nyírósebesség

Shear rate, known in Hungarian as nyírósebesség, is a fundamental concept in fluid mechanics and rheology. It quantifies the rate at which layers of a fluid slide past each other. Imagine a fluid confined between two parallel plates, where one plate is moving and the other is stationary. The fluid layer in contact with the moving plate moves at the same speed as the plate, while the layer in contact with the stationary plate remains still. The fluid layers in between move at progressively slower speeds as they get closer to the stationary plate. The shear rate is the change in velocity across the fluid depth divided by that depth.

Mathematically, shear rate is often represented by the Greek letter gamma dot ($\dot{\gamma}$). For a simple Couette flow, where the velocity gradient is linear, the shear rate is constant throughout the fluid. It is typically measured in units of inverse seconds (s$^{-1}$). A higher shear rate indicates that the fluid layers are moving past each other more rapidly. This has significant implications for the behavior of fluids, particularly non-Newtonian fluids, whose viscosity can change with shear rate. For instance, some fluids become less viscous at higher shear rates (shear thinning), while others become more viscous (shear thickening). Understanding shear rate is crucial for designing and analyzing processes involving fluid flow, such as pumping, mixing, and extrusion.