Calculating Wall shear Stress and Mapping value to isosurface skin.

Greetings all, I’m new to VTK. I’m trying to calculate Wall Shear Stress on a 3D MRI Velocity Dataset. I’ve done the noise filtering, and generated a 3D velocity volume and the construction of vessel skin by using vtkMarchingCube, but I’m lost now. I have 3 directional velocities for each point, but I don’t know how to apply or what to apply to calculate the wall shear stress.