  Seedless velocimetry is gaining interest in many industrial and research
applications. We report on a comparative study of time-resolved optical
velocimetry using traditional, mirror-type knife-edge schlieren optics versus
Background-Oriented Schlieren (BOS) of subsonic round turbulent helium jets in
air at Red = 5,890 and 11,300. Digital images with 1024 pixels streamwise
resolution (0 < x/d < 200) were captured at 6000 frames/s in large ensembles.
Velocimetry was performed on these results by digital image correlation (DIC)
using OpenPIV software, and by streak-schlieren analysis of x-t diagrams
(kymography). Limited PIV data were also collected for verification of the
schlieren velocimetry results. Both BOS and traditional schlieren show partial
success in measuring the mean-flow helium jet self-similarity in terms of the
1/x decay of centerline velocity, Gaussian-shaped radial velocity profiles, and
linear spreading rate of the jet. Visualized turbulent eddies, used as tracers
in schlieren velocimetry, are observed to last longer than is necessary for
this purpose in the present helium jets. Also, the measured convective velocity
appears to be sufficiently robust to sum to the jet mean velocity in some of
the results. Kymography yields better overall results than DIC, which we
attribute to kymography's spatiotemporal "spectrum" of jet velocities, enabling
the discrimination of fast eddies near the jet centerline from slower ones near
the jet periphery. DIC and other analysis methods suffer from a path-averaging
bias which negatively affects the results. The reduction of kymographic data
for velocimetry was done manually and also by a Fourier-transform
image-feature-orientation code, both yielding equivalent results.
