By SPH, I suppose you imply "Smoothed-particle hydrodynamics".
If abbreviations are avoided in questions (at best, given in parentheses), it will help people to give more useful answers. As a general comment, I beseech all those who post questions to stop for a moment and consider if the question, obviously very clear to the poser of the question, will also be clear to the potential reader before hitting the 'send' button.
too much severity in my opinion. Acronyms are easily identifiable by 'experts'. Otherwise you are not an 'expert' in that field. To the point: Spheric community is involved in the issue you raised since a while. Try to google 'inlet outlet conditions sph' or have a reading here: https://www.researchgate.net/publication/227718639_Permeable_and_non-refecting_boundary_conditions_in_SPH
Article Permeable and non-refecting boundary conditions in SPH