1. 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 13 th October 2011

2. Main aim Investigation on streamwise vorticity developing in a round jet and its effect on mixing processes

3. 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?

4. Mixing and efficiency at entraining fluid is decisive in many industrial applications Streamwise vorticity takes part to this process Why secondary structures?

5. Water, fully developed turbulent round jet. Streamwise (up to x/D=7) and crosswise acquisitions Reynolds number: 3000-30000 Experimental setup

6. Mean velocity field at x/D=1.5 and Reynolds number 4800 and 19300 Results: average fields

7. Time evolution of secondary structures at x/D=4.5 and Reynolds number 4800 Instantaneous fields

8. Re=4800 For x/D>2.5 peak vanishes Coherency spectra

9. Size decreases Asymptotic behaviour for Re>14500 Spatial correlation functions

10. At low x/D (<1.5) vortex rings effect on entrainment is relevant Entrainment Rate

11. At x/D=4.5 sheer entrainment rate increase Steady state shifts towards higher radii Entrainment Rate

12. 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)

13. Fibre-laden case Phase discrimination and orientation estimate Pre- processing Object identification Discriminate objects Fitting rod to an ellipse

14. Fibre-laden case Experimental results from pipe flow: near wall mean orientation

15. 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

16. Thank you for your attention! Questions