Measurements of Wall Formation Forces in an Acoustic Resonator

Acoustic or electromagnetic shaping in resonators can form thin walled structures from pulverized materials. The technology is applicable to a wide variety of materials, particle shapes, and sizes. While radiation force models adequately capture the transport of particles towards the nodal surfaces where walls form, the actual wall formation process involves complex particle-field and interparticle forces. A finite element computation is used along with pulsed laser particle image velocimetry for air movement, and particle tracking velocimetry for particle movement, to close the gap between predictions and measurements. Non-intrusive force measurement is attempted by deriving the acceleration field of particles, from velocity field data. Results are compared with particle acceleration computed from velocity calculations using the finite element code. The predicted acoustic velocity field from the standing wave pattern in the resonator, is compared with measurements. The difference between the computed particle velocity and the acoustic velocity is used to assess the relative roles of fluid dynamic drag and radiation forces on the acceleration of the particles. The measured particle acceleration does differ substantially and consistently from the computed values, showing the effect of the unmodeled near-field particle-field interaction forces.