Particles in Turbulence Preliminary 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 - Clustering Enhancement of settling velocity Dispersion … 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 Ultrasonic Emitter Doppler shift : Scattering vector :,

Acoustic velocimetry principle Receiver Ultrasonic Emitter Doppler shift : Scattering vector : ~ 160 o 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 ~ 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 6 mm bubbles

Lagrangian velocity Increments statistics 2 mm bubbles 6 mm bubbles von Karman flow at La Porta et al., Nature, 409, p.1017 Lagrangian tracers in a PDF

Lagrangian velocity Increments statistics 2 mm bubbles 6 mm bubbles 2 mm bubbles 6 mm bubbles 2 mm bubbles von Karman flow at La Porta et al., Nature, 409, p.1017 Lagrangian tracers in a 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 (15  m) 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