Velocity Field Measurements in Rarefied, Hypersonic Flows of Nitrogen Using Laser-Induced Fluorescence of Iodine

Velocity fields are measured in the shock layer and boundary layer on a plate with a cylindrical fin immersed in a hypersonic, free jet of nitrogen, using laser-induced fluorescence (LIF) of iodine. A sheet beam from a single-mode argon laser at 514 nm is used to excite hyperfine components of the P(13), R(15) and P(48), P(103) blended rotational-vibrational lines in the B-X electronic transition for iodine seeded in the flow. The Doppler broadening and shift of these lines, and the relative rotational line strengths are determined for excitation spectra recorded in a planar grid.;Using this measurement technique, estimates for iodine of the mass velocity component and kinetic temperature of translation in the direction of laser propagation, rotational temperature, and relative number density are determined at each point. Sectional planes of the flow over the body are investigated at a spatial resolution on the scale of the molecular mean-free-path in the free jet near the plate leading edge. Two directions within each plane are examined, to determine the velocity vector and to investigate translational non-equilibrium. Predictions from two direct simulation Monte Carlo computations of the flow are compared with the measurements.;Large values of slip velocity and temperature jump at the plate surface are observed for iodine. Measurements and DSMC predictions indicate strong translational non-equilibrium effects for the iodine in the shock wave and the thick boundary layer on the plate, and are qualitatively consistent with a bimodal velocity distribution function. As a consequence of the ratio of molecular masses, the translational non-equilibrium of iodine is much greater than for nitrogen.