On the convective velocity of large-scale structures in compressible axisymmetric jets

The role of compressibility on the convective velocity of large-scale structures in axisymmetric jets is studied using a home-built pulse burst laser system and newly developed high-repetition rate experimental diagnostics. In the last decade and a half, a number of researchers have made measurements of the convective velocity of large-scale structures within compressible free shear layers. These measurements, based entirely on flow visualization images, indicate a departure of the convective velocity from its theoretically expected value. This work attempts to explore this in further detail through the acquisition of time-correlated planar velocimetry data in two compressible axisymmetric flow fields.; A pulse burst laser system was designed and constructed with the ability to produce a burst of short duration (10 nsec) pulses over a ∼150 microsecond period with inter-pulse timing as low as 1 microsecond (1 MHz). Pulse energies were increased by a factor of five through the addition of a phase conjugate mirror, which eliminates a gain robbing low-energy pedestal superimposed on the burst of pulses. The laser can produce individual pulse with energies over 100 mJ/pulse and 2nd and 3rd harmonic conversion efficiencies reaching 50% and 40%, respectively. In addition, the frequency of the laser is found to fluctuate less than 12 MHz, making it ideal for spectroscopic applications.; The application of the pulse burst laser for flow measurements was investigated through the development of MHz rate flow visualization and MHz rate planar Doppler velocimetry (PDV). MHz rate flow visualization is achieved by using a high-repetition rate camera to image laser light scattered from particles in the flow. MHz rate PDV is a spectroscopic technique that uses the laser output at 532 nm, two ultra-high framing rate cameras and a molecular iodine vapor filter to measure the Doppler frequency shift of laser light scattered from seed particles contained in the flow field. The technique produces 28 time-correlated realizations of the velocity over a plane with a maximum repetition rate of up to 1 MHz. Velocity measurements obtained using MHz rate PDV are found to have accuracies on the order of 5% and are primarily speckle noise limited.; Both flow visualization and MHz rate PDV were used to make time-correlated measurements within Mach 1.3 and Mach 2.0 flow fields. Space-time correlations were used to track structures within the flow field and determine their convective velocity. Data produced using flow visualization images agrees with previous research and indicates a strong departure of the convective velocity from theory. Data produced using velocity data, however, shows starkly different trends and does not produce the same measurements of convective velocity. This difference in measurement is attributed to a misinterpretation of the use of space-time correlation for tracking structures. It is shown that space-time correlations will follow features exhibiting the highest degree of spatial variation. (Abstract shortened by UMI.)...