reynolds number calculation

i am a total newbie to lattice boltzmann
i am using d3q19 model to solve 3D pipe flow
i use bounce back condition at the boundaries
wrap around in flow direction
and forcing to impose direction of flow(add .001 to feq for the concerned direction)
in calculation of nu the eqn nu=(tao-.5)/3 is used and comes out to be .16666 for tao =1
i know the diameter of the pipe in lattice units depending on my initialisation of boundary
vavg is calculated from the result
i assumed speed of sound © to be 1
with this when i calculate re=vavg d/nu
i generally get a 2 digit reynolds number
even if i take a large mesh of 50
i get a parabola which fits the analytical result of V/Vmax=(1-(r/R)^2) on making everything dimensionless
but i am starting to get deviations from my plot at a small re of around 35 which is for a mesh of 51
any and all help will be greatly appreciated
thanks in advance
pls help guys am really desperate