[Ivanjica]

I consider steady compressible axisymmetric isothermal flow of Newtonian fluid-gas in conical tube at low Re numbers.
According to problem I have governing equations in cylindrical coordinates. Because of symmetry one component of velocity is $u_\phi=0$ and because it is steady flow $\dfrac{\partial}{\partial t}=0$.
I am not sure what is happening with radial velocity $u_r$, is it with little variability, so can I ignore terms which consist $u_r$ and derivative of $u_r$ or not? And is there any other simplification according to type of flow?

Couple steps after these I need to make NS equations simpler using Mach number, so is it possible define Mach number according to both velocities $u_r$ and $u_z$, because I will have it both? $u_z$ is longitudinal velocity and it exist.