Analysis of jet entrainment mechanism in the transitional regime by time-resolved PIV

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?''