= Viscometric functions
{title2=$\sigma_{xy}=\mu(\dot\gamma)\dot\gamma,\quad N_i=\psi_i(\dot\gamma)\dot\gamma^2$}
The three viscometric functions describe steady homogeneous <simple shear flow>: its <shear viscosity> and the first and second <normal-stress differences> per squared shear rate. Take x as the flow direction, y as the gradient direction and z as the vorticity direction, so $N_1=\sigma_{xx}-\sigma_{yy}$ and $N_2=\sigma_{yy}-\sigma_{zz}$. Isotropic pressure drops out of these differences.
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