Experimental investigations on secondary structures

in a fully developed turbulent jet and extension to fibre-laden case

Alessandro Capone, Alfredo Soldati

Department of Energetics, University of Udine, Italy

Giampaolo Romano

Department of Mechanics and Aeronautics, University of Rome La Sapienza, Italy

Cost Action FP1005 – Nancy 13th October 2011

Main aim

Investigation on streamwise vorticity developing in a round jet and its effect on

mixing processes

• Turbulence develops in 3D giving rise to so-called secondary structures

• Main feature is streamwise vorticity

secondary structures

Courtesy Prof. A.Soldati Courtesy Prof. GP Romano

Why secondary structures?

• Mixing and efficiency at entraining fluid is decisive in many industrial applications

• Streamwise vorticity takes part to this process

Why secondary structures?

• Water, fully developed turbulent round jet.

• Streamwise (up to x/D=7) and crosswise acquisitions

• Reynolds number:

3000-30000

Experimental setup

• Mean velocity field at x/D=1.5 and Reynolds number 4800 and 19300

Results: average fields

• Time evolution of secondary structures at x/D=4.5 and Reynolds number 4800

Instantaneous fields

• Re=4800• For x/D>2.5

peak vanishes

Coherency spectra

• Size decreases• Asymptotic

behaviour for Re>14500

Spatial correlation functions

• At low x/D (<1.5) vortex rings effect on entrainment is relevant

Entrainment Rate

• At x/D=4.5 sheer entrainment rate increase

• Steady state shifts towards higher radii

Entrainment Rate

Fibre-laden case

Synthetic plastic fibres (nylon)

• Rod orientation contains information about velocity gradients tensor

• Parsa et al. Rotation and alignment of rods in two-dimensional chaotic flow Phys. Fluids 23 (2011)

Fibre-laden case• Phase discrimination and orientation estimate

Fibre-laden case• Experimental results from pipe flow: near wall

mean orientation

• Time-resolved PIV allowed statistical and spectral analysis

• Streamwise vorticity take control of entrainment as x/D and Re increase

• The size of secondary vortical structures appears to be strictly connected to entrainment

• Rod rotation calculations bring useful info about vorticity

Conclusion and developments

Thank you for your attention!

Questions