Dynamic renormalization-group approach to diffusive flow in   heterogeneous systems
  Conventional methods for the simulation of diffusive systems are quite slow when applied to strongly inhomogeneous systems. We present a new hierarchical approach based on dynamic renormalization-group ideas and on the Walsh transform (or Haar wavelet) of signal-processing theory. The method is very efficient for simulation of petroleum reservoirs or other strongly inhomogeneous diffusive or pressure-driven flow systems. In a test case, the hierarchical method is found to achieve 1.5% accuracy roughly 25 times faster than conventional finite difference methods. 
Yuksel Gunal and P B Visscher