What is wall shear stress and shear strain rate?
 

Correct me if wrong, i am just guessing the meaning from notes i study on material science, and i not sure if the same applied on fluid as well.

wall shear stress is the force the 'tear' the fluid near the wall due to no slip boundary condition.

shear strain rate is the rate of fluid being 'teared'. higher shear strain rate means the fluid is being 'teared' more compare to lower shear strain rate when the time is constant.

Thanks in advanced.

somehow you are right.
in solid mechanics shear stress is proportional to shear strain. in fluids there is no constant shear strain for a constant shear stress which is the definition of fluid. instead, the shear stress is proportional to rate of shear strain and the required factor is viscosity. it is described in the first chapter of Fluid Mechanics books.

Thanks. I did saw the equation, where viscosity is a constant. :)