Natural phenomena

Kelvin-Helmholtz instability

In this blogpost we try and explore a fundamental phenomena called the Kelvin-Helmholtz instability and exemplify it with a paperweight toy.

What is the Kelvin-Helmholtz instability

First of all, lets try and understand what is the Kelvin-Helmholtz instability. The Kelvin-Helmholtz instability is not a person with a mental health issue, rather it is the naturally occurring instabilities which develop when two fluids of opposite nature try and pass each other.

Video 1: Numerical solution of the Kelvin-Helmholtz instabilities utilizing Discontinous Galerkin methods for interface fluid-dynamics simulations.

These instabilities play an important role in everything from the development of clouds, hurricanes, tornadoes and wind gusts down to mixing of saline sea-water, temperatures and development of waves.

What is the role of viscosity

Swirls and vortices are needing to be dissipated by internal frictional forces on scales not visible for the human eye. This is where the viscos part of the equations come into play.

Viscosity is the internal resistance of fluids mixing arising from natural phenomena such as Kelvin-Helmholtz instabilities. The more viscos fluids resist mixing and visa versa.

Related Read: how do one measure the viscosity of fluids? The Buckingham-Pi Theorem

Toy world example

The Kelvin-Helmholtz instability is explained through large-scale phenomena above, such as the creation of clouds. However, the phenomena appears on all scales including more tangible scales such as paperweights.

By injecting a dense fluid with dye and filling up the container with water, it is possible to create the illusion of a drifting ship within the ocean. See Gallery 1.

By tilting the resulting mixture of colored (dense fluid) and lighter (water) it is possible to create a turbulent seas which then creates waves inside the container.

This creation of waves is also caused by the Kelvin-Helmholtz instability within the fluid interfaces. The viscosity and top floating pressure of fluid helps to stabilize the system after a short while. The result is an interactive toy boat paperweight bouncing around in disturbed seas.

How do I get my own toy?

In order to buy the paperweight toy follow the amazon link below, I also get some affiliate commissions so not only do you get an amazing toy, you are also helping the site out directly.

I have chosen four different variations of the conceptually same toy. The unifying characteristic is that all of them are showcasing the same fun natural phenomena. The toys vary in expression from the titanic and an iceberg, Dinosaur and volcano, Penguins surfing and a surfer with a shark.


In this blogpost we have explored the naturally occurring phenomena that is the Kelvin-Helmholtz instability. Furthermore we have exemplified it with a toy paperweight.