D[xi] du dv dw / du dv dw \
----- - [xi]-- - [eta]-- - [zeta]-- + [xi]( -- + -- + -- ) = 0, (9)
dt dx dx dx \ dx dy dz /
and combining this with the equation of continuity
1 D[rho] du dv dw
----- ------ + -- + -- + -- = 0, (10)
[rho] dt dx dy dz
we have
D /[xi] \ [xi] du [eta] dv [zeta] dw
-- ( ----- ) - ----- -- - ----- -- - ------ -- = 0, (11)
dt \[rho]/ [rho] dx [rho] dx [rho] dx
with two similar equations.