Acoustic design at a conceptual level
The team “design-to-noise” of the Engine Acoustics Department develops methods in order to improve aircraft engines with respect to noise early in the design process. It
also aims at investigating innovative technological solutions for noise mitigation.
One should distinguish in the design between three different levels.
The integration of the engines into the aircraft is the result of an iterative process converging to the best compromise. Nevertheless the work organisation of the aircraft industry reflects the fact that engines are to a large extend designed separately from the rest of the aircraft: on the one side, airframe specialists like Airbus or Boeing; on the other side, engine manufacturers like Rolls-Royce, MTU, SNECMA, General Electric, Pratt and Whitney.
In fact an engine product is optimised for some given specifications or requirements. Several criteria are considered in the optimisation: consumption, emission, noise, weight, maintenance costs, safety, etc. Some of those criteria are correlated: e.g. consumption and emission.
Our institute is highly interested in the design process of engine components. Our work is currently restricted to fan components and open rotors; it should be extended to other components in the future
A fan design can be achieved at different levels of approximation.
A first approach based on meanline aerodynamic models developped for pre-design tools form the base of the 'stand-alone' version of PropNoise, which does not need any external programms (such as a flow solver) to generate the input necessary to the acoustic models. The aerodynamic and acoustic impact of fundamental design modifications (for example the design inflow Mach number of the rotor) is investigated and evaluated as potential solutions for reducing the noise emissions of future turbofan engines.
A second approach relies on the coupling of steady RANS calculations (for aerodynamics) to the acoustic models of PropNoise (which are analytical models based on the acoustic analogy). The CFD solver TRACE dedicated to turbomachinery applications is used for RANS whilst the analytical tool PropNoise is employed for the acoustical prediction. The input data for the models (geometry, flow) are automatically extracted from the RANS, completed and saved into a datafile whose format can be read by PropNoise. First published results about this approach are promising. Should the acoustic models properly capture physical trends and be sufficiently accurate, they will be integrated as acoustic cost function in the design optimisation loop developed at the Fan and Compressor Department.
The 'design-to-noise' method proposed by DLR has found some industrial applications. One of the best examples is the DNW-NWB wind tunnel in Braunschweig for which a stator with low blade count and highly swept blade geometry was designed in order to reduce both tonal and broadband noise generated by the ventilator.
Department Fan and Compressor