Abstract
In the previous paper, a signal detection method (Homodyne detection method) of flow rate measurement by using optical fiber was proposed. But it was impossible to realize the reproducibility of signal detection by the method, because of extreme signal intensity change in case of a little change of surrounding temperature.
In this paper, a new signal detection method (Heterodyne detection method) is proposed. A series of wind tunnel tests was carried out using the new method. The results of the tests are summarized as follows :
(1) The wind tunnel tests were carried out in order to confirm the proposed new method to be appropriate. The results made it clear that the reproducible and stable operation of the method independent on the surrounding conditions would be possible.
(2) A smaller tension of an optical fiber is desirable because a high signal-to-noise ratio can be obtained. An improvement of a signal-to-noise ratio of the new method could be possible by a proper vortex generator.
(3) A level of a signal-to-noise ratio of the new detection method showes periodic and steep changes with regard to wind speed. The phenomenon happens by the hydromechanical resonance of an optical fiber.
(4) In the resonant phenomenon, a vortex shedding frequency locks in the resonant frequency of the optical fiber. The lock-in phenomenon makes it very difficult to measure a flow rate from a vortex shedding frequency. An experiment was carried out in smaller tension condition of optical fiber, and the resonant phenomenon were proved to be suppresed considerably.
The basis for optimal design of the optical fiber flow rate sensor was obtained by the results of the wind tunnel tests.
