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Particles in Turbulence Preliminary results from Lagrangian Acoustic Velocimetry

This study presents preliminary results on the effects of particle size, density ratio, and concentration in turbulence. The research focuses on preferential concentration, clustering, and dispersion of inertial particles, with a detailed outline of the acoustic velocimetry technique used for measurements. The study examines the impact of particle properties on velocity statistics and dynamics at various scales, emphasizing the potential for simultaneous Eulerian measurements. Gas-filled soap bubbles are used as neutrally buoyant particles with adjustable parameters, enabling the exploration of Stokes number effects and inertial particle behavior. The research concludes with insights on particle dynamics, intermittency, and future perspectives for further investigations.

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Particles in Turbulence Preliminary results from Lagrangian Acoustic Velocimetry

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  1. Particles in Turbulence Preliminary results from Lagrangian Acoustic Velocimetry M. Bourgoin, P. N. Qureshi, A. Cartellier, Y. Gagne, C. Baudet,

  2. 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

  3. 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

  4. Acoustic velocimetry principle Ultrasonic Emitter , Receiver Scattering vector : Doppler shift :

  5. Acoustic velocimetry principle L ~ 50 cm Ultrasonic Emitter ~ Ø 10 cm , ~ 100 kHz ~ 160o Receiver Scattering vector : Doppler shift :

  6. 3D Acoustic velocimetry • 4 independent projections • Better SNR • Well adapted for measurements • in open flows with a (large) mean • velocity • Possibility for simultaneous • Eulerian measurements (hot wire)

  7. Particles : Gas filled soap bubbles Inner gas Using Hellium as inner gas, we can compensate the weight of soap Air Neutrally buoyant particles Soap Air flow D ~ 2 - 6 mm (Disp. < 6 %) Air flow • Adjustable parameters : • soap, gas and air flow rates • inner gas type Bubbles density, size and production rate adjustable Stokes number effects : Lagrangian tracers  inertial particles

  8. Data Acquisition - Processing Receiver Complex downmixed signal (90 kHz) Emitter Time-frequency analysis [a.u.] Time [ms]

  9. Inertial particles • effect of particles finite size ? • effect of particle to fluid density ratio ? • effect of particles concentration (collective effects) ? - Wind tunnel grid turbulence

  10. 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

  11. Lagrangian velocity Increments statistics 2 mm bubbles 6 mm bubbles PDF PDF

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

  13. von Karman flow at La Porta et al., Nature, 409, p.1017 Lagrangian velocity Increments statistics Lagrangian tracers in a 2 mm bubbles 6 mm bubbles PDF PDF 8 6 3 2 mm bubbles 2 mm bubbles 6 mm bubbles 6 mm bubbles

  14. 6 mm 2 mm Non-normalized acceleration PDFs PDF Acceleration [a.u.]

  15. Conclusions • Acoustic Lagrangian Velocimetry technique (3D) • Well suited for individual particle tracking in open flows • Possibility of silmultaneous Eulerian measurements • Tracking of soap bubbles inflated with gas - density and size easily adjustable • Size effects on large neutrally buoyant isolated particles (preliminary) - 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 • heavy particles dynamics • Clustering-Collective effects (many particles)

  16. Ultrasonic transducers Sell-type transducers (electro-acoustical circular piston) Mylar sheet (15m) Zync plate Ø 1 cm  30 cm Reciprocal Linear 200 V Large spectral band width (20kHz  150 kHz) Directional Home made

  17. Data Acquisition - Processing Receiver Complex downmixed signal (90 kHz) Emitter Time-frequency analysis [a.u.] Time [a.u.]

  18. Data Acquisition - Processing Receiver Complex downmixed signal higher bubbles seeding density (90 kHz) Emitter 2 particles

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