The correct option is (a) \(\frac{∂\overline{u}}{∂t}=C_{vr} (\frac{\overline{u}}{∂r^2}+\frac{1}{r}\frac{∂\overline{u}}{∂r})\)
Easiest explanation: In three dimensional consolidation of sand drain, having radial symmetry, the governing consolidation equation is,
\(\frac{∂\overline{u}}{∂t}=C_{vr} (\frac{\overline{u}}{∂r^2}+\frac{1}{r}\frac{∂\overline{u}}{∂r})+C_{vz}\frac{∂^2 \overline{u}}{∂z^2},\) in this the radial flow part of equation is,
\(\frac{∂\overline{u}}{∂t}=C_{vr} (\frac{\overline{u}}{∂r^2}+\frac{1}{r}\frac{∂\overline{u}}{∂r}).\)