Recently, NASA Goddard released a visualization of aerosols in the Atlantic region. The simulation uses real data from satellite imagery taken between August and October 2017 to seed a simulation of atmospheric physics. The color scales in the visualization show concentrations of three major aerosol particles: smoke (gray), sea salt (blue), and dust (brown). One of the interesting outcomes of the simulation is a visualization of the fall Atlantic hurricane season. The high winds from hurricanes help pick up sea salt from the ocean surface and throw it high in the atmosphere, making the hurricanes visible here. Fires in the western United States provide most of the smoke aerosols, whereas dust comes mostly from the Sahara. Tiny aerosol particles serve as a major nucleation source for water droplets, affecting both cloud formation and rainfall. With simulations like these, scientists hope to better understand how aerosols move in the atmosphere and how they affect our weather. (Image credit: NASA Goddard Research Center, source; submitted by Paul vdB)
Tag: computational fluid dynamics
Lighting Engines
Combustion is complicated. You’ve ideally got turbulent flow, acoustic waves, and chemistry all happening at once. With so much going on, it’s a challenge to sort out the physics that makes one ignition attempt work while another fails. The animations here show a numerical simulation of combustion in a turbulent mixing layer. The grayscale indicates density contours of a hydrogen-air mixture. The top layer is moving left to right, and the lower layer moves right to left. This sets up some very turbulent mixing, visible in middle as multi-scale eddies turning over on one another.
Ignition starts near the center in each simulation, sending out a blast wave due to the sudden energy release. Flames are shown in yellow and red. As the flow catches fire, more blast waves appear and reflect. But while the combustion is sustained in the upper simulation, the flame is extinguished by turbulence in the lower one. This illustrates another challenge engineers face: turbulence is necessary to mix the fuel and oxidizer, but turbulence in the wrong place at the wrong time can put out an engine. (Image, research, and submission credit: J. Capecelatro, sources 1, 2)
Adaptive Meshing
The use of numerical simulations in fluid dynamics has exploded over the past half century with new computational techniques being developed constantly. Most methods involve solving the equations of motion (or an approximation thereof) on a grid of points known as a mesh. To accurately capture the physics, meshes must often be quite closely packed in areas where detail is needed, but they can be more widely spaced in areas where the flow is not changing quickly. An increasingly common technique is adaptive meshing in which the mesh of grid points shifts between time steps; this places more grid points where the flow requires them and removes them from less important areas in order to reduce computational time.
An example of adaptive meshing is shown above. On the left particles are falling into salt water. The colors show the concentration of particles. The right side shows the solid particles and the fluid mesh around them. Notice how the grid shifts as the particles fall. (Image credit: C. Jacobs et al., source)
Inside Earth’s Core
Without our magnetic field, life as we know it could not exist on Earth. Instead, our atmosphere would be stripped away and the surface would be bombarded by charged particles in the solar wind. Relatively little is known about the dynamo process that governs our magnetic field, though it’s thought to be the result of liquid iron moving in the Earth’s outer core. The video above shows a slice of a recent 3D simulation of this liquid iron segment of our core. The colors show variations in the temperature, revealing vigorous convection in the core. This motion, combined with the spinning of the Earth, is the likely source of our magnetic field. Researchers hope that simulations like these can help us understand features we observe in our magnetic field – like local variations in field strength and the pole reversals in our geological record. (Video credit: N. Schaeffer et al.; CNRS via Gizmodo)
Break-Up of the Chelyabinsk Meteor
In 2013, a meteor about 20-meters in diameter broke up over Chelyabinsk, Russia in a dramatic display that damaged buildings within 100 km and injured more than 1200 people. To better understand the threat presented by such objects, NASA has been conducting 3D, hypersonic simulations like the one shown here. The meteor material is shown in gray and black. Brighter colors like red and yellow indicate the hot, high-pressure shock wave caused when the meteor slams into the atmosphere. Aerodynamic effects quickly erode the meteor, ripping it into pieces that disperse energy explosively in the atmosphere. While you might think the meteor breaking up is good for us, it’s actually the blast waves from its break-up that cause the most damage. (Image and video credit: NASA, source; via Gizmodo)
How Cycling Position Affects Aerodynamics
New FYFD video! How much does a rider’s position on the bike affect the drag they experience? To find out I teamed up with folks from the University of Colorado at Boulder and at SimScale to explore this topic using high-speed video, flow visualization, and computational fluid dynamics.
Check out the full video below, and if you need some more cycling science before the Tour de France gets rolling, you can find some of my previous cycling-related posts here. (Image and video credit: N. Sharp; CFD simulation – A. Arafat)
ETA: Please note that the video contained in this post was sponsored by SimScale.
Simulating Thunderstorms
With today’s supercomputing power, it’s possible to simulate entire thunderstorms to study how and why some of them can spawn deadly tornadoes. The animation above comes from a computer simulation of a supercell thunderstorm. The simulation uses initial conditions from a 2011 storm that produced an EF-5 tornado – the highest category of tornado, based on its wind speeds. To see more of the simulation, check out the video below. One thing that might surprise you is just how enormous the towering supercell clouds are compared to the tornado produced in the simulation. Often what we can see of a storm from the ground is only the tiniest part of what goes into producing it. (Image credit: L. Orf et al., source; GIF via @popsci; video credit: UWSSEC)
Creating Moana’s Ocean
Hopefully by now you’ve had an opportunity to see Disney’s film Moana. Fluid dynamics play a central role in the movie, and Disney’s animators faced the challenge of hundreds of shots requiring special effects to animate water, lava, waves, and wind. Science Friday has a great segment interviewing a couple of Moana’s animators, in which they discuss the process of turning the ocean itself into a character.
Because the physics of fluids is so complex, scientists and animators differ in the way they approach simulations. Scientists usually try to capture a full physical representation of a flow, simulating every detail to the smallest scale and time step. Animators, on the other hand, are interested in capturing a realistic feel for a flow. For an animator, the simulation should be exactly as complex as necessary to make the water move in a way a person believes it should. With Moana, animators had the extra challenge of melding the ocean character’s actions with appropriate water physics–think bubbles, drops, and splashes. The results are impressive and exceptionally fun. (Image credits: Disney/Science Friday; via Jesse C.)
Simulating the Earth
Computational fluid dynamics and supercomputing are increasingly powerful tools for tracking and understanding the complex dynamics of our planet. The videos above and below are NASA visualizations of carbon dioxide in Earth’s atmosphere over the course of a full year. They are constructed by taking real-world measurements of atmospheric conditions and carbon emissions and feeding them into a computational model that simulates the physics of our planet’s oceans and atmosphere. The result is a visualization of where and how carbon dioxide moves around our planet.
There are distinctive patterns that emerge in a visualization like this. Because the Northern Hemisphere contains more landmass and more countries emitting carbon, it contains the highest concentrations of carbon dioxide, but winds move those emissions far from their source. As seasons change and plants begin photosynthesizing in the Northern Hemisphere, concentrations of carbon dioxide decrease as plants take it up. When the seasons change again, that carbon is re-released.
These visualizations underscore the fact that these carbon emissions impact everyone on our planet–nature does not recognize political borders–and so we share a joint responsibility in whatever actions we take. (Video credit: NASA Goddard; h/t to Chris for the second vid)
A Molecular View of Boiling
All matter is made up of molecules. But most of the time we treat fluids as materials with given properties – like density, viscosity, and surface tension – without worrying about the individual molecules responsible for those material characteristics. Now that we have much more powerful computers, though, we can begin to simulate fluid behavior in terms of molecules.
The animations above show some examples of this. In the top animation, we see a gas condensing into a liquid. As the temperature decreases, molecules start clumping together, and eventually settle into a droplet on the solid surface. The lower animation shows the opposite situation – boiling – in which bubbles of vapor nucleate next to the solid surface and grow as more liquid changes phase. To see more examples, including droplets pinching off, check out the full video. (Image credit: E. Smith et al., source; submitted by O. Matar)
