  Implementation of an outlet boundary condition is challenging in the context
of the weakly-compressible Smoothed Particle Hydrodynamics method. We perform a
systematic numerical study of several of the available techniques for the
outlet boundary condition. We propose a new hybrid approach that combines a
characteristics-based method with a simpler frozen-particle (do-nothing)
technique to accurately satisfy the outlet boundary condition in the context of
wind-tunnel-like simulations. In addition, we suggest some improvements to the
do-nothing approach. We introduce a new suite of test problems that make it
possible to compare these techniques carefully. We then simulate the flow past
a backward-facing step and circular cylinder. The proposed method allows us to
obtain accurate results with an order of magnitude less particles than those
presented in recent research. We provide a completely open source
implementation and a reproducible manuscript.
