| | All I wanna know about Viscous Stress |
 | | In a nutshell, viscous stress try to stop relative motion between different small part of the fluid
I can even say whenever there is a strain rate (i.e., deformation that changes with time and is caused by the velocity gradient) within the fluid, a viscous stress act to reduce that rate-of-strain. |
 | | For example, it can be sometimes seen in some (bad) books that the stress tensor is defined positive in compression but the rate-of-strain is defined negative in compression or that the stress tensor is positive in compression and in the next chapter positive in extension, all this is horribly confusing, complicate and pure non-sense. |
 | | This means, for a given normal applied stress and a given volumetric grain concentration, it is impossible to have shear stress within a frictional granular flow higher than its yield shear stress (i.e., the flow is at yield and flow plastically). |
| www.granular-volcano-group.org /viscous_stress.html (4861 words) |