Filtered Rayleigh Scattering: Toward multiple property measurements (Invited)

In molecular/atomic-filter diagnostics, the filter-which contains within a glass cell selected vapor-phase molecules or atoms (e.g., I₂ or Hg)-is placed in front of the detector to modify the frequency spectrum of radiation scattered by flow-field constituents (i.e., molecules/atoms and/or particles). With the Filtered Rayleigh Scattering (FRS) technique, and techniques derived from it, the molecular/atomic Rayleigh scattering is transmitted through the vapor filter and then focused on a detector, typically a charge-coupled device (CCD) camera or photomultiplier tube. The filter can be used simply to suppress background surface/particle scattering, and thereby enhance flow visualization, or to make quantitative measurements of thermodynamic properties (e.g., temperature). This manuscript summarizes the background needed to employ FRS when the flow-field molecules/atoms are the scattering media. Three groups of FRS diagnostic techniques are reviewed. First, to demonstrate the ability to suppress unwanted scattering from walls and flow-field particles, imaging of i) the temporal evolution of the flow field produced by laser-induced optical breakdown in quiescent air and of ii) an underexpanded Mach-2 jet is demonstrated. Next, the efficacy of FRS for planar thermometry is shown with measurements in i) a premixed, laminar stagnation flame and of ii) the thermal field above a heated cylinder. Finally, the potential for point/planar measurements of density, temperature, pressure, and velocity in a supersonic jet is discussed and then demonstrated.