Numerical simulations of directed sandpile models
  Directed sandpile models with different toppling rules are studied by means of numerical simulations in two dimensions, with the purpose of determining the different universality classes. It is concluded that the random-threshold directed model is in the same universality of the Manna directed model where multiple toppling events plays a determinant role. The BTW model with uniform and noisy driving are found to display the same critical behavior. Moreover it is observed that the Zhang model does not satisfy simple finite size scaling. 
Alexei Vazquez