How can it be shown that the
(i) power component of a fluid due to shear stress $S_\tau$
$\dot{W}_{s_\tau} = -2\mu\int_A e_n \cdot (\vec{V} \cdot \dot{S}) dA + 2\mu \int_{\forall} (\dot{S} \cdot \nabla)\vec{V} \: d\forall$
(ii) and power component of a fluid due to normal stress $S_n$
$ \dot{W}_{s_p} = - \oint e_n \cdot \vec{V} \: p \: dA$
where
$\mu $- Kinematic Viscosity
$e_n $- Normal Vector
$\vec{V}$ - Velocity
$dA$ - Area component
$\dot{S}$ - Rate of Strain Dyadic
$p$ - pressure of the fluid