What forces act on fluid moving through a pipe to make it spiral?

Ron Dippold San Diego, California, US 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. Advertisement 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. When the liquid hits a tiny defect, or even if there’s a slight temperature variation – since hot fluid is less dense and rises – a small amount is deflected. This creates a small eddy, or swirl. If the liquid is moving slowly enough, the boundary layer acts as a damper and these just die out. Once the fluid reaches a critical speed, however, the damping can’t keep up, and this is where the non-linearity comes in. Since the fluid closer to the centre of the pipe is moving faster than the fluid at the edges, it stretches these swirls down the pipe. And, like a skater pulling in their arms, a stretched, narrower swirl spins faster and harder, pulling energy from the fluid to grow longer and narrower. These swirls don’t cancel each other out; they just feed on each other and twist into crazy shapes. You get what’s called turbulent flow, for obvious reasons. You may see spiralling, but also unpredictable thrashing and surging around. Any trapped gas bubbles make things much worse as they tumble around wildly. So, how do you stop this? Slowing down the liquid helps, but you usually don’t want much less flow. If it’s below the critical turbulence speed, you can run it through a long, straight pipe and it will settle down thanks to the drag from the boundary layer. In the best case, this length is about 15 times the diameter of the pipe after a single curve, but can be much longer – it depends on the viscosity of your liquid, the roughness of the pipe and how convoluted your pipes are. Like a skater pulling in their arms, a stretched, narrower swirl spins faster, pulling energy from the fluid to grow If we really need laminar flow immediately, we use flow conditioners. These divide the big pipe into many smaller pipes for a bit or just add flat plates with carefully designed holes cut in them. These kill any large swirls, which have most of the energy, because the swirls are much larger than the holes. And in smaller pipes, 15 times the diameter of the pipe is a much shorter distance. The downside is that you are slowing the fluid by cutting down the pipe volume – so, if you need to get the final flow rate higher, you have to pump harder, which means, yes, more turbulence upstream. So just invent frictionless pipes to solve all this, and please remember me when your royalty cheques arrive. To answer this question – or ask a new one – email [email protected]. Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address. The use of AI tools (e.g. ChatGPT, Claude) to write either questions or answers is forbidden, and any submissions found to have been written using AI tools will be withdrawn.