Application of a laser-Doppler scanning technique to the unstable flow field of a heated jet

Results of rapidly scanned velocity profiles in the initial region of a heated circular jet are described. The technique uses a specially developed optomechanical scanning device. The Doppler-shifted frequency or wavelength of scattered light of a single laser beam in the seeded flow field is analyzed by a particular type of laser Doppler velocimeter. This instrument is a Michelson-type interferometer which is zero-controlled by means of a Pockels cell with respect to the wavelength variations to be analyzed. With a scanning rate of about 2000 velocity profiles per second the instability fluctuations in the hot jet are resolved in such a way that the periodicity of the developing coherent structures becomes visible in different kinds of diagrams. Due to the density gradient between the hot jet and the cold surrounding fluids the coherence of the fluctuations is higher than in a comparable cold-jet flow. The locally measured structure frequencies deviate slightly from the Strouhal number 0.4 which is valid for cold jets. An associated subharmonic frequency develops temporarily which seems to arise on account of vortex shedding processes, in which case the spectral effect is detected already upstream relatively near the nozzle exit