This dissertation describes the development of holographic interferometry techniques that rely on various tuning schemes of the recording laser sources to provide a range of measurements for gas flows. The techniques are intended for the ultimate use of providing two-dimensional diagnostics on the flows produced by high-velocity impulsive test facilities, such as shock and expansion tubes.
The main techniques developed in this dissertation are near resonant holographic interferometry (NRHI) and two wavelength holographic interferometry (TWHI). NRHI utilises the large variation in refractive index that occurs near an atomic or molecular transition as the basis for a range of measurements. The refractive index is related to the number density of the transition of interest, and interferograms recorded using NRHI techniques provide quantitative information about species concentration. TWHI exploits the situation that the refractive index for free electrons in a partially ionised plasma has a different wavelength dependence than the refractive index for neutral atoms. The technique relies on recording two interferograms simultaneous at two widely spaced wavelengths. The electron concentration and total density can be obtained by a simultaneous solution of a series of equations, using data from the interferogram pair.
The dissertation describes the techniques developed by the author for successful implementation of basic holographic interferometry on expansion tube facilities. Details relating to the design of practical interferometers for these facilities are outlined, as are details of a system for reconstructing interferograms for data processing.
A series of image processing techniques have been developed by the author to enable the phase from the recorded interferograms to be obtained. The procedures used for image conditioning, phase calculation, phase unwrapping, and background subtraction are all detailed, along with discussions on processing errors.
The development of NRHI techniques for use on flows seeded with ionic species is described. A theoretical model for the refractivity due to the seed species is developed, and an approximation to this model is also introduced which is shown to be valid for practical regimes of interest, and allows the number density of the species to be determined without knowledge of line broadening effects.
The details of quantitative number density experiments performed on an air-acetylene flame are given, and a comparison with an alternative absorption-based experiment is made. The NRHI technique is applied to seeded expansion tube flows and quantitative number density measurements for this flow are provided. The use of NRHI techniques to enhance the visualisation of density gradients is also demonstrated for a variety of models exposed to expansion tube flow.
The possibility of determining temperature using several NRHI schemes is also outlined.
The implementation of NRHI techniques to naturally occurring molecular species is discussed. An exploration of the issues for performing NRHI measurements on the hydroxyl radical (OH) produced in an air-acetylene flame is made, and the results of experiments to measure the number density are provided. A temperature measurement scheme based on the knowledge of the populations of two rotational transitions is outlined, and an experimental measurement of the temperature for an air-acetylene flame is also presented.
The concept of TWHI is introduced, and the basis for the simultaneous electron concentration and total density measurements is outlined for high enthalpy flows that generate significant free electrons. Theoretical models for relating the quantities of interest to the interferogram data are developed for both molecular nitrogen and air flows. The design of interferometers for production of TWHI interferograms is discussed, along with the design of an interferogram reconstruction apparatus. The results from several experiments on different expansion tube flows are presented, along with comparison with computational fluid dynamics (CFD) simulations.