Raman scattering measurements have the advantage that they provide single-point measurements of multiple species and temperature using a single laser source. It is well-known that the scattering cross-sections scale with the fourth power of the frequency. Hence, the proposed near-infrared technique suffers from a disadvantage relative to Ar-ion laser based methods or pulsed ultra-violet lasers. In addition to the practical advantages of diode lasers cited above, the laser source can be tuned to an alkali resonance transition which provides a very efficient method of rejecting elastic scattering interferences when performing Raman measurements. Because the laser operates cw there is the potential of obtaining temporally correlated measurements of multiple species at a point. 

I have developed a novel variation on filtered Rayleigh scattering using solid-state diode lasers that we call Modulated Filtered Rayleigh Scattering (MFRS). We exploit the tuning capability of the diode laser to set it to the D2 line of Rb at 780 nm and perform filtered Rayleigh scattering measurements. The continuous tuning is also exploited to modulate the laser frequency rapidly, which provides the MFRS technique with some unique properties. Because the laser operates cw one can obtain continuous velocity and density measurements. In principle, two velocity components can be measured with a single laser beam, and simultaneous multi-point measurements of velocity and density or temperature are possible.

A tunable infrared diode laser was used for absorption tomography in an axisymmetric atmospheric pressure flat-flame burner. Profiles of temperature and CO2 mole fraction were measured simultaneously. Experiments were conducted in methane-air flames with stoichiometries ranging from lean to rich. Absorption measurements were made just short of the R-branch band head at 4.17 µm to minimize interferences with other species, while providing good temperature and concentration sensitivity at flame conditions. The tomographic reconstructions were successful when spectra were taken just short of the nu-3 band head. Spectra that include the band head show large line shifts of the R(110) ­ R(122) lines in flames. The shift varies rapidly with rotational quantum number, but is not observed in measurements of pure CO2 in a cell. The flame measurements also showed an anomalously intense absorption in the region of the R(116) ­ R(124) transitions. The cause of this anomaly has not been determined and requires further investigation. 

We are developing a high power (~100 W) Raman scatterring instrument that will operate at 10-15 kHz in order to obtain simultaneous measurements of several major species (fuel, oxidiser, products) and temperature at rates two to three orders of magnistude faster than conventional Raman systems.

A study is being conducted of plasma-induced ignition of solid propellants. Work to date has included the characterization of a pulsed plasma jet resulting from a capillary discharge. An image of the visible emission of the plasma jet, as captured by a fast shuttering intensified camera, is shown at right.

Planar laser induced fluorecence image of copper atom in a capillary discharge

Flows with significant continuum and rarefied zones are important in a number of practical situations. The application of immediate interest is the impingement of plumes on the solar panels of the International Space Station during Space Shuttle docking maneuvers. We use Direct Simulation Monte Carlo (DSMC) in the regions of significant gradients and a discrete velocity method in the equilibrium zone. The two regions are coupled and the boundaries are continuously adapted to track moving shocks, expansion waves, boundary layers, etc. 
Additional details on Mixed Continuum and Rarefield Flow Modeling 

Io has a tenuous atmosphere that arises from volcanic activity. We model the rarefied plumes on Io and compute radiation from the plumes for comparison to observations that can be made from ground and space-based telescopes. Plume features include a canopy shock, supersonic gravity driven flows, and regions of extreme non-equilibrium where the classical thermodynamic or kineitc definitions of temperature are misleading.

Io was studied by NASA's Galileo probe which took some great pictures and movies.

This work is an extension of the Delta-Epsilon method (Z. Tan and P. L. Varghese, J. Computational Physics 110, pp. 327-340, 1994) in which one uses a discrete velocity approximation in the infinite velocity space, but considers only those distribution function points which that exceed a threshold value. The distribution function points may occur anywhere in the infinite discrete velocity space and are not constrained to a pre-specified region. A fourth-order finite difference is used for the convection terms. A Monte Carlo-like method is applied to the discrete velocity model of the collision integral. The effort of the method is proportional to the number of discrete points. Currently the method is being extended to include rotational and vibrational energy modes of molecules and gas mixtures