Laser-induced thermal acoustic velocimetry

Laser-Induced Thermal Acoustics (LITA) is a non-intrusive, remote, four-wave mixing laser diagnostic technique for measurements of the speed of sound and of the thermal diffusivity in gases. If the gas composition is known, then its temperature and density can be inferred.; Beam misalignments and bulk fluid velocities can influence the time history and intensity of LITA signals. A closed-form analytic expression for LITA signals incorporating these effects is derived. The magnitude of beam misalignment and the flow velocity can be inferred from the signal shape using a least-squares fit of this model to the experimental data. High-speed velocimetry using homodyne detection is demonstrated with NO₂-seeded air in a supersonic blow-down nozzle. The measured speed of sound deviates less than 2% from the theoretical value assuming isentropic quasi-1D flow. Boundary layer effects degrade the velocity measurements to errors of 20%. Heterodyne detection is used for low-speed velocimetry up to Mach number M = 0.1. The uncertainty of the velocity measurements was ∼0.2 m/s. The sound speed measurements were repeatable to 0.5%. The agreement between theory and experiments is very good.; A one-hidden-layer feed-forward neural network is trained using back-propagation learning and a steepest descent learning rule to extract the speed of sound and flow velocity from a heterodyne LITA signal. The effect of the network size on the performance is demonstrated. The accuracy is determined with a second set of LITA signals that were not used during the training phase. The accuracy is found to be better than that of a conventional frequency decomposition technique while being computationally as efficient. This data analysis method is robust with respect to noise, numerically stable, and fast enough for real-time data analysis.; The accuracy and uncertainty of non-resonant LITA measurements is investigated. The error in measurements of the speed of sound and of the thermal diffusivity initially decreases with increasing signal intensity (excitation beam pulse energy) and increases again after passing a minimum. The location of the minimum error for the speed of sound and for the thermal diffusivity coincide. The errors at the minimum are 0.03% and 1%, respectively. The uncertainties for the speed of sound and the thermal diffusivity decrease monotonically to 0.25% and 5%, respectively. The increased error for high excitation beam pulse energies results from finite-strength waves that cannot be treated using the linearized equations of motion.