Between the shear stress τ = F/A and the velocity gradients
in a moving fluid (Figure D 93) the following relationship applies:
The proportionality factor η is called the dynamic viscosity. It is a measure of the internal resistance that opposes displacement of neighbouring layers in the fluid. As such the dynamic viscosity has a very significant influence on the friction of the flowing medium and also the leakage losses in gaps.
The dynamic viscosity is a specific variable for every fluid and that for Newtonian fluids is dependent on the temperature and the pressure. Its unit is Pa · s.
The old unit P (cP) is no longer allowed. Conversion: 1 cP = 1 mPa · s.
Figure D 93: Drag flow in a parallel gap