1. A system for noninvasively determining at least one physiological characteristic of a patient, the system comprising: at least one computer system configured to: using a three-dimensional surface mesh model created using patient-specific imaging data, create a three-dimensional combined surface and volume mesh model, including at least a first model portion that has a different spatial resolution than at least a second model portion; input the three-dimensional surface and volume mesh model into a fluid simulation system; and determine a measurement of the physiological characteristic, using the fluid simulation system.