Eric R. Lachney, Mike F. Smith and Noel T. Clemens
Mach 3 turbulent wakes originating from flat plates with bluff trailing edges were studied using planar laser-induced fluorescence (PLIF) imaging. Laser-induced fluorescence involves adjusting the laser wavelength until it coincides with an electronic resonance of a molecular species present in the flow. The molecules then absorb the laser light, putting them into a higher energy state. Some of the molecules will transition to a lower energy state by emitting light as fluorescence. The amount of fluorescence can be related to the thermodynamic state of the gas, allowing us to make measurements of the pressure, temperature, or density. Nitric oxide (NO) is used as the fluorescent molecule because it provides high signal levels and because it is easily seeded into large flows.
A schematic and photograph of the experimental setup are shown above. The laser light that excites the NO molecules is generated with tunable dye laser, which is pumped with a frequency doubled Nd:YAG laser. The output of the dye laser is then frequency doubled in a nonlinear crystal to about 285 nm and is mixed in another crystal with infrared laser light to form the excitation frequency near 225 nm. The fluorescence is imaged with an image-intensified CCD camera.
A schematic of the supersonic near wake flow field is shown above. A thick splitter plate separates the Mach 3 free streams. The turbulent boundary layers on each side of the plate detach from the bluff trailing edge, and form a recirculation region at the base of the plate. At the edge of the plate, the flow turns toward the centerline through expansion fans and then turns parallel to the centerline further downstream, forming recompression shocks.
Two sets of NO PLIF images were acquired for excitation of two different absorption lines. The ratio of the images resulting from excitation of the two different lines can be used to infer the temperature of the flow. A resulting mean temperature image is shown at left. When the NO is seeded into nitrogen, the fluorescence signal is directly proportional to the static pressure, and this property is used to obtain the mean pressure image shown at right.
Publications:
PLIF imaging of mean temperature and pressure in a supersonic bluff wake, Lachney, E.R. and Clemens, N.T., Experiments in Fluids, Vol. 24, No. 4, pp. 354 - 363, 1998.