The aerodynamic generation of sound during phonation was studied using direct numerical simulations of the airflow and the sound field in a rigid pipe with a modulated orifice. Forced oscillations with an imposed wall motion were considered, neglecting fluid–structure interactions. The compressible, two-dimensional, axisymmetric form of the Navier–Stokes equations were numerically integrated using highly accurate finite difference methods. A moving grid was used to model the effects of the moving walls. The geometry and flow conditions were selected to approximate the flow within an idealized human glottis and vocal tract during phonation. Direct simulations of the flow and farfield sound were performed for several wall motion programs, and flow conditions. An acoustic analogy based on the Ffowcs Williams–Hawkings equation was then used to decompose the acoustic source into its monopole, dipole, and quadrupole contributions for analysis. The predictions of the farfield acoustic pressure using the acoustic analogy were in excellent agreement with results from the direct numerical simulations. It was found that the dominant sound production mechanism was a dipole induced by the net force exerted by the surfaces of the glottis walls on the fluid along the direction of sound wave propagation. A monopole mechanism, specifically sound from the volume of fluid displaced by the wall motion, was found to be comparatively weak at the frequency considered (125 Hz). The orifice geometry was found to have only a weak influence on the amplitude of the radiated sound.

The results are described of the second part of an ongoing study aimed at performing direct numerical simulations of translaryngeal flows during phonation. The use of accurate numerical schemes allows the radiated sound to be calculated directly, without the need for acoustic analogy models. The goal is to develop a better understanding of this class of flow, and of the basic sound generation mechanisms involved in phonation. In the present study, the effects of subglottal pressure and of glottal oscillation frequency on the near-field flow and farfield sound were investigated. The effects of the presence of the ventricular folds downstream of the oscillating glottal region were also examined. The results highlighted the effects of subglottal pressure and oscillation frequency on the jet vortical structure, wall pressure and shear stress, and sound radiation. Jet impingement on the ventricular folds introduced additional dipole sources similar to those observed in problems involving grazing flows over cavities.

Traditional magnetic field-induced particle separations take place in water-based diamagnetic solutions, where magnetic particles are captured while diamagnetic particles flow through without being affected by the magnetic field. We demonstrate that replacing the diamagnetic aqueous medium with a dilute ferrofluid can significantly increase the throughput of magnetic and diamagnetic particle separation. This enhancement is attributed to the simultaneous positive and negative magnetophoresis of magnetic and diamagnetic particles, respectively, in a ferrofluid. The particle transport behaviors in both ferrofluid- and water-based separations are predicted using an analytical model.

Concentrating particles and cells for measurement or removal is often essential in many chemical and biological applications. Diamagnetic particle concentration has been demonstrated in magnetic fluids using two repulsive or attracting magnets, which in almost all cases are symmetrically positioned on the two sides of the particle-flowing channel. This work studies the effects of magnet asymmetry on the pattern and flow rate of diamagnetic particle concentration in ferrofluid flow through a straight rectangular microchannel. Two attracting permanent magnets with a fixed distance are each embedded on one side of the microchannel with a symmetric or an asymmetric configuration. A pair of symmetric counter-rotating circulations of concentrated particles is formed in the microchannel with a symmetric magnet configuration, which is found to grow in size and progress up the flow. In contrast, the single asymmetric circulation of concentrated particles formed in the microchannel with an asymmetric magnet configuration nearly maintains its size and position. Moreover, the magnet asymmetry is found to increase the ferrofluid flow rate for particle trapping, which is predicted by a three-dimensional theoretical model with a reasonable agreement.

Electrokinetic instability (EKI) is a flow instability that occurs in electric field-mediated microfluidic applications. It can be harnessed to enhance sample mixing or particle trapping but has to be avoided in particle separation. Current studies on EKI have been focused primarily on the flow of Newtonian fluids. However, many of the chemical and biological solutions exhibit non-Newtonian characteristics. This work presents the first experimental study of the EKI in viscoelastic fluid flows with conductivity gradients through a T-shaped microchannel. We find that the addition of polyethylene oxide (PEO) polymer into Newtonian buffer solutions alters the threshold electric field for the onset of EKI. Moreover, the speed and temporal frequency of the instability waves are significantly different from those in the pure buffer solutions. We develop a three-dimensional preliminary numerical model in COMSOL, which considers the increased viscosity and conductivity as well as the suppressed electroosmotic flow of the buffer-based PEO solutions. The numerically predicted threshold electric field and wave parameters compare favorably with the experimental data except at the highest PEO concentration. We attribute this deviation to the neglect of fluid elasticity effect in the current model that increases with the PEO concentration.