$$\frac{\mathrm{\partial }\mathbf{u}}{\mathrm{\partial }t}=\frac{K+\frac{4}{3}G}{f}\frac{\mathrm{\partial }}{\mathrm{\partial }r}\left\{\frac{1}{r^2}\left[\frac{\mathrm{\partial }}{\mathrm{\partial }r}\left(r^2\mathbf{u}\right)\right]\right\}$$ (19)