MHz Rate Planar Doppler Velocimetry in Supersonic Jets

A pulse burst laser and two high-speed CCD cameras were used to perform one-component MHz Rate Planar Doppler Velocimetry (PDV) measurements in an ideally expanded Mach 2.0 jet. The pulse burst laser produced 28 0.532 micron pulses at 250 kHz with ~9 mJ/pulse. Both two-camera and a single-camera experimental set-ups were used to measure a single component of velocity on a streamwise plane passing through the jet centerline and covering ~6 – 12 jet heights downstream of the jet exit. Velocity image sequences consisting of 28 frames showed dynamics of the velocity field over a time span of 108 microseconds (~4.5 non-dimensional time scales). A typical sequence of images is presented, which demonstrates the process of entrainment as low-speed fluid rolls up into the high-speed portion of the jet. Mean and standard deviation statistics of the velocity calculations produced expected trends and showed good agreement between the single- and two-camera experiments. An error analysis revealed speckle as the predominant source of noise, as in a conventional PDV technique. Accuracy is estimated to be 16 m/s for the single-camera system and 24 m/s for the two-camera system.