Looking at convective cells, it’s easy to think that they are still and unmoving. But when you add particles, their inner flow becomes obvious. Warm, light fluid moves up through the center of each cell, skims along the surface, and then sinks at the edges of the cell after losing its heat at the cooling surface. Below, the fluid moves back toward the cell center, getting warmer as it’s heated by the lower surface. Once it reaches the middle of the cell, it’s light enough to rise up and start the process again. Convective cells like these are typical in cooking – watch for them forming in your miso soup or hot chocolate – but they can also be found on the sun and even in situations without heating! (Image credit: G. Kelemen, source)
Tag: flow visualization
Rheoscopic Flow Vis
One of the great challenges in visualizing fluid flows is the freedom of movement. A fluid particle – meaning some tiny little bit of fluid we want to follow – is generally free to move in any direction and even change its shape (but not mass). This makes tracking all of those changes difficult, and it’s part of why there are so many different techniques for flow visualization. The technique an experimenter uses depends on the information they hope to get.
Often a researcher may want to know about fluid velocity in two or more directions, which can require multiple camera angles and more than one laser sheet illuminating the flow. An alternative to such a set-up is shown above. The injected fluid – known as a rheoscopic fluid – contains microscopic reflective particles, in this case mica, that are asymmetric in shape. Imagine a tiny rod, for example. By illuminating the rod from different directions with different colors of light, you can determine the particle’s orientation based on the color it reflects. Since the orientation of the particle depends on the surrounding flow, you can infer how the flow moves. (Image credit and submission: J. C. Straccia; research link: V. Bezuglyy et al.)
Sunglinting Seas
Sunlight reflecting off the Earth can reveal a remarkably rich picture of our planet’s activity. The silver-gray areas seen in this satellite image are sunglint, where lots of light is reflected back to space. Sunglint occurs in regions with very few waves; more waves – like in the bluer areas – mean more directions in which light can be scattered. The reason for these rough and smooth waters is atmospheric: the prevailing summer winds blow across the Aegean from the north. In open water, that wind drives up the waves, but rocky islands disrupt the flow, leaving “wind shadows” on their southern, leeward sides where the waves are smaller. (Image credit: J. Schmaltz; via NASA Earth Observatory)
Sunset Vortices
Often our atmosphere’s transparency masks the beautiful flows around us. This spectacular image shows a flight landing in Munich just after sunrise. Low-hanging clouds get sliced by the airplane’s passage and curl into its wake. The swirls are a result of the plane’s wingtip vortices, which wrap from the high-pressure underside of the wing toward the low-pressure upperside. The vortices stretch behind in the plane’s wake, creating turbulence that can be dangerous to following planes. In fact, these vortices are a major determining factor in the frequency of take-off and landing on a given runway. The larger a plane, the larger its wingtip vortices and the more time it takes for the turbulence of its passage to dissipate to a safe level for the next aircraft. (Image credit: T. Harsch; submitted by Larry S.)
Songs in Soap
There are many beautiful ways to visualize sound and music – Chris Stanford’s fantastic “Cymatics” music video comes to mind – but this is one I haven’t seen. This visualization uses a soap film on the end of an open tube with music playing from the other end. You can see the set-up here. The result is a fascinating interplay of acoustics, fluid dynamics, and optics. As sound travels through the tube, certain frequencies resonant, vibrating the soap film with a standing wave pattern (3:20). At the same time, interference between light waves reflecting off the front and back of the soap film create vibrant colors that show the film’s thickness and flow.
When the frequency and amplitude are just right, the sound excites counter-rotating vortex pairs in the film (0:05), mixing areas of different thicknesses. With just a single note, the vortex pairs appear and disappear, but with the music, their disappearance comes from the changing tones. Watching the patterns shift as the film drains and the black areas grow is pretty fascinating, but one of the coolest behaviors is how the acoustic interactions are actually able to replenish the draining film (2:15). Because the tube was dipped in soap solution, some fluid is still inside the tube, lining the walls. With the right acoustic forcing, that fresh fluid actually gets driven into the soap film, thickening it.
There are several more videos with different songs here – “Carmen Bizet” is particularly neat – as well as a short article summarizing the relevant physics for those who are interested. (Video and research credit: C. Gaulon et al.; more videos here)
Watching Flow Inside Rock
Flow through porous substances has been a major interest in fluid dynamics for the last hundred years because rocks are porous. For most of that period, we’ve used Darcy’s law to calculate how a fluid flows through pores in a rock. (Incidentally, it can also be used for determining the perfect length of time for dunking a cookie in milk.) Often, however, there is more than one fluid in a pore – for example, both a liquid and a gas could be trapped there. In that case, researchers made a few assumptions that allowed them to extend Darcy’s law for these multiphase situations. For a long time, that was the best anyone could do because it was impossible to observe what’s actually happening in the pores inside an actual rock.
Recently, however, scientists have begun observing these multiphase flows inside sandstone pores using x-ray imaging. They’re only able to take an image every 45 seconds or so, but even that is frequent enough to show that the flow is surprisingly unsteady. An example image is shown above. The colored areas show pores filled with nitrogen inside the rock. Brine is also being injected into the rock but not being shown. The colors indicate how connected the nitrogen-filled pores are to other pores nearby. Red areas are highly connected; blue have moderate connections; and green areas are smaller and have fewer connections. The network connections inside the rock change relatively rapidly, even with steady-state injection conditions. That varying connectivity implies that some of the injection energy is going into shifting interfaces around rather than actually moving the fluids through the pores. More work will be needed to unravel what’s really happening inside the porous network, but the results have far-reaching implications for understanding groundwater filtration, fossil fuel extraction, and, in the future, the possibility of carbon sequestration. (Image credit: C. Reynolds et al., source; submitted by Simon D.)
Flow Around a Cylinder
A cylinder standing upright in a flow creates a complicated system of vortices and recirculation. In the photo above, the flow is left to right. The cylinder itself is somewhat hard to see but is located in the center of the image; we see it from above. The colored streaks of dye show the flow path around the cylinder. In yellow, we see a spiraling vortex that forms just ahead of the cylinder and stretches downstream on either side. Because of its shape, this is called a horseshoe vortex. Its sense of rotation is such that it tends to pick up loose material in front of the cylinder; in other words, it can erode that area. This is often seen around the pilings of bridge supports and must be accounted for in designs. You also see the effects of this horseshoe vortex digging out material at the base of trees after snowfalls in areas with a dominant wind direction, and here’s an example with a snow roller. (Image credit: H. Werlé; via eFluids)
Impressionist Gibraltar
Swirls of phytoplankton make this satellite image of Gibraltar look like an Impressionist painting. The photo is a composite of data from several instruments, with colors enhanced to highlight features of the phytoplankton blooms. The tiny plankton act as tracer particles that reveal some of the complex flow between the North Atlantic and the Mediterranean. Although narrow, the Strait at Gibraltar has deep and complex terrain that was formed during a breach flood event millions of years ago. Water flowing through that terrain sets up enormous and complicated waves well beneath the ocean surface. These drive some of the turbulence that we see here as the blue swirls east of the Strait. (Image credit: NASA/N. Kurig; via NASA Earth Observatory)
Kelvin-Helmholtz Instability
Sixty Symbols has a great new video explaining the laboratory set-up for demoing a Kelvin-Helmholtz instability. You can see a close-up from the demo above. Here the pink liquid is fresh water and the blue is slightly denser salt water. When the tank holding them is tipped, the lighter fresh water flows upward while the salt water flows down. This creates a big velocity gradient and lots of shear at the interface between them. The situation is unstable, meaning that any slight waviness that forms between the two layers will grow (exponentially, in this case). Note that for several long seconds, it seems like nothing is happening. That’s when any perturbations in the system are too small for us to see. But because the instability causes those perturbations to grow at an exponential rate, we see the interface go from a slight waviness to a complete mess in only a couple of seconds. The Kelvin-Helmholtz instability is incredibly common in nature, appearing in clouds, ocean waves, other planets’ atmospheres, and even in galaxy clusters! (Image and video credit: Sixty Symbols)
How Smoke Rings Work
Vortices are a ubiquitous part of life, whether they’re draining down your bathtub or propelling underwater robots. In the latest video from the Lib Lab project, you can learn about how vortex rings form, what makes them last so long, and even make a vortex generator of your own. I can personally attest that vortex cannons are good for hours of entertainment, no matter your age. They’re even more fun with friends, as the Oregon State drumline demonstrates in the video. Want even more vortex fun? Check out leapfrogging vortices, vortex rings colliding head-on, and a giant 3 meter wide vortex cannon in action. (Video and image credit: Lib Lab)
