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"A Practical Method of Measurements System for the improving Stealth Bomber and Fighter Acoustic Passive Performances (Turbofan Vibro-Acoustics)" So Far, in the previous articles we have been working on finding a practical way to increase the passive acoustic performance of the nacelles, duct, and nozzles of a Bomber or a Stealth Military Fighter from the sound emissions produced by the propulsion engines and therefore from a closely related point of view to the acoustic sources; Now we have to think about the vibrations transmitted by the same engines to the carriages. For example, thinking of the structural vibrations of the fan blades (same as the turbine blades) that propagate through the structure and finally radiate acoustically from the surfaces of the nacelles; We know that vibrations in the fan blades are the result of the combination of many vibration sources;One of the main  
sources is that caused by the fluctuating lift force that acts on the aerodynamic profiles (airfoil) of the blades when they rotate. Other sources include atmospheric turbulence and viscous wake interaction with the original turbulence and mechanical components of the propulsor. All of these factors result in vibration of the fan blades to a wide variety of resonant frequencies that depend mainly on the design of the propulsor. For this reason, it is not possible to design a vibration damping system for a specific frequency only; Various methods have been designed to dampen the above-mentioned structural vibrations, the first being to add layers of highly absorbent material to the vibrating surfaces of the structure in order to increase the most flexural energy dissipation of propagating waves, the second method Is given by the suppression of resonant  
vibrations of structures by reducing the reflection of structural elastic waves from their free edges; One of the most recent methods is called "acoustic black hole effect" and consists mainly of adding small pieces of absorbent material even in the form of absorbent ribbons to apply to the trailing edges of the fan blades, which have proven to be very effective in damping the Flexural waves. However, even if these methods reduce the vibrations transmitted to the nacelles engine surfaces, in a Stealth project we must limit the acoustic surface irradiation as much as we do not have to allow the listening stations to detect our presence and, above all, we must absolutely stop the Low-frequency waves sound radiation that reaches long distances and is easily detectable by the acoustic sensors of enemy listening stations. So proceed this way; An excitation force is applied through an electromagnetic shaker fed with a broadband signal with the excitation shaft directly glued to the axis of our turbofan; A scanning laser vibrometer is simultaneously used to obtain a vision  
of the nacelles engine's surface vibration nodes and antinodes and the relative resonance dominant frequencies. At this point, setting up the measurement chain, we use the degree reduction of the phenomenon called "Sounding Board Effect"; It consists of a reinforcement or sound amplification that is directly related to structural noise irradiation. In practice, the efficiency of a sound source differs in a proportional manner directly with the ratio of its surface area and the wavelength of the produced sound. In fact, the greater the area of the radiator surface and the greater its efficiency. A sound source with a small radiant surface will produce a small air noise but will still radiate a high-frequency sound more efficiently than a low-frequency sound at the same conditions. Therefore, after attempts with the laser scanning vibrometer, the irradiated positions and dominant frequencies are identified, after which the assemblies are assembled, making them with a ridge-like plate structure, gluing (visco-elastic joints) between them the various surfaces of the plates which make them have obviously calculated areas to reduce the efficiency of the radiated sound at the frequencies of interest (obviously the low frequencies will result in the use of small radiant surfaces); It proceeds for attempts until the scanning laser vibrometer analysis shows a significant reduction in structural nodes and antinodes dominant resonance frequencies.
 In the photo on the right is shown the modal analysis carried out by Angelo Teobaldelli in the far year 1996, with scanning laser vibrometer on the engine suction system of the Ferrari 550 Maranello where vibrational nodes and antinodes are highlighted in order to Characterize the vibroacoustic behavior of the system, excited in this case particularly acoustically with a loudspeaker connected directly to the intake manifold inlet and powered by a white noise signal generator.The next Step, in our development of Acoustics Stealth Program Technology, will be a software simulation with ANSYS Fluent (Aeroacoustics). 
 
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