W increasing environmental pressures on the aviation industry, there is a renewed interest in the open rotor (or Counter Rotating Open Rotor) concept to reduce aircraft emissions. Due to their high by-pass ratio and without the drag of a duct, the open rotor can offer significant fuel savings, and hence, reductions in emissions when compared to modern turbofan engines [1]. Further efficiency gains are achieved by the counter-rotation of the aft rotor removing the residual swirl from the flow. However, with the lack of a duct to limit noise propagation, the noise emissions of the open rotor have so far limited their integration on modern aircraft. Open rotor noise is a complex problem; it is highly tonal with significant noise in all directions over a wide frequency range. Therefore, an understanding and appreciation of the noise emissions are required from the early stages of the design. In order to address this, this contribution presents a parametric study of open rotor noise at the preliminary design stage. Open rotor noise comprises broadband and tonal components. Despite the importance of the broadband component [2], the tonal component is generally considered the most significant and annoying. Open rotor noise is characterised by tonal peaks at sum and differences of the fore and aft blade passage frequencies. As a result, the noise emitted from the open rotor is very sensitive to the fore and aft blade count and tip speed combinations. Therefore, this parametric study is primarily concerned with both blade count and rotor tip speed configuration. To further this, a number of optimisations are performed to minimise open rotor noise under a number of additional constraints. In addition to the self-noise, open rotor noise is also comprised of a number of interactions sources. The interaction noise can be further classified as acoustic or aerodynamic interactions. Acoustic interactions result from the constructive and destructive addition of the pressure signals from fore and aft rotors [3]. Aerodynamic interactions comprise a number of sources. In particular, the potential interactions travelling both up and downstream, and the interaction of the front rotor wake and tip vortices with the aft rotor. For take-off and approach conditions, these interactions sources are dominant.

A number of concepts have been proposed to reduce these interaction components and hence achieve a total noise reduction. In order to reduce the fore wake contraction and hence weaken the downstream interaction, Zachariadis et al.[4] proposed changes to the rotor rotational speed and setting angles at take-off. Despite the reduction in interaction noise, a subsequent increase in self-noise resulted. Weckmüller and Guérin [5] demonstrated that the use of serrated trailing edges used to increase the mixing of the fore rotor wake and hence minimise the downstream interactions was successful in reducing open rotor noise. Akkermans et al.[6] achieved a reduction in the interaction component by using trailing edge blowing on the fore rotor in order to reduce the fore wake deficit and hence its interaction with the aft rotor. After quantifying the open rotor interaction mechanisms, Peters and Spakovszky [7], achieved a reduction in open rotor noise with an optimised geometry in addition to aft rotor clipping and an increased axial spacing. These concepts have demonstrated a number of unique methods of reducing open rotor noise. However, as they use high-order models, they can only consider small changes to existing geometries and a limited number of cases. This contribution will use low-order models that allow for a wide design space to be explored. In addition, a number of operating points are also …