250 kHz three-component Doppler velocimetry at 32 simultaneous points: a new capability for high speed flows

Abstract

Time-resolved Doppler global velocimetry (TRDGV) is used to demonstrate three-component velocity measurements of 32 planar points, acquired simultaneously at 250 kHz. Photomultiplier tube arrays are used to detect the flow signal, with each array having eight rows of four pixels. The system utilizes frequency scanning of a continuous wave laser, allowing for simple calibration of the signals used to determine the incident and Doppler-shifted frequency. The TRDGV system is used to measure an unheated, Mach 0.91 free jet downstream of the potential core collapse. Axial mean velocities were measured within a root-mean-square error of 7 m s−1 compared to Pitot probe validation measurements. Additionally, axial turbulence intensities showed agreement to hot wire validation measurements and exhibited typical magnitudes reported in the literature. Velocity spectra were obtained for the three components of velocity, revealing a − 5/3 decay in the axial velocity spectra in the inertial subrange frequencies, and generally less decay present in the transverse component spectra. Hot wire measurements and spectra from literature were used for validation of the TRDGV spectral results, showing broadband spectral agreement. The instrument described in this work shows promising capability for multi-point, time-resolved velocity vector measurement in high speed compressible flows with acceptable levels of uncertainty.