Acoustics Stealth Program Technology - at-aerospaceservice

Vai ai contenuti

Menu principale:

"A Practical Method of Measurements System for the improving Stealth Fighter and Bomber Acoustic Passive Performances" So far, skipping for now the thermal invisibility of the Stealth Fighter that is operated adequately cooling the gas flow from the exhaust outlet of the jet propulsor, such way is effective only if the fighter travels to an altitude to which the cooled gases are otherwise indeed more cold of the surrounding air, or the infrared sensors they will trace the temperature gradient of the gas flow, let me explain here a first practical method, to begin with, study the passive acoustics performance of the nacelle and duct of the jet propulsor. 
The noise radiated by the jet propulsor that is able to be traced by the sensors of the possible station of listening, is composed of three contributions, the first one is the radiated noise both from inlet and from outlet duct of the jet propulsor, the second contribution is due to the structural vibrations of the nacelle of the jet propulsor, the third contribution it is due to the interaction of the gas flow of unloading with the surrounding atmosphere in quiet. In this article we will speak only about the first and the second of the three contributions and that is to improve the passive acoustics performance of the jet propulsor; for the first contribution to the radiated noise we can be begin to study a chain of measure that is realized with a strong loudspeaker to position next to the entry of the inlet fed by 
a generator of signal with white noise (in this way, noise includes all the phases and regimes of operation of the jet propulsor being wide band), following microphones are inserted for the whole length of the nacelle connected to a multi-channel spectrum analyzer (the first entry is the microphone positioned next to the loudspeaker that will measure the signal in entry, the following microphones will measure the signal on the whole length of the jet propulsor up to the exit where the last microphone is positioned) obviously it is evident that in this phase we proceed with the jet propulsor turned off and so the spectrum analyzer besides giving in output the FFT transfer function (it is the function that multiplied for the signal of entry it gives me the signal of exit) it will furnish me indications on the shape of the standing waves that is formed inside the duct of the jet propulsor, and hence of the position of antinodes and nodes of sound pressure (resonant elements must obviously be positioned in proximity of the antinodes or they don't work); 
obviously for to characterize acoustics of the jet propulsor, all the experimental measurements they will begin first with the empty principal duct then they will totally be inserted every time, to every following phase of experimental measurement, all the mechanical components of the jet propulsor up to the final assembly. For the second contribution to the noise radiated due to the structural vibration of the nacelle of the jet propulsor it will be begin in a first time with measurements of Laser Scanning Vibrometry so that to identify nodes and antinodes of vibration and/or speed, where the dumpers will be positioned (what opportune local stiffening can be also) for then to always continue with the Laser Scanning Vibrometry on the jet propulsor fully operative to the various regimes.
 This practical method is fundamental for to identify the spectrum components of radiated noise and especially those with low frequency that because of the sound wave diffraction, they can travel up long distances overcoming obstacles with non-comparable length with their wavelength and therefore to be also easily traced from the stations of listening and from their sensors (obviously the shape of the nacelle is also important for the sound wave diffraction, even if to the low frequencies the jet propulsor can be considered as a sound wave source punctiform). The next step, about the third contribution, begin with ANSYS Fluent (CFD)
Torna ai contenuti | Torna al menu