  The propulsion of colloidal particles via planar traveling ultrasound waves
has attracted increasing attention in recent years. A frequently studied type
of particles is bullet-shaped and cup-shaped nano- and microparticles. Based on
acoustofluidic simulations, this article investigates how the propulsion of
bullet-shaped particles depends on their length and diameter, where cup-shaped
particles are included as limiting cases corresponding to the smallest particle
length. The structure of the flow field generated by the particles is discussed
and it is shown that the particles' propulsion strength increases with their
length and diameter. When varying the diameter, we observed also a sign change
of the propulsion. This work complements previous experimental studies that
have addressed such particles only for particular aspect ratios, and the
provided understanding of how the propulsion of the particles depends on their
dimensions will prospectively be helpful for the choice of particle shapes that
are most suitable for future experimental studies.
