The wall shear stress is a quantity of profound importance for clinical
diagnosis of artery diseases. The lattice Boltzmann is an easily parallelizable
numerical method of solving the flow problems, but it suffers from errors of
the velocity field near the boundaries which leads to errors in the wall shear
stress and normal vectors computed from the velocity. In this work we present a
simple formula to calculate the wall shear stress in the lattice Boltzmann
model and propose to compute wall normals, which are necessary to compute the
wall shear stress, by taking the weighted mean over boundary facets lying in a
vicinity of a wall element. We carry out several tests and observe an increase
of accuracy of computed normal vectors over other methods in two and three
dimensions. Using the scheme we compute the wall shear stress in an inclined
and bent channel fluid flow and show a minor influence of the normal on the
numerical error, implying that that the main error arises due to a corrupted
velocity field near the staircase boundary. Finally, we calculate the wall
shear stress in the human abdominal aorta in steady conditions using our method
and compare the results with a standard finite volume solver and experimental
data available in the literature. Applications of our ideas in a simplified
protocol for data preprocessing in medical applications are discussed.Comment: 9 pages, 11 figure