Semi-empirical prediction of tone noise due to counter-rotating open rotors
Counter-rotating open rotors (CROR) were extensively studied to power medium-size aircraft in the 80s aft er the first increases in fuel costs. Indeed, their efficiency i s greater than that of turbofans and of single prop fans. They have again become a topical subject due to the new incre ase of fuel costs and to the risk of oil shortage. They however raise a serious acoustic issue because noise is gen erated by the two rotors and by their interactions, and due to the lack of any shielding nacelle. An original and fast semi-empirical method is proposed to predict radia ted sound levels. The objective is to rapidly assess if certification rules can be fulfilled, and to estimate the possib le impact of a future fleet on noise contours around airports. This work is thus focused on takeoff and approach conditions (advancing Mach number lower than 0.3). It is shown that direc tivity of a tone is mainly determined by a Bessel fu nction, and a parabolic pattern is suggested for overall sound pr essure levels. The shape of third-octave spectra is based on the large number of interaction tones which are present in each frequency band. Finally, some information is given on overall sound levels.
Low- Speed Aerodynamics and Aeroacoustics of CROR Propulsion Systems
Contra Rotating Open Rotor (CROR) propulsion systems have come back into focus as a possible economic and environmentally friendly powerplant for future transport aircraft. 
Having been widely applied to the simulations of single rotation propellers, the DLR CFD code TAU and the aeroacoustic analysis tool APSIM have been employed for the analysis of the complex aerodynamics and aeroacoustics of this type of aircraft propulsion system. 
In order to demonstrate the codes applicability to these types of simulations as well as to develop an understanding of the impact of configuration variations, a generic 8x8 and 10x8 pusher CROR powerplant are studied here at typical sea-level take-o conditions of M=0.2. The results allow for a detailed analysis of the aerodynamic interactions between the two rotors as well as the noise generation mechanisms, allowing for an improved understanding of interaction tone sources. In addition, the impact of the numerical approach to the aeroacoustic simulations on the results is studied with a focus on determining the importance of the quadrupole contributions.
On a New Subclass of p-Valent Close-to-Convex Mappings Defined by Two-Sided Inequality
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