Particles in TurbulencePreliminary results from

Lagrangian Acoustic Velocimetry

M. Bourgoin,

P. N. Qureshi, A. Cartellier, Y. Gagne, C. Baudet,

Inertial particles in turbulence

effect of particles finite size ?

effect of particle to fluid density ratio ?

effect of particles concentration (collective effects) ?

Preferential concentration - ClusteringEnhancement of settling velocityDispersion…

Lagrangian measurements : to characterize particles dynamics at large and small scales

Outline

Acoustic velocimetry technique

- principle of Acoustic Doppler velocimetry

- data acquisition and processing

« Inertial » particles dynamics (preliminary)

- wind tunnel measurements

- finite size effects on velocity increments statitics

Acoustic velocimetry principle

Receiver

UltrasonicEmitter

Doppler shift :

Scattering vector :

,

Acoustic velocimetry principle

Receiver

UltrasonicEmitter

Doppler shift :

Scattering vector :

~ 160o

L ~ 50 cm

~ Ø 10 cm, ~ 100 kHz

3D Acoustic velocimetry

4 independent projections

Well adapted for measurements in open flows with a (large) mean velocity

Possibility for simultaneous Eulerian measurements (hot wire)

Better SNR

Particles : Gas filled soap bubbles

Using Hellium as inner gas,we can compensate the weight of soap

Neutrally buoyant particles

Inner gas

Soap

Air

Air flow

D ~ 2 - 6 mm (Disp. < 6 %)

Bubbles density, size and production rate adjustable

Air flow

Stokes number effects : Lagrangian tracers inertial particles

Adjustable parameters :- soap, gas and air flow rates- inner gas type

Emitter

Receiver

Complex downmixed signal

Data Acquisition - Processing

(90 kHz)

Time-frequency analysis

Time [ms]

[a.u.]

Inertial particles

effect of particles finite size ?

effect of particle to fluid density ratio ?

effect of particles concentration (collective effects) ?

- Wind tunnel grid turbulence

Inertial particles

effect of particles finite size ?

effect of particle to fluid density ratio ?

effect of particles concentration (collective effects) ?

- Wind tunnel grid turbulence

- isolated neutrally buoyant particles

Lagrangian velocity Increments statistics

2 mm bubbles

PDF PDF

6 mm bubbles

Lagrangian velocity Increments statistics

2 mm bubbles 6 mm bubblesvon Karman flow at La Porta et al., Nature, 409, p.1017

Lagrangian tracers in a

PDF PDF

Lagrangian velocity Increments statistics

2 mm bubbles 6 mm bubbles

6 mm bubbles2 mm bubbles

6 mm bubbles2 mm bubbles

von Karman flow at La Porta et al., Nature, 409, p.1017

Lagrangian tracers in a

3

6

8

PDF PDF

Acceleration [a.u.]

PDF

6 mm

2 mm

Non-normalized acceleration PDFs

Acoustic Lagrangian Velocimetry technique (3D)

- density and size easily adjustable

Conclusions

Tracking of soap bubbles inflated with gas

- Well suited for individual particle tracking in open flows- Possibility of silmultaneous Eulerian measurements

Size effects on large neutrally buoyant isolated particles (preliminary)

- heavy particles dynamics - Clustering-Collective effects (many particles)

- Intermittency - weaker than for fluid tracers

- Smaller bubbles have larger acceleration variance

- Surprisingly, we find a larger acceleration flatness for the larger bubbles

Perspectives- repeat the measurements for other sizes of bubbles

Directional

Large spectral band width

Home made

Sell-type transducers (electro-acoustical circular piston)

Ultrasonic transducers

200 V

Mylar sheet (15m)Zync plate Ø 1 cm 30 cm

Reciprocal

Linear

(20kHz 150 kHz)

Emitter

Receiver

Complex downmixed signal

(90 kHz)

Time-frequency analysis

Time [a.u.]

[a.u.]

Data Acquisition - Processing

Emitter

Receiver

Complex downmixed signal

(90 kHz)

2 particles

higher bubbles seeding density

Data Acquisition - Processing