Yedhu Krishna

Baseline correction for stray light in log-ratio diode laser absorption measurements.

Log-ratio detection is a convenient technique for making temperature and concentration measurements using sensors based on tunable diode laser absorption spectroscopy. In many practical sensing applications, it is difficult to avoid stray light falling on the signal photodiode of the sensor. This stray light acts as noncommon-mode interference and introduces a systematic error in absorption measurements, which is not removed by baseline subtraction. This paper analyzes the factors that determine this systematic error and also presents a calibration method that can correct for it. This correction method is verified using a simple experiment.

Flight test of a rugged scramjet-inlet temperature and velocity sensor

The performance of a gas temperature and velocity sensor based on tuneable diode laser absorption spectroscopy (TDLAS) of oxygen in a scramjet engine inlet during a flight test is discussed. This is the first flight-tested TDLAS sensor designed specifically for measuring gas velocity of a scramjet inlet flow under flight conditions. The robustness and stability of the sensor design in dynamic load conditions were made evident by the quality of the data delivered after a very unstable and high-g launch scenario. The scramjet did not reach the targeted test altitude-velocity regime. However, analysis of the low-altitude data determined pressure and temperature in the inlet and showed that an adequate signal-to-noise ratio was obtained for high-speed velocity measurements at altitudes up to at least 30 km.

Diode-Laser-Based Driver Gas Detector for Hypersonic Shock Tunnels

Measurement of the useful flow duration in ground-based hypersonic flow facilities, especially for impulse facilities which have relatively short output flow duration, poses significant measurement challenges.