Dynamics of vortex interaction and entrainment characteristics in inclined elliptic jets using sparse track-enhanced volumetric measurements

This work is a further discussion about elliptical jet flows (Zeng et al. in Exp Fluids 64:142, 2023) by exploring how variations in nozzle shape enhance flow control efficiency. The study examines the dynamic evolution of vortex interactions and their positive effects on the entrainment of inclined elliptical jets (with inclination angles of 30° in both the major and minor planes) compared to non-inclined elliptical jets. Experiments were conducted at an aspect ratio (AR) of 2 and a Reynolds number of approximately 3000. Time-resolved tomographic particle image velocimetry (Tomo-PIV) and three-dimensional Lagrangian particle tracking (3D LPT) measurements were employed to capture the flow dynamics in detail. The Eulerian velocity field datasets obtained from Tomo-PIV are enhanced using high-precision tracks from 3D LPT measurements, improving the accuracy of velocity field reconstruction. Statistical analyses indicate that inclined elliptical jets exhibit greater mass entrainment characteristics and higher momentum flux compared to non-inclined elliptical jets. The power spectral densities (PSDs) and time-domain spectral proper orthogonal decomposition (td-SPOD) results reveal that for the major-plane inclined-A nozzle, both the leading and trailing vortex rings share the same dominant frequency of St = 0.28, suggesting that vortex pairing and merging in the 30° inclined-A jet may be a periodic process. After vortex merging at approximately Y/D_e ≈ 3.5, the first axis-switching is completed, indicating that axial switching is suppressed in the major-plane inclined-A elliptical jet compared to the non-inclined case. In contrast, for the minor-plane inclined-B jet, the dominant frequencies of the leading and trailing vortex rings are St = 0.28 and St = 0.61, respectively. In some instances, two consecutive large-scale vortex rings corresponding to SPOD mode (3,4) do not merge downstream; instead, they develop independently and eventually break down separately. At the axis-switching plane (Y/D_e = 2.5), during both the merging and axis-switching processes of vortex rings in the two inclined nozzles, a significant number of streamwise vortex structures are generated. This phenomenon substantially contributes to an increased mean entrainment rate, further enhancing the mixing characteristics of inclined elliptical jets.

Graphical Abstract