Contra-rotating Open Rotor Research Articles

Investigation of Flutter Mechanisms of a Contra-Rotating Open Rotor

The growing pressure to reduce fuel consumption and cut emissions has triggered renewed interest in contra-rotating open rotor (CROR) technologies. One of their potential issues is self-excited or forced vibration of the unducted, light-weight, highly swept blades. This paper presents a numerical study into the flutter behavior of a CROR rig at take-off conditions. The study presented in this paper aimed to validate the numerical approach and provide insights into the flutter mechanisms of the open rotor under investigation. For the initial validation, pressure profiles and thrust coefficients from steady-state mixing plane calculations were compared against rig measurements. A full domain unsteady analysis predicted front rotor instability at low advance ratios. Flutter occurred in the first torsional mode in 0 and 1 nodal diameter (ND) which agreed with experimental observations. Subsequent unsteady computations focused on the isolated front rotor and first torsional mode. The flow field and aerodynamic damping over a range of advance ratios were studied. It was found that minimum aerodynamic damping occurred at low advance ratios when the flow was highly three-dimensional on the suction side. A correlation between the quasi-steady loading on the blade and aeroelastic stability was made and related to the numerical results. The effects of variations in frequency were then investigated by linking local aerodynamic damping to the unsteady pressure on the blade surface.

Active Flow Control for Interaction Noise Reduction of Contra-Rotating Open Rotors

A possible way to decrease interaction tone noise of a contra-rotating open rotor is through the application of trailing-edge blowing by reducing the momentum deficit of the front rotor wake, and therefore its interaction with the aft rotor. In this contribution, an assessment of front-rotor trailing-edge blowing is presented for the reduction of contra-rotating open-rotor interaction noise. For this purpose, the German Aerospace Center designed a generic contra-rotating open rotor that has been modified to include trailing-edge blowing at the front rotor blades. With the German Aerospace Center computational fluid dynamics code TAU, unsteady Reynolds-averaged Navier–Stokes simulations have been made of the baseline and the trailing-edge blowing configuration. Subsequently, an aeroacoustic analysis has been performed with the Ffowcs-Williams/Hawkings tool APSIM+ for both configurations. The results show negligible differences of the aerodynamic performance, with significantly lower unsteady loading of the aft rotor when the trailing-edge blowing technique is active. The aeroacoustic results illustrate that the interaction tones are notably ameliorated; that is, a reduction over almost the complete polar angle is seen when trailing-edge blowing is active. With respect to the A-weighted overall sound pressure level, a reduction of approximately 2.5 dBA in the maximum level for the trailing-edge blowing configuration is achieved for the considered geometry. Noise reductions up to 5 dBA are displayed in the up- and downstream directions, which mainly arise from reduced interaction tones caused by the trailing-edge blowing technique. The decreased noise radiation with trailing-edge blowing mainly stems from a reduction in the first two interaction tones.

Validation of time-domain single-passage methods for the unsteady simulation of a contra-rotating open rotor

The growing pressure to reduce fuel consumption and cut emissions has triggered renewed interest in contra-rotating open rotor technologies. Open rotors achieve a significant reduction in fuel consumption by allowing large increases in bypass ratio without the weight and drag penalties which arise from simply scaling up a turbofan bypass duct. However, in addition to potential noise issues, open rotors are susceptible to self-exited or forced vibration because of their unducted, light-weight, highly swept blades. The prediction of aeroelastic instabilities requires numerical methods which can accurately model the unsteady environment. Compared to full circumference simulations, single-passage multi-row methods with phase-lagged boundary conditions can offer significant savings in computational time but need to be validated for contra-rotating open rotor geometries operating at angle of attack. This paper presents the application of a time-domain single-passage method with multi-frequency phase-lagged boundary conditions for the unsteady simulation of a contra-rotating open rotor rig at take-off operating conditions. The single-passage solution is compared to the reference full circumference solution as well as experimental measurements. It is demonstrated that the single-passage multi-row approach provides a good approximation for open rotors operating at angles of attack at significantly reduced computational cost.

The effect of long-range propagation on contra-rotating open rotor en-route noise levels

The purpose was to calculate the en-route noise level produced by an advanced contra-rotating open rotor (CROR) powered aircraft. The en-route noise is defined as the noise produced by an aircraft in high altitude operation (>3000 m) measured by a microphone 1.2 m above ground level. Calculations were performed for three different aircraft operating conditions—cruise, climb, and descent. For each calculation, the aircraft noise source was modeled as an isolated CROR engine. This noise model was determined from experimental measurements made in a transonic wind tunnel using a 1/6th-scale open rotor rig. En-route noise levels were calculated using the whole aircraft noise prediction code SOPRANO. The CROR noise model were input into SOPRANO and long-distance propagation was calculated using the ray-tracing code APHRODITE, which is implemented within SOPRANO. This ray tracing code requires atmospheric wind speed, wind direction, temperature, and humidity profiles, which were collected from historical data around Europe. The ray tracing method divides the atmosphere up into a number of layers. Meteorological parameters were assumed to vary linearly between the values specified at the boundaries of each layer. Numerous simulations are conducted using different atmospheres in order to assess the impact of atmospheric conditions on the en-route noise levels.

Performance prediction and progress towards multi-disciplinary design of contra-rotating open rotors

AbstractAt DLR’s Institute of Propulsion Technology, the prediction tools and multi-disciplinary optimisation strategies developed for turbofan engines have been extended to contra-rotating open rotors (CROR). Thereby the objective has been to appraise and improve the performance of CROR engines and thus to reduce their environmental impact. The present paper reviews the intermediate progress achieved in this scope. The prediction is based on analytical and CFD methods and covers the fields of engine performance analysis, aerodynamics and acoustics. The aerodynamic and acoustic results could be partly validated through the comparison to experimental data obtained from wind-tunnel tests. In a multi-disciplinary approach the aforementioned aspects are optimised together. First results of an aero-acoustic optimisation are presented. Furthermore this paper undertakes some comparison between high-bypass ratio turbofan engines and open-rotor concepts.

Multidisciplinary analysis of CROR propulsion systems: DLR activities in the JTI SFWA project

In the frame of the EU 7th Framework Joint Technology Initiative Smart Fixed Wing Aircraft project, the DLR Institute of Aerodynamics and Flow Technology (DLR-AS) is participating as an associated partner in the Airbus-led studies of the Contra-Rotating Open Rotor (CROR) as possible powerplant for future transport aircraft. Due to significant technical challenges in terms of noise emissions, installation effects and certification that still need to be addressed for this novel propulsion system, the numerical activities require the use of sophisticated multidisciplinary analysis tools and approaches covering aerodynamics, aeroacoustics and aeroelastics. In this paper an overview of the DLR-AS work in the project is given, which covers the numerical assessment of a novel noise reduction technology, an initial study of blade aeroelasticity as well as some in-depth studies on isolated and installed pusher-configuration CROR engine configurations. The first results of a validation of the numerical simulations using experimental test data that is being generated in Airbus-led low-speed wind tunnel tests are also presented.

In-Plane Forces Prediction and Analysis in High-Speed Conditions on a Contra-Rotating Open Rotor

Due to the growing interest from engine and aircraft manufacturers for contra-rotating open rotors (CROR), much effort is presently devoted to the development of reliable computational fluid dynamics (CFD) methodologies for the prediction of performance, aerodynamic loads, and acoustics. Forces transverse to the rotation axis of the propellers, commonly called in-plane forces (or sometimes 1P forces), are a major concern for the structural sizing of the aircraft and for vibrations. In-plane forces impact strongly the stability and the balancing of the aircraft and, consequently, the horizontal tail plane (HTP) and the vertical tail plane (VTP) sizing. Also, in-plane forces can initiate a flutter phenomenon on the blades or on the whole engine system. Finally, these forces are unsteady and may lead to vibrations on the whole aircraft, which may degrade the comfort of the passengers and lead to structural fatigue. These forces can be predicted by numerical methods and wind tunnel measurements. However, a reliable estimation of in-plane forces requires validated prediction approaches. To reach this objective, comparisons between several numerical methods and wind tunnel data campaigns are necessary. The primary objective of the paper is to provide a physical analysis of the aerodynamics of in-plane forces for a CROR in high speed at nonzero angle of attack using unsteady simulations. Confidence in the numerical results is built through a code-to-code comparison, which is a first step in the verification process of in-plane forces prediction. Thus, two computational processes for unsteady Reynolds-averaged Navier–Stokes (URANS) simulations of an isolated open rotor at nonzero angle of attack are compared: computational strategy, open rotor meshing, aerodynamic results (rotor forces, blades thrust, and pressure distributions). In a second step, the paper focuses on the understanding of the key aerodynamic mechanisms behind the physics of in-plane forces. For the front rotor, two effects are predominant: the first is due to the orientation of the freestream velocity, and the second is due to the distribution of the induced velocity. For the rear rotor, the freestream velocity effect is reduced but is still dominant. The swirl generated by the front rotor also plays a major role in the modulus and the direction of the in-plane force. Finally, aerodynamic interactions are found to have a minor effect.

A numerical insight into contra-rotating open rotor in-plane loads

This paper presents the comparison of results obtained from two codes with different degrees of accuracy and computational time on the case of a contra-rotating open rotor (AI-PX7). The main goal is to assess the capacity of a code based on singularity methods to predict global open rotor performance, the in-plane loads (also called 1P loads), and the blade load distribution during a full blade cycle. Moreover, installation and unsteady correction models have been implemented in this code in order to evaluate the possible improvements of initial predictions. Finally, a numerical method to quantify some of the mechanisms governing open rotor thrust and 1P loads is proposed.

Analysis of simulated flyover Contra Rotating Open Rotors noise data by using beamforming techniques for moving sources

The relevance of beamforming techniques to analyze conventional airframe noise sources from flyover noise measurements is now well-known. With the development of Contra Rotating Open Rotors (CROR), the performance of such techniques needs to be assessed. To this aim, realistic ground CROR noise data are simulated. First, the flow is simulated by a U-Rans approach in the CROR frame. The front rotor consists of 11 blades and the rear rotor is composed of 9 blades. In addition, the incoming flow Mach number is M = 0.2. Then, the hydrodynamic blade pressure is propagated toward a ground microphone array by using the Ffowcs-Williams Hawkings' equation. The CROR follows an horizontal trajectory at constant speed (M = 0.2) and 150 m altitude. Finally, the microphone array data are analyzed by a conventional beamforming-based deconvolution technique for moving sources, DAMAS-MS. The results show that the amplitude of the harmonics of each blade passage frequency is correctly recovered. However, the amplitude of the interaction tones is badly estimated. To overcome this difficulty, the DAMAS-MS methodology, based on an acoustic source model constituted of uncorrelated monopoles, should be modified in order to introduce the correlation between the monopoles.

Contrarotating Open Rotor Operation for Improved Aerodynamics and Noise at Takeoff

The contrarotating open rotor is, once again, being considered as an alternative to the advanced turbofan to address the growing pressure to cut aviation fuel consumption and carbon dioxide emissions. One of the key challenges is meeting community noise targets at takeoff. Previous open rotor designs are subject to poor efficiency at takeoff due to the presence of large regions of separated flow on the blades as a result of the high incidence needed to achieve the required thrust. This is a consequence of the fixed rotor rotational speed constraint typical of variable pitch propellers. Within the study described in this paper, an improved operation is proposed to improve performance and reduce rotor-rotor interaction noise at takeoff. Three-dimensional computational fluid dynamics (CFD) calculations have been performed on an open rotor rig at a range of takeoff operating conditions. These have been complemented by analytical tone noise predictions to quantify the noise benefits of the approach. The results presented show that for a given thrust, a combination of reduced rotor pitch and increased rotor rotational speed can be used to reduce the incidence onto the front rotor blades. This is shown to eliminate regions of flow separation, reduce the front rotor tip loss and reduce the downstream stream tube contraction. The wakes from the front rotor are also made wider with lower velocity defect, which is found to lead to reduced interaction tone noise. Unfortunately, the necessary increase in blade speed leads to higher relative Mach numbers, which can increase rotor alone noise. In summary, the combined CFD and aeroacoustic analysis in this paper shows how careful operation of an open rotor at takeoff, with moderate levels of repitch and speed increase, can lead to improved front rotor efficiency as well as appreciably lower overall noise across all directivities.

The nonlinear harmonic method: from single stage to multi-row effects

The nonlinear harmonic method (NLH) and its extension to the modelling of the clocking effects can simulate the unsteady flows in multistage machines at much lower CPU cost. The bladerow interactions that can be reproduced are those between adjacent rotors and stators and also between successive rotors or successive stators, including the contra-rotating open rotors (CROR). However, the simulation of the unsteady effects of the engine pylon on the flow in the rear rotor requires the modelling of the time-varying disturbance from a stator of periodicity 1 (the pylon) into a rotor that is not adjacent, requiring an extension of the current NLH method. In addition, it has been observed that in some stator1/rotor/stator2 configurations, the amplitude of the perturbation associated with a blade passing frequency (BPF) in a row is not only created by the adjacent row but is also modified in space by the interaction of the periodic disturbances coming from the other two rows. For instance, for a struts/rotor/stator machine, the spatial frequencies of the periodic disturbance in the stator could be provoked not only by the interaction with the adjacent rotor but also by an additional interaction with the periodic flow coming from the struts. The present paper introduces a method that enables the identification of these spatial frequencies for a general row1/row2/row3 configuration. Besides, any row can be a stator or a rotor and the time-varying disturbances are not only produced by the interaction with the adjacent row but also with the other rows. A more general NLH method is therefore obtained. The proposed method was validated against a three-cylinder test case, a CROR test case, a struts/rotor/stator case and an impeller/bladed diffuser/volute case.

Broadband Noise Predictions from Uninstalled Contra-Rotating Open Rotors

This paper presents broadband noise predictions from uninstalled contra-rotating open rotors (CRORs). CRORs are being increasingly investigated as an alternative to single propellers and turbofan engines for power plants on aircraft, since they yield a significant reduction in fuel burn for short-haul flights. However, there is the need to develop schemes by which CROR noise can be predicted and reduced. Using semi-analytical prediction schemes, the principal broadband noise source mechanisms (rotor trailing edge noise and rotor-rotor interaction noise) are investigated, leading to strategies for their mitigation. For the configuration considered, trailing-edge broadband noise is predicted to be dominant at low rotor loading conditions (typical of cruise and approach) whereas rotor-rotor interaction broadband noise is predicted to be dominant at high rotor loading conditions (typical of take-off). A systematic parameter study is also presented in which the dependence of CROR broadband noise on the rotor-rotor gap, rotor speeds and blade numbers are investigated at constant engine power, torque split and solidity.

Experimental and Numerical Investigation of a Contra Rotating Open-Rotor Flowfield

Contrarotating open rotor propulsion systems have seen renewed interest as a possible economic and environmentally friendly powerplant for future transport aircraft. While the potential efficiency benefits are well accepted, concerns persist regarding the probable rotor-to-rotor interaction-driven noise penalty this type of engine would have in comparison to modern ducted turbofan engines. This paper presents results of a collaborative experimental and numerical study to quantify and study in-depth the complex flowfield of a generic contrarotating open rotor model at wind-tunnel scale. The model has 10 front blades and 8 aft blades, with blade design similar to modern propellers for high-disk loadings. The comparison of flow visualization results obtained through the use of modern stereoscopic particle image velocimetry and unsteady Reynolds-averaged Navier–Stokes simulations helps to improve understanding of the interactions of front-rotor-blade wakes and tip vortices with the aft rotor, which is an important aspect to guide the design of future efficient and quiet contrarotating open rotor engines. The generally good match between the experimental and numerical slipstream results gives confidence in the utility for their analysis capabilities in this field.

Prediction of Near-And Far-Field Noise Generated by Contra-Rotating Open Rotors

An innovative aeroacoustic prediction method for Contra-Rotating Open Rotors (CROR) based on the nonlinear harmonic method (NLH) for the CFD computations and on the Ffowcs Williams and Hawkings (FW-H) equations for the far-field acoustic propagation is described. This method is tested on a generic 8 × 8 puller CROR at typical take-off conditions. The outstanding efficiency of the prediction method in terms of CPU cost is demonstrated in this study. An analysis of the aerodynamic interactions between the two rotors as well as of the aeroacoustic field radiated in the far-field is presented, allowing for an improved understanding of noise generation mechanisms.

Acoustic Measurements of a Contra-Rotating Open Rotor in an Open Jet Wind-Tunnel

Acoustic measurements of a scaled model of a contra-rotating open rotor engine were performed in an open wind tunnel facility. The motivation for the work presented here was to analyse the possibilities for quantitative acoustic measurements in an environment that was not designed for acoustic measurements and where reverberations, shear layer refraction and turbulence distort the sound field. Extensive acoustic measurements were performed using a linear microphone array with 104 microphones on the floor of the hall. Room acoustics effects were investigated with a reference sound source and the sound field of the rotors was measured for different configurations. The results show that reflections from the ceiling and side walls interfere with the direct sound field and influence the measured directivity due to strong local variations of the sound pressure level from one microphone position to the next. General trends and differences between the configurations can be shown from spatially averaged frequency spectra.

Installation impact on pusher CROR engine low speed performance and noise emission characteristics

The renewed interest in the contra-rotating open rotor (CROR) as a potential highly efficient powerplant for future transport aircraft has prompted a study into the aerodynamic and aeroacoustic performance of this type of engine at DLR. With significant technical challenges related to the noise emissions and installation effects remaining to be addressed prior to realisation of a viable engine of this type, a study of a generic pusher-configuration CROR propulsion system is presented, which focuses on the impact of installation effects on the rotor aerodynamics and noise emissions. The two primary installation effects angle of attack and the pylon wake are studied utilising a high-fidelity coupled analysis based on unsteady Reynolds averaged Navier-Stokes (uRANS) simulations with the CFD solver TAU and the aeroacoustic analysis tool APSIM, both developed at DLR, highlighting the importance of these aspects on CROR in-plane loads and rotor noise emissions.

Aero-mechanical optimization of a contra-rotating open rotor and assessment of its aerodynamic and acoustic characteristics

Within the frame of the ongoing 7th Framework European Project DREAM, this article presents a synthesis of Cenaero, DLR, and Onera's joint effort to demonstrate the aerodynamic and acoustic optimization potential of a contra-rotating open rotor. Within WP 3.2 led by Snecma, the objective was to maximize the propellers efficiency at top-of-climb conditions and to minimize the noise emission at take-off focusing on interaction noise while fulfilling the thrust and torque split specifications at both operating points. These objectives were successfully met by the development and exploitation of efficient multi-objective three-dimensional Reynolds-averaged Navier–Stokes (RANS)-based surrogate-assisted optimization strategies. In order to assess the aerodynamic and acoustic characteristics of both baseline and optimized geometries, coupled unsteady RANS (URANS) simulation and farfield prediction based on an integral Ffowcs Williams–Hawkings approach were then carried out. The results demonstrate that although the acoustic criterion driving the optimization process did not lead to an improvement of the noise characteristics over the whole directivity range, it may be regarded as a cost-effective way to incorporate noise-related aspects into the design intent.

Broadband Noise Due to Rotor-Wake/Rotor Interaction in Contra-Rotating Open Rotors

A semi-analytical model for the prediction of the broadband noise due to the interaction between turbulent rotor wakesandarotorincontra-rotatingopenrotorsispresented.Theunsteadyloadingoftherearrotorismodeledusing classicalisolated flat-platetheory.Striptheoryisusedtotreatthespanwisevariationsofaerodynamicquantitiesand blade geometry. The turbulent wake is assumed to be homogeneous and isotropic turbulence that is modulated by a train of wake profiles. The model is presented in detail and insight into its modal behavior is provided. A parameter study is conducted to investigate the effects of blade number, rotor–rotor gap and rotor speeds on broadband noise emissions due to rotor–wake/rotor interaction in contra-rotating open rotors. Scaling laws for sound power levels have been established analytically and show good agreement with the results of the parameter study. Nomenclature a = empirical wake parameter Bi = blade number of theith rotor bW = half-wake width, m Cd = drag coefficient of front airfoils c0 = speed of sound, m:s � 1 ci = blade chord, m cl