Two-point radiation statistics from large-scale turbulent structures within supersonic jets

The noise from large-scale coherent turbulent structures within jets remains the dominant source. For the purpose of developing future control systems for the large-scale noise source, we investigate the statistics between upstream and downstream radiating waves. We investigate two off-design supersonic jet flows with instability theory and associated noise radiation, large-eddy simulation (LES), and experiments. We compare the auto-correlation, cross-correlation, coherence, and other statistics predicted by aeroacoustic instability theory. As instability waves are closely connected with the formation of large-scale turbulent structures, they yield insight into large-scale noise statistics. We investigate two nozzles at two supersonic off-design conditions. The first is a biconic nozzle operating at an unheated condition, and the second is a NASA nozzle operating at a heated condition. We find that for these jets, the noise from instability waves is coherent between 0.40 to 0.70 at large-scale radiation frequencies between the downstream and upstream radiation directions.

Evaporating Interface Oscillation of Subcooled Flow Boiling in Locally Heated Microchannels

An investigation was conducted to understand flow boiling of de-ionized water in locally heated parallel microchannels. High speed visualization technology was employed to visually observe the transient phase change process in an individual microchannel. The process at locally heated condition was different from those at entirely heated condition where vapor column(s) stayed quasi-stable for a long time without venting out. Intensive interface oscillation occurred during phase change. Thin film evaporation appeared to be the main evaporation mode at the upstream vapor column cap, whereas nucleation happened occasionally at favorable flow, heat transfer and local wall conditions. Continuous condensation at the downstream vapor column cap led the flow back to single liquid phase regime out of the microchannel. A signal analysis method was introduced to study the interfacial oscillation behavior at the evaporating cap, which showed marked impact of the evaporation mode on the characteristic frequency.