$A$ pure number which determines the type of flow of a liquid through a pipe is known as

$A$ highly viscous liquid of viscosity coefficient $\eta$ flows through a fixed horizontal cylindrical tube (fixed from the outer surface) of internal radius $r$,thickness $t$ $(t \ll r)$,and length $l$. The volume of liquid flowing per second is $Q$ and the pressure difference across the tube is $P$. If the modulus of rigidity of the material of the tube is $\beta$,the shear strain in the tube will be:

When a liquid flows through a tube,the Reynolds number is $900$. The flow of the liquid is:

Show that the Reynolds number is dimensionless.

What is critical velocity? Write a formula for critical velocity.

Two different liquids are flowing in two tubes of equal radius. The ratio of coefficients of viscosity of liquids is $52:49$ and the ratio of their densities is $13:1$,then the ratio of their critical velocities will be