Abstract: A model-order reduction system and methods are described for providing a fast fluid flow simulation model of fluid flow in a conduit such as a blood vessel with stenosis. A first method is described for generating a reduced generic numerical model (20) for predicting fluid flow characteristics of fluid flowing through a subject conduit (7?). In steps 11, 12 and 13, geometric and fluid-flow parameter data are derived from CT image scans for each sampled conduit (7) in a reference set. A 3D model is generated (13) for each sampled conduit (7). The geometric parameter data, the 3D model and the fluid flow parameter data are used to generate solutions to fluid dynamics (such as the Navier-Stokes) equations for each sampled conduit (7), and a full order model is created comprising the geometric parameters data, the fluid flow parameter data and the Navier Stokes solutions.