Chevrons: Looks Simple in Design but Reduces Engine Noise Significantly
Till now, Aeroplane is the fastest mode of transportation but it is not quiet enough for people’s comfort living inside the passenger cabin and vicinity of the airports. Many components of the Aeroplane contribute to noise pollution. The majority of the noise comes from the engine, particularly the nozzles of jet engines. The given bar diagram shows that exhaust jet is the major source of engine noise.
During the 1990s and early 2000s, NASA along with other Partners carried out research on the noise reduction technology and they got the best solution from Chevrons. Chevrons are the saw tooth patterns seen on the trailing edges of some jet engine nozzles. During the research, they tried on various configurations of engine nozzle as shown in figure. They found the Chevrons giving optimal solution with least reduction of thrust coefficient. The Chevrons are being implemented on many of today’s commercial aircraft.
Source: NASA Innovations in Aeronautics
What it makes engine nozzle so Noisy and how the simple looking Chevrons helps to reduce the noise!
Well we have a hot gas at high speed leaving the engine core and another gas coming from bypass fan at higher speed than the external air but much slower than the core hot gas. On the top, we have the cold external airflow. Due to the high velocity difference(or other properties), hot and cold air mixing creates the shear layers which are instable and that lead to highly turbulent vortices which in turn generate the pressure fluctuations that is responsible for sound. Here is a typical image of simulation of downstream flow from jet engine nozzle.
The higher the difference between properties of hot air and external cold air form nacelle, the higher will be the shear between the mixing layers and higher will be the turbulence. If the difference in properties between the mixing layers can be made smaller, the shear between the layers becomes lesser and ultimately lesser vortices or turbulence causes lesser noise. High bypass also helps to reduce the engine noise.
The saw tooth pattern or triangular pattern at the trailing edge of nacelle allows certain part of the external cold air to premix (or mixes earlier) with high velocity hot exhaust air jet. This premix helps to create the extra flow layers between the cold and hot gases so that it reduces the differences of properties of air between the layers. So less difference in velocities or other properties between the layers of air results lesser shear between the layers and reduces turbulence that creates noise.
The Boeing 787 aircraft engines with Chevrons is able to reduce noise by 69% and Boeing 747-8 is able to reduce noise by 29%.
In simple and concluding words, Chevrons helps to mix the hot air from engine core and ambient air smoothly reducing friction between the mixing layers, which reduces turbulence or vortices that creates noise.
Article: Shekhar Thapa
Department of Mechanical Engineering Pulchowk Campus