3D Wall Shear Stress

I realise this may have been asked before, but I'm running a CFD simulation of blood flow through a 3D cerebral aneurysm model. Initially, I have a laminar flow study in which I would like to visualise the wall shear stress within the aneurysm sac. As a new COMSOL user, I am unsure of whether an expression exists for this, or how I would approach writing an expression?

Any help would be greatly appreciated!