An experimental study of turbulent vortex rings using particle image velocimetry
In this dissertation, the early development of turbulent vortex rings at two Reynolds numbers is studied using two-dimensional and Stereoscopic Particle Image Velocimetry. In the late 1980s’ a similarity theory of turbulent vortex rings was proposed and tested primarily using a twochannel tracking Laser Doppler Velocimeter. However, due to the limitations of the experimental technique the tests were inconclusive and important assumptions could not be checked. Since single-point measurements were used, turbulent vortex ring structures could only be inferred using a complex signal-analysis technique. In the present study, two-dimensional and stereoscopic Particle Image Velocimetry techniques provide spatial and temporal resolved measurements of the full field of the cross-section of turbulent vortex rings, from which a more rigorous investigation of the similarity theory is possible. Since the region over which the similarity theory appears to hold starts at about 2.5 orifice diameters downstream, this study focusses on the early development region from this point to ten diameters downstream. Finally, the ensembleaveraged turbulent ring velocity contours, vorticity contours, pressure field contours, as well as Reynolds stresses and turbulence production contours, are presented. The effects of the turbulent vortex ring position dispersion and tilting angle variation on the measurement results are also studied and quantified. An effort is also made to reconstruct a three-dimensional turbulent vortex ring velocity field by adopting Taylor’s hypothesis. Some important features are successfully captured. An azimuthal-averaging method is also developed in an attempt to estimate the turbulence quantities in cylindrical coordinates. However, because of various limitations, the three-dimensional reconstruction method is not perfect, and room for future improvement is discussed.
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