Schlieren and BOS velocimetry of a round turbulent helium jet in air

Gary S. Settles*, Alex Liberzon

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

6 Scopus citations


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 = 5890 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.

Original languageEnglish
Article number107104
JournalOptics and Lasers in Engineering
StatePublished - Sep 2022


  • Background-Oriented Schlieren (BOS)
  • Digital image correlation
  • Kymography
  • Round turbulent jets
  • Schlieren velocimetry


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