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Proc. DAGA 98, Fortschritte der Akustik, pp 144-
Acoustic Sensor To Measure Wind Velocity And
Snowdrift For Snow Avalanche Forecasting
(page 2)
2. Principle of the acoustic anémodriftometer
The so-called Flow CaptTM acoustic anémodriftometer determines both wind velocity and snow particles flux. The detection principle is based on mechanical-acoustical coupling. The sensor is composed of closed pipes containing electro-acoustic transducers and a powering, filtering and amplifying unit. When the sensor is placed into a snow particles flux, the particles shock the sensor pipes, inducing acoustical pressure (fig. 2). The pressure is picked-up by piezo-electric microphones. The electrical outputs are filtered and time-averaged in given frequency ranges to provide a signal proportional to particles flux Q (kg.m2.s). The formal relation between the measured acoustic pressure and the snow particles flux Q requires the determination of the mechanical-acoustical coupling equations for the sensor, according to suitable hypothesis about particle impacts. The wind velocity is determined on a similar principle : the wind interacts with the body of the sensor and generates acoustic pressure into the air enclosures (fig. 3). Suitable sensitivity can be obtained optimising the body shape and structure to the expected wind velocities.
Fig. 2: Left - Saltation ice particles shocking a pipe. (from V. Chritin). Right - Visualisation of a turbulent flux around a cylindrical obstacle. (from H. Werlé, ONERA).
Because snowdrift happens during windy periods, it is necessary that the sensor strongly discriminates wind from snowdrift. This property can be obtained by an appropriate design of the mechanical-acoustical coupling. With no mobile components and full protected transducers (microphones inside closed cavities), the Flow Capt is very suitable for stringent topographical and climatic environments.
3. Prototyping of the anémodriftometer
Theoretical and experimental campaigns have been carried-out at the Swiss Federal Institute for Technology (EPFL) to develop Flow Capt prototypes (fig. 3).
Fig. 3: Top left - Prototypes tested in wind tunnel, at LASEN-EPFL. The acoustic response of cylindrical and spherical forms excited by wind were characterised in the 0 - 12.5 m/s range (from Th. Castelle). Top right - Calibration with controlled particles flux on test-bench, at LEMA-EPFL. (from Th. Melly). Bottom - Prototype tested at Anzère ski resort (2400 m).
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