#boundary layer

[fuckyeahfluiddynamics]

The Tea Leaves Effect

If you've ever stirred a cup of tea with loose leaves in it, you've probably noticed that the leaves tend to swirl into the center of the cup in a kind of inverted whirlpool. (Video and image credit: S. Mould)

[awkwardbotany]

The Flight of the Dandelion

The Flight of the Dandelion

The common dandelion (Taraxacum officinale) comes with a collection of traits that make it a very successful weed. Nearly everything about it screams success, from its asexually produced seeds to its ability to resprout from a root fragment. Evolution has been kind to this plant, and up until the recent chemical warfare we’ve subjected it to, humans have treated it pretty well too (both…

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[fuckyeahfluiddynamics]

Peering Into the Gap

This video offers a glimpse into turbulence developing in a classic flow set-up, a Taylor-Couette cylinder. The apparatus consists of two upright, concentric cylinders; the outer cylinder is fixed, and the inner one rotates. (Video credit: D. van Gils)

[fuckyeahfluiddynamics]

Tokyo 2020: Baseball Aerodynamics

For a long time, people thought baseball aerodynamics were simply a competition between gravity and the Magnus effect caused when a ball is spinning. But the seams of a baseball are so prominent that they, too, have a role to play.  (Image credit: top - Pixabay, others - B. Smith; research credit: B. Smith; see also Baseball Aerodynamics) Today wraps up our Olympic coverage, but if you missed our earlier posts, you can find them all here. Read the full article

[fuckyeahfluiddynamics]

Tokyo 2020: Volleyball Aerodynamics

Like footballs and baseballs, the trajectory of a volleyball is strongly influenced by aerodynamics. When spinning, the ball experiences a difference in pressure on either side, which causes it to swerve, per the Magnus effect. But volleyball also has the float serve, which like the knuckleball in baseball, uses no spin.  (Image credit: game - Pixabay, volleyballs - U. Tsukuba; research credit: S. Hong et al., T. Asai et al.; via Ars Technica) Stick around all this week and next for more Olympic-themed fluid physics! Read the full article

[fuckyeahfluiddynamics]

For many engineering students, their first exposure to fluid dynamics comes in a heat transfer class. The problem is that the picture they get is overly simplified. (Image and research credit: J. Lienhard; via phys.org)

[fuckyeahfluiddynamics]

Embers fly through the Kincade wildfire leaving streaks of light that reveal the strong winds helping drive the fire. This unintentional flow visualization mirrors techniques used by researchers to understand how flows are moving. The shutter of the camera remains open for a fixed time, so the length of each streak tells us about the speed of the flow. Longer streaks occur where embers moved faster. 

Here we see the longest streaks in the upper left side of the image, which tells us that the wind was moving faster there than it did at lower heights, like near the photographer in the picture. That’s in keeping with what we would expect. In general, winds move faster above the ground than they do near the surface. That speed difference is one of the reasons wildfires are so difficult to contain; a single ember caught by high winds is easily carried to unburnt areas, allowing the fire to spread more quickly than if it had to burn along the ground. (Image credit: J. Edelson/Getty Images; via Wired)

[fuckyeahfluiddynamics]

There are many ways to repel droplets from a surface: water droplets will bounce off superhydrophobic surfaces due to their nanoscale structures; a vibrating liquid pool can keep droplets bouncing thanks to its deformation and a thin air layer trapped under the drop; and heated surfaces can repel droplets with the Leidenfrost effect by vaporizing a layer of liquid beneath the droplet. But all of these methods will only work for certain liquids under specific circumstances. 

More recently, researchers have begun looking at a different way to repel droplets: moving the surface. The motion of the plate drags a layer of air with it; how thick that layer of air is depends on the plate’s speed. (Faster plates make thinner air layers.) Above a critical plate speed, a falling droplet will impact without touching the plate directly and will rebound completely. This works for many kinds of liquids -- the researchers used silicone oil, water, and ethanol -- across many droplet sizes and speeds. The key is that the air dragged by the plate deforms the droplet and creates a lift force. If that lift force is greater than the inertia of the droplet, it bounces. (Image and research credit: A. Gauthier et al., source)