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Abstract
This thesis addresses three different topics, all related to the application of tunable diode laser absorption spectroscopy (TDLAS) for the diagnosis of hypersonic air flows. Firstly, a numerical method to retrieve angle of attack of a scramjet inlet from a single path-integrated absorption spectrum of oxygen in a simple hypersonic inlet flow is derived. A sensor design that can be incorporated into a double-wedge inlet or a conical inlet is proposed. A nonlinear least-squares distribution fitting method is used to simultaneously retrieve the free-stream Mach number and the angle of attack in a simple two-dimensional inlet and the angle of attack with known free-stream Mach number in a conical inlet using simulated absorption spectrum. The advantages of using the distribution technique in spatially non-uniform flows, particularly for applications that are restricted in space, such as for flight testing, is explained in detail.
The second topic concerns a systematic error involved with measurements that use log-ratio detection techniques. When log-ratio detectors are used in luminous environments and when the stray light intensity falling on the photodiodes of the detector is significant relative to the intensity of the laser used in the system, the baseline of the absorption spectrum measured is distorted. The conventional method of baseline subtraction introduces a systematic error in the absorption spectrum. It is shown that this systematic error is significant when the intensity of interfering stray light is comparable to the intensity of the laser beam. A method of calibrating the log-ratio detection system to minimise this effect is proposed and verified using a simple experiment.
The third topic involves the diagnosis of the free-stream flow in a free-piston shock tunnel using a sensor developed based on TDLAS. Room air is used as the test gas and water vapour absorption peaks near 1392 nm are targeted. The free-stream temperature and velocity of the flow is measured at two different enthalpy conditions and the results are compared with the corresponding simulated values. The sensor is also proposed as a non-intrusive driver gas detector convenient for free-piston shock tunnels. Its effectiveness as a driver gas sensor is investigated.