  Nonreciprocal signal operation is highly desired for various acoustic
applications, where protection from unwanted backscattering can be realized so
that transmitting and receiving signals are processed in a full-duplex mode.
Here we present the realization of a class of nonreciprocal circulators based
on simply structured acoustic metagratings, which consist only of a few solid
cylinders and a steady fluid flow with low velocity. These innovative
metagratings are intelligently designed via a diffraction analysis of the
linearized potential flow equation and a genetic-algorithm-based optimization
process. Unitary reflection efficiency between desired ports of the circulators
are demonstrated through full-wave numerical simulations, confirming
nonreciprocal and robust circulation of the acoustic signal over a broad range
of flow velocity magnitude and profile. Our design provides a feasible degree
of tunability, including switching from reciprocal to nonreciprocal operation
and reversing the handedness of the circulator, presenting a convenient but
efficient approach for the realization of nonreciprocal acoustic devices from
wavelength-thick metagratings. It may find applications in various scenarios
including underwater communication, energy harvesting, and acoustic sensing.
