  In this work we investigate the issue of non-physical slip at wall of lattice
Boltzmann simulations with the bounce-back boundary scheme. By comparing the
analytical solution of two lattice models with four and nine discrete
velocities for the force-driven Poiseuille flow, we are able to reveal the
exact mechanism causing the issue. In fact, no boundary condition is defined by
the bounce-back scheme for the the discrete velocities parallel to wall. Other
factors, such as initial conditions and inlet and outlet boundary conditions,
can play the role and induce the non-physical slip velocity. Therefore, the
issue is not related to the single-relaxation-time scheme. Naturally the key
for resolving it is to specify the definition for these velocities. Through a
lid-driven cavity flow, we show that the solution can be as easy as no extra
effort required for simple geometries, although further study is necessary for
complex geometries.
