  Modeling blood flow in larger vessels using lattice-Boltzmann methods comes
with a challenging set of constraints: a complex geometry with walls and
inlet/outlets at arbitrary orientations with respect to the lattice,
intermediate Reynolds number, and unsteady flow. Simple bounce-back is one of
the most commonly used, simplest, and most computationally efficient boundary
conditions, but many others have been proposed. We implement three other
methods applicable to complex geometries (Guo, Zheng and Shi, Phys Fluids
(2002); Bouzdi, Firdaouss and Lallemand, Phys. Fluids (2001); Junk and Yang
Phys. Rev. E (2005)) in our open-source application \HemeLB{}. We use these to
simulate Poiseuille and Womersley flows in a cylindrical pipe with an arbitrary
orientation at physiologically relevant Reynolds (1--300) and Womersley (4--12)
numbers and steady flow in a curved pipe at relevant Dean number (100--200) and
compare the accuracy to analytical solutions. We find that both the
Bouzidi-Firdaouss-Lallemand and Guo-Zheng-Shi methods give second-order
convergence in space while simple bounce-back degrades to first order. The BFL
method appears to perform better than GZS in unsteady flows and is
significantly less computationally expensive. The Junk-Yang method shows poor
stability at larger Reynolds number and so cannot be recommended here. The
choice of collision operator (lattice Bhatnagar-Gross-Krook vs.\ multiple
relaxation time) and velocity set (D3Q15 vs.\ D3Q19 vs.\ D3Q27) does not
significantly affect the accuracy in the problems studied.
