Strobe Observations of Turbulent Jet Modes

We experimentally measure spatial modes of a subsonic turbulent jet at dynamic resonance frequencies. Using water mist as a passive tracer and a strobe light for illumination, we isolate and observe the periodic appearance of coherent structures. Examples include symmetric vortex rings near the nozzle, the internal potential core, unstable modes in the transition region, and characteristic signatures of turbulence. The latter include the development of Kelvin-Helmholtz modes at the jet edge, self-similar mushroom dynamics as the jet expands, core flickering into a von Karman vortex street, and helical streak modes. Both density structures and velocity profiles are measured. Relations between these measurements, the dynamics of spectral proper orthogonal decomposition (SPOD) modes, and the potential for aeroacoustic control will be discussed.