Coherent Structrues in the Far Field of a Turbulent Jet

Coherent structures (CS) in the far field of a turbulent jet have not received as much attention as the near-field vortex structures. CSs play an important role in transport of mass, and momentum in turbulent flows, making their study important. We herein characterize the CSs in the far field of a fully developed round turbulent jet for Re_D=2640, 5280 & 10700, using conditional averaging. The 3-D velocity field is measured using the Scanning Tomographic Particle Image Velocimetry introduced by Casey et al. [1]. In this technique a volume-slice of laser light is scanned with a galvanometer to obtain a larger total measurement volume, compared to the conventional Tomographic PIV. Visualization of instantaneous CS shows that the region primarily consists of C-shaped azimuthal and axial tube-like structures. Conditional velocity field obtained by imposing conditions on azimuthal vorticity at characteristic reference locations, shows horseshoe structures, which are representative of the C-shaped instantaneous loops. The structures are averaged over 10-15 independent experimental runs for better convergence. In the inner region (r~D), the horseshoes appear with head closer to the axis and legs pointing downwards, where D is the nozzle diameter. In the outer region (r>4D), they had their legs pointing upwards towards the axis with head farther away from the axis. They are inclined at 40-60^o with the mean flow direction, and the distance between their legs lie in the range measured to be 0.2r_0.5-0.4r_0.5, where r_0.5 is the jet half-radius. Conditional velocity field obtained using appropriate conditions on axial vorticity, shows tube-like structures with some slight inclination with the mean flow direction. We also present resolved measurements of the turbulent kinetic-energy dissipation rate e and its radial profile using zoomed-in experiments. We use higher spatial resolution, to estimate e using all velocity gradient terms and get better measure of the Kolmogorov scale.

[1] T. A. Casey, J. Sakakibara, and S. T. Thoroddsen (2013). Scanning tomographic particle image velocimetry applied to a turbulent jet. Phys. Fluids, 25(2):025102.