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

A stereoscopic particle image velocimetry velocity field measurement technique was used to investigate the flow structure and entrainment rate of a turbulent elliptic jet of aspect ratio AR = 2. A 3D calibration procedure was established to compensate for the distortion and refraction effects embedded in the captured particle images; the results of this procedure were used to calculate three orthogonal velocity components. The instantaneous velocity fields of three orthogonal components in several cross sections perpendicular to the jet axis were measured and the spatial distributions of the turbulent statistics were obtained by ensemble averaging 500 instantaneous velocity fields. The variation of the entrainment rate along the jet axis was evaluated from the mean velocity fields and compared with that of the round jet of the same equivalent diameter. At x/D_e = 2, the jet widths along the major and minor axes are almost identical and axis-switching occurs. As the flow goes further downstream, the jet width grows much faster along the minor axis than along the major axis. After axis-switching, the flow structure of the elliptic jet shows two critical points on the minor axis, which are not observed in the round jet. The total entrainment rate of the elliptic jet is about 1.5% larger than that of the round jet. In addition, entrainment of the surrounding fluid is more active on the minor axis as a result of the fast expansion of the jet along this axis due to the self-induction of the elliptic vortical structure in the near field.