LES of turbulent jet in cross flow: Part 2 – POD analysis and identification of coherent structures

The paper presents results of a Proper Orthogonal Decomposition (POD) investigation of the LES based numerical simulation of the jet-in-crossflow (JICF) flowfield with Reynolds number based on the cross flow velocity and jet diameter Re=2400 and the velocity ratio of R=3.3. LES results are validated against pointwise time averaged Laser Doppler Anemometry (LDA) measurements in PART1 of this study.

In PART2 of the presented study – a planar (2D) LES based snapshot POD analysis is first conducted on two mutually perpendicular planes located in the jet-to-crossflow entrance zone. The obtained results are directly compared and found to be in close agreement with results of a Particle Image Velocimetry (PIV) based planar (2D) snapshot POD analysis by Meyer et al. (JFM 583, p. 199–227, 2007), indicating that LES is able to predict the same large scale flow dynamics as that captured by PIV.

Some differences are also observed, but appear to be directly connected to the differences in levels of the resolved turbulent kinetic energy (TKE) between LES and PIV datasets. Those differences proved to be linked to the process of filtering out the small-scale fluctuations implicit to the PIV measurement technique.

Comparisons of TKE captured by the first POD modes showed that they are not affected by this implicit filtering.The LES based POD analysis was also conducted in 3D. The 3D POD analysis results based on the first two POD modes show a full ability to directly visualize details of the relationship between the counter-rotating vortex pair (CVP), the hanging vortex and the wake vortices.

POD reconstruction shows that the CVP originates from the hanging vortex formed at the lateral sides of the jet. It also shows that the shedding process involving oscillation of the jet core is responsible for the creation of wake vortices and that the wake vortex originates from the hanging vortex, but grows quickly by “sucking up” the wall boundary layer fluid and vorticity.