We use an Argon laser with power 8 Watts, with optical fibres and an optical system to generate the laser sheet. Double pulsed Yag lasers are classically used in PIV technique (for an application to a large hydraulic facility, the 100 m long wave channel of INHA at Barcelona, see http://lim-ciircupc.es/eng/CIEM/index.htm). They provide an intense peak power on a very short time, so that fast velocities can be captured. However, the repetition rate is small (typically 20 Hz), optical fibres would be damaged by excessive pulse power, synchronisation with camera is delicate, and a careful alignment of the two laser beams is needed.
Practical operations are thus much easier with a steady laser, and the flow evolution can be resolved in time, or the 3D structure obtained by rapid scanning of the laser sheet. The steady laser is therefore quite appropriate for our flows with moderate velocity, up to typically 10-20 cm/s.
We produce the laser sheet with an oscillating mirror (model 6850
of Cambridge Technology), at frequency 120 Hz, synchronized with the
images by the electronic pulses emitted by the camera. The total angular
spread in water can reach 50
, but it can be adjusted to
optimize luminosity in a given field of view. Producing such a wide
and uniform laser sheet by scanning is another advantage of operating
with a steady laser.
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