Introduction to Aeroacoustics Fernando Lau

Introduction to Aeroacoustics Noise Sound travels as a wave: in air sound travels at a speed of 340 m/s in water it travels at a speed around 1500 m/s Audible human range: 20 Hz – 20 KHz Human ear more sensible around 1Khz – 5 KHz Acoustic Power: large range!! Human whisper: 𝟏𝟎 −𝟏𝟎 watt Human shout: 𝟏𝟎 −𝟓 watt Large jet at take-off: 𝟏𝟎 𝟕 watt Introduction to Aeroacoustics

Noise Logarithmic scale used : decibel (dB) Sound Pressure Level (SPL) 𝑝 𝑟𝑚𝑠 - Randon Mean Square (rms) of sound pressure perturbation 𝑝 𝑟𝑒𝑓 - Reference Pressure: Atmospheric Air: 𝑝 𝑟𝑒𝑓 = 10 −5 N /m 2 In water: 𝑝 𝑟𝑒𝑓 = 10 −6 N /m 2 SPL = 10 𝐿𝑜𝑔 10 𝑝 𝑟𝑚𝑠 2 𝑝 𝑟𝑒𝑓 2 =20 𝐿𝑜𝑔 10 𝑝 𝑟𝑚𝑠 𝑝 𝑟𝑒𝑓 Introduction to Aeroacoustics

Introduction to Aeroacoustics Noise Pain Threshold: 130 – 140 dB Sound Pressure Level above 150 dB: Acoustic Fatigue Continuous Exposition to 75 dB noise: - Stress; - Hipertension & Heartbeat increase. Introduction to Aeroacoustics

Introduction to Aeroacoustics Aircraft Noise Aircraft Noise Predictions: - Steady growth rate of 2 – 3% per year - From 1990 to 2015 the number of airplanes will double in number. Necessary to reduce take-off and landing noise. Aircraft Noise reduction less that Air traffic Increase Introduction to Aeroacoustics

Introduction to Aeroacoustics Aircaft Noise Easy way to reduce noise in aircraft: - Take-off weight limitation, by reducing number of passengers, payload and/or fuel. - Less engine powwer and/or speed Flights less profitable (more fuel stops and less passengers) Efficient Solution: Prediction Methods of aircraft noise generation during early design phase. Introduction to Aeroacoustics

Subjective Units of Noise Noise nuisance depends on its duration, periodicity, etc. Human response to noise varies with the noise frequency Units used in Aviation, taking into account the human ear sensibility and the specific noise characteristics: dBA – “A – weighted decibel” Does not take into account time variations PNdB – “Perceived Noise decibel” Used to meadure Aircraft Noise (in particular, it considers the high pitch noise components of jet engines). EPNdB – “Effective Perceived Noise decibel” Uses as penalty not only the fundamental frequencies of jet engines, but also the time duration of the Aircaft flight. Resposta relativa (em dB) da escala dBA Introduction to Aeroacoustics

Aircraft Noise Certification Worldwide, three Organizations can be singled out for aircraft certification: ICAO – International Civil Aviation Organization United Nations Agency, for international certification FAA – Federal Aviation Agency FAR – Federal Aviation Regulations, divided em parts JAA – Joint Aviation Authorities (European Space) JAA – Joint Aviation Requirements EASA – European Aviation Safety Agency Replaced the former JAA Created in 2003; in full operation in 2008 Aircraft Authority in European Union (as opposed to JAA, that depended on each European Nation) For noise: FAR – 36 JAR – 36 EASA - Form 45 Introduction to Aeroacoustics

Introduction to Aeroacoustics Noise Certification As Aircraft noise become less popular, FAA proposed in the 70s a noise certification scheme for all Aircraft, later adopted by ICAO, using three noise certification reference positions: Noise under approach flight; Noise under take-off (also called flyover); Sideline noise, with engines at full power. Noise limits are dependent on aircraft weight and on the number of engines. Noise could not exceed 108 EPNdB, which was 6 -10 DB less than the noise generated by the operating aircraft. Introduction to Aeroacoustics

Stage IV has been introduce, which will force Aircraft to be quieter. Noise Certification Current three stages for Noise Certification: A Stage I aircraft generates noise levels during take-off, approach and sideline, that exceed Stage II limits. - A Stage II aircraft obeys to the limits given by FAR 36 - C36.5 (a) (2), tendo contudo (Noise levels above those imposed by limites impostos pelo Stage III.) A Stage III aircraft generate noise within the limits given by FAR 36 - C36.5 (a) (3). Stage IV has been introduce, which will force Aircraft to be quieter. Introduction to Aeroacoustics

Noise Certification for Stages III and IV Stage 4 Certification Noise Limits, EPNdB Approach Limit: 98.7 dB Sideline Limit: 94.8 dB Takeoff Limit: 89 dB Measured Approach: 89.8 dB Measured Sideline: 89.1 dB Measured Takeoff: 82.4 dB Cumulative Margin = 21.2 dB Measured Global Express, EPNdB Stage 3 Certification Noise Limits, EPNdB 282.5 dB Total 261.3 dB Total Approach Max Limit: 98.7 dB Sideline Max Limit: 94.8 dB Takeoff Max Limit: 89 dB Maximum = 272.5 dB Total Total Reduction = 10 dB Cumulative Margin =11.2 dB Introduction to Aeroacoustics

Number of current Aircraft Introduction to Aeroacoustics

The Green Aviation Challenge Aviation currently accounts for 2-3% of man’s contribution to global warming. Air travel is predicted to increase by 4-5% each year over the next 20 years. Aviation will have to find ways to meet the growing demand for civil air transport whilst reducing its environmental impact, specifically to the level of carbon emission. The International Air Transport Association vision is for: carbon neutral growth by 2020, 50% absolute reduction in carbon emission by 2050 and zero-emission aircraft within 50 years. The aviation industry in Europe has long recognised this challenge.

EU 2020 targets: Horizon 2020 In 2001 the Advisory Council for Aeronautical Research in Europe (ACARE) established the following targets for new aircraft entering service in 2020 compared to those entering service in 2000: reduce fuel consumption and CO2 emissions by 50% per passenger kilometre reduce NOx emissions by 80% reduce perceived noise by 50% (from ‘European Aeronautics: A Vision for 2020’, January 2001)

Aircraft Noise Sources The Aircraft flight forces the movement of considerable amounts of air, which generate high noise levels. Noise Sources Airframe Noise Sonic Boom Propeller noise Jet Noise Helicopter Noise Air flow interacts with all aircraft structures: Windows structures (dimensions in centimeters) Wings and Fuselage (dimensions in meters) Broad frequency range (broadband noise), sendo o Significant Noise increase when landing gear is down. Introduction to Aeroacoustics

Aircraft Noise Sources Sonic Boom Noise created by Aircraft flying at supersonic speed. Nowadays, only used by military Aircraft (Even the Concord only flew at Mach >1 over the Oceans). Supersonic flights over Land are not allowed by most Countries: Even a sonic boom of a small aircraft, at a long distance, can have a significant impact on the human and wild welfare. Introduction to Aeroacoustics

Aircraft Noise Sources Propeller Noise Although first Propellers were very noisy, it was no cause for concern untill WWII. With the increase of Turbine Propulsion, Jet Noise became much more relevant. However the noise restriction increase Brought a new concern in noise research, when generated by Propeller Engines. Noise characterized by well defined harmonics, that are multiple of BPF (Blade Passing Frequency) Monopolar Noise (Thickness Noise) and Dipolar Noise (Loading Noise): Efficient Sound Sources Introduction to Aeroacoustics

Aircraft Noise Sources Jet Noise Considerable increase in efficiency, as larger volumes of air are drawn at considerable less speed. Bypass cycle: requires 50% more air per unit of Thrust unit, which reduces the mean exhaust velocity. Divides the larger entry airflow into two almost equal streams, using only one to supply a smaller but more efficient engine core, which then has the same fairly exhaust as in pure jet. - Turbo-Fan: Another step forward in efficiency by even higher levels of bypass flow via a large fan. Lighthill Law: Introduction to Aeroacoustics

Aircraft Noise Sources Jet Noise As bypass ratio rises and jet velocity falls, so the external jet mixing noise falls also. Introduction to Aeroacoustics

Aircraft Noise Sources Helicopter Noise Several characteristics that are different from Aircraft: - The rotor is aligned with the flight direction, rather than being normal to it. - BVI (Blade Vortex Interaction) – characteristic noise generated by the blade interaction with the flow coming from the previous blade. - Tail Rotor Noise. - Asymmetric noise field: substantial difference in relative tipspeed between the blade on the advancing side and on the retreating side of the rotor disc plane, with respect to flight direction. Introduction to Aeroacoustics

Introduction to Aeroacoustics Future Perspectives Number of passengers tends to increase 4% per year. Given the continuous increase on the aircraft dimensions, we can predict an increase of 2-3 % per year on the global number of aircraft. Given the number of flights, noise increases 1 dB each year. However: Individual Aircraft Noise tends to decrease. Twin-engine Aircraft have been replacing Four-Engine Aircraft. (The reliability of the former even led to their use in transatlantic flights.) Introduction to Aeroacoustics

Introduction to Aeroacoustics Questions?