In physiological fluid flows, except for in the heart and the aorta, the Reynolds numbers are moderate (below 1000). This is far below the typical point of transition in pipe flow, which occurs around Reynolds number 2300. Because of this, laminar flow is commonly assumed in the modelling of these flows, resulting in computational methods tailored for this flow regime.

This chapter presents a critical review of this assumption, and both clinical and numerical evidence of transitional physiological flows are presented. The pulsatility and complex geometries in physiological flow are highlighted as the main reasons for a lower transition point in physiological flows.

Furthermore, we discuss the threshold of transition in a particular case of an aneurysm with respect to resolution, Reynolds number and non-Newtonian viscosity modelling.