Supersonic Mass-Flux Measurements via Tunable Diode Laser Absorption and Nonuniform Flow Modeling

Measurements of mass flux are obtained in a vitiated supersonic ground-test facility using a sensor based on line-of-sight diode laser absorption of water vapor. Mass flux is determined from the product of measured velocity and density. The relative Doppler shift of an absorption transition for beams directed upstream and downstream in the flow is used to measure velocity. Temperature is determined from the ratio of absorption signals of two transitions (lambda(1) = 1349 nm and lambda(2) = 1341.5 nm) and is coupled with a facility pressure measurement to obtain density. The sensor exploits wavelength-modulation spectroscopy with second-harmonic detection for large signal-to-noise ratios and normalization with the 1f signal for rejection of non-absorption-related transmission fluctuations. The sensor line of sight is translated both vertically and horizontally across the test section for spatially resolved measurements. Time-resolved measurements of mass flux are used to assess the stability of flow conditions produced by the facility. Measurements of mass flux are within 1.5% of the value obtained using a facility predictive code. The distortion of the wavelength-modulation spectroscopy lineshape caused by boundary layers along the laser line of sight is examined and the subsequent effect on the measured velocity is discussed. A method for correcting measured velocities for flow nonuniformities is introduced and application of this correction brings measured velocities within 4 m/s of the predicted value in a 1630 m/s flow.