Simultaneous temperature, pressure, and velocity line imaging in an underexpanded jet with a combined CARS-FLEET instrument

Abstract

We demonstrate a new capability to study scalar transport, combining coherent anti-Stokes Raman scattering (CARS) and femtosecond laser electronic excitation tagging (FLEET) for simultaneous measurements of temperature, pressure, and velocity along a one-dimensional (1D) measurement volume. Multi-parameter measurements were performed along a ~ 2-mm line in an underexpanded sonic jet on a single-laser-shot basis at 20 Hz. The femtosecond–picosecond (fs-ps) CARS instrument with an early and late probe pulse captured collision-free and collision-altered pure-rotational CARS spectra for measurements of temperature and pressure, respectively. The femtosecond pump/Stokes pulse from the CARS measurement was re-used as the FLEET excitation source and aligned with the one-dimensional CARS measurement volume at a time delay of several nanoseconds to eliminate interference with the CARS measurement. Radial profiles of temperature, pressure, and axial velocity were recorded at axial locations of 6.5, 15, and 19 mm, corresponding to measurements in the cold isentropically expanding gases before the Mach disk, the edge of the slip line downstream of the Mach disk, and through the inner shear layer developed from the slip line, respectively. From measurements performed in the nearly steady flow upstream of the Mach disk, estimates of measurement precision for temperature, pressure, and velocity were found to be 2%, 7%, and 3% about the mean, respectively. Flow downstream of the Mach disk was seen to have higher fluctuations, particularly near the inner shear layer. Measured flow properties match previous experimental and theoretical results in underexpanded jets. This study demonstrates the feasibility of the combined CARS-FLEET instrument for scalar transport measurements and provides a baseline for future applications in high-speed wind tunnel facilities.