Planar Velocity and Temperature Measurements in Rarefied Hypersonic Flow Using Iodine LIF

Abstract

A planar laser-induced fluorescence (PLIF) technique is discussed and applied to measurement of time-averaged values of velocity and temperature in an I2-seeded N2 hypersonic free jet facility. A low temperature, non-reacting, hypersonic flow over a simplified model of a reaction control system (RCS) was investigated using the PLIF technique. Data are presented of rarefied Mach 12 flow over a sharp leading edge flat plate at zero incidence, both with and without an interacting jet issuing from a nozzle built into the plate. The velocity profile in the boundary layer on the plate was resolved. The slip velocity along the plate, extrapolated from the velocity profile data, varied from nearly 100% down to 10% of the freestream value. These measurements are compared with results of a DSMC solution. The velocity variation along the centerline of a jet issuing from the plate was measured and found to match closely with the correlation of Ashkenas and Sherman. The velocity variation in the oblique shock terminating the jet was resolved sufficiently to measure the shock wave thickness. I. Iodine fluorescence measurement technique Numerous methods have been reported in the literature for measurement of thermodynamic properties in supersonic flows using laser-induced fluorescence. This paper presents results of a technique based on narrowband excitation of fluorescence from nuclear hyperfine components in the B↔X electronic transition of molecular iodine. This approach was developed to study low temperature flows of I2-seeded N2 gas. A tunable single frequency laser beam is expanded and collimated into a thin sheet and passed into a low density wind tunnel flow, causing I2 molecules to fluoresce within a selected planar section of the flow-field. A series of images are taken of the fluorescence with a charge-coupled device (CCD) array as the laser frequency is incrementally tuned over selected lines in the I2 absorption spectrum. The fluorescence signal versus frequency is thereby obtained for any point in the plane of the laser sheet, within the resolution of the digital image. These absorption spectra can provide information on the local thermodynamic conditions in the flow. This paper describes planar laser-induced fluorescence (PLIF) measurements of velocity and temperature made in a low density, low temperature, hypersonic flow over a simplified model of a reaction control system (RCS) jet, 1 intended to support assessment of a hybrid numerical solution method for such flows. 2, 3