  Transport of Brownian particles in a two-dimensional asymmetric tube is
investigated by applying a polarized field. From the Brownian dynamics
simulations we find that the longitudinal current can be enhanced remarkably by
applying a weakly transversal drive. Multiple current reversals can be realized
by altering the driving frequency of the polarized field. By coupling the
longitudinal and transversal forces together, one can control the particle
transport flexibly.
