Engine integration of high aspect ratio rectangular jet nozzle (unheated subsonic flow)

Kurzfassung

Jet noise is a crucial component in the entire mix of aircraft-relevant noise sources. A quiet aircraft requires a low noise engine integration. Compared to the conventional round jet engine, the jet potential core length is shorter for a rectangular jet engine with same nozzle outlet area. This is especially true for high aspect ratio rectangular jets (here AR 13.3). Hence, the engine integration of the rectangular nozzle onto a wing could be conducted in a way where jet noise is shielded from an observer at the ground. Yet, the question is how such a low noise engine integration should be designed. Therefore, an experimental setup was built at the Aeroacoustic Wind tunnel Braunschweig (AWB) where rather large engine integration lengths and heights between engine lip and plate trailing edge can be studied. This allows to test shielding effects of embedded engine systems, e.g. short and long aft-decks, backward-facing steps as well as non-embedded/poled engines. The contributions of this paper are made in terms of the aero-geometric characterization of the problem physics, the evaluation of the installation effect for observers on the ground (along flyover arc or overhead position) as well as the evaluation of the shielding effect. The aero-geometric analysis helps to predict acoustical effects which occur for three general installation problems: the asymmetric nozzle, jet-surface interaction as well as wide-angle installations. Shielding benefits due to the installation of a plate can be determined by a shielding frequency criterion. Unfortunately, the high-frequent noise reduction comes with a lowfrequent installation penalty, thereby making noise reduction a diffcult design mission: All in all, only one tested configuration shows an overall installation noise benefit. The general results of rather achieving design penalties for long bevel/aft-deck design and step configurations is in agreement with previous studies conducted by NASA and Georgia Tech. Future low noise rectangular engine integrations may therefore consist of rather small engine integration lengths.