The physics
A liquid flows in layers. Each layer slides over the next one, and the liquid resists this sliding. This resistance is viscosity, η.
F = ηAv / d 1 poise = 0.1 Pa·s
A plate on a thin film drags the top layer along. The bottom layer stays still. The drag grows with the area and the speed. A thicker film gives less drag.
The shear stress is the drag on each square metre: τ = ηv / d.
In a thin tube the middle moves fastest. The layer at the wall stays still. The flow rate goes as the fourth power of the radius.
Q = πr⁴ΔP / 8ηl
Tubes in a line carry the same flow. So r⁴ΔP / l is the same for each tube.
Slow flow is smooth, and dye stays in neat lines. This is streamline flow. Fast flow is turbulent, and the dye breaks into swirls. The Reynolds number decides which.
Re = ρvD / η smooth below 2000, turbulent above 4000
The flow each second is the speed times the pipe's area.