The present invention relates to the general field of turbine engines, and it applies more particularly to turbojets with unducted propulsive propellers.
The present trend concerning civil aircraft engines seeks to reduce in particular their specific consumption and their discharge of atmospheric pollutants. 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 aircraft engine. On this topic, several turbojet architectures have been proposed, including turbojets having a pair of contra-rotative open rotors (CRORs), which constitute good candidates for replacing present turbojets, in particular on aircraft used on medium-haul flights.
With a conventional turbojet architecture, the nacelle channels the secondary stream so as to produce the majority of the thrust. With a CROR architecture, the nacelle is removed and the propulsion system comprises an upstream propeller that drives the flow, and a downstream propeller mounted to contrarotate relative to the upstream propeller, having the purpose of straightening out the flow (the downstream propeller could also be stationary in certain other types of architecture). The propulsion efficiency of the engine is improved by recovering the rotary energy more effectively than with a stationary wheel, and the diameter of the propellers is also greatly increased in order to enable a greater quantity of air to be driven.
Nevertheless, in the absence of a nacelle, sound emissions represent a major drawback of that 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 the installation of the engine on the airplane).
When the turbojet is mounted on the fuselage of an aircraft by means of an attachment pylon fastened upstream from the propellers, the assembly is said to be of the “pusher” type. In such a configuration, several sources of noise are associated with the presence of the attachment pylon, and the greatest is constituted by the interaction between the upstream propeller and the wake created downstream from the pylon (and corresponding to a lack of speed of the flow).
This interaction between the wake and the upstream propeller gives rise to two types of noise in particular:                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 is present at the natural frequencies of the propeller; and        broadband type noise corresponding mainly to the interaction between the turbulent structures in the wake and the upstream propeller, with the source being localized at the leading edges of the blades of the upstream propeller and covering a broad band of frequencies.        
Various solutions have been proposed to reduce the sound nuisance produced by interactions between the wake from the pylon and the upstream propeller. By way of example, Document FR 2 968 634 proposes compensating for the speed deficit of the wake downstream from the pylon in order to reduce the impact of the wake by means of a pylon that is provided with a trailing edge having two tiltable faces between which air can be blown over the entire span of the pylon. Nevertheless, such a solution presents the drawback of being active and of requiring a large amount of air under pressure to be taken off from the turbine engine, which can in particular reduce its performance.