Analysis of jet entrainment mechanism in the transitional regime by time-resolved PIV |
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Authors: | A Meslem M El Hassan I Nastase |
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Institution: | 1. LEPTIAB, Université de La Rochelle, La Rochelle, France 2. Building Services Department, Technical University of Civil Engineering in Bucharest, Bucharest, Romania
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Abstract: | Abstract The entrainment mechanism in the near field of daisy-shaped and circular orifice jets have been investigated in the transitional
regime using time-resolved 2D PIV measurements. The objective is to improve the knowledge from one previous investigation
at initial Reynolds number of 800, based on the construction of a pseudo-time resolved PIV fields using the combination of
non time-resolved PIV measurements and time-resolved visualizations (Nastase and Meslem J Vis 11(4):309–318, 2008). As expected
in the previous work, the entrainment in the circular jet is correlated to the periodic Kelvin–Helmholtz (K–H) ring passing
and the entrainment is produced in the braid region where the streamwise structures develop. In the daisy jet, we found that
the entrainment rate is not correlated to the periodic K–H vortex passing. The observed small variation of the entrainment
rate amplitude in the daisy jet could be related to the K–H dynamics. However, at the studied low Reynolds number the contribution
of the K–H vortex on the daisy jet entrainment seems negligible comparing with the streamwise structures role. Furthermore,
the real-time resolved measurements allow an indepth analysis of the role played by the K–H ring in the entrainment of circular
jet. It is shown that the entrainment is not only produced in the braid region but is also present in the upstream part of
the K–H ring. In the downstream part of the ring, the entrainment is dramatically reduced. This new observation opens a question
which still has to be answered with time-resolved 3D PIV measurements. The question is “Whether the depression formed due
to the ring passing or the streamwise structures rolled-up on the ring is responsible for entrainment at the upstream part
of the ring?” |
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