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.
Study of the Contra-Rotating Open Rotor Noise under Static Conditions
The paper presents the main results of the experimental study of acoustic characteristics of the propfan engine with a high-loaded contra-rotating open rotor. The measurements are made during the engine operation under static conditions. The power, directivity, and spectral characteristics of the power plant acoustic field are obtained.
A NEW BOUNDARY INTEGRAL FORMULATION FOR THE PREDICTION OF SOUND RADIATION
Abstract A new boundary integral formulation is presented for the evaluation of the noise radiated in a uniform medium by generic sources. To use the method one requires knowledge of the pressure, velocity, and density disturbances on a smooth closed surface surrounding the source, and to assume that the propagation is linear outside the surface itself. When applied to the prediction of transonic rotor noise the method can be used in the same manner as the Kirchhoff approach, but the new integral equations are derived without requiring the non-penetration condition in the Ffowcs Williams–Hawkings equation. The method is therefore referred as the Kirchhoff–FWH. The main advantage of the proposed formulation in respect of the Kirchhoff method is that it does not require the knowledge of the surface pressure normal derivative. Additionally, it can be applied also for bodies with permeable surface, while the classical FWH equation is not valid in this case. Two different formulations are presented, which differ in the way in which a time derivative is handled, and some general issues on the numerical efficiency of the two formulations are addressed. Comparisons with experiments, and with Kirchhoff and FWH methods, are presented for a hovering rotor in transonic conditions at various tip Mach numbers.