Here’s a potential flow field with heart-shaped streamlines, made just for you. Thank you to everyone for having helped made FYFD such a success over these 700 posts, whether by liking, reblogging, tweeting, or telling a friend. Happy Valentine’s Day!
For the curious among you, the flow is a superposition of uniform flow, two sources, and two sinks. The Matlab code is here. Have fun!
The video above shows vortex rings of smoke ejected from the burning tire of a moving truck. Without seeing the damaged tire, it’s tough to pinpoint the cause with certainty, but here are a couple of ideas. Typically vortex rings are formed with a burst of air through a narrow orifice; this is, for example, how humans, dolphins, vortex cannons, and volcanoes all make smoke rings. If air is escaping the tire through small holes, this could cause rings. Unlike in those situations, though, the tire is spinning, which means its motion is already imparting vorticity to the flow, so that any air escaping the tire forms a vortex ring. (Video credit: The Armory; submitted by eruditebaboon)
ETA: Others are suggesting the vortex rings are due to a failure of the engine, with unsteady exhaust velocities resulting in the vortex structures. I think this might still depend on the exhaust pipe’s geometry. Regardless of the exact cause, the video remains an interesting bit of fluid dynamics.
Here a ferrofluid climbs a spiral steel structure sitting on an electromagnet. Magnetic field lines emanating from the sculpture’s edges tend to push the ferrofluid out into long spikes–part of the normal field instability–but surface tension resists. The short, somewhat squat spikes we see are the balance struck between these opposing forces. Though known for their wild appearance, ferrofluids appear many in common applications, including hard drives, speakers, and MRI contrast agents. Researchers have also recently suggested they might help understand the behavior of the multiverse. (Photo credit: P. Davis et al.)
Aeroelastic flutter occurs when fluid mechanical forces and structural forces get coupled together, one feeding the other. Usually, we think of it as a destructive mechanism, but, for hummingbirds, it’s part of courtship. When a male hummingbird looks to attract a mate, he’ll climb and dive, flaring his tail feathers one or more times. As he does so, air flow over the feathers causes them to vibrate and produce noise. Researchers studied such tail feathers in a wind tunnel, finding a variety of vibrational behaviors, including a tendency for constructive interference–in other words two feathers vibrating in proximity is much louder than either individually. For more, check out the original Science article or the write-up at phys.org. (Video credit: C. Clark et al.)
This image shows oil-flow visualization of a cylindrical roughness element on a flat plate in supersonic flow. The flow direction is from left to right. In this technique, a thin layer of high-viscosity oil is painted over the surface and dusted with green fluorescent powder. Once the supersonic tunnel is started, the model gets injected in the flow for a few seconds, then retracted. After the run, ultraviolet lighting illuminates the fluorescent powder, allowing researchers to see how air flowed over the surface. Image (a) shows the flat plate without roughness; there is relatively little variation in the oil distribution. Image (b) includes a 1-mm high, 4-mm wide cylinder. Note bow-shaped disruption upstream of the roughness and the lines of alternating light and dark areas that wrap around the roughness and stretch downstream. These lines form where oil has been moved from one region and concentrated in another, usually due to vortices in the roughness wake. Image © shows the same behavior amplified yet further by the 4-mm high, 4-mm wide cylinder that sticks up well beyond the edge of the boundary layer. Such images, combined with other methods of flow visualization, help scientists piece together the structures that form due to surface roughness and how these affect downstream flow on vehicles like the Orion capsule during atmospheric re-entry. (Photo credit: P. Danehy et al./NASA Langley #)
This super high resolution video (check the original on YouTube) by filmmaker Jacob Schwarz features slow motion diffusion of ink into water. The subtle differences in density between the ink and the water promote instabilities such as the Rayleigh-Taylor instability and its distinctive cascade of mushroom- or umbrella-like shapes. The mixing of two fluids seems like a simple concept, but the reality is beautiful, complex, and always fascinating. (Video credit: J. Schwarz; submitted by Rebecca S.)
Surface tension creates a glassy, smooth layer of water over U.S. swimmer Tyler Clary the instant before he surfaces as he competes in the backstroke. Surface tension arises from intermolecular forces between water molecules. In the bulk of the liquid, any given water molecule is being pulled on in every direction by the surrounding molecules, which results in zero net force. At the surface, however, molecules only experience forces from those to the side and below them. As a result, these molecules are pulled inwards, forcing the liquid to take on a form with minimal area. (Photo credit: Getty Images; submitted by drhawkins)
Prior to reaching terminal velocity, a falling droplet typically oscillates between a prolate shape (like an American football about to be kicked) and an oblate one (like that same football when thrown or carried). As explained by Minute Laboratory, this oscillation behaves very similarly to a mass on a spring. For a spring/mass system, the frequency of oscillation is related to the spring’s stiffness; for the falling droplet, it is instead governed by surface tension. If only high schools had high-speed cameras, this would make a fantastic fluids lab experiment! (Video credit: Minute Laboratory; submitted by Pascal W.)
FYFD is sporting a new look and new about and resources pages for those who want more fluidsy goodness.
Reader Mike L asks:
Why do I never see frost on my car when I park in a detached garage or under a carport?
Great question! Frost forms on surfaces when their temperature drops below the freezing point of water and the dew point of the surrounding air. The water vapor in the air gets deposited as a solid directly; this is called deposition. This means that the surface–in this case your car–has to be colder than the nearby air. Neither conduction nor convection of heat between your car and the surrounding air can cause this drop; heat transfer between your car and the surrounding air would tend to make them the same temperature, not make the car colder than the air. The third–and typically least effective–type of heat transfer, radiation, is the answer because it allows heat transfer between two objects that are not in direct contact like the air and car are.
Frost typically forms on still, clear nights with little clouds or wind. A car sitting beneath a clear night sky will radiate heat out into space. Since space is much, much colder than the air, this radiation cooling to space allows the car’s surface temperature to drop below that of the surrounding air, which is not a good radiator by comparison. On a night with little wind (and thus little convection), this radiation cooling can be quite effective. Frost will tend not to form on one’s car under a carport because the car is sheltered from the night sky, blocking such radiative cooling. Having a tree or house blocking the car from the night sky is also effective at preventing frost formation. (Photo credit: N. Sharp; with thanks to Keri B and Jerry N for the meteorological assistance)
When a droplet falls through an air/water interface, a vortex ring can form and fall through the liquid. In this video, the researchers investigate the effects of a stratified fluid interface on this falling vortex ring. In this case, a less dense fluid sits atop a denser one. Depending on the density of the initial falling droplet and the distance it travels through the first fluid, the behavior and break-up of the vortex ring when it hits the denser fluid differs. Here four different behaviors are demonstrated, including bouncing and trapping of the vortex ring. (Video credit: R. Camassa et al.)