  Simulation of the cosmic clustering of massive neutrinos is a daunting task,
due both to their large velocity dispersion and to their weak clustering power
becoming swamped by Poisson shot noise. We present a new approach, the
multi-fluid hybrid-neutrino simulation, which partitions the neutrino
population into multiple flows, each of which is characterised by its initial
momentum and treated as a separate fluid. These fluid flows respond initially
linearly to nonlinear perturbations in the cold matter, but slowest flows are
later converted to a particle realisation should their clustering power exceed
some threshold. After outlining the multi-fluid description of neutrinos, we
study the conversion of the individual flows into particles, in order to
quantify transient errors, as well as to determine a set of criteria for
particle conversion. Assembling our results into a total neutrino power
spectrum, we demonstrate that our multi-fluid hybrid-neutrino simulation is
convergent to $<3\%$ if conversion happens at $z=19$ and agrees with more
expensive simulations in the literature for neutrino fractions as high as
$\Omega_\nu h^2 = 0.005$. Moreover, our hybrid-neutrino approach retains
fine-grained information about the neutrinos' momentum distribution. However,
the momentum resolution is currently limited by free-streaming transients
excited by missing information in the neutrino particle initialisation
procedure, which restricts the particle conversion to z $\gtrsim 19$ if
percent-level resolution is desired.
