The present invention relates to the general field of reducing jet noise from a turbine engine having a nozzle of the type with separate streams.
Sound pollution has nowadays become a subject of concern for engine manufacturers, who are being confronted more and more about the sound nuisance of their engines. There are numerous sources of noise in a turbine engine, but it has been found that the jet noise at the outlet from the nozzle of an engine is the noise that predominates during airplane takeoff stages. Since certification authorities are becoming more and more demanding concerning sound emissions from turbine engines, engine manufacturers are being required to make efforts to reduce the noise from their engines, and in particular jet noise.
Typically, an engine having a nozzle with separate streams comprises a central body centered on the longitudinal axis of the engine, an inner cover arranged coaxially around the central body so as to co-operate therewith to define an annular channel (or passage) for passing a core stream (or hot stream) from the engine and an outer cover arranged axially around the inner cover to co-operate therewith to define an annular channel for passing a bypass stream (or cold stream) coming from the engine.
In such an engine, the jet noise comes in particular from the mixing between the bypass stream and the air stream outside the engine flowing along the outside wall of the outer cover. That noise has a broad frequency band generated by two types of sound source: high frequency noise coming from small turbulent structures in the mixing between the streams and perceived essentially close to the point of ejection, and low frequency noise coming from large turbulent structures that appear far away from the jet.
In order to reduce jet noise, one of the means used consists in specifically increasing the mixing between the gas streams. For this purpose, it is known to take air flowing in the flow channel for the bypass stream and to eject it further downstream at the trailing edge of the outer cover so as to act on the shear layers that form in the zone where mixing takes place between the bypass stream and the air stream outside the engine. By way of example, reference may be made to Document WO 2006/013243, which describes an implementation of this principle in which air jets taken by feed ducts and ejected at the trailing edge of the outer cover converge in pairs in order to form interaction triangles.
In order to feed the air jets that form disturbances in line with the trailing edge of the outer cover, that solution requires air to be taken from the flow in the flow channel for the bypass stream, which presents the advantage of avoiding having recourse to an additional air supply system. Nevertheless, taking air in that way from the bypass stream necessarily gives rise to impacts on the performance of the engine, which impacts may turn out to be critical if they are not optimized sufficiently.
The solutions proposed in the prior art thus do not always make it possible to obtain a significant reduction in jet noise. Specifically, the presence of feed ducts together with their inlets and outlets gives rise to impacts on the performance of the engine that may turn out to be critical if they are not optimized sufficiently. Likewise, those ducts lead to significant noise penalties at medium and high frequencies, in particular as a result of the aerodynamic noise caused by the flow of the stream at the trailing edge of the outer cover.