Noise is becoming a major obstacle to growth in air transport as increasing numbers of airports place restrictions on the amount of noise that can be generated by an aircraft during various phases of flight. Airbus is working hard to reduce aircraft noise such as by improving the nacelle acoustic liners used to minimize the fan noise radiated from the engine. The company has dramatically reduced the time required to design and evaluate optimized acoustic liners by moving to a simulation-based process using Actran acoustic simulation software developed by Free Field Technologies (FFT), MSC Software Company.
The acoustic liners that are built into the engine nacelle are fundamental in controlling fan noise. Acoustic liners present a major design challenge because they must address a large number of conflicting design requirements. Liners must provide high levels of noise reduction over a wide range of engine operating conditions and frequencies. Liners must also meet tight space restrictions and need to be as light as possible in order to limit fuel consumption. The liner is typically designed at a point when aspects of the airframe and engine are not completely defined so the liner design must be flexible enough to adapt to changes. The liner must be able to survive exposure to heat, cold, water, oil, and maintenance operations. Finally, the liner must be durable enough to deliver decades of service in the highly demanding aircraft engine environment.
Liners are typically manufactured in two or three curved segments that are assembled with longitudinal splices. Simulation with Actran and other numerical tools helped to reveal the substantial impact of splices on forward fan noise and these simulations were confirmed with physical testing. These simulations made it possible to compute the radiated noise fields under all relevant engine operating conditions and predict the noise reduction in certification conditions. The design of the zero-splice concept, through numerical simulation, made it possible to significantly reduce the fan noise and the acoustic discomfort.
Airbus developed an integrated numerical chain for Actran in order to streamline its use by acoustics engineers who are not numerical experts. The chain, called Automated Liner Optimization Chain for Nacelles Air Inlets and Exhausts (ANaNax), automates the simulation process from engine geometry to Actran results including prompting the user for all required information and performing validation checks on the data entered by users. “A typical optimization loop for the nacelle liner requires evaluation of 80 liner iterations and three flight conditions at a frequency range from 125 Hz to 5650 Hz which means we need to simulate several thousand different cases,” Suratteau said. “Robustness and accuracy of the simulations is critical so realistic 3D shapes, flows and boundary conditions are a must. ANaNax greatly reduces the time required for non- analytical experts to perform simulations and to check their work to be sure inputs are realistic. Computation time has also been drastically decreased by the implementation of a high performance computing (HPC) platform based on Westmere X5670 Infiniband technology with 5312 cores combined with the high scalability of Actran.