  Flow-induced shear stresses have been found to be a stimulatory factor in
pre-osteoblastic cells seeded in 3D porous scaffolds and cultured under
continuous flow perfusion. However, due to the complex internal structure of
the scaffolds, whole scaffold calculations of the local shear forces are
computationally-intensive. Instead, representative volume elements (RVEs),
which are obtained by extracting smaller portions of the scaffold, are commonly
used in literature without a numerical accuracy standard. Hence, the goal of
this study is to examine how closely the whole scaffold simulations are
approximated by the two types of boundary conditions used to enable the RVEs:
"wall boundary condition" (WBC) and "periodic boundary condition" (PBC). To
that end, Lattice-Boltzmann Method fluid dynamics simulations were used to
model the surface shear stresses in 3D scaffold reconstructions, obtained from
high resolution microcomputed tomography images. It was found that despite the
RVEs being sufficiently larger than 6 times the scaffold pore size (which is
the only accuracy guideline found in literature), the stresses were still
significantly under-predicted by both types of boundary conditions: between 20
and 80% average error, depending on the scaffold's porosity. Moreover, it was
found that the error grew with higher porosity. This is likely due to the small
pores dominating the flow field, and thereby negating the effects of the
unrealistic boundary conditions, when the scaffold porosity is small. Finally,
it was found that the PBC was always more accurate and computationally
efficient than the WBC. Therefore, it is the recommended type of RVE. Overall,
this work provides a previously-unavailable guidance to researchers regarding
the best choice of boundary conditions for RVE simulations.
