The present trend for civilian aeroengines seeks to reduce both their specific fuel consumption and the pollutants they reject to the atmosphere. One of the technical solutions adopted by engine manufacturers consists in increasing the bypass ratio between the primary stream (or “hot” stream) and the secondary stream (or “cold” stream) of the aeroengine. In this respect, several turbojet architectures have been proposed, including turbojets having pairs of contrarotating propellers (also known as “contrarotating open rotors” (CROR)), which are good candidates for replacing present turbojets, in particular on aircraft that perform medium-haul flights.
In another conventional turbojet architecture, a nacelle channels the secondary stream so as to produce the majority of the thrust. With the CROR architecture, the nacelle is removed and the propulsion system comprises an upstream propeller driving the flow and a downstream propeller that is contrarotating relative to the upstream propeller, and that has the function of straightening or guiding the flow (it being possible for the downstream propeller to be stationary in other types of architecture). The propulsion efficiency of the engine is improved by recovering rotary energy more effectively than with a stationary wheel, and the diameter of the propellers is also greatly increased in order to enable a larger quantity of air to be entrained.
Nevertheless, in the absence of a nacelle, sound emissions represent a major drawback for this architecture, and more particularly the noise generated by the propellers, and by various interactions between the propellers and the components associated with mounting the engine on the aircraft (also referred to as effects associated with installing the engine on the aircraft).
When the turbojet is mounted on the fuselage of an aircraft by means of an attachment pylon located upstream from the propellers, the configuration is said to be of the “pusher” type. In such a configuration, the presence of the attachment pylon is associated with several sources of noise, of which the major source is constituted by interaction between the wake (corresponding to a deficit of flow speed) created downstream from the pylon and the upstream propeller.
This interaction between the wake and the upstream propeller leads in particular to two types of noise:
a tonal type noise, corresponding to the interaction between the mean wake (constituted by a speed deficit downstream from the pylon) and the upstream propeller, which noise is present at the frequencies specific to the propeller; and
a broadband type noise, corresponding mainly to the interaction between the turbulent structures of the wake and the upstream propeller, with the source of this noise being located at the leading edges of the blades of the upstream propeller, this noise covering a wide range of frequencies;
Several solutions have been proposed for reducing the sound nuisance produced by interaction between the wake from the pylon and the upstream propeller. By way of example, Document FR 2 968 634 proposes compensating the speed deficit downstream from the pylon in order to reduce the impact of the wake by using a pylon that has a trailing edge fitted with two tiltable faces, between which air can be blown over the entire span of the pylon. Nevertheless, such a solution presents the drawback of requiring a large amount of air to be taken from the engine, thereby reducing performance.