Kelvin-Helmholtz Instability Between Two Fluids - Simulation Example


Kelvin-Helmholtz (KH) instability typically occurs between two immiscible fluids with a velocity difference across their interface. This instability can be observed, for example, as a natural phenomenon of certain cloud patterns. These patterns are characterized by the generation of small-scale motion at the interface, which later leads to the formation of an unstable interfacial vortex street. This generates spiral structures at the fluid-fluid interface. Wave formation on still water due to blowing wind is another common example of this instability.

In this simulation example, we will look at the formation of KH instability structures between a cloud and the surrounding air. Because of very small density differences, we will use the properties of air to model both the cloud and the air surrounding it.


The primary objective of this example is to learn how to set up this model in Ansys Fluent and to visualize the interfacial behavior between the two fluids with the help of post-processing tools. The top and bottom fluids are moving in the same direction with velocities of 0.25 m/s and 0.1 m/s respectively.


Download the Mesh file needed for setting up the simulation and the associated Case & Data files from here. Follow the instructions below to set up this simulation in Ansys Fluent starting with a Mesh file. In case you face any issues setting up or running the simulation, then please use the corresponding initial and final Case and Data files.

Alternate video link.

Results & Discussion

Let’s now analyze the simulation results and understand the physics of Kelvin-Helmholtz instability at the interface of the two fluids.