# Velocity of a viscous fluid through a tube

When a viscous liquid flows through a tube in a laminar flow, why is its velocity highest at the center?

I understand the concept of shear viscosity and why the liquid in contact with moving plate has highest velocity. That's because of viscosity itself. But, I cannot understand how it explains liquid flow in a tube or whether it explains at all.

$$v = - \frac{1}{4 \eta} \frac{\Delta P}{\Delta x} (R^2 - r^2)$$
This equation uses $r$ for distance from the center. When that is zero, you're at the center-line, and that above expression has the highest value. You can go look up the exact steps for how to get this expression from the fluid momentum equation itself, which is more-or-less a statement of the definition of viscosity.