After the initial transients have dispersed and the flow has settled, the mesh may be refined. The criterion used is based on the gradient of the pressure for deciding which elements are to be refined. The user specifies a percentage of elements which are to be refined, and a criterion
is calculated for each element. A value of this criterion is found such that the given percentage of elements have a value of greater than , and those elements are refined. The criterion is not simply the gradient of the pressure, because the strongest shock in the simulation would soak up all the refinement leaving weaker shocks unresolved. With the element area in the criterion, regions will ``saturate'' after sufficient refinement, allowing weaker shocks to be refined.