On the cost of null-control of an artificial advection-diffusion problem
  In this paper we study the null-controllability of an artificial advection-diffusion system in dimension $n$. Using a spectral method, we prove that the control cost goes to zero exponentially when the viscosity vanishes and the control time is large enough. On the other hand, we prove that the control cost tends to infinity exponentially when the viscosity vanishes and the control time is small enough. 
Pierre Cornilleau, Sergio Guerrero
math.OC