particles in turbulence preliminary results from lagrangian acoustic velocimetry m. bourgoin, p. n....
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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 :
~ 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 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
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
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