  We study the hydrodynamical stability of the laminar flows associated with
warped astrophysical discs using numerical simulations of warped shearing
boxes. We recover linear growth rates reported previously due to a parametric
resonance of inertial waves, and show that the nonlinear saturated state can
significantly reduce the laminar flows, meaning that the warp would evolve on
much longer time scales than would be concluded from the internal torques due
to these laminar flows. Towards larger warp amplitudes, we find first of all a
reversal of angular momentum flux, indicating that the mass distribution would
evolve in an anti-diffusive manner, and second that the linear growth rates
disappear, possibly because of the very strong shear in the laminar flows in
this regime. For discs with small enough viscosity, a nonlinear state can still
be found when linear growth rates are absent by introducing a large enough
perturbation, either by starting from a nonlinear state obtained at smaller
warp amplitude, or by starting from a state with no laminar flows.
