AEROACOUSTICS: LESS NOISE ON AIR AND ON THE GROUND
In the early history of aviation, noise was considered a minor problem, or even a necessary consequence. However, this is not the case anymore, especially at a time when drones are becoming increasingly popular in our cities. It´s in this context that CEiiA has developed expertise in aeroacoustics, an innovative area with a high potential.
The sustainability of the aviation industry is an increasing challenge due to its continued expansion (it´s estimated that air traffic will double in the next 15 years). This requires a new approach to safety and emissions, both exhaust emissions and noise, of commercial airplanes.
The expected introduction of drones in our cities, particularly in various logistics operations, will broaden the issue of noise to areas thus far sheltered from this phenomenon as they are away from airports.
World Health Organization data shows that about 40% of the EU population is exposed to noise levels above 55dB (a level comparable to a busy office) and up to 1.6 million healthy lives are prematurely lost due to exposure to excessive noise levels. With the increase in air traffic and the expansion of airports, the aviation industry is under an increasing pressure to reduce emissions.
Innovation as a response
ACARE (Advisory Council for Aviation Research and Innovation in Europe) has set as environmental targets the reduction of noise perception by observers on the ground by 50% until 2020 and by 65% until 2050. These targets are ambitious and require a cooperative effort among aviation stakeholders, universities and research centres in different initiatives. The X-Noise initiative, for example, funded by the European Commission, which began in 1995 and whose second phase ends this year, contributed decisively to the H2020, for example, with projects such as SILENCE (R), which decreased total noise generated from an aircraft by 5db.
CEiiA, as an active member of EREA (European Research Establishments in Aeronautics), takes part in the Future Sky project, which aims to make possible air mobility 24/7, taking into account four key areas: safety, noise emissions, air transport integration and new energy concepts.
The Future Sky Safety project started in early 2015 and CEiiA participates in four topics: P1-Coordination of institutionally Funded Safety Research, P2-Dissemination Exploitation and Communication, P4-Total System Risk Assessment and P7-Mitigating the Risk of Fire, Smoke & Fumes.
It´s already under preparation the Future Sky Quiet Air Transport project focused on noise reduction, which is why CEiiA started one aeroacoustics project in 2014.
As a result, CEiiA has developed one aeroacoustics analysis tool initially focused on the noise generated by the helicopter rotors. This tool was developed in the languages C and CUDA C in order to parallelize processes and allow real-time noise calculation.
Initially, the acoustic tool was validated for stationary and non-stationary manoeuvring of a Finmeccanica helicopter. Along with the aerodynamic tool developed internally, CEiiA masters the complete computational chain for aeroacoustics analysis of rotating surfaces. Given that the physical phenomenon is the same, it can also analyse, for example, the noise generated by a propeller or a wind turbine.
CEiiA´s tool application in a UAV
It´s precisely to test the process adaptability to different types of noise source that CEiiA has advanced to a second stage in which the tool reliability was tested in a quadrator UAV.
Noise is not the most valued factor for most UAVs manufacturers because problems such as autonomy, payload and control are considered more relevant. However, in UAVs for military purposes, noise is a significant variable since it has direct implications in the success of a mission and, therefore, there is a large investment in the area. Additionally, with the expected widespread use of UAVs in urban areas, noise will have an increasing importance.
The electric quadrotor is an interesting object of study for acoustic analysis due to its electric propulsion, with the rotors standing out as the biggest source of noise. Therefore, CEiiA analysed the UX-SPYRO quadrotor of UAVision.
The acoustic analysis was divided into two phases. The first one was an initial validation in a controlled environment of a single rotor of the quadrotor, in order to prove that the acoustic analysis process is valid in the new conditions (the dimensions and the angular velocity of the rotor differ from typical cases). The second phase was based on the acoustic analysis of the quadrotor in flight, performing different manoeuvres. The flight tests intended to study the effect of interference between rotors in the acoustic footprint and the influence of atmospheric effects.
This project also involved the Instituto Superior Técnico (IST). The reverse engineering process to obtain the geometry of the blade was carried out at CEiiA and the first analysis, in a controlled environment, was performed in an anechoic chamber of IST. The flight tests were carried out in the UAVision facilities. The results of the first phase showed, as expected, that rotors are the largest contribution to the acoustic footprint of the quadrotor.
As rotors are the largest source of noise, it means there is room to optimise the geometry of the blades. Acoustic factors can be taken into account in the UAV design phase, being a distinguishing feature in this competitive market. Future developments may focus on the acoustic optimization process of the blades without compromising the aerodynamic performance.
Causes of noise
On an aircraft, noise sources can be related with the fuselage and the engine. The noise produced by the fuselage is generated by the interaction between a turbulent flow and the surfaces (landing gear, supportive surfaces, etc.). The noise produced by the engine is originated by the fan, the combustion, or the compressor, among others. In fixed wing aircrafts these two noise sources have a comparable importance. However, in the case of helicopters, rotors are the largest source of noise.
A noise component present in all aircrafts is the one generated by rotating surfaces (rotor fans, propellers), which may be the main source of noise even in fixed wing aircrafts with non-traditional propulsion, as is the case of the open-rotor.
While determining the acoustic footprint of an aircraft, it is essential to have an appropriate computational procedure, and most institutions develop their own acoustic tools, such as NASA (PSU-WOPWOP) and Airbus (PANEM). This is because the commercial software available is not conceptualized for each specific case, which makes it computationally heavier, specially in the case of aeroacoustic analysis, that requires a high numerical resolution, becoming too time consuming.