turbulence effects on particle dispersion in a free-surface flow salvatore lovecchio, cristian...

Turbulence effects on particle dispersion in

a free-surface flowSalvatore Lovecchio, Cristian Marchioli, Alfredo Soldati

Università degli Studi di Udine

Dipartimento di Ingegneria Elettrica Gestionale e Meccanica

Oceanographical applications

Settling of organic and inorganic matter

Dispersion of drifters or floaters

Motion of phytoplankton

Motivation

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

Physical Problem andModelling Approach

• 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)

Free-slip wall

No-slip wall

Physical Problem andModelling 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)

Physical Problem andModelling Approach

Physical Problem andModelling Approach

1. Flow at surface

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

1. Flow at surface

Streamwise energy spectrum of the streamwise 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

1. Flow at surface

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

2. Particles segregation

2. Particles segregation

x+ x+

y+

y+

∇ 2D ∇ 2D

Clustering is intermittent

3. Surface cluster renewal

Intermittency is due to

near-bottom turbulence

Upwellings

∇ 2D

x +

y+

Clustering is intermittent

3. Surface cluster renewal

Intermittency is due to

near-bottom turbulence

Upwellings

∇ 2D

x

y+

Lagrangian time scale:

3. Cluster renewalFlow time scale

Structures at Surfacepersiste for long time!!

3. Correlation DimensionFractal dimension of cluster

3. Correlation DimensionFractal dimension of cluster

Particles distributeduniformly over surface:

3. Correlation DimensionFractal dimension of cluster

Particles distributeduniformly over surface:uniformly into a line:

3. Correlation DimensionFractal dimension of cluster

Particles distributeduniformly over surface:uniformly into a line:

Generally:

3. Correlation DimensionFractal dimension of cluster

3. Correlation DimensionFractal dimension of cluster

3. Forcing at surface: the effect ofwind

A:WIND opposite the direction of flow

AMean streamwise velocity

3. Forcing at surface: the effect ofwind

B:WIND in the same direction of flow

BMean streamwise velocity

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

Thank you for your

kind attention