Diode-Laser Sensor for Air-Mass Flux 1: Design and Wind Tunnel Validation

The design and construction of a fiber-coupled diode-laser sensor to determine air-mass flux via separate measurements of the velocity and density of oxygen in air is reported. The density is determined from the spectroscopic absorption of an oxygen feature in the A-band near 764 nm, while the velocity is obtained from the Doppler frequency shift between the absorption features from two crossed paths within the flowfield (wind-tunnel path length 1.07 m). A hybrid operation is used for this sensor, in which rapidly alternating (50 Hz) measurements are made: direct absorption for the density and wavelength modulation for velocity. The sensor's performance is validated using a well-characterized low-speed wind tunnel which provided uniform flows up to 23 m/s. The results showed that the sensor is able to measure a time-averaged velocity to within 0.25 m/s and a time-averaged density to within 0.7% of the tunnel set point, with corresponding 1-s standard deviations of 0.22 m/s and 0.76%, respectively. The mean mass flux accuracy was assessed and a 1-s standard deviation of 1.3% was measured at the highest flow rates.