why is density preferred over viscosity while choosing repeating variables during dimensional analysis?

Question

anonymous · Answer

This question makes no sense as it is presented.  Need more information and/or a context.

anonymous · Answer

ok.
here is the context.
we have to prove that velocity through a small orifice in a tank is$$V=\sqrt{gh}*\phi(D/H,\mu/\rho*V*H)$$
where D is the diameter of the orifice,H is the head causing the flow,rho id density,mu is dynamic viscosity and g is gravitational acceleration.

anonymous · Answer

I still don't  quite get the question.  viscosity and density have different dimensions and are not interchangeable. Phi is a dimensionless quantity and I see all the arguments of phi are dimensionless,  so do you mean why is rho important compared to viscosity? Density  helps determine the pressure at the orifice while viscosity tends to restrict the flow.  I  do not have any expertise with viscous flow   beyond knowing what viscosity is.  Sorry