameters are used to estimate the turbulent flow recovery in the wake, as well as the interaction of the counter-rotating rotor wakes. The observed performance characteristics of the dual rotor configuration in the present study are found to be similar to those for single rotor investigations in other studies. Similarities between dual and single rotor far-wake characteristics are also observed.