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Basic Concept
Anemos is a Greek word which means
“wind”. Consequently, a Laser Doppler Anemometer (LPA) is a
“wind meter” using a laser based on the physical effect
Christian Doppler discovered in 1842. However, the LDA cannot
detect pure wind as a clean air stream; it needs to have
particles moving with the wind. These particles move through two
crossing laser beams. Due to the coherence of the laser, a
spatial interference pattern appears within the crossing zone
which look like Zebra stripes. When particles are moving through
the stripes, they scatter the light in preferred directions.
When children play by striking the vertical bars of a picket
fence with a stick, it gives a characteristic burst-like noise.
The faster they run the shorter the burst is, however, the
frequency of the strikes is higher. Yeh and Cummins exploited
the same principle in1964 when they invented their laser doppler
anemometer.
Instead of children playing, the set-up uses an ultra sonic
particle seeder. An avalanche photodetector combined with a
telescope is used to detect the scattered light. A storage
oscilloscope is required to display and store the individual
burst for subsequent analysis.
Experimental Set-up
The set-up of this experiment on the left. The beam of the laser
is transfered by means of an optical fibre to the beam splitter
where it is divided into two beams. Beam A is reflected and the
other one (B) transmitted. Also with patch fibres, the output of
the beam splitter unit is connected to the LDA optics head.
Inside the head, both beams A and B are converted back to
parallel beams. The main lens 1 bends and focuses the beams to a
crossing point, producing the required spatial interference
pattern. By means of an ultra sonic device, particles are seeded
which pass the crossing beams. During their travel, they produce
back scattered light which is collimated by lens 1 and imaged by
lens 2 into the entrance of an optical fibre. This fibre is
connected via a fibre patch cable to the SiPIN detector unit
providing an fibre optical input. The optical signal is
converted into an electronic one and subsequently conditioned
and amplified to be displayed on an storage oscilloscope. |
