Numerically Determining Mach Number and Orientation in Hypersonic Inlets Using Absorption Spectroscopy

A numerical method is presented for measuring angle of attack and Mach number in the inlet of a supersonic combustor using a sensor design based on tunable diode laser absorption spectroscopy. A nonlinear least-squares fitting routine was applied to simultaneously retrieve the freestream Mach number and the angle of attack in a simple two-dimensional inlet and angle of attack with a known freestream Mach number in a conical inlet using simulated absorption spectrum. The spectrum spanned over five transitions in the A-band of molecular oxygen. In the presence of 15% white Gaussian background noise, the fitting routine converged to the actual value of Mach number with a standard deviations of 0.5% for the two-dimensional inlet and to the actual angle of attack with a standard deviation of 3% for both the inlets. It was also found that, at the conditions used in the simulation for a conical inlet, a systematic error of 34K would occur in freestream temperature if the usual two-line-ratio method were used and a systematic error of 0.25 would occur in freestream Mach number if the Doppler shift of the absorption peaks were used without considering the flow nonuniformity in the inlet.