# Science Journey: Fluid Dynamics: From Disturbances to Turbulence

The storms we see on Jupiter and the motions in a coffee cup can be explained by analyzing the equations of fluid dynamics. These equations are used to design airplanes, predict the weather, and even make art!

How do engineers and scientists solve these equations?

In this video, Salvador Gomez explains how the important information can be identified to simplify the problems so that engineers and scientists can make predictions in fluid flows.

### Pop Quiz

A fluid is anything that flows because of motion, it can be a liquid, gas, or plasma.

They are the Navier Stokes equations.

They break up the domain into small squares. The equations are then solved by looking at each square and its neighbors.

They trace path lines of the fluid by letting smoke, particles, or dye flow around the fluid. They measure the fluid flow by taking pictures of it or taking measurements.

The computations and experiments fail because you need to model things that are too small or things that are too big. It is hard to capture both.

Density. The density of oil is less than the density of water.

Viscosity. The honey is more viscous than water.

If the fluids do not mix, or the interface between the two fluids stays nice and separated.

It is when the fluid has a lot of mixing and vortices with different sizes.

About the thickness of a human hair. If you want to model the shark, you need to model its body *and* its tiny features to understand why the sharkskin is important.

You can determine snapshots and rank them by how much they represent the photo. By choosing a few of these snapshots, you can reconstruct a less detailed image of the cat.

Because you would then be able to test various designs rapidly and not have to solve all of the problem.

### Q&A with Salvador

Fluid mechanics is many scales, down to biological scales and all the way up to galactic scales. In a black hole there is matter orbiting a really heavy object. This matter can be gases. So there are fluid mechanics at play around black holes with all the swirling gases around the black hole, but what goes on *inside* the black hole is at the limits of physics because the matter is so compact in the black hole.

A characteristic length is a length that we expect to be important to the physics of the problem. Like if you want to model a ball flying, the characteristic length would be its radius.

Sometimes it's evident like with the example of a ball, but other times its more complicated. In the more complicated cases they are found from the equations or found in experiment. When they find it in experiment, there are usually strong measurements in the sensors that say there are vortices of a certain size, for example.

Engineers have done a lot of experiments where they use wind tunnels to see what's going on at the wing. They see that when they have turbulence over the wing, there is more drag. But that idea of sensors is something that people are attempting, to do flow control to reduce the turbulence on the wing.

### About the Speaker

Salvador Gomez grew up in Hayward, California, just south of Oakland in the Bay Area. He became the first in his family to ever go to college when he went to school at UC Berkeley to study mechanical engineering. At Berkeley, Salvador got the chance to teach middle and high school students at summer camps and teach college students in the club Hispanic Engineers & Scientists during the winters. His interest in teaching, along with the exciting possibilities in research, led Salvador to apply to Caltech for a PhD in Aeronautics.

Salvador is currently a second year graduate student at Caltech, earning a degree in aeronautics (the science of understanding how airplanes fly). He is studying turbulence to understand the processes that increase drag on vehicles. He uses math and coding to identify structures that can help reduce the complexity of the turbulent flows. Outside of research, Salvador is also an officer in Club Latino at Caltech.

Salvador hopes to one day be a professor to continue studying fluid dynamics and help develop programs that can increase retention of underrepresented students in STEM.

## About the Series

Science Journeys online programs are recommended for grades 8 and up, but everyone is welcome to attend.

*These programs are made possible through the generosity of the* *Friends of Beckman Auditorium*

If you have questions, please email Mary Herrera at mhh@caltech.edu.