The simple collision of two liquid jets can form striking and beautiful patterns. Here the two jets strike one another diagonally near the top of the animation. One is slanted into the screen; the other slants outward. At their point of contact, the liquid spreads into a sheet and forms what’s known as a fishbone pattern. The water forms a thicker rim at the edge of the sheet, and this rim destabilizes when surface tension can no longer balance the momentum of the fluid. Fingers of liquid form along the edge, stretching outward until they break apart into droplets. Ultimately, this instability tears the liquid sheet apart. Under the right conditions, all kinds of beautiful shapes form in a system like this. (Image credit: V. Sanjay et al., source)
Tag: flow visualization
Delta Wing Flow Viz
Designing new aerodynamic vehicles typically requires a combination of multiple experimental and numerical techniques. The photo above shows a model for an unmanned flying wing-type vehicle. Here it’s tested in a water tunnel with dye introduced to the flow to highlight different areas. The model is at a high angle of attack (18 degrees) relative to the oncoming flow. This puts it in danger of flow separation and stall, the point where a wing experiences a drastic loss in lift. The smooth flow over the front of the model indicates it hasn’t reached this point yet, but notice how both the green and red dyes are separating from the model and becoming very turbulent over the back of the wing. If the model were pushed to an even higher angle of attack, that separation point would move further forward, bringing stall that much closer. (Image credit: L. Erm and J. Drobik; research credit: R. Cummings and A. Schütte)
Rolling Along
Leidenfrost drops – droplets deposited onto a surface much hotter than their boiling point – are known for their mobility. With the right surface, they can be propelled, trapped, and even guided through a maze, typically by directing the vapor layer that cushions them. But new work shows that these drops have internal dynamics that also contribute to their propulsion.
By adding tracer particles to each droplet, researchers can visualize flows inside the droplet. Large drops tend to have a flatter shape and contain two or more rotating vortices. Such drops won’t propel themselves without another force in play. But smaller droplets are more spherical and contain only a single rotating flow. Once these drops detach, they roll away! Despite the similarity to wheels, these liquid drops aren’t moving the same way. Remember that the drop is not actually in contact with the surface. To see what sets the drop’s direction, researchers examined the shape of the bottom of the drop. They found that it sits at a slant on its vapor cushion. That pushes evaporating gases out one side, propelling the drop the other way. (Image and video credit: A. Bouillant et al., source)
Atmospheric Aerosols
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)
Microfluidic Chips in Action
Earlier this year, The Lutetium Project explored how microfluidic circuits are made, and now they are back with the conclusion of their microfluidic adventures. This video explores how microfluidic chips are used and how microscale fluid dynamics relates to other topics in the field. Because these techniques allow researchers very fine control over droplets, there are many chemical and biological possibilities for microfluidic experiments, some of which are shown in the video. Microfluidics in medicine are also already more common than you may think. For example, test strips used by diabetic patients to measure their blood glucose levels are microfluidic circuits! (Video and image credit: The Lutetium Project; submitted by Guillaume D.)
Symmetric Wakes
Nature is full of remarkable patterns and moments of symmetry. This image shows the wake behind two rotating cylinders. Half of the cylinders are visible at the far left. The flow moves left to right. The cylinders are rotating at the same rate but in opposite directions, clockwise for the cylinder on top and counter-clockwise for the bottom one. At this speed relative to the freestream, there is a beautiful symmetry to the vortices in the wake, but the researchers found that even a slight deviation from this condition quickly destroyed the pattern. The flow is visualized here by introducing tiny hydrogen bubbles via electrolysis. The bubbles are small enough that their buoyancy has no appreciable effect. (Image credit: S. Kumar and B. Gonzalez)
Peering Between Particles
Turbulence is not the only way to mix fluids. Even a steady, laminar flow can be an effective mixer if geometry lends a hand. Above, two dyes, fluorescein (green) and rhodamine (red), are injected into a porous flow through packed spheres. The flow runs from bottom to top in both images. Seeing the flow in such a crowded geometry is challenging. Here researchers used spheres with an index of refraction that matches water – that helps them avoid refraction that would prevent them from looking through spheres to the flow on the other side. They also lit a narrow plane of the flow using a laser sheet to isolate it. Together, this allowed the researchers to track the mixing of the two initially separate streaks of dye as they randomly mix in the spaces between spheres. (Image and research credit: M. Kree and E. Villermaux)
Flow Inside Convection Cells
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)
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)
