Shear rate

Short description: Rate of change in the shear deformation of a material with respect to time

In physics, shear rate is the rate at which a progressive shearing deformation is applied to some material.

Simple shear

The shear rate for a fluid flowing between two parallel plates, one moving at a constant speed and the other one stationary (Couette flow), is defined by

$\dot{\gamma }=\frac{v}{h},$

where:

• $\dot{\gamma }$ is the shear rate, measured in reciprocal seconds;
• v is the velocity of the moving plate, measured in meters per second;
• h is the distance between the two parallel plates, measured in meters.

Or:

${\dot{\gamma }}_{ij}=\frac{\partial v_i}{\partial x_j}+\frac{\partial v_j}{\partial x_i}.$

For the simple shear case, it is just a gradient of velocity in a flowing material. The SI unit of measurement for shear rate is s⁻¹, expressed as "reciprocal seconds" or "inverse seconds". However, when modelling fluids in 3D, it is common to consider a scalar value for the shear rate by calculating the second invariant of the strain-rate tensor

$\dot{\gamma }=\sqrt{2\epsilon :\epsilon }$.

The shear rate at the inner wall of a Newtonian fluid flowing within a pipe is

$\dot{\gamma }=\frac{8v}{d},$

where:

• $\dot{\gamma }$ is the shear rate, measured in reciprocal seconds;
• v is the linear fluid velocity;
• d is the inside diameter of the pipe.

The linear fluid velocity v is related to the volumetric flow rate Q by

$v=\frac{Q}{A},$

where A is the cross-sectional area of the pipe, which for an inside pipe radius of r is given by

$A=\pi r^2,$

thus producing

$v=\frac{Q}{\pi r^2}.$

Substituting the above into the earlier equation for the shear rate of a Newtonian fluid flowing within a pipe, and noting (in the denominator) that d = 2r:

$\dot{\gamma }=\frac{8v}{d}=\frac{8\left(\frac{Q}{\pi r^2}\right)}{2r},$

which simplifies to the following equivalent form for wall shear rate in terms of volumetric flow rate Q and inner pipe radius r:

$\dot{\gamma }=\frac{4Q}{\pi r^3}.$

For a Newtonian fluid wall, shear stress (τ_w) can be related to shear rate by ${\tau }_w={\dot{\gamma }}_x\mu$ where μ is the dynamic viscosity of the fluid. For non-Newtonian fluids, there are different constitutive laws depending on the fluid, which relates the stress tensor to the shear rate tensor.