} \partial _t f + { \mathrm{div}_x}(fv) + { \mathrm{div}_v}\left \{ f { \left ( I - \frac{v \otimes v}{|v|^0} \right ) }a \right \} = { \mathrm{div}_v}\left \{ { \mathrm{div}_v}\left [ f { \left ( I - \frac{v \otimes v}{|v|^0} \right ) }\right ] + 0 f \frac{v}{|v|^0}  \right \}$$ 