Velocimetry using scintillation of a laser beam for a laser-based gas-flux monitor

This paper describes a velocimetry system using scintillation of a laser-beam with spatial filters based on sensor arrays for a laser-based gas flux monitor. In the eddy correlation method, gas flux is obtained by mutual relation between the gas density and the flow velocity. The velocimetry system is developed to support the flow velocity monitor portion of the laser-based gas flux monitor with a long span for measurement. In order to sense not only the flow velocity but also the flow direction, two photo diode arrays are arranged with difference of a quarter period of the weighting function between them; the two output signals from the sensor arrays have phase difference of either pi/2 or -pi/2 depending on the sense of flow direction. In order to obtain the flow velocity and the flow direction instantly, an electronic apparatus built by the authors extracts frequency and phase from crude outputs of the pair of sensors. A feasibility of the velocimetry was confirmed indoors by measurement of the flow-velocity vector of the convection. Measured now-velocity vector of the upward flow agreed comparatively with results of an ultrasonic anemometer.