Wind Tunnel Tests for the Evaluation of the Installation Effects of Noise Emissions of an Open Rotor Advanced Regional Aircraft

Executive Summary:

Throughout the 1980s significant research was conducted on a new propulsion technology, at that time referred to as prop-fan, with the objective of significantly reducing fuel consumption. The technology was successfully flight tested but never widely applied partly due to excess noise emissions. In recent years, research has begun on a new generation of propulsion system referred to as counter rotating open rotors with the aim of achieving the already demonstrated reduction in fuel consumption combined with a reduction in noise emissions to address environmental concerns. Modern advances in aerodynamic design tools have made possible the development of open rotor systems with decreased noise emissions while maintaining their fuel burn advantage. This achievement has resulted in a better reception of the updated technology; however, optimal acoustic performance has still not been achieved. Numerous unanswered questions exist regarding optimum blade profiles, interactions between the front and rear blade planes and installation effects. Large scale testing of open rotor systems will play an important role in evaluating counter rotating open rotor systems.

The WENEMOR project has been developed in response to the requirements of the European Clean Sky Joint Technology Initiative to assess the aero-acoustic noise emissions for an advanced regional open rotor aircraft configuration. The consortium consists of 7 partners including two universities (Trinity College Dublin and Universita Politecnica delle Marche), a large European wind tunnel facility (Pininfarina) and several SMEs (Eurotech, Teknosud, MicrodB and Paragon S.A.) with specific competences in design, manufacture, noise measurement and data analysis. The project utilises a proposed design for an advanced regional aircraft configuration, developed within the Clean Sky Green Regional Aircraft project, for the aircraft model geometries. The propulsion system simulators were based on an advanced Counter Rotating Open Rotor (CROR) design and featured realistic modern blade profiles provided by SNECMA.

The aircraft model was manufactured at 1:7.5 scale and was designed to be parametric following a modular approach. The aircraft model was configurable with interchangeable tail pieces, variable fuselage length, engine pylon rotation and elongation and was controllable for angle of attack. The test campaign consisted of aero-acoustic measurements in an open test section of a large low speed wind tunnel for a wide ranging set of configurations. All aircraft configurations were tested including an installed open rotor system operated in both pusher and tractor modes. The propulsion system simulators featured adjustable blade pitch and thrust settings to simulate take-off and approach conditions.

A considerable array of instrumentation was deployed for each of these test set ups. The aircraft model was instrumented with flush mounted pressure sensors in the engine blades and aircraft fuselage. Near field noise measurements were made using a microphone array on a traversing arm. Far field sound measurements were acquired on 3 microphone beam forming arrays and also on a far field linear array of microphones. In total, over 250 near and far field measurement sensors were deployed for the test campaign. The data produced by this extensive measurement campaign have been used to generate the most comprehensive database of installed and isolated CROR noise emission currently available.

The aircraft noise emission was investigated as a function of tail geometry, engine pylon length, engine pylon rotation and wing to engine distance. It was generally found that the pusher engine configurations were between 5-7dBA quieter than the equivalent tractor configurations. The best performing configuration overall featured a U-tail design with the pusher engine configuration. This design produced a further reduction in far field sound of up to 5dB over the baseline pusher configuration.

Project Context and Objectives:

The project consisted of 5 work packages with specific tasks in design, manufacture, testing, analysis and management. The project successfully met all of the technical objectives despite a considerable delay due to changes in inputs from the GRA and an under estimate of the complexity of the engine simulator design and manufacture. The following is a breakdown of activity by work package.

WP1 - Multi-configuration model design

The objective of this work package was the design of the wind tunnel model of at 1:7.5 scale following the indication and the geometrical characteristics provided by Alenia. The model was designed to be parametric following a modular approach to geometry changes. FEM structural analysis was performed in order to insure the absence of significant deformations and stress level that could be dangerous during WTT, in particular, in addition to static and dynamic analysis (modal analysis), GVT and flutter analysis will be also carried out. A decision was taken in the GRA to incorporate realistic SNECMA blade geometries in the engine simulators; this introduced significant delay to the design phase. Prior to this decision only representative blade geometries had been planned. However the updated blade geometries were not supplied until M8 of the project. All design activities were successfully completed by M10 of the project.

WP2 - Multi-configuration model manufacturing

The objective of this work package was the manufacturing of model subcomponents and model assembly. This included the modular wind tunnel model, the wind tunnel interfaces and the engine simulators. The manufacture of the modular wind tunnel model and interfaces proceeded according to the original estimated timeframe. The manufacture time of the engine simulators was underestimated in the original project schedule. A number of significant challenges needed to be overcome in order to complete the manufacturing process. All manufacturing tasks were completed in M20 of the project.

WP3 -Wind Tunnel Test Campaign

The objective of WP3 was to execute wind tunnel tests and aero-acoustic measurements of noise emitted by the scaled model of the CROR aircraft. This work package defined the test program, instrumentation setup, data analysis procedures as well as the experimental validation and characterization of the test articles. The final test program was completed in M25 of the project. All test requirements were met successfully.

WP4 - Aeroacoustic measurement analysis

The object of WP4 was to execute the processing and the organisation of data collected during the test campaign completed in WP3. The output of the test program was a very large and extensive database of CROR noise emission. Measurements results were used to create a database of CROR engines noise emission and investigate installation effects on a/c, for far-field and near-field noise. The conclusions of the parametric study identified the PS-E configurations (U, L and T-tail) as the optimum low noise configurations.

WP5 - Management and JTI GRA Interaction

The project was managed within this work package. The coordination of recovery actions in the project timeline and financial aspects was an on-going activity from early in the project. TCD conducted monthly teleconference reviews with all partners, held face to face review meetings at critical milestones and during recovery action planning. Project documents in the form of Co-ordination Memos and Technical Notes were used to communicate design changes, modifications and status updates within WENEMOR to the project management and ITD leader. Data transfer was successfully managed through the use of a secure online cloud storage system which enabled the project coordinator to control the distribution of project material. These activities ensured the successful completion of WENEMOR with minimal over-run in cost despite considerable extension to the project time frame from 15 to 26 months.

Project Results:

1. Overview

The main objective of the WENEMOR project was the establishment of an experimental database for open rotor noise emission. This database will be used for the validation of numerical codes simulations being developed and utilised in the larger Green Regional Aircraft program. The project has successfully completed this goal. The test campaign consisted of aero-acoustic measurements in an open test section of a large low speed wind tunnel for a wide ranging set of aircraft configurations.

The database is currently the most comprehensive dataset of counter rotating open rotor noise emission available worldwide. The database will be utilised by the GRA partners as well as members of the WENEMOR consortium for the validation and development of numerical codes and advanced data analysis techniques as well as for the development of future aircraft designs in the GRA program.

2. Principle Scientific and Technical Results

The influence of the aircraft geometry on the noise emission of the counter rotating open rotor has been investigated using a parametric study. The aircraft configurations were grouped to investigate the effects of tail geometry, engine pylon length, engine pylon rotation and wing to engine distance. The conclusions of the parametric study are quite clear and successfully identified the best performing low noise configurations. It was generally found that the pusher engine configurations were between 5-7dBA quieter than the equivalent tractor configurations. The best performing configuration overall featured a U-tail design with the pusher engine configuration. This design produced a further reduction in far field sound of up to 5dB over the baseline pusher configuration.

3. Model and Engine Simulators

The partners Eurotech and Teknosud developed considerable expertise in the design and manufacture of wind tunnel mock ups. The design of the wind tunnel model and interface required considerable iterations through a process of design, FEM and redesign in order to achieve a model suitable for wind tunnel testing. In particular, a problematic low frequency mode was identified in the front fuselage section which corresponded to a frequency likely to be excited during wind tunnel testing. This was addressed through the use of alternative materials and structural changes to the front fuselage. This process involved the exchange of data, drawings and staff resulting in close ties between the design teams of both companies.

An innovative engine simulator design was achieved which allowed for easy adjustments of the blade pitch for take-off and approach conditions. This design featured a single control for adjusting all blade pitches simultaneously. This dramatically reduced change times during engine calibration and wind tunnel testing. The blade design has been done according to a profile agreed with GRA and suggested by SNECMA, so the model’s representativeness is well assured. Eurotech developed manufacturing skills necessary to machine complex blade designs with exacting tolerance requirements. One instrumented set of rotor blades (by kulites sensors) were installed on the right side of the model, to measure the flow filed pressure on the blade profiles during the test. Eurotech developed expertise with the use of kuilte pressure sensors. These kulite sensors installed over 10 blades for each rotor have the scope to monitor the pressure distribution in frequency at given cord and span position over the blade, when running at operative RPM.

4. Wind tunnel Facility

The Pininfarina wind tunnel facility has further demonstrated its suitability for large scale aerodynamic testing of this nature. The WENEMOR partners gained considerable experience interfacing additional measurement equipment, sensors and automated control systems with the existing systems of Pininfarina.

5. Data analysis

The project consortium consists of several partners with a diverse set of competencies in terms of data analysis. One of the principle outputs of the data analysis was a database of linear and A-weighted OASPL and 1/3 Octave Band spectra on all sensors. TCD developed a custom National Instruments software suite to process all of the spectra in accordance with IEC 1260:1995 and ANSI S1.11-2004 standards. In order to demonstrate the suitability of the software approach taken to the analysis a comparison was made with the Brüel & Kjær and LMS hardware implementations of the 1/3 O.B. filters and equivalent results from all three approaches were found. TCD will make use of the software approach in future Clean Sky projects of this nature.

In order to fulfil the objectives of the project a number of different beamforming approaches were deployed to analyse the test data. There were different approaches taken by the consortium (TCD, MicrodB, UNIVM and Paragon) in order to fully characterise the CROR noise source in the challenging environment presented by installation on the airframe.

The concept of the average beamforming, based on conventional (frequency domain) beamforming calculation was applied by UNIVPM to improve the spatial resolution of the beamforming results. This technique increases the overall array aperture and reduces the effects of reflections from the fuselage to better localize the real sources on the CROR.

TCD utilised several advanced beamforming techniques in the time domain to increase the contrast of the noise maps. The time domain approach has the ability to directly deal with broadband signals. The standard delay and sum beamformer using diagonal deletion with the effect of removing microphone self noise was combined with a subtractive signal decomposition technique, which can be used to reveal weaker sources. In this iterative process the reconstructed signal of the strongest source is sequentially translated by appropriate negative time delays and subtracted from all original microphone signals. Therefore the strongest signal is excluded from the noise map and further sources which had previously been masked become visible.

TCD also utilised a frequency domain robust narrowband beamforming adapted to identify locations of noise sources. It is based on eigenvalue decomposition of the cross spectral matrix and uses beamforming in the signal modal subspace, where individual eigenvalues and eigenvectors of the signal subspace are linked to the noise sources. The eigenvectors reveal the location of noise source while the eigenvalues contain information about the strength of the source and the largest eigenvalues should be considered to get a number of potential uncorrelated sources.

MicrodB focused on the front microphone array to localize rotor sources (on first or second blade row) and to localise and quantify stationary sources including those resulting from interactions with the engine pylon. Due to the fact that the antenna plane is not parallel to the CROR plane, an off-plane extension of the ROSI algorithm has been developed. This approach was validated using a small scale study on a rotating cylinder. Calculating sources rotating in the inclined plane ensures more robust source localisation. The application of this technique to the CROR, in both clock-wise and anti-clockwise direction, can produce a comparison between the first and rear blade row source contributions and the results will appear in a future study.

UNIVPM conducted the data collection and post processing tasks. There were 3 separate data acquisition systems deployed during the measurement campaign. The wind tunnel featured a Brüel & Kjær DAQ system for the top and lateral microphone arrays, a National Instruments system was added for the front array, linear far field array and the blade kulites, a LMS system was added for the fuselage sensors and blade kulites. The data from the three systems was post-processed and integrated to a common Matlab format which enabled all of the data analysis partners easy access to the data.

The MicrodB contribution in WENEMOR project is mainly concentrated on 2 aspects regarding project constraints: algorithm to identify fixed and rotating sources and computing power in post-processing. For rotating sources, the efforts have been concentrated on taking into account the circular motion of the blades (application of this technique to the CROR, in both clock-wise and anti-clockwise direction, in order to produce a comparison between the first and rear blade row source contributions). Because of many experimental configurations were to be analysed during the project, an advanced software platform for efficient computation has been developed and used: a new software architecture for calculation in batch mode and powerful result comparison post-processing, and an optimized core calculation program (which can host all inverse identification methods, existing ones and new ones including the rotating source method) by using parallel computing capabilities (multi-core and GPU). This software platform formed the basis of the new products of MicrodB marketed and will be marketed in the coming years.

6. Coordination and Management

The research group in the Department of Mechanical and Manufacturing Engineering at TCD has a long history of engagement in aeroacoustic research at a European level. WENEMOR was the 16th European research project undertaken by the group. Previous co-ordination experience included the FP-5 project Jet Exhaust Aerodynamics & Noise (JEAN). The WENEMOR project is the first Clean Sky project to be coordinated by the research group with the notable difference of the public private partnership funding mechanism in the JTI program. The experience gained has allowed the group to compete for and coordinate other Clean Sky calls featuring large scale wind tunnel test programs. The group now has an even stronger skill set enabling participation in and management of large scale research projects.

Potential Impact:

The work within this project has and will continue to generate research publications within the fields of experimental aero-acoustics, noise source identification and novel noise reduction technologies. This will produce a number of international conference level publications and the AIAA/CEAS conferences on aero-acoustics have been identified as promising events for dissemination during the project. The initial results of the first pusher configuration test campaign were presented at this conference in Berlin, May, 2013.

The work will also produce journal level publications suitable for the highest level research publications. The track record of the consortium for presentation at the AIAA and the publication history will insure the success of these activities. TCD will utilise its role as a national contact point within the X-Noise thematic network to promote and disseminate the outputs of this research project to the European aero-acoustics community.

The results of the WENEMOR project will integrate with research activities in the wider Clean Sky and Green Regional Aircraft programs and pave the way for flight testing of the demonstrated low noise technology. In addition, the project will serve to sustain the momentum of a number of national programmes in Ireland, France, Italy and Greece from which the consortium partners have been drawn. This has led to the further development of specific expertise in advanced experimental techniques, numerical modelling and sound propagation. Interaction and exchange of knowledge between the project partners supports the development of a synergistic approach to the design of novel airframe concepts and the development of new analysis tools for the European aerospace community.

The outputs of this project will integrate with world leading graduate level education at TCD and enable graduate level researchers to develop skills targeted at the needs of the European aerospace community. The SME’s involved in the WENEMOR project have further developed their skill set for engagement in advanced European aerospace research.

Dr. Gareth Bennett

Assistant Professor

Parsons Building

Department of Mechanical and Manufacturing Engineering

School of Engineering

Faculty of Engineering, Mathematics and Science

Trinity College Dublin

Dublin 2

Ireland

T +353(0)1896-1383/3878

F +353(0)16795554

gareth.bennett@tcd.ie

http://people.tcd.ie/bennettg