  In this paper we implement a simple strategy, based on Jin and Braza's
method, to deal with nonreflecting outlet boundary conditions for
incompressible Navier-Stokes flows using the method of smoothed particle
hydrodynamics (SPH). The outflow boundary conditions are implemented using an
outflow zone downstream of the outlet, where particles are moved using an
outgoing wave equation for the velocity field so that feedback noise from the
outlet boundary is greatly reduced. For unidirectional flow across the outlet,
this condition reduces to Orlanski's wave equation. The performance of the
method is demonstrated through several two-dimensional test problems, including
unsteady, plane Poiseuille flow, flow between two inclined plates, the
Kelvin-Helmholtz instability in a channel, and flow in a constricted conduit,
and in three-dimensions for turbulent flow in a $90^{\circ}$ section of a
curved square pipe. The results show that spurious waves incident from the
outlet are effectively absorbed and that steady-state laminar flows can be
maintained for much longer times compared to periodic boundary conditions. In
addition, time-dependent anisotropies in the flow, like fluid recirculations,
are convected across the outlet in a very stable and accurate manner.
