Authors: Dr John Kennedy and Prof Gareth J Bennett, Department of Mechanical and Manufacturing Engineering, School of Engineering, Trinity College Dublin The Fluids, Vibration & Acoustics Research Group in the School of Engineering in Trinity College Dublin (TCD) has pioneered research for many years in modelling and analysis of flow/structure interaction including aeroacoustics and vibroacoustics. The group has participated in all of the European framework programmes and, to date, it has been involved in 18 European research projects relating to aircraft noise reduction. Most recently, the group has become involved in the most ambitious aeronautical research programme ever launched in Europe, namely the Clean Sky Joint Technology Initiative. Clean Sky’s mission is to develop breakthrough technologies to significantly increase the environmental performance of aircraft and air transportation, resulting in quieter and more fuel-efficient aircraft, hence making a key contribution to achieving the Single European Sky environmental objectives. Within the group, Dr Gareth J Bennett and Dr John Kennedy lead the Clean Sky research team. The team is currently co-ordinating three major international research projects issued as calls for proposals from the Green Regional Aircraft domain of the Clean Sky programme. The first of these projects, WENEMOR, has investigated the noise implications of an innovative, fuel-efficient engine design that employs a counter-rotating open rotor (CROR). This technology was rejected in the past partly due to noise-level concerns, but with advances in airframe and engine design, is it now possible to reap the benefits of this more environmentally friendly technology. The experimental database produced by the project is the most extensive available and is currently being used to validate advanced numerical codes simulating CROR acoustics. In addition to the redesign of aircraft engines, it is also possible to reduce noise through the application of advanced noise-abatement technologies to the airframe. ARTIC and ALLEGRA are currently ongoing projects co-ordinated by Trinity, which aim to reduce landing-gear noise. This contributes up to 30% of the overall noise emission of an aircraft during the take-off and approach phases. These projects are investigating multiple low-noise technologies applied to full and half-scale landing gear models in two of Europe’s leading aeroacoustic wind-tunnel facilities.

Developing novel aircraft concepts

[caption id="attachment_18893" align="alignright" width="640"]TCD ARCTIC Gareth Bennett Profr Gareth J Bennett and the team take time out after testing their new design[/caption] Development of novel aircraft concepts requires a complex compromise between contradictory requirements in safety, exhaust emissions, noise, performance and price. Exterior noise of aircraft will, most likely, be subject to further regulation in the future and therefore require additional technological advances for airframe, wing and engine design. To overcome the challenges of providing ultra-light, energy-efficient aircraft with acceptable exterior noise levels, concepts based on smart materials and structures are currently being investigated in the European JTI Clean Sky. The purpose of the Clean Sky programme is to advance novel technologies through to full-scale demonstrator and in-flight testing phases. It is only through these novel technologies that the European objectives, as stated in the ACARE and Flight Path 2050 reports, can be met. These European objectives require that perceived noise emissions of flying aircraft and rotorcraft should be reduced in 2050 by 65% relative to 2000 technologies. This goal should be achieved through a significant and balanced research programme aimed at developing novel technologies and enhanced low-noise operational procedures, complemented by a co-ordinated effort providing industry, airports and authorities with better knowledge and impact-assessment tools to ensure that the benefits are effectively perceived by the communities exposed to noise from air transport activities. More specifically, this amounts to developing technological and operational solutions by 2050, aimed at a 15dB reduction per fixed-wing aircraft operation (departure and arrival) with a medium-term target of 11dB by 2035. The specific activities that will be conducted in the ALLEGRA and ARTIC projects are the testing of a full-scale main and nose-landing gear (MLG and NLG), fuselage mounted and sized for a high-wing regional aircraft configuration. The presence of a full-scale fuselage model in the wind-tunnel testing, in particular, sets these tests apart from previous research into landing-gear noise problems. A range of novel low-noise technologies – including perforated fairings, meshes, hub caps and bay cavity treatments – will be utilised to achieve a noise reduction over the baseline configuration. Extensive aero-acoustic tests will be performed in two of Europe’s leading wind tunnels, DNW’s Large Low-speed Facility, the Netherlands and Pininfarina’s Aeroacoustic Research Facility, Italy. The DNW facility is the only aero-acoustic wind tunnel in Europe with a test section of sufficient size to complete the investigation of the full scale MLG.

Landing-gear noise problem

Such advanced testing facilities are required due to the nature of the landing-gear noise problem. Landing-gear noise is an example of airframe noise caused by turbulent airflow around aircraft components. This type of noise is highly dependent upon the aircraft configuration, i.e. approach, take-off or cruising. Typically, aircraft noise is at a minimum when cruising at high altitude, yet is most significant when closest to the ground – therefore having a significant impact on community noise near airports. The requirement for high drag on approach with slats extended and flaps down and the undercarriage lowered results in high levels of flow-induced noise. Landing gear is very complex and primarily designed to support the load of a landing aircraft. In order to ease the stringent requirements on inspection and maintenance, the aerodynamic design is not as refined. As a result, many components such as hydraulic cables, electrical wiring, torque links, front and rear braces are exposed to the airflow. It is the flow separation over these landing-gear components that constitute the primary noise source mechanisms through unsteady wake flow and large-scale vortex instability and deformation. The magnitudes of these sources are determined by their aerodynamic load, a function of the 6th power of flow velocity. The broadband nature of landing-gear noise can be attributed to the various different scales of the components responsible for the aerodynamic noise. These features include large-scale structures such as the wheels, medium-scale structures such as the support struts and small-scale features such as holes or grooves used for weight reduction. The action of any noise-reduction technology may not uniformly affect all of these various noise components. Of the technologies to be investigated in ALLEGRA and ARTIC for landing-gear noise reduction some are appropriate as retrofit solutions on existing designs, whereas others would require a more fundamental redesign of the landing-gear components. The aerospace industry has already begun to explore retrofit solutions; the most common approaches here are to add fairings or elastic membranes to the landing gear. Full-scale models of fuselage mounted landing gear are rarely tested. Most experimental airframe noise research has been performed using small-scale models. The difficulty of using model-scale results for full-scale noise predictions arises from the lack of details in the geometrical modelling. One of the significant contributions of these projects is that full representation of the landing-gear detail and associated structures (e.g. dressing cabling, bay cavity, bay doors etc.) will be included and addressed at full-scale size.

Noise-reduction technology and frequency ranges

[caption id="attachment_18886" align="alignright" width="640"]TCD ARCTIC Gareth Bennett Prof Gareth J Bennett is coordinating three EU consortia with the aim of producing quieter, greener planes that will be in the skies soon[/caption] Previous work has taken a semi-empirical approach to this problem by dividing the far-field spectra into low-, mid- and high-frequency ranges and assessing the effect of the noise-reduction technology on each of these frequency ranges. While this can provide some useful insights, it does not represent a true source separation and therefore does not allow a complete understanding of the mechanisms of noise reduction. The project will utilise a polar microphone array to assess basic measures such as directivity of the noise in 1/3 octave bands, overall sound-pressure level (OASPL) and effective perceived noise level (EPNL), as well as utilise more advance microphone array and signal processing techniques to identify the mechanisms behind the noise reduction. The microphone array technique is a perfect tool for determining and classifying the partial noise sources found on a landing gear model with and without a low noise technology. The output of this technique consists of separated noise source components clearly identified on the structures of the landing gear. This approach presents an opportunity to investigate a potentially cost-saving design for future full-scale landing gear testing. ARTIC will use the concept of an acoustic mirror along the centre plane of half the model. The influence of this approach on the near and far field sound and on the microphone array results will be investigated and quantified. If this approach can successfully capture the influence of a full-scale model, then the size requirements for full scale testing in a wind tunnel facility will be lowered significantly. The potential benefits for future research in terms of cost reduction and increased validity of test data are significant. The results of the ARTIC 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. The outputs of these projects are integrated 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. These programmes have led to the further development of specific expertise in advanced experimental techniques, noise source identification and sound propagation, which can further contribute to the advancement of European aerospace technologies. Contact Details: Dr John Kennedy (kennedj@tcd.ie) and Prof Gareth J Bennett (gareth.bennett@tcd.ie), Department of Mechanical and Manufacturing Engineering, School of Engineering, Trinity College Dublin, Dublin 2. +353 (0) 1 896 3878
WENEMORCall - SP1-JTI-CLEAN SKY-2010-4Topic - JTI-CS-2010-4-GRA-05-005ALLEGRACall - SP1-JTI-CLEAN SKY-2011-3Topic - JTI-CS-2011-1-GRA-02-017ARTICCall - SP1-JTI-CS-2013-01Topic - JTI-CS-2013-01-GRA-02-021