1. A system for non invasively determining at least one physiological characteristic of a patient, the system comprising: at least one computer system configured to: use a hardware processor of the at least one computer system to create a three-dimensional combined surface and volume mesh model of the patient using patient-specific imaging data of the patient by forming multiple elongated shaped elements of the combined surface and volume mesh model along a direction of fluid flow through the combined surface and volume mesh model, the 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, the first model portion including shaped elements that are elongated relative to shaped elements of the second model portion; input the three-dimensional combined surface and volume mesh model of the patient into a fluid simulation system; and determine a measurement of the physiological characteristic of the patient, by running the fluid simulation system on the three-dimensional combined surface and volume mesh model of the patient.