Three-dimensional flow features of underexpanded jets emerging from an elliptic convergent nozzle

Understanding the fundamental structure of shock-containing elliptic jets is of great academic and engineering interest, but there are still many unknowns. The three-dimensional flow features of an underexpanded jet emerging from an elliptic convergent nozzle with an aspect ratio of 4.0 at the exit face are experimentally investigated by rainbow schlieren tomography (RST). The elliptic jet is discharged into quiescent air using an intermittent blowdown wind tunnel. The Reynolds number based on the equivalent diameter and flow properties at the nozzle exit is \(3.0\times 10^{5}\). Multi-view rainbow schlieren images of the elliptic jet are taken by rotating the nozzle around its longitudinal axis, and the density field is reconstructed using the convolution back-projection (CBP) method. The three-dimensional density field of the elliptic jet is acquired with a nominal spatial resolution of approximately 13 \(\upmu\)m. The flow characteristics of shock-containing elliptic jets, such as the shock-cell length, the supersonic length, the switchover location, and the axis-switching location, are quantitatively revealed from the streamwise density profiles, the density contour plots in the minor-axis and major-axis planes where a method is proposed to quantitatively estimate the switchover and axis-switching locations. The shock-cell and supersonic lengths are quantitatively compared with the recently introduced analytical solution and scaling law, respectively. In addition, the shock structures and topology showing the spatial evolution in the streamwise direction of the near-field shock system within the elliptic jet are experimentally demonstrated for the first time.