In flight, airplane wings produce dramatic wingtip vortices. These vortices reduce the amount of lift a 3D wing produces relative to a 2D one. How much they influence the lift depends on both the strength and proximity of the vortex. The stronger and closer it is, the more detrimental its effect. One way airplane designers reduce the effects of wingtip vortices is by adding an extra section, called a winglet, to the end of the wing. Among other effects, the winglet moves the wingtip vortex further away from the main wing, which reduces its influence and allows the airplane to regain some of the lift that would otherwise be lost. (Image credits: A. Wielandt et al., source)
Earlier this month, an international team launched a successful hypersonic flight test in Australia. The Hypersonic International Research Experimentation (HIFiRE) Flight 5b was launched atop a two-stage rocket and reached its maximum speed of Mach 7.5, well above Mach 5, which defines the start of the hypersonic regime. The purpose of this particular flight test was not to test new propulsion technologies – there was no scramjet engine on this flight. Instead, researchers wanted to study aerodynamics at high Mach number, specifically the behavior of the air very close to the vehicle, its boundary layer.
The payload being tested was an elliptical cone mounted on the front of the vehicle and shown in images above. The shape of the payload is such that flow will curve around the cone rather than following straight lines. The image on the lower right contains black streamlines that show how air twists around the cone. This complex flowfield complicates the physics of the boundary layer near the cone’s surface and increases the likelihood that the boundary layer will transition from laminar flow to turbulent flow, thereby increasing heating on the payload. Ideally, the data from the test flight will let engineers test their ability to understand and predict this boundary layer transition in the future. For more on boundary layer transition and its effects at hypersonic speeds, check out my latest FYFD video. (Image credit: Australia Department of Defense, R. Kimmel et al., F. Li et al.; topic requested by Guido)
You may have noticed when baking that fluids don’t always behave as expected when you agitate them. If you put a spinning rod into a fluid, we’d expect the rod to fling fluid away, creating a little vortex that stirs everything around. And for a typical (Newtonian) fluid, this is what we see. The fluid’s viscosity tries to resist deforming the fluid, but the momentum imparted by the rod wins out. With a viscoelastic fluid, on the other hand, the story is much different. As before, the spinning of the rod deforms the fluid. But the viscoelastic fluid contains long chains of polymers. As those polymers get stretched by the deformation, they generate their own forces, including forces parallel to the rod. Instead of being flung outward, the viscoelastic fluid starts climbing up the rod, with the stretchy elasticity of the polymers helping pull more fluid up and up. (Image credit: Ewoldt Research Group, source)
Our atmosphere is full of aerosols – extremely tiny particles and droplets of salt, dust, pollutants, and other substances. Wind’s effects alone cannot account for the sizes and quantities of aerosols we measure. Another potential source is the bursting of bubbles; more specifically, the bubbles that form at the oceans’ surface. Frothy, crashing waves often capture pockets of air. When these bubbles burst, the thin film of their surface ruptures into long filaments that break into tiny droplets. Such droplets can be small enough to get carried on the breeze, eventually evaporating and leaving the particulates that were once in the water to ride the winds. (Image credit: H. Lhuissier & E. Villermaux; see also: Y. Couder)
Photographer Ernie Button discovered that whiskey left behind intriguing patterns after it evaporated. Unlike coffee rings, the whiskey leaves behind a more uniform residue. Curious, he contacted researchers at Princeton, who were eventually able to explain why whiskey and coffee dry so differently. They observed three major effects in drying whiskey mixtures. Firstly, the alcohol in whiskey evaporates faster than other components, creating differences in concentration and, therefore, surface tension along the droplet. These variations in surface tension create Marangoni flow, which tends to mix the droplet. Coffee, being non-alcoholic, does not do this.
Whiskey also contains surfactants, low surface tension chemicals, which help pull particulates away from the edge of the droplet so they aren’t trapped there like in coffee. And finally, they found that the polymers in whiskey helped glue particles to the glass so that they were less likely to be carried by the flow. Taken together, these three ingredients – alcohol, surfactants, and polymers – all help make the whiskey stain more uniform. For more, watch the video below, see Button’s website, or check out the research paper. (Image credit: E. Button; research credit: H. Kim et al.; video credit: C&EN; submitted by @tommyjwilson)
Evolution often requires compromise between competing effects. Large-eared bats, for example, rely on the size of their ears to aid their echolocation, but such large ears can hurt them aerodynamically, thus limiting their flight. Results from a recent experiment, however, suggest that large ears are not a total loss aerodynamically speaking. Researchers used particle image velocimetry to study the wakes behind free-flying, large-eared bats and found significant downward flow behind the bats’ bodies. This indicates that the bats generate some lift with their ears, body, and/or tail. The position and tilt of the ears in flight is similar to forward swept wings, which the authors suggest could help contract the wake behind the ears and reduce its negative influence on flow over the wings. Although the evidence is not yet conclusive, the study does suggest that large ears may be more aerodynamically beneficial than they appear. (Image credit: L. Johansson et al./Lund University, source; via Jalopnik)
The next FYFD webcast will be this Saturday, May 21st at 1pm EDT. My guests will be Professor Jean Hertzberg of the University of Colorado at Boulder and Professor Kate Goodman of the University of Colorado at Denver. Dr. Hertzberg is the creator of the course Flow Visualization, an interdisciplinary course combining engineering, art, and fluid dynamics. It’s a class (and website) that’s been an inspiration for me and FYFD since the early days! Dr. Goodman, an expert in engineering education, earned her PhD studying the Flow Viz course and its impact. This will be wide-ranging discussion – with everything from experimental fluid dynamics and engineering education to art, photography, and hopefully even cardiac fluid dynamics!
Mezcal is a traditional Mexican liquor distilled from agave. (The more commonly known tequila is actually a special type of mezcal.) As a part of the production process, distillers pour a stream of mezcal into a bowl, creating a flotilla of small bubbles called pearls. Strange as it sounds, these pearls let the distiller judge the alcohol content of the liquor! When the ratio of alcohol and water in the mixture is just right, the bubbles will have a longer lifetime before they coalesce. If there’s too little or too much alcohol, the bubbles won’t last as long. The effect depends on both the viscosity and the surface tension of the liquor, but it’s the odd way that viscosity changes in water/alcohol mixtures that creates this Goldilocks behavior. It’s a fascinating demonstration of how traditional techniques often have true scientific underpinnings. (Video credit: M. Wilhelmus et al.)
Movement in Earth’s oceans is driven by a complicated interplay of many factors like temperature, salinity, and Earth’s rotation. Above are results from a numerical simulation of the top 100 meters of ocean contained within a 1 km x 1 km box. The colors indicate surface temperature. Two major processes create the motion we see. The first is convection, in which water at the surface releases heat to the atmosphere and cools, causing it to then sink due to its greater density. Warmer water rises to replace it. This process happens quickly and dominates the early part of the simulation where we see the puffy convection cells shown on the left animation.
A slower process is in effect as well. Because of variations in the water temperature, the density of the fluid at a given depth is not constant. We can already see that at the water surface, where the temperature (and thus density) is varying significantly. Those variations in density at the same depth combined with gravity’s tendency to shift fluids create what is known as a baroclinic instability. Put simply, this instability will cause warmer water to slide horizontally past colder water. The result is the large, spinning eddy motion seen in the animation on the right. To see how the whole system develops, check out the full video below. (Image/video credit: J. Callies)
Time for another fluids round-up! Here are some of the best fluids-related links I’ve seen around:
– Above The Brain Scoop tells us about beetles that spend their whole lives underwater. They carry a little bubble of air with them in order to breathe!
The roll cloud, or Morning Glory cloud, is a rare phenomenon that looks rather like a horizontal tornado. In reality, it is part of a soliton wave traveling through the atmosphere. At its leading edge, moist air is forced upward, causing water vapor to condense, and, at the trailing edge, air moves downward, dissipating the cloud. These clouds are most frequently observed in Australia near the Gulf of Carpentaria, where local geography and sea breezes promote their growth during springtime. The clouds do appear elsewhere on occasion; the photos above show rolls clouds in Calgary, Alberta and coastal Uruguay, respectively. (Image credits: G. E. Nyland, D. M. Eberl; see also: Z. Ouazzani)
