Summary description of project context and objectives

While a considerable number of European research projects conducted in the past and current Framework Programmes have addressed the exterior noise issues posed by aircrafts during take-off and landing, insufficient effort has been applied to understand and reduce the aerodynamic noise generated by the aircraft internal cooling and ventilation systems (grouped below under the acronym ECS for Environmental Control System). The intrinsic complexity of confined turbulent flows raises very specific challenges related to near-wall modeling and the prescription of adequate boundary conditions. Moreover, many interactions take place between sub-components of a given ECS: the blower unit, temperature and pressure regulating devices or simply duct connections. For these reasons, it proves difficult to devise the most promising noise reduction strategies and achieve the objectives of the Work Programme related to passenger comfort, and to airport personnel health and occupational safety.
ECS components are indeed strong contributors to the noise perceived within the cockpit and cabin, and to the acoustic environment of the personnel servicing the aircraft when grounded. Clearly, some research is needed to understand in detail the noise generating mechanisms within interacting ECS components, and propose better design and sound mitigation guidelines. The present project is tackling these important issues, in order to lead to an effective reduction of the cabin and ramp noise. Two key ingredients are still missing to provide a reliable ECS acoustic prediction model able to drive the design of quieter systems. Firstly, the interactions between subcomponents are crucial. Many of the studies carried out on individual components of an ECS system have indeed demonstrated a quite large sensitivity of the unsteady flow field, and thereby sound generation, with respect to their environment. For system designers, it is quite important to develop a system model assembly, composed of various duct component description and acoustic prediction laws. To enable this model development, sub-components need to be thoroughly tested and modeled, alone and coupled with their most recurring parts, defined as generic for a range of system distributions. To the present consortium’s knowledge, the multi-components interactions of the ECS have not been addressed previously, while being quite important. These effects will be investigated experimentally in WP2 and through detailed simulations in WP4.
Secondly, the numerical implementation of modern aeroacoustic prediction schemes still presents a prohibitive cost if design or optimization are targeted. Stochastic methods or statistical approaches present much lower computational costs and give quite satisfactory results as long as the statistical
models that are required as input are properly tuned. The combination of scale-resolved methods, based on time-resolved CFD, with statistical/stochastic methods is an important objective of this project, in order to permit studying and optimizing interactions effects with the current computational resources. The development of this innovative approach will be conducted in WP3. Simulating the ECS aero-acoustic performance is an important goal of this project, but the ultimate purpose is the reduction of the acoustic levels. To this end, innovative noise control approaches will be first assessed on simplified configurations in WP5, and on a production system in a second stage in WP6. A critical analysis of the gains to be expected for the installed ECS system will be finally checked against the targeted impacts of the Work Programme 2012 for cabin and ramp noise and relevant regulations (for ramp noise in particular), in WP6 as well.

Description of work performed and main results

The activities carried out in IDEALVENT for the first 18-months period have been mainly focused on the experimental work planned under the WP2: “Experimental characterization of flow-acoustic and muti-component interactions”. Within this WP, relevant simplified Environmental Control System (ECS) configurations have been specified in collaboration with the industrial partners of the project, designed and manufactured. After a preliminary acoustic survey aimed at down-selecting the most interesting combinations of ECS components (blower unit, valves, diaphragms, bends, T-junctions and rectangular-circular duct transitions), detailed flow and acoustic measurements have been performed. The flow measurements include hot-wire radial and azimuthal scans at various locations within the duct system, Particle Image Velocimetry measurements and classical mass flow rate and static pressure measurements. The acoustic campaign has seen the development of a novel multi-port microphone array technique, aimed at quantifying the installation effects through a modal
decomposition of the acoustic field up to the first radial mode. These measurements are providing a considerable insight into the installation effects responsible for extraneous noise generation. Besides, they are used to define the boundary conditions that are necessary for conducting the Computational
Fluid Dynamics (CFD) and Computational Acoustics (CA) modeling work planned in WP3 and WP4. Finally, they offer validation data for these new techniques. An important last component of the WP2 research has consisted in the measurement, by an acoustic beamforming method, of the ramp noise emitted by a full-scale Embraer aircraft. The results have clearly confirmed the importance of the noise emitted by the ECS compared to the Auxiliary Power Unit (APU) over a large portion of the servicing and boarding area, thereby comforting the consortium about the relevance of the work being conducted in IDEALVENT. The research planned in WP2 is now being completed, with some delays that are however not impacting the overall critical path of the project. In WP3 “Innovative and combined simulation tools”, innovative prediction methodologies are developed for the integrated ventilation system. A first Task thereto, has consisted in the detailed specification of the different modeling approaches offered by the consortium, of their field of applicability and interfacing strategies. Then, the required interfaces have been specified and developed where needed. Another important research component deals with the development of low-CPU CFD and acoustic modeling of the sound generation and propagation within the ventilation system. The application of the aeroacoustic analogy on the basis of Large Eddy Simulation or Detached Eddy Simulation (scale-resolved approaches) yields relatively low frequency limits. In order to cover the higher frequency range, alternative approaches based on statistical/stochastic turbulence models will be developed and combined with the low-frequency scale-resolved techniques. Finally, 1D network models are being developed and refined on the basis of the results obtained by the 3D approaches.
Some work has already started in WP4 “Simulation of the interactions”, somewhat in advance of phase with respect to planning. This has permitted an early identification of CAD and meshing issues for the simulation of the blower unit, which are now being resolved. Overall, it can be concluded that while some experimental tasks in WP2 have been somewhat delayed due to unexpected hardware issues that are now under control, the research in other WPs is progressing according to plan and in some cases even slightly anticipated. The project is thus globally progressing according to plans.

Expected final results and potential impacts

The IDEALVENT project contributes directly to several of the objectives of the ACARE Strategic Research Agenda for air transport, and Flightpath 2050 vision for aviation. The project outcomes will lead to novel aircraft integrated ventilation concepts for the reduction of cabin and ramp noise, which is being recognized as a growing issue for passenger comfort and safe aircraft operation and maintenance.
Regarding ramp noise first, our consortium estimates that for the positions farther from the APU (entrance door, some servicing positions), a reduction of the ECS noise sources will reduce the overall ramp noise by the same level. In other terms, designing a system 10 dB(A) quieter, the overall noise level at the regions specified above will also be reduced by 10 dB(A) as well. At the tail of the aircraft the noise levels will be still dominated by the APU, but EMB estimates that for this case a reduction of 3 dB(A) can still be expected. This clearly demonstrates that the IDEALVENT project can potentially have a very significant positive impact on the occupational health and safety of the personnel servicing the aircraft.
Then, cabin comfort is more than a question of convenience. The adverse effects that noise can have on health have been documented for quite some time, and include hearing impairment, hypertension, ischemic heart disease, annoyance, sleep disturbance and loss of concentration. Furthermore, the quality of the cabin acoustic environment is becoming a more pressing constraint since the spread in economy class of personalized flat screens for the in-flight entertainment, games, and services of all sorts that are nowadays making the concept of office-in-the-sky come true. Beyond the comfort issue, the economic success of a complete newly envisioned range of comfort items and customization concepts is at stake with cabin noise. Passenger comfort has become a challenging competitive selling point. In quantitative terms, EMB estimates that a reduction of 6 dB(A) in the
ECS noise sources inside the cabin will lead to a decrease of 3 dB(A) in the overall cabin noise. This represents a very significant impact as well.
Beyond the aeronautical field, the tools and methods developed in the project will have very tangible impact on other fields such as ground transportation, where the noise emitted by train and automotive air-conditioning and cooling systems is a concern. Building ventilation systems are concerning a very large community as well, having a significant impact on the quality of the workspace, health and performance of the employees. In these various fields, the level of technological readiness of some modelling and mitigation strategies is somewhat lagging behind the level of expertise reached by the aeronautical industry. The presence of these non-aeronautical important societal actors within the Users Committee will greatly enhance the dissemination of mature tools and best practises developed within the project towards a wide industrial community affecting the life of many European citizens.
One of the important outcomes of the IDEALVENT project will be providing the aeroacoustic simulation community with tools and recommendations on the best modelling practises for ventilation systems. The extensive and detailed experimental campaign that will be performed in WP2 on the one hand, and the advanced simulation tools that will be developed in WP3 and applied in WP4 on the other hand, will be employed to better understand the type and degree of interaction existing between the different components ventilation systems, at both aerodynamic and acoustic levels. Ventilation systems designers are eagerly expecting numerically efficient simulation strategies to reach optimized designs in a cost-effective and reliable way. This accumulated knowledge will be also very instrumental to devise control strategies in WP5, in order to manipulate the transient flow features and reduce noise production and transmission.