Vortex dynamics and entrainment mechanisms in low Reynolds orifice jets

Abstract

Classical planar 2D-PIV measurements and time-resolved visualizations enriched by low-level processing are used for the reconstruction of the Kelvin-Helmholtz vortex passing in the near field of a circular and a 6-lobed orifice jet flow. In the circular jet, the entrainment is produced in the braid region, being interrupted in the presence of the Kelvin-Helmholtz ring. The latter compresses the streamwise vortices and alters their self-induction role. Conversely, the 6-lobed orifice geometry allows the cutting of the Kelvin-Helmholtz structures into discontinuous ring segments. Consequently, into these discontinuity regions streamwise large scale structures are developing. These streamwise structures are permanent thus controlling and enhancing the jet entrainment which is not altered by the Kelvin-Helmholtz structures passing.

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

“…In order to obtain the same initial conditions as in the previous study (Nastase and Meslem 2008), air jet facility is conserved (Fig. 1a).…”

“…A comparison between images from classical PIV measurements and from TR visualizations, using a low-level image-processing algorithm, allowed the reconstruction of a pseudo-time resolved PIV fields sequence corresponding to one period of K-H vortex passing. This procedure allowed a quantitative approach of a particular phenomenon, in the circular jet and in the daisy jet having the same low initial Reynolds number (Nastase and Meslem 2008). For the circular jet, we found that the entrainment is produced in the braid region where the streamwise structures develop.…”

“…The recent availability of a high-speed PIV system allowed us in this study to check whether the proposed reconstruction method does not introduce any artefact in the previous analysis. In the same jet flow conditions, comparison of the previous (Nastase and Meslem 2008) and the new results are confronted. Also, TR PIV fields provide more details of K-H vortex periods for longer duration, while observing for the interested periodic phenomenon.…”

“…The previous reconstruction method (Nastase and Meslem 2008) provided a good analysis of the streamwise vortex effect on jet entrainment. However, it is believed that this method limited the answer to our question on the direct role of the K-H vortex dynamics on jet entrainment.…”

“…K-H and streamwise structures) in the entrainment mechanism. The previous investigation of this phenomenon was performed using classical PIV (non timeresolved PIV) and time-resolved (TR) visualization (Nastase and Meslem 2008). A comparison between images from classical PIV measurements and from TR visualizations, using a low-level image-processing algorithm, allowed the reconstruction of a pseudo-time resolved PIV fields sequence corresponding to one period of K-H vortex passing.…”