1. Turbulence effects on particle dispersion in a free-surface flow
Salvatore Lovecchio, Cristian Marchioli, Alfredo Soldati
Università degli Studi di Udine
Dipartimento di Ingegneria Elettrica Gestionale e Meccanica

2. Motivation Oceanographical applications
Dispersion of drifters or floaters
Motion of phytoplankton
Settling of organic and inorganic matter

3. Outline of the Presentation
Physical Problem and Modelling Approach
Part 1: Flow at surface
1.1 Energy spectra
1.2 Structures at surface
Part 2: Particles segregation
2.1 Surface divergence
2.2 Intermittency in clustering
2.3 Time scales
Conclusions and future developments

4. Physical Problem and Modelling Approach
Free-slip wall
No-slip wall
3D turbulent water flow field at shear Reynolds number: Re= 171, 509
Channel size: Lx x Ly x Lz = 4 h x 2 h x 2h
Pseudo-spectral DNS: Fourier modes (1D FFT) in the homogeneous directions (x and y)‏, Chebyschev coefficients in the wall-normal direction (z)‏
Time intergration: Adams-Bashforth (convective terms), Crank-Nicolson (viscous terms)

5. Physical Problem and Modelling Approach
Free-slip wall
No-slip wall
3D turbulent water flow field at shear Reynolds number: Re=171, 510
Channel size: Lx x Ly x Lz = 4 h x 2 h x 2h
Pseudo-spectral DNS: Fourier modes (1D FFT) in the homogeneous directions (x and y)‏, Chebyschev coefficients in the wall-normal direction (z)‏
Time intergration: Adams-Bashforth (convective terms), Crank-Nicolson (viscous terms)

6. Physical Problem and
Modelling Approach
Physical Problem and
Modelling Approach

7. 1. Flow at surface Komori et al. : [...] Large fraction of the
Near-wall burtsting events result in surface
renewal events [...]
JFM (1989)

8. 1. Flow at surface
Komori et al. : [...] Large fraction of the
Near-wall burtsting events result in surface
renewal events [...]
JFM (1989)
Streamwise energy spectrum of the streamwise velocity

9. 1. Flow at surface Streamwise energy spectrum of the spanwise velocity
Komori et al. : [...] Large fraction of the
Near-wall burtsting events result in surface
renewal events [...]
JFM (1989)
Streamwise energy spectrum of the spanwise velocity

10. 2. Particles segregation

11. 2. Particles segregation+ y + y + + x x

12. Intermittency is due to near-bottom turbulence
3. Surface cluster renewal
Clustering is intermittent
Upwellings + y
Intermittency is due to
near-bottom turbulence + x

13. Intermittency is due to near-bottom turbulence
3. Surface cluster renewal
Clustering is intermittent
Upwellings + y
Intermittency is due to
near-bottom turbulence x

14. 3. Cluster renewal Lagrangian time scale: Flow time scale
Structures at
Surface
persiste for
long time!!

15. 3. Correlation Dimension
Fractal dimension of cluster

16. 3. Correlation Dimension Fractal dimension of cluster
Particles distributed
uniformly over surface:

17. 3. Correlation Dimension Fractal dimension of cluster
Particles distributed
uniformly over surface:
uniformly into a line:

18. 3. Correlation Dimension Fractal dimension of cluster
Particles distributed
uniformly over surface:
uniformly into a line:
Generally:

19. 3. Correlation Dimension
Fractal dimension of cluster

20. 3. Forcing at surface: the effect of wind
A:WIND opposite the direction of flow A
Mean streamwise
velocity

21. 3. Forcing at surface: the effect of wind
B:WIND in the same direction of flow B
Mean streamwise
velocity

22. 4. Conclusion and Future Developments
It was perfomed DNS of open channel flow at Re =171 e Re=509
Flow at surface was characterised by energy spectra and lagrangian time scale (LTS)
Particles tend to cluster into fractal structures with correlation dimension less than one
The segregation at surface is intermittent (cluster surface renewal) due to bursts of bottom layer turbulence and cluster have life-time greater than LTS

23. Thank you for your
kind attention