Research and Development of Fiber L2F

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A new type laser-2-focus velocimeter (L2F) was developed to materialize a small size, a low price and an easier operation. An optical system and a signal processing system based on an optical fiber or based on a diode laser were designed and were built up. The design specifications of new systems were tested applying to a flow measurement in a wind tunnel or in a rotating impeller.At the first step, a fiber L2F with an optical fiber of 8 m in length used between the laser source and the optical head was developed. The size of the fiber L2F was reduced to a half in weight and a third in volume compared with the original L2F. The optical S/N ratio of the fiber L2F was not deteriorated for application to a flow measurement in a rotating impeller, when the optical shutter was installed ahead of the optical fiber according to the behavior of polarized light in an optical fiber.At the second step, a diode L2F with a transmitter optics of 48 mm in diameter and of 120 mm of measuring distance … More was developed, the size of which is 324 mm in length, 98 mm in width and 140 mm in height, and the weight is 5kg. A diode laser of 840 mm in wave length was used as a light source, and the scattered light from a particles was detected using an avalanche photodiode. An electric amplifier was designed with high sensitivity against the diode laser. The measuring volume of the diode L2F was measured using a fine wier of 5 mum in diameter with a mechanical traversing system.It was possible to make a flow measurement at I mm close to the bottom wall even when the laser beam was inserted into a small-sized wind tunnel normal to the wall, and this confirmed a high S/N ratio of the diode L2F system. In the case of low turbulence flow, the maximum sampling rate was 2kHz and the probability of a particle traveling through the two foci was beyond 80 percent so that a measuring time was reduced significantly by narrowing the distance between the two foci to 0.1 mm. Velocity distribution in a two-dimensional wake behind a circular cylinder was measured. The diode L2F was applicable to a flow measurement even in a high turbulence flow with 20 % in transverse turbulence intensity. Less