The Effect of Frequency Sweeping and Fluid Flow on Particle Trajectories in Ultrasonic Standing Waves

• Published in: IEEE sensors journal 2008-06, Vol.8 (6), p.667-677
• Main Authors: Lipkens, B., Costolo, M., Rietman, E.
• Format: Article
• Language: English
• Subjects: Acoustic radiation; Acoustic radiation force; Acoustic waveguides; Acoustic waves; Acoustics; Atoms & subatomic particles; Buildings; Computational fluid dynamics; Drag; Fluid dynamics; Fluid flow; Fluids; Mathematical models; particle concentration; particle separation; Particle trajectories; Piezoelectric transducers; Predictive models; Resonant frequency; Resonators; Sweeping; Ultrasonic imaging; ultrasound
• ISSN: 1530-437X; 1558-1748
• DOI: 10.1109/JSEN.2008.922675

Particle concentration and separation in ultrasonic standing waves through the action of the acoustic radiation force on suspended particles are discussed. The acoustic radiation force is a function of the density and compressibility of the fluid and the suspended particles. A two-dimensional theoretical model is developed for particle trajectory calculations. An electroacoustic model is used to predict the acoustic field in a resonator, driven by a piezoelectric transducer. Second, the results of the linear acoustic model are used to calculate the acoustic radiation force acting on a particle suspended in the resonator. Third, a particle trajectory model is developed that integrates the equation of motion of a particle subjected to a buoyancy force, a fluid drag force, and the acoustic radiation force. Computational fluid dynamics calculations are performed to calculate the velocity field that is subsequently used to calculate fluid drag. For a fixed frequency excitation, the particles are concentrated along the stable node locations of the acoustic radiation force. Through a periodic sweeping of the excitation frequency particle translation is achieved. Two types of frequency sweeps are considered, a ramp approach and a step-change method. Numerical results of particle trajectory calculations are presented for two configurations of flow-through resonators and for two types of frequency sweeping. It is shown that most effective particle separation occurs when the fluid drag force is orthogonal to the acoustic radiation force.