1. A method for simulating fluid flow within a mold cavity, the method comprising the steps of: (a) providing a surface representation for a three-dimensional volume associated with a mold cavity; (b) separating the surface representation into at least a first portion and a second portion, the first portion of the surface representation being associated with at least one section of the volume having at least one of (i) a substantially invariant thickness and (ii) a gradually varying thickness along a length thereof; (c) discretizing a first portion of a solution domain bound on an exterior thereof by the first portion of the surface representation; (d) discretizing a second portion of the solution domain bound on an exterior thereof by the second portion of the surface representation; (e) defining a plurality of interface elements for the solution domain that connect at least part of the first portion of the solution domain to at least part of the second portion of the solution domain; (f) obtaining values of at least one process variable for the first portion of the solution domain using a first set of governing equations; and (g) obtaining values of the at least one process variable for the second portion of the solution domain using a second set of governing equations.