Survival of a Diffusing Particle in a Transverse Flow Field
  We consider a particle diffusing in the y-direction, dy/dt=\eta(t) where \eta(t) is Gaussian white noise, and subject to a transverse flow field in the x-direction, dx/dt=f(y), where x \ge 0 and x=0 is an absorbing boundary. We discuss the time-dependence of the survival probability of the particle for a class of functions f(y) that are positive in some regions of space and negative in others. 
Alan J. Bray, Panos Gonos