Viscosity is typically thought of as how easily a fluid will flow, more viscous [[Fluid|fluids]] require greater [[Force|force]] to move. We can think of the viscous effect and [[Friction|friction]] as one and the same for a fluid flow. Viscosity is defined for Newtonian fluids (such as air and water) as the coefficient that relates [[Stress|stress]] and [[Strain|strain]] in a fluid: $\tau_{xy}=\mu\dfrac{du}{dy}$ Viscosity is sensitive to [[Temperature|temperature]] and [[Pressure|pressure]] but is typically relatively constant for most applications with small temperature fluctuations. Fluid flows can be classified into two types regarding viscosity: - **Viscous:** Viscosity is **non-negligible** and must be accounted for. - **Inviscid:** Viscosity is **negligible** and assumed to be zero.