1,721,141 research outputs found
Flow pressure fluctuation measurement by means of interferometric technique combined with tomographic reconstruction
No full textFlow pressure fluctuation measurement by means of interferometric technique combined with tomographic reconstruction
No full text
A new laser vibrometry-based 2D selective intensity method for source identification in reverberant fields: part II. Application to an aircraft cabin
No full textThe selective intensity technique is a powerful tool for the localization of acoustic sources and
for the identification of the structural contribution to the acoustic emission. In practice, the
selective intensity method is based on simultaneous measurements of acoustic intensity, by
means of a couple of matched microphones, and structural vibration of the emitting object. In
this paper high spatial density multi-point vibration data, acquired by using a scanning laser
Doppler vibrometer, have been used for the first time. Therefore, by applying the selective
intensity algorithm, the contribution of a large number of structural sources to the acoustic
field radiated by the vibrating object can be estimated. The selective intensity represents the
distribution of the acoustic monopole sources on the emitting surface, as if each monopole
acted separately from the others. This innovative selective intensity approach can be very
helpful when the measurement is performed on large panels in highly reverberating
environments, such as aircraft cabins. In this case the separation of the direct acoustic field
(radiated by the vibrating panels of the fuselage) and the reverberant one is difficult by
traditional techniques. The work shown in this paper is the application of part of the results of
the European project CREDO (Cabin Noise Reduction by Experimental and Numerical
Design Optimization) carried out within the framework of the EU. Therefore the aim of this
paper is to illustrate a real application of the method to the interior acoustic characterization of
an Alenia Aeronautica ATR42 ground test facility, Alenia Aeronautica being a partner of the
CREDO project
Subsonic jet pressure fluctuation characterization by tomographic laser interferometry
No full textThis paper describes the application of a nonconventional experimental technique based on optical interferometry for the characterization of aeroacoustic sources. The specific test case studied is a turbulent subsonic jet. Traditional experimental methods exploited for the measurement of aerodynamic velocity fields are laser Doppler anemometer and particle image velocimetry which have an important drawback due to the fact that they can measure only if the flow is seeded with tracer particles. The technique proposed, by exploiting a laser Doppler interferometer and a tomographic algorithm for 3D field reconstruction, overcomes the problem of the flow seeding since it allows directly measuring the flow pressure fluctuation due to the flow turbulence. A laser Doppler interferometer indeed is sensitive to the density oscillation within the medium traversed by the laser beam even though it integrates the density oscillation along the entire path traveled by the laser. Consequently, the 3D distribution of the flow density fluctuation can be recovered only by exploiting a tomographic reconstruction algorithm applied to several projections. Finally, the flow pressure fluctuation can be inferred from the flow density measured, which comprehends both the aerodynamic pressure related to the turbulence and the sound pressure due to the propagation of the acoustic waves into the far field. In relation to the test case studied in this paper, e.g., the turbulent subsonic jet, the method allows a complete aeroacoustic characterization of the flow field since it measures both the aerodynamic "cause" of the noise, such as the vortex shedding, and the acoustic "effect" of it, i.e., the sound propagation in the 3D space. The performances and the uncertainty have been evaluated and discussed, and the technique has been experimentally validated
A new laser vibrometry-based 2D selective intensity method for source identification in reverberant fields: part I. Development of the technique and preliminary validation
No full textThe selective intensity technique is a powerful tool for the localization of acoustic sources and
for the identification of the structural contribution to the acoustic emission. In practice, the
selective intensity method is based on simultaneous measurements of acoustic intensity, by
means of a couple of matched microphones, and structural vibration of the emitting object. In
this paper high spatial density multi-point vibration data, acquired by using a scanning laser
Doppler vibrometer, have been used for the first time. Therefore, by applying the selective
intensity algorithm, the contribution of a large number of structural sources to the acoustic
field radiated by the vibrating object can be estimated. The selective intensity represents the
distribution of the acoustic monopole sources on the emitting surface, as if each monopole
acted separately from the others. This innovative selective intensity approach can be very
helpful when the measurement is performed on large panels in highly reverberating
environments, such as aircraft cabins. In this case the separation of the direct acoustic field
(radiated by the vibrating panels of the fuselage) and the reverberant one is difficult by
traditional techniques. The first aim of this work is to develop and validate the technique in
reverberating environments where the location and the quantification of each source are
difficult by traditional techniques. The reverberant field is clearly challenging also for the
proposed technique, affecting the achievable accuracy, mainly due to the fact that coherence
between radiated and reverberated fields is often unknown and may be relevant. Secondly, the
applicability of the method to real cases is demonstrated. A laboratory test case has been
developed using a large wooden panel. The measurement is performed both in anechoic
environment and under simulated reverberating conditions, for testing the ability of the
selective intensity method to remove the reverberation
Signal processing techniques for gears quality control and for specific defects detection
No full textExploiting Continuous Scanning Laser Doppler Vibrometry in timing belt dynamic characterisation
No full textPerformance analysis of continuous tracking laser Doppler vibrometry applied to rotating structures in coast-down
No full textIn this paper a performance analysis of the so-called tracking continuous scanning laser
Doppler vibrometry (TCSLDV) exploited in coast-down has been performed. This non-contact
measurement system is able to scan continuously over a rotating surface during coast-down
and to determine vibration operational deflection shapes (ODSs) and natural frequencies in
short time, i.e. the temporal extent of the coast-down. The method is based on a laser Doppler
vibrometer (LDV) whose laser beam is driven to scan continuously over the whole rotor
surface synchronously with its rotation, so that the LDV output is modulated by the structure’s
ODSs. This technique has a full-field nature that enables it to measure simultaneously the time
and spatial dependence of the vibration in a unique measurement. However, the TCSLDV
presents some criticalities in practical applications, especially when applied to rotary transient
and fast processes. In fact, if the vibration is transient and decays very fast, then the laser beam
could not have had the time to scan the complete structure surface and the modulation of the
ODS could be partial. An analytical model reproducing a representative experiment has been
developed in order to evaluate the sensitivity of results to testing conditions. The laser beam
trajectory in both the fixed and rotating reference systems has been synthesized showing its
dependence on experimental parameters as the rotation speed variation during coast-down. It
has been demonstrated the decrease in speed induces the deformation of the laser trajectory
influencing the LDV output time history, spectrum and consequently the recovered ODS
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