The DIME Laboratory of experimental Aeroacoustics (DIME-LAA)
The LAA activity is mainly aimed at the study of low-speed fans. These machines are widely employed in cooling, heating, ventilation and air conditioning systems and cover many fields, e.g. automotive, electronic, civil, industrial, and maritime. The current market trend towards higher performance and smaller size units contrasts with the parallel request for more silent, less annoying fans. Likely, this has made the noise reduction as important as the performance improvement.
Aerodynamic noise usually provides the major contribution and, indeed, its reduction is the final goal of aeroacoustic studies. The basic theory of aerodynamic noise generation is well established: it is generated by pressure and velocity fluctuations in the flow and does not require the presence of solid surfaces vibration. As in many acoustic problems, the noise depends on the details of the acoustic source, in the present case the flow which, in turn, depends on the geometry of the fan assembly. Therefore, the main difficulty in the aerodynamic noise analysis and prediction is constituted by the lack of knowledge of the instantaneous features of the flow. Indeed, the actual geometries and operating conditions are quite complicated and, although the aerodynamic prediction methods have strongly improved in the last decade, experimental tests and investigations are fundamental.
From the practical point of view, the purpose of experimental studies is twofold:
- Identifying the active noise generating mechanisms, and
- Identifying the geometry modifications which may reduce the radiated noise.
The received noise depends on the combination of two effects:
- The noise generating mechanisms (e.g. inlet turbulence, inlet flow distortion, rotor-stator interaction, tip-gap flow), and
- The interaction of the radiated waves with the propagation environment (e.g. reflection and diffraction from blades and other solid surfaces, acoustic treatment, and so on).
Therefore, both aspects must be adequately considered. Furthermore, as the noise generating mechanism is aerodynamic in nature, also anemometric measurements are necessary in order to better understand the underlying physics. In fact, the final aim should the noise annoyance reduction, rather than the sound power reduction. Hence, the behaviour of the human hearing system must be taken into account, which means dealing with psychoacoustic aspects. At DIME-LAA, the main facilities, instrumentation, and software for such a kind of investigations are available:
- Hemi-anechoic chamber
- Microphone arrays and accelerometers
- Bruel&Kjaer spectrum analysers
- Bruel&Kjaer Sound Quality SW
- Dantec hot wire anemometers, laser Doppler velocimeter, and particle image velocimeter
Test-rig for fans (ISO 10302 standards), Variable speed servomotors