What you're seeing. Blue particles trace the flow. In the wide section, flow is laminar — smooth, parallel layers. At the restriction, flow accelerates (same volume must pass through a smaller area) and pressure drops — that's Bernoulli's principle. After the restriction, the sudden expansion creates turbulence — chaotic eddies that waste energy as heat. This is why sudden pipe expansions cause pressure loss in your plumbing.

The pressure graph shows static pressure along the pipe. Notice the pressure drop at the restriction — this is where Bernoulli converts pressure energy into kinetic energy (speed). After the restriction, pressure partially recovers but never fully — the lost energy became heat through turbulence. In your home, this pressure drop means weaker flow at distant taps. That's why plumbers use 22mm for main runs and 15mm for branches.