Open Rotor Research Articles

Study of the Contra-Rotating Open Rotor Noise under Static Conditions

The paper presents the main results of the experimental study of acoustic characteristics of the propfan engine with a high-loaded contra-rotating open rotor. The measurements are made during the engine operation under static conditions. The power, directivity, and spectral characteristics of the power plant acoustic field are obtained.

Noise reduction of a Counter Rotating Open Rotor through a locked blade row

Counter Rotating Open Rotors (CROR) have the potential to significantly reduce aircraft emissions. However, there are a number of concerns surrounding their noise levels. In this contribution, we propose a number of novel configurations to reduce the noise of CROR for general aviation aircraft. In particular, we lock either fore or aft blade row on departure and approach, and keep the CROR trimmed at the same thrust as in design configuration (both rotors running). The CROR is switched back to design configuration at cruise to regain the high propulsive efficiency. To further reduce noise, two additional cases are investigated: 1) folded aft rotor; 2) increased diameter fore rotor (with locked-aft blade row). A low-order CROR trim code that has been developed to compute the aerodynamic and aeroacoustic performance of new design configurations is used in the numerical analysis. The results demonstrate significant noise gains for a number of blade count combinations during a constant altitude flyover. The analysis found that using a locked-fore and locked-aft configuration resulted in a maximum noise reduction of around 3 dB(A) (or 3 dB (EPNL)) and 3 dB(A) (or 6 dB (EPNL)) respectively. Folding the aft rotor backward could result in a maximum noise gain of around 12 dB(A). Increasing the fore rotor diameter by 30% was shown to reduce noise by a maximum of 8 dB(A) against an operative CROR of the same tip extension.

Full-scale experiments on the reduction of propeller-induced aircraft interior noise with active trim panels

Results are presented of full-scale experiments on the active reduction of rotor-induced passenger cabin noise in a Dornier Do728 aircraft. Two active sidewall panels (smart linings) are used to reduce the sound pressure in a control region in front of the linings. The fuselage pressure distribution associated with the first five harmonics of a generic counter-rotating open rotor (CROR) engine is emulated with a loudspeaker array. Each smart lining is equipped with two inertial force actuators. It uses up to eight error microphones and implements an adaptive feedforward controller. The two smart linings are driven in parallel. A maximum SPL reduction of 11.3 dB is achieved in the controlled area. The mean SPL reduction over 18 microphones is 6.8 dB. At the most critical second frequency, a mean SPL reduction of 9.3 dB is achieved by the two smart linings working in parallel. It is observed that the SPLs near the lining (at the window seat positions) are significantly higher than the SPLs near the aisle. This leads to the conclusion that the major part of the sound energy is transmitted through the linings, which is seen as an argument for the suitability of the smart lining concept.

Current Status and Future Challenges for Small Horizontal Axis Wind Turbines

Abstract This paper examines the category of small wind turbines. Numerous definitions are found in the literature. However, this paper's focus is on fixed pitch, small horizontal axis wind turbines, with a direct drive DC generator in the 1–10 kW class. Small wind turbine growth world-wide is analyzed for trends and predicted development. It is necessary to discuss design tools available for design, including computational fluid dynamic models and experimentally testing both open rotors and wind tunnel models. Wind turbines must be optimized for peak performance to include startup/cut-in speeds and other modifications. These wind turbines will rely on new and purposely designed airfoils; however, for low-Reynolds number conditions, computational tools do not accurately predict separation. An analysis of noise generation as well as design techniques for reducing noise is necessary for future designs. Discussions on the technologies particular to small wind turbines should include the topics of aerodynamics and structures/materials. Small wind turbines are contributing to the concept of distributed generation. Urban applications are leading to studies of flow fields in and around buildings. Interest in hybrid systems, which combine wind with other energy generation systems such as solar, internal combustion engines, and diesel engines, is growing. These systems are advantageous for the homeowner, small business, cell phone towers, remote locations, and backup emergency power systems (to include lighting). Finally, the concept of energy storage must be addressed in the context of small wind turbines, especially those turbines used in an isolated application.

Investigation of radial turbines for wind energy harvesting

Wind energy has been a research focus of the last few decades, promoting the topic to a much more matured level. However, the body of literature is built around a limited number of turbine types and it is still beneficial to increase turbine diversity for the opportunity to select optimal turbine for a given purpose. The radial (inflow) turbines known in gas and hydro turbine industries for pressurized pipe flows are undiscovered turbines for free-stream wind harvesting both in air and underwater. In such case, the wind tangentially interacts with each blade by an external casing; therefore, all blade angular positions may, at least in theory, contribute equally to power production. Enclosed compact architecture with low rotational speeds and narrow intake ports may possess advantages in enabling the use of much weaker but more recyclable and long-lasting blades and in applications such as integrated wind harvesting for blunt bodies like buildings, vessels and devices or for any application where an open rotor is not feasible. Using experimentally validated Reynolds-averaged Navier–Stokes simulations, mass swallowing capacities and corresponding efficiencies of such turbines are parametrically investigated at low Reynolds numbers, Re chord=11 × 103, and finally enhancement is shown in 88 × 103. Findings provide first insights into such turbines. Considering the investigated very low Reynolds numbers and three-dimensional effects, comparable performance with other small turbines are observed. Moreover, undiscovered potential for performance optimization persists.

Integrated Modelling and Static Experiment of a Novel Tandem Ducted Fan Flying Robot

This paper puts forward a novel tandem ducted fan flying robot which has nice passivity and compact structure aiming at operating in complicated and dangerous environment. The structure of its ducted coaxial twin-rotor with reverse rotation is unconventional and make its aerodynamic characteristics much more complex. To solve the problem, the modelling method of the traditional open rotors with the blade element theory and the momentum theory is extended to the ducted coaxial twin-rotor structure, and the integrated dynamic model of the ducted fan flying robot is established. After the modelling, a static lift experiment has been carried out, and the aerodynamic model of the ducted coaxial twin-rotor structure is modified based on the test results.

로터 공력 해석을 위한 Actuator Surface Method 개선 연구

In this study, improvements on up-to-date actuator surface method are proposed to enhance the prediction capability and universality for rotor aerodynamic analysis. In order to improve accuracy for analysis of high-solidity rotor, the correction equation for reference line velocity was newly derived to take into account influences of neighboring blades. To describe more realistic flow field and wake around blades, source-doublet panel method was adopted to calculate the momentum source distribution along chord-wise direction which reflects the characteristics of sectional airfoil and flow condition. The validity of proposed methods is evaluated by carrying out the analyses for rectangular wing and open rotor, and comparing with available experimental and computational results. It is confirmed that suggested methods can provide improvement on prediction of velocity field and wake trajectory, as well as aerodynamic performance.

Predicting the noise of an open rotor in a wind tunnel

A study of the impact of reverberation on cruise regime acoustic testing of contra-rotating open rotors in a wind tunnel is presented. Models for the rotor tone noise generated in closed cross-sect...

MUSIC-haic: 3D Multidisciplinary Tools for the Simulation of In-Flight Icing due to High Altitude Ice Crystals

<div class="section abstract"><div class="htmlview paragraph">Icing is a major hazard for aviation safety. Over the last decades an additional risk has been identified when flying in clouds with high concentrations of ice-crystals where ice accretion may occur on warm parts of the engine core, resulting in engine incidents such as loss of engine thrust, strong vibrations, blade damage, or even the inability to restart engines. Performing physical engine tests in icing wind tunnels is extremely challenging, therefore, the need for numerical simulation tools able to accurately predict ICI (Ice Crystal Icing) is urgent and paramount for the aeronautics industry, especially regarding the development of new generation engines (UHBR = Ultra High Bypass Ratio, CROR = Counter rotating Open Rotor, ATP = Advanced Turboprop) for which analysis methods largely based on previous engines experience may be less and less applicable. The European research project MUSIC-haic has been conceived to fill this gap and has started in September 2018. MUSIC-haic brings together the main European research institutions working on icing modelling as well as engine manufacturers and aircraft manufacturers. The project will develop advanced ice crystal icing models, implement them in existing industrial 3D multi-disciplinary tools, and finally perform extensive validation of the new ICI numerical capability through comparison of numerical results with both academic and industrial experimental data. The aim of the present paper is to provide an overview of the project technical objectives, scientific ambition and methodology, and work breakdown structure.</div></div>

Computational Study on the Steady Loading Noise of Drone Propellers: Noise Source Modeling with the Lattice Boltzmann Method

In the present study, a new computational methodology is explored to compute the acoustic field of drone propellers using noise source modeling with the lattice Boltzmann method. A simple mathematical model of steady loading noise for predicting the blade passing frequency (BPF) tone and harmonics at low frequencies (100–1000 Hz) is proposed and tested for various types of drone propellers. The computed result is in a reasonably good agreement with NASA’s measured sound pressure level (SPL) for APC-SF and DJI-CF two-blade single drone propellers rotating at 3600–6000 revolutions per minute. It replicates well the feature of an even number of BPF harmonics for the tested model propellers, showing the decaying slope of $$-\,6$$ for the first two BPF and harmonic peaks in the SPL spectrum. Notably, the proposed steady loading noise model shows all components of RPS harmonics with different magnitudes for different blade sizes and rotor arrangements, such as tricopter and quadcopter. The proposed method can be used for predicting and analyzing tones at low frequencies for various types of open rotor systems, such as multicopters and distributed electric propulsion vehicles.

Thermodynamic analysis of nutating disc engine topping cycles for aero-engine applications

Within the next thirty years the evolutionary approach to aero engine development will struggle to keep abreast with increasingly stringent environmental targets. Therefore radical approaches to aero-engine development in terms of energy savings need to be considered. One particular concept involves the inclusion of a pressure-rise combustion system, within the architecture of an aero-engine, to provide additional shaft power. The nutating disc engine concept is a strong contender due to its power density. The feasibility of the nutating disc engine has been previously investigated for unmanned vehicle applications. However, this paper investigates the performance benefits of incorporating a nutating disc core in a larger geared open rotor engine for a potential entry in to service in 2050. In addition, a methodology is presented to estimate the size and weight of the nutating disc core. This methodology is pivotal in determining the overall performance of the novel aero-engine cycle. The outcome of this study predicts a potential 9.4% fuel burn benefit, over a state of the art geared open rotor in the year 2050. In addition, the sensitivity of the nutating disc design variables highlights the possible fuel burn benefits compared against a comparable year-2000 aircraft mission.

Aerodynamic performance and analysis of a hovering micro-scale shrouded rotor in confined environment

The aerodynamic characteristics of open rotor hovering in confined environment are extensively studied, but few papers focus on shrouded rotor. In order to study aerodynamic performance and flowfie...

Reverse Thrust Aerodynamics of Variable Pitch Fans

Abstract Variable pitch fans are of interest for future low-pressure ratio fan systems since they provide improved operability relative to fixed pitch fans. If they can also be re-pitched such that they generate sufficient reverse thrust they could eliminate the engine drag and weight penalty associated with bypass duct thrust reversers. This paper sets out to understand the details of the 3D fan stage flow field in reverse thrust operation. This study uses the Advanced Ducted Propulsor variable pitch fan test case, which has a design fan pressure ratio of 1.29. Comparison with spanwise probe measurements show that the computational approach is valid for examining the variation of loss and work in the rotor in forward thrust. The method is then extended to a reverse thrust configuration using an extended domain and appropriate boundary conditions. Computations, run at two rotor stagger settings, show that the spanwise variation in relative flow angle onto the rotor aligns poorly to the rotor inlet metal angle. This leads to two dominant rotor loss sources: one at the tip associated with positive incidence and the second caused by negative incidence at lower span fractions. The second loss is reduced by opening the rotor stagger setting, and the first increases with rotor suction surface Mach number. The higher mass flow at more open rotor settings provide higher gross thrust, up to 49% of the forward take-off value, but is limited by the increased loss at high speed.

Development of immersed boundary computational aeroacoustic prediction capabilities for open-rotor noise

A sharp immersed boundary computational aeroacoustic simulation approach for open-rotor direct noise predictions is presented. A specific feature of the finite-difference based immersed boundary method is that the stencil coefficients in the vicinity of the immersed boundary are determined in such a way that the stability of the numerical scheme is improved. The characteristics of this immersed boundary method are discussed for acoustic scattering and the treatment of moving boundaries, such as accounting for freshly cleared and dead cells, efficient geometry queries and efficient computation of irregular boundary stencils and point clouds in the vicinity of the immersed boundary. It is shown that the numerical error when considering moving boundary problems consistently scales with the order of accuracy of the boundary discretization. Finally, the method is applied to simulate the flow around a contra-rotating open rotor at take-off and cruise conditions where experimental data is available for comparison. Excellent agreement for the noise predictions of the contra-rotating open rotor system are obtained between the numerical simulations and the experimental noise measurement.

The Influence of an Upstream Pylon on Open Rotor Aerodynamics at Angle of Attack

The aerodynamic impact of installing a horizontal pylon in front of a contra-rotating open rotor engine, at take-off, was studied. The unsteady interactions of the pylon's wake and potential field with the rotor blades were predicted by full-annulus URANS CFD calculations at 0 deg and 12 deg angle of attack (AoA). Two pylon configurations were studied: one where the front rotor blades move down behind the pylon (DBP), and one where they move up behind the pylon (UBP). When operating at 12 deg AoA, the UBP orientation was shown to reduce the rear rotor tip vortex sizes and separated flow regions, whereas the front rotor wake and vortex sizes were increased. In contrast, the DBP orientation was found to reduce the incidence variations onto the front rotor, leading to smaller wakes and vortices. The engine flow was also time-averaged, and the variation in work done on average midspan streamlines was shown to depend strongly on variation in incidence, along with a smaller contribution related to change of radius.

Numerical Assessment of Open-Rotor Noise Shielding with a Coupled Approach

In this work, scattering and shielding of the tonal noise from contrarotating open rotors by surrounding parts are investigated. The computational tool chain is based on unsteady Reynolds-averaged Navier–Stokes simulations providing permeable integration surface data for a Ffowcs Williams and Hawkings-type evaluation (“Sound Generated by Turbulence and Surfaces in Arbitrary Motion,” Philosophical Transactions of the Royal Society of London, Series A: Mathematical and Physical Sciences, Vol. 264, No. 1151, 1969, pp. 321–342), which is coupled with a fast multipole boundary-element method to account for shielding by the airframe. The effect of a background mean flow is taken into account by applying a Lorentz transform to the boundary-element problem and using the Möhring analogy instead of the Ffowcs Williams and Hawkings formulation to obtain the incident acoustic field. Validation of the Möhring/boundary-element coupling method is shown on the basis of a model contrarotating open rotor with shielding geometries and a mounting pylon. The tool chain is then applied to a full-scale open-rotor engine installed in different aircraft tail configurations (T tail, L tail, and U tail) to assess their influence on flyover and lateral noise in takeoff conditions.

Development of Composite Propeller Blade Model for High Velocity Impacts on Aircraft Fuselage using Finite Element Analysis

An open rotor blade failure and release event can result in a high energy impact on an aircraft fuselage that can reduce the strength of the structure and challenge the safe continuation of the flight and landing. This work highlights the development of a numerical approach and methodology in order to improve the assessment of the damage predictions of a composite propeller blade impact against the fuselage of an aircraft to be able to estimate a minimum thickness of shielding for the full protection of the airframe. A number of dynamic simulations were carried out, from rigid up to deformable and frangible projectiles at different angles of incidence, varying the material and the thicknesses using Abaqus/Explicit. The finite element (FE) models for blade and target were calibrated and validated separately allowing to capture the right behavior and failure modes. Impact tests of partial blade fragments against stiffened composite panels were correlated with simulations and the obtained results show a good agreement regarding deformations and delaminated area. Finally, a full blade FE model was generated and used for the fuselage impact numerical analysis. This was done within the frame of the Open Rotor project funded by Clean Sky European research programme.

Evaluation of research initiatives on the problem of reducing the aviation transport noise rate in accordance with the tasks of Flightpath 2050 goal 9

The technical solutions to reduce noise level in engines of power plants were reviewed in the work. The review on priority of the projects directed to solve the problems of air vehicle acoustic reduction proceeding from evaluation of the main requirements of Strategic Research and Innovation Agenda of ACARE was presented. One of the main directions of research within Flightpath 2050 (goal 9) is a rational integration of airframe elements and power plant. The evaluation results of research initiatives as for the problem to reduce noise of GTE were substantiated according to program Flightpath 2050. On the grounds of the analytical data it was estimated that electrical airplanes and Open Rotor engines characterized by higher efficiency compared to bypass engines with a high bypass ratio (m=50...70) would further develop to a new level by 2050. It was marked that the aircraft with a distributed electrical power plant could essentially influence the solution of the problem to reduce noise of power plants in future.

Rotor–duct aerodynamic and acoustic interactions at low Reynolds number

Ducted single and multi-rotor UAVs (unmanned aerial vehicle) offer better crash worthiness and safety compared to open rotor UAVs. The interactions that affect performance and acoustics are of interest. High speed stereo particle image velocimetry (SPIV), performance, and acoustic measurements were done on a rotor with removable protective duct to study rotor–duct aerodynamic interactions over a range of hover conditions at low Reynolds number. Instantaneous and averaged SPIV velocity field and performance results, as well as acoustic signatures, are compared to baseline un-ducted rotor cases. Increase in rotor figure of merit is strongly correlated with the level of duct interaction with rotor tip vortex cores. Efficiency change is higher for the cases where thrust induced losses take a larger share of the overall aerodynamic losses. While the overall acoustic sound pressure level was not affected significantly, cases with higher vortex core/duct interactions showed higher tonal content. Graphical abstract

Investigation of a Pylons Effect on the Character of Counter-Rotating Open Rotor Noise Using Beamforming Technology

Investigation of a Pylons Effect on the Character of Counter-Rotating Open Rotor Noise Using Beamforming Technology