BREAK-UP OF AGGREGATES IN TURBULENT CHANNEL FLOW 1 Università degli Studi di Udine Centro...

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BREAK-UP OF AGGREGATES

IN TURBULENT CHANNEL FLOW

1Università degli Studi di UdineCentro Interdipartimentale di Fluidodinamica e Idraulica

2Università di Roma “Tor Vergata”Dipartimento di Fisica

3Eindhoven University of TechnologyDept. Applied Physics

Eros Pecile1, Cristian Marchioli1, Luca Biferale2,

Federico Toschi3, Alfredo Soldati1

Session TS036-1 on “Multi-phase Flows”

ECCOMAS 2012

September 10-14, 2012, University of Vienna, Austria

Premise

Aggregate Break-up in Turbulence

What kind of application?

Processing of industrial colloids• Polymer, paint, and paper industry

Premise

Environmental systems• Marine snow as part of the oceanic carbon sink

Premise

Environmental systems• Marine snow as part of the oceanic carbon sink

Aerosols and dust particles• Flame synthesis of powders, soot, and nano-particles• Dust dispersion in explosions and equipment breakdown

Premise

What kind of aggregate?

Aggregates consisting ofcolloidal primary particles

Schematic of an aggregate

What kind of aggregate?

Aggregates consisting ofcolloidal primary particles

Break-up due toHydrodynamics stress

Schematic of break-up

Premise

Problem Definition

Description of the Break-up Process

Focus of this work!

SIMPLIFIEDSMOLUCHOWSKIEQUATION (NOAGGREGATIONTERM IN IT!)

• Turbulent flow laden with few aggregates (one-way coupling)

• Aggregate size < O(h) with h the Kolmogorov length scale

• Aggregates break due to hydrodynamic stress, s

• Tracer-like aggregates:

s ~ ( / )m e n 1/2

• scr = scr( )x

• Instantaneous binary break-up once > s scr( )x

Problem Definition

Further Assumptions

Problem Definition

Strategy for Numerical Experiments

• Consider a fully-developed statistically-steady flow

• Seed the flow randomly with aggregates of mass x at a given location

• Neglect aggregates released at locations where > s scr( )x

• Follow the trajectory of remaining aggregates until break-up occurs

• Compute the exit time, =t tscr (time from release to break-up)

Problem Definition

For jth aggregatebreaking afterNj

time steps:

x0=x(0)

x t =x(tcr)

dtn n+1

tj=tcr,j=Nj·dt

Problem Definition

• The break-up rate is the inverse of the ensemble-averaged exit time:

For jth aggregatebreaking afterNj

time steps:

x0=x(0)

x t =x(tcr)

dtn n+1

tj=tcr,j=Nj·dt

Characterization of thelocal energy dissipationin bounded flow:

Wall-normal behavior of mean energy dissipation

Flow Instances and Numerical Methodology

Channel Flow

• Pseudospectral DNS of 3D time- dependent turbulent gas flow

• Shear Reynolds number:

Ret = uth/n = 150

• Tracer-like aggregates:

Wall Center

• Wall-normal behavior of mean energy dissipation

Whole Channel

Channel Flow

Choice of Critical Energy Dissipation

• PDF of local energy dissipation

PDFs are strongly affected by flow anisotropy (skewed shape)

Whole Channel Bulk

Channel Flow

Bulk ecr

Whole Channel Bulk Intermediate

Channel Flow

Bulk ecr

Intermediate ecr

Whole Channel Bulk Intermediate Wall

Channel Flow

Wall ecr

Bulk ecr

Intermediate ecr

Different values of the critical energy dissipation level requiredto break-up the aggregate lead to different break-up dynamics

• PDF of the location of break-up when ecr = Bulk ecr

errorbar = RMS

Channel Flow

For small values of ecr break-up events occur preferentially in the bulk

Bulk ecr

Wall Center Wall

errorbar = RMS

Channel Flow

Wall ecrWall Center Wall

Different values of the critical energy dissipation level requiredto break-up the aggregate lead to different break-up dynamics

• PDF of the location of break-up when ecr = Wall ecr

For large values of ecr break-up events occur preferentially near the wall

Evaluation of the Break-up Rate

Results for Different Critical Dissipation

Measured Expon. Fitcr

crff cr

/)(ln)(

])(exp[)(

Exp. Fit

Exponential fit works reasonably for small values of the critical energy dissipation…

Measuredf(ecr) fromDNS

-c=-0.52

crcrf )( Exp. Fit

crff cr

/)(ln)(

])(exp[)(

Exponential fit works reasonably for small values of the critical energy dissipation… and a power-law scaling is observed!

-c=-0.52

crcrf )( Exp. Fit

crff cr

/)(ln)(

])(exp[)(

Exponential fit works reasonably for small values of the critical energy dissipation… and away from the near-wall region!

How far do aggregates reach before break-up?

Analysis of “Break-up Length”

Consider aggregates released in regions of the flow where > s scr( ) x with scr( )x ~ (m ewall/ )n 1/2

Wall distance of aggregate’s release location: 0<z+<10

Channel lengths covered in streamwise direction

Conclusions and …

… Future Developments

• A simple method for measuring the break-up of small (tracer-like) aggregates driven by local hydrodynamic stress has been applied to non-homogeneous anisotropic dilute turbulent flow.

• The aggregates break-up rate shows power law behavior for small stress (small energy dissipation events). The scaling exponent is c ~ 0.5, a value lower than in homogeneous isotropic turbulence (where 0.8 < < 0.9c ).

• For small stress, the break-up rate can be estimated assuming an exponential decay of the number of aggregates in time.

• For large stress the break-up rate does not exhibit clear scaling.

• Extend the current study to higher Reynolds number flows and heavy (inertial) aggregates.

Cfr. Babler et al. (2012)

Thank you for your kind attention!

errorbar = RMS

Whole Channel Intermediate Bulk Wall

Channel Flow

Wall ecr

Bulk ecr

Intermediate ecr

Estimate of Fragmentation Rate

Two possible (and simple…) approaches

Exponential fit works reasonably away from the near-wall region and for small values of the critical energy dissipation

-0.52 (slope)

Problem Definition

• In bounded flows, the break-up rate is a function of the wall distance.

Problem Definition

BREAK-UP OF AGGREGATES IN TURBULENT CHANNEL FLOW 1 Università degli Studi di Udine Centro...

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