Low Noise Aeroacoustic Design for Turbofan Powered NATO Air Vehicles

Kurzfassung

Military air vehicles cover a broad range of size and function, from large transports to small U(C)AVs to the turbojets of high-powered fighters. The typically intense exterior noise generated by all of these is of aerodynamic origin and this noise represents a problem in peace times during training operation and at wartime due to the acoustic detectability of operating combat aircraft. The state of the art of aircraft noise prediction and reduction is aligned with civil transport aircraft, while much less effort has been invested in noise reduction of military aircraft. Noise reduction concepts can only partially be adopted from the civilian side, because of the largely different aircraft configurations in the military domain. Military aircraft noise is practically entirely determined by engine noise. While turbofan engine noise reduction for civil aircraft is a very active topic of research and technology, any progress in this field is completely insufficient for military aircraft. The engine integration in combination with the aircraft configuration at military aircraft is vastly different which changes the relevant sound source mechanisms and also provides significant potential for noise reduction, not available at typical tube-and-wing civil aircraft. For instance, conceptually, certain techniques exploited to reduce the infrared signature of combat aircraft may be transferred to reduce sound radiation. AVT-318 “Low Noise Aeroacoustic Design for Turbofan Powered NATO Air Vehicles, focused on the MULDICON vehicle and added the jet noise source from the rectangular nozzle including the jet-trailing edge interaction arising from the highly integrated nozzle/airframe as well as a characterization of the inlet acoustic radiation. Experimental campaigns included a monopole point source generated inside the inlet duct and exhaust ducts. Considerable CFD and CAA studies were performed of the powered vehicle. An initial effort was performed to develop a noise reduction device, the Shielding Flap, that could be feasible to integrate with the MULDICON. Experimental and prediction studies were performed to understand the design parameters of the Shielding Flap and their acoustic impacts. Of particular interest is a first time aeroacoustic characterization of intake and exhaust sound radiation of a UCAV-type military vehicle and a series of experiments on the Shielding Flap concept. A low noise concept in the form of a trailing edge deployable devices used to shield aft radiated jet noise. The database assembled in AVT-318 consists in wind tunnel investigations of the DLR-F24 MULDICON model conducted in the acoustic test section the DNW-NWB facility as well as generic investigations of the Shielding Flap concept in the Quiet Flow Facility at NASA. While the tests were coordinated respectively by DLR and NASA, all partners have contributed with various activities to the design and realization of the model, including flow simulations. Each group has done predictions of either the installed jet and/or the intake/exhaust diffraction problem with their respective aeroacoustic prediction suites. A dedicated code-to-code and codes-to-experiments study was accomplished.