"There’s a quantity called “wall shear stress,” which is just a frictional force on the wall. We’ve shown that when doctors can visualize that wall shear stress

"There’s a quantity called “wall shear stress,” which is just a frictional force on the wall. We’ve shown that when doctors can visualize that wall shear stress at different parts of the artery, they may actually change the length of the stent that they choose. It really informs their decisions." This sounds pretty good to me? The point of fluid dynamics and any other science of physical process is about refining a set of those simplifying assumptions. You don't model every atom or molecule, that's what fluid dynamics is for.