A software tool has been developed that allows one dimensional and three dimensional fluid flow simulation packages to interact in a heterogeneous (UNIX -NT/2000/98) environment. This allows for the implementation of a “co-simulation” methodology in which the internal flow in a component simulated in three dimensions is incorporated into a network containing components represented by bulked loss coefficients in a one dimensional analysis. This methodology enables the details of the internal flow to be retained whilst conserving overall mass flow in the network, thus eliminating uncertainties in boundary values prescribed in the three dimensional model and reducing the overall simulation time.