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Intuition

# The Physics of Vortex Cannons

## A Deep-dive into the Principles of Fluid Dynamics

By Valkyrie Holmes

Over the summer, my team and I embarked on a new assignment: Project Firefly. Over the course of a few months, we became familiar with vortex cannons. Vortex cannons can be used for all sorts of things but in our project, we were looking at how we could use them to extinguish fires and attach them to drones. You can read all about our project here.

But I decided to go a bit further into how vortex cannons actually work. We’ve been trying to come up with a way of explaining them for a while now and the closest thing we could come up with was “fighting fire with sound”. When talking to the CEO of ARSAC Technologies, Suchinder Dhillon, he told us that it’s far from the truth but it may very well be the easiest way of bringing this concept into the light. But through my research, I think I may have found a simpler way of describing how the technology works.

We are simply “throwing air”.

# MECHANICS

Before I describe what that means, I have to first break down the parts of a vortex cannon and how it all comes together. First, we have the subwoofer, a speaker-like pump that emits low-frequency oscillations into the barrel, also known as the collimator. The collimator makes sure that the air inside of the barrel travels in a laminar stream (straight and not chaotic flow). Finally, the baffle is a relatively small opening that the air is forced through.

Now, how does this work?

When I refer to the mechanics of the vortex cannon as “throwing air”, I mean that the subwoofer is applying force to air molecules to get them to move in one direction. This is in response to the force and exert force on the cannon itself. When the subwoofer collides directly with air molecules, it accelerates them through the collimator and towards the baffle in a series of high-speed collisions. They all combine once they exit the barrel, which forms a gaseous projectile known as a toroidal vortex or a vortex ring.

Vortex rings are caused by the friction of the air colliding with the edges of the baffle. The air traveling inside brushes against the hard circular opening and the slow-moving air outside slows down the outer molecules, causing them to curl back away from the center of the jet to form a donut shape, also known as a torus.

The air around the vortex ring usually has higher pressure, which stabilizes the ring as it travels forward, keeping its shape. Toroidal vortices are extra cool because while the air motion is stable, it doesn’t require any outside intervention to keep it going, which means the ring travels as far as it can until something disperses the air molecules.

These kinds of rings are actually more common than you think. Whenever air is shot under fluid, we can see a visible ring, like with whales and dolphins and even special mosses underwater. You can also think of it as a spinning flow, like a tornado. It forms toroidal funnels to create a giant massive storm. Another example of this could be water forming rings as it goes down the drain in order to adapt to its space.

# FLUID DYNAMICS

All this talk about rings and vortices threw me down a rabbit hole of fluid dynamics. Fluid dynamics is a “branch of applied science concerned with the movement of liquids and gases” according to the American Heritage Dictionary. Despite what you may think, the motion of gases and liquids are very similar. In fact, in both gases and liquids, swirling is called a vortex, and twisting is referred to as a toroid.

This field helps us understand things like currents, weather patterns, plate tectonics, the evolution of planets and stars, and more, which helps engineers build better rocket engines, wind turbines, pipelines, and AC systems.

In a vortex cannon, we have poloidal flow. This kind of flow is characterized by a mass of fluid or gas being pushed through a small opening by some impulse. Flow refers to how fluids behave when they interact with their surroundings and is usually either laminar or turbulent (think smooth vs chaotic). Turbulent refers to mixing fluids or equalizing temperature while laminar is mostly used when discussing drainage. Laminar is also way more efficient and less energy is lost in the process.

Based on these definitions, vortex cannons would be more laminar since the shape formed comes from a constant flow of energy and then forms into a toroidal vortice. This flow of energy is referred to as “eddies”, which is the amount of turbulent motion present in a certain reaction. This is according to Korogomov’s theory of inertial turbulence, which states that “as energy is transferred from larger to smaller eddies, the transfer of kinetic energy is converted to heat in an energy cascade, which fuels turbulence.”

There’s one more thing you have to understand about fluid dynamics. Because gas is compressible where liquid isn’t and is hardly affected by gravity, it makes it difficult to manipulate in most cases. We use Reynolds number (Re) to communicate the ratio of inertial to viscous forces, which is basically the ratio of fluid resistance in relation to the change of motion to the amount of friction due to the thickness of the fluid. Basic conditions of flow operate on this scale, with low Re tending to be more laminar while high follows a more turbulent pattern.

Because air is so malleable with fluid dynamics, it’s extremely difficult to get a laminar pattern without forming tons of eddies and vortices. That’s where the vortex cannon comes into play. By using low-frequency oscillations to move air through the collimator, the air comes out the other end in a vortice but continues on a laminar flow because of the barrel’s guidance in a certain direction. Without this system, the pattern would be chaotic and wouldn’t work.

Re can be used to predict flow speed and flow transition speed from laminar to turbulent patterns, which can be useful for predicting how the vortex cannon reacts to things like temperature, weather patterns, fire weather, etc. By using CFD (computational fluid dynamics) to create simulations, we can determine how flow will change over time. Although it’s a difficult system to set up and we’d have to account for specific rules for the computer to follow, it would make it a lot easier to regulate all of the nonlinear and chaotic happenings that occur.

You can review more of the fundamental principles of fluid dynamics here.

Bottom line: vortex cannons are extremely complicated and are still very new. We can better understand the world around us with fluid dynamics and learn how to manipulate those molecules but only once we understand how they work on a very simplistic level. You can actually build your own vortex cannons and see how you can manipulate the air around you! I’ve linked some articles below!

The more we know about the world, the more we’ll be able to change it.