Contra-rotating open rotors (CRORs) show promise for use in future aircraft propulsion systems, but suffer from high noise emissions. Both tonal and broadband noise emissions are produced by CRORs. Historically, tonal noise has been more extensively studied because it was thought to be the dominant noise type. However, recent studies have shown that broadband noise is also significant. Therefore, research is needed to better understand the generation of CROR broadband noise.
The project is concentrates on two areas of CROR broadband noise. Firstly, analytic models are being developed to improve industrial modelling of the noise. Secondly, the assumptions which are made in the analytic models are being tested with the use of high-order computational aeroacoustic (CAA) methods.
Analytic prediction tools are being developed in collaboration with Airbus, to allow the prediction of the major broadband noise sources that are present in a CROR engine
These sources are:
- Noise at the airfoil trailing edge due to interactions between the boundary layer and the sharp trailing edge of an airfoil.
- Noise at the leading edges of the rear rotor, due to unsteady aerodynamic interactions with the wakes generation by the front rotor.
- Noise due to aerodynamic interactions between an installation pylon and the rotor blades.
The model development is providing greater insight into the underlying mechanisms which cause CROR broadband noise. Predictions from models of this type also allow parameter studies to be performed, which will enable noise reduction to be included into the aircraft design process in future.
In addition to the development of analytical models, computational aeroacoustic (CAA) methods are being used to validate the assumptions which are made in industrial prediction models. A high-order CAA code known as SotonCAA has been developed at Southampton University, and is currently being used to test the effects of real blade geometry on CROR broadband noise. Analytic models usually assume the rotor blades to be unloaded flat plates, and therefore do not include any effects on the noise due to the real blade shape.