>[!Definition] >Shear stress in a fluid is proportional to shear velocity. The constant of proportionality is the fluid's [[dynamic viscosity]] $\tau = \mu\frac{\delta u}{\delta y}$ where: $\tau$ = shear stress (Force/Area) $\mu$ = [[dynamic viscosity]] $\frac{\delta u}{\delta y}$ = shear velocity = velocity gradient = rate of change of velocity in the direction perpendicular to applied stress ![[Untitled 26.jpg|lg]] In this model, the internal shear stress and the velocity gradient of a fluid are proportionally related by the constant [[dynamic viscosity]]. Fluids that don't behave this way are [[Non-Newtonian fluids|non-Newtonian]] ## Couette flow In this theoretical flow, a fluid is between two infinite plates. One plate is stationary, one is moving at constant velocity. ![[Untitled 25 1.jpg|grid]]