Investigating the Influence of Swirl on an Axisymmetric Shear Flow using Proper Orthogonal Decomposition*

Understanding the effects of the addition of swirl to the turbulence structure of axisymmetric shear flows such as wakes and jets is important from both practical and fundamental points of view. Studies have shown that the addition of swirl creates a secondary direction of shear that interacts with the primary shear, leading to higher turbulence levels and increased decay rates. In this study, spectral-spatial Proper Orthogonal Decomposition (POD) is used to identify the large-scale structures[JWN1] in an axisymmetric jet flow with and without swirl. The dataset used in this study consists of Stereoscopic Particle Image Velocimetry (SPIV) measurements of three components of velocity at various cross-sections downstream of the jet exit. The axisymmetric nature of the flow was exploited to simplify the interpretation of the POD results and cross-case comparisons. The results show that the addition of swirl has a significant impact on both the shape of the modes and the energy distribution across the modes. Specifically, the introduction of a tangential component of velocity seems to slightly bend the modes in the direction of swirl, while also shifting the energy towards lower azimuthal modes.