  Bulk-driven acoustic (Eckart) streaming is the steady flow resulting from the
time-averaged acoustic energy flux density in the bulk of a viscous fluid. In
simple cases, like the one-dimensional single standing-wave resonance, this
energy flux is negligible, and therefore the bulk-driven streaming is often
ignored relative to the boundary-driven (Rayleigh) streaming in the analysis of
resonating acoustofluidic devices with length scales comparable to the acoustic
wavelength. However, in closed acoustic microcavities with viscous dissipation,
two overlapping resonances may be excited at the same frequency as a double
mode. In contrast to single modes, the double modes can support a steady
rotating acoustic energy flux density and thus a corresponding rotating
bulk-driven acoustic streaming. We derive analytical solutions for the double
modes in a rectangular box-shaped cavity including the viscous boundary layers,
and use them to map out possible rotating patterns of bulk-driven acoustic
streaming. Remarkably, the rotating bulk-driven streaming may be excited by a
non-rotating actuation, and we determine the optimal geometry that maximizes
this excitation. In the optimal geometry, we finally simulate a horizontal
2-by-2, 4-by-4, and 6-by-6 streaming-roll pattern in a shallow square cavity.
We find that the high-frequency 6-by-6 streaming-roll pattern is dominated by
the bulk-driven streaming as opposed to the low-frequency 2-by-2 streaming
pattern, which is dominated by the boundary-driven streaming.
