Investigation of the near-field structure of an elliptic jet using stereoscopic particle imagevelocimetry

Abstract

A stereoscopic particle image velocimetry velocity field measurement technique was used toinvestigate the flow structure and entrainment rate of a turbulent elliptic jet of aspect ratioAR = 2.A 3D calibration procedure was established to compensate for the distortion and refractioneffects embedded in the captured particle images; the results of this procedure were used tocalculate three orthogonal velocity components. The instantaneous velocity fields of threeorthogonal components in several cross sections perpendicular to the jet axis weremeasured and the spatial distributions of the turbulent statistics were obtainedby ensemble averaging 500 instantaneous velocity fields. The variation of theentrainment rate along the jet axis was evaluated from the mean velocity fieldsand compared with that of the round jet of the same equivalent diameter. Atx/De = 2,the jet widths along the major and minor axes are almost identical and axis-switchingoccurs. As the flow goes further downstream, the jet width grows much faster along theminor axis than along the major axis. After axis-switching, the flow structure of theelliptic jet shows two critical points on the minor axis, which are not observed inthe round jet. The total entrainment rate of the elliptic jet is about 1.5% largerthan that of the round jet. In addition, entrainment of the surrounding fluid ismore active on the minor axis as a result of the fast expansion of the jet alongthis axis due to the self-induction of the elliptic vortical structure in the nearfield.