What forces are acting on a particle of fluid that makes it start spiralling or moving helically down a pipe instead of flowing smoothly? The main players are the inertia of the fluid, the friction and adhesion between the pipe and liquid, and any turbulence due to variations in the material or temperature of the pipe wall. Liquid and gas are both fluids and their flow is governed by the Navier-Stokes equations, which are non-linear.
Microscopic defects cause much larger vortices, similar to how a fast-moving truck can be sent flying in another direction after hitting a pebble. The simplest factor to understand is the inertia of all the fluid travelling down the pipe from gravity or pumping. In a perfectly temperature-controlled, frictionless pipe, it would be smooth and straight-flowing everywhere, even with kinks in the pipe.
This is called laminar flow. But nothing is frictionless, and chemical and electrical forces attract the liquid to the pipe wall. So, fluid at the edges is slowed by contact with the pipe – this cylinder of fluid that is slowed is called the boundary layer.
If the pipe is perfectly straight and long, this actually helps calm things, but as soon as there’s a curve, it creates a spiral. Liquid tends to hug the inside curve because of that boundary layer – this is called the Coanda effect. You can see this for yourself with a sink and spoon.
Run a small, steady stream of water from your faucet, then slowly move the back of the spoon into the stream. In the absence of friction and the Coanda effect, you would expect to see the water just fall straight down, but instead, you will see it hug the surface of the spoon and come off the tip of the spoon at an angle. Back in our pipe, that means the fluid isn’t going straight down the pipe, which means it starts spiralling.
Any curve, valve or pump will give you this spiralling. And no pipe is completely smooth. Even “smooth” glass has microscopic defects.
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