This thesis describes the development of two laser based methods for measurements of flow properties in ground based supersonic and hypersonic impulse facilities. The techniques are resonant degenerate four wave mixing (DFWM) and velocimetry by laser enhanced ionisation (LEI) flow tagging. Degenerate four wave mixing in gaseous, molecular iodine (I2) was theoretically and experimentally investigated. An extensive review of the DFWM method was presented and the stationary absorber model was chosen for DFWM experiments in iodine. As the pressure can vary greatly in many practical flows of interest this model was modified to determine the effect of buffer gas pressure on the DFWM signal. To model the spectral features of DFWM in iodine, the dipole moments for the transitions of interest were calculated from potentials constructed from spectroscopic data. Extremely good agreement between the calculated and experimental absorption and DFWM spectra were found.
The influence of molecular collisions on the production of the DFWM signal in I2 is presented. Measurements were performed on gaseous molecular I2, contained in a glass cell where pressure, temperature and species concentration are easily and independently varied. Frequency doubled output from a seeded Nd: YAG laser and an excimer pumped dye laser were both used as excitation sources. The dependence of signal strength versus buffer gas pressure, with pump intensity as a third parameter was studied. It is evident from the results that for pump intensities less than 1 MW/cm2 the pressure dependence of the signal follows that given by a simple two level model in the homogeneously broadened regime. In this regime collisional de-excitation becomes significant, leading to changes in saturation intensity evidenced by a reduction in the signal with an increase in buffer gas pressure. At higher pump intensities, the signal increased with pressure and this behaviour could not be described by the simple two level model. The implications of these results to flow diagnostics is discussed.
A new flow tagging technique for measurements of gas velocities over a wide range of flow conditions was developed. This method employs the laser-based spectroscopic techniques of Laser Enhanced Ionisation (LEI) and Planar Laser Induced Fluorescence (PLIF) to tag a flow by ionising a significant fraction of a neutral atomic species contained in that flow. Models to predict levels of neutral depletion by LEI were developed and were subsequently employed to investigate processes such as collisional quenching, pump rate and electron-ion recombination on the tagging process. Tagging by depletion of neutral sodium contained in a flame was performed and velocities were measured with an accuracy of better than 10%. The diffusion coefficient of neutral sodium in the flame was determined and electron-ion recombination rates were measured for different seed concentrations. Finally the technique was applied to the measurement of supersonic velocities generated in a shock tube. The freestream velocity of 1.68 km/s was measured in the shock tube and also the x-component of the velocity in the wake of a cylinder placed in the freestream flow.
The design and construction of laser and data acquisition equipment for both parts of this thesis are described.