1. A method for non-invasive assessment of coronary artery stenosis, comprising: extracting patient-specific anatomical measurements of coronary arteries of a patient from medical image data of the patient; determining a model of coronary circulation comprising a reduced-order model that defines a blood pressure drop across at least one stenosis region of the coronary arteries of the patient as a function of blood flow velocity through the stenosis region; determining first patient-specific boundary conditions of the model of coronary circulation representing the coronary arteries using the patient-specific anatomical measurements and non-invasive clinical measurements of the patient; determining second patient-specific boundary conditions of the model of coronary circulation using the first patient-specific boundary conditions; simulating blood flow and pressure across the at least one stenosis region of at least one coronary artery using the model of coronary circulation and the second patient-specific boundary conditions; and determining a characteristic of blood flow through the at least one stenosis region using the simulated blood flow and pressure.