So, as we learned previously, sound can actually travel through space. But the recordings our spacecraft send us from other planets or from the edge of the Solar System aren’t really that kind of sound. Acoustic waves require a medium; they travel when particles bump into one another, which, given the sparseness of space, means that only very low frequency sounds can travel. But space has a lot of ions and plasmas – charged particles like electrons and protons – and those particles can interact without physically contacting one another. Instead their motion causes a changing magnetic field that affects nearby particles, which in turn affect more particles (and so on). This transmits what’s called ionic sound. Check out the video above to hear some awesome examples of the ionic sounds of our solar system! (Video credit: The Point Studios)
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Inside a Cello
At first glance, Adrian Borda’s photograph seems to show an old room. In reality, this is the interior of a cello with light shining through the f-holes. Dust particles in the air trace out pathlines that reveal the turbulent movements of air inside the instrument. Both the camera’s perspective and the visible flow try to trick our minds into seeing something larger than reality. It’s a reminder that the patterns and forms of fluid flow repeat across an enormous range of scales, from millimeters to light-years. (Image credit: A. Borda; via Joseph S./CU Boulder Flow Viz)
Giant Vortex Cannon
Playing with a vortex cannon is a ton of fun, and they are remarkably easy to make. You can knock over cups or card houses, create art, or just try your best Big Bad Wolf impression. Or you can supersize things like one group in the Czech Republic did and build a 3m vortex cannon capable of firing 100m! (Seriously, watch it in action here.) And if you’d like to learn more about how vortex rings form and why they’re useful in nature and engineering, check out my vortex ring video. (Image credit: Laborky Cz, source; via Gizmodo)
Making Droplets
If you’ve ever wondered how fluid dynamicists create those tiny perpetually bouncing droplets they study, wonder no further. A typical method, shown here, is to use a simple toothpick. First, you take a shallow container of silicone oil and vibrate it vertically. Then you dip the tip of the toothpick into the oil and pull it out, stretching the oil into a long filament. When it detaches from the toothpick, a droplet will start to form at the tip of the filament as it falls back toward the pool. But the bouncing of the surface is enough to keep the new drop from coalescing back into the pool, leaving the little drop to bounce along on its liquid trampoline. (Image credit: S. Lapointe)
The Law of Urination
Tonight is the 26th Ig Nobel Prize ceremony. As I’ve covered previously, the subject of fluid dynamics has been quite successful at winning these awards designed to “make people LAUGH, then THINK,” and last year’s ceremony was no exception. Georgia Tech researchers won the Physics Prize last year for explaining why mammals of very different sizes all urinate for roughly 21 seconds.
Urination is a gravity-driven process, and larger animals have longer urethras, which means that gravity will have more time to accelerate fluid flowing from the the bladder to, well, the exit. Thus, larger animals will have higher flow rates. This allows them to empty their bigger bladders in essentially the same amount of time as a smaller animal. Recognizing this pattern can be helpful to both veterinarians diagnosing problems in animals and to engineers designing systems to move fluids efficiently.
There’s no way to know whether fluid dynamics will win another Ig Nobel Prize tonight, but I can guarantee that subject will come up. I’ll be giving a 24/7 lecture on Fluid Dynamics during tonight’s Ig Nobel Prize ceremony. You can see me – and find out this year’s winners – by watching the ceremony webcast here starting at 5:40pm EDT. (Video credit: DNews; research credit: P. Yang et al.)
Jovian Poles
We’re used to viewing Jupiter from its equator, where bands of light and dark clouds dominate the picture. From its poles, Jupiter looks very different, as these recent images from Juno show. Jupiter’s north pole is shown on the left and its south pole on the right. Both are awash in vortices. There’s another great black-and-white image of the south pole here, where the vortices really stand out. Jupiter’s atmosphere contains both cyclones, which rotate counterclockwise in the northern hemisphere and clockwise in the southern hemisphere, and anticyclones, which behave in the reverse. Unlike in Earth’s atmosphere, anticyclones dominate on Jupiter, especially among storms more than 2000 km across. (Image credit: NASA/JPL/Juno Mission; via APOD)
P.S. – Tomorrow night is the Ig Nobel Prize Ceremony, and I’ll be giving one of their 24/7 lectures. If you’d like to tune in and hear me describe fluid dynamics in 24 seconds + 7 words, there will be a webcast here.
Shark Tooth Instability
Imagine that you partially fill a horizontal cylinder with a viscous fluid, like corn syrup or honey. If that cylinder is still, the fluid will simply pool along the bottom. On the opposite extreme, if you spin it very fast, that cylinder will become coated in an even layer of fluid that rotates along with the cylinder thanks to centrifugal force. Between those two extremes in rotational velocity, some interesting fluid behaviors occur. Start spinning the cylinder and some of the pooled fluid will be pulled up the sides, eventually forming a thicker film with a straight front along the bottom of the cylinder. Spin faster and that straight front starts to break down, forming sharper cusp-like waves known as shark teeth. (Image credit: S. Morris et al., source; research credit: S. Thoroddsen and L. Mahadevan)
Reader Question: Rudders
Reader le-mec writes:
My question involves “fenestrated rudders”, a Chinese invention that
involved cutting diamond-shaped holes in the rudders of ancient Chinese
sailing ships (known as Junks). According to several articles (on the
internet, ha ha), it reduces the amount of effort required to steer the
ship at higher speeds with “no loss of function”. All I can find is
anecdotal evidence and I’d like to know if these claims hold water or if
they’re just steering us in the wrong direction.First off:
Now, I’m no expert on ships or sailing, but let’s talk rudders. Ships use rudders for steering. The rudder is completely submerged and turning it deflects water and creates a side force that helps steer a boat. In essence, it’s an underwater wing that generates lift in the side-to-side direction. Modern rudders even have the same shape as airfoils. That’s clearly not the case with the rudders of Chinese junks, but flat plates are a lot easier to make.
There’s another key feature of the junk’s rudder, and that’s the way it’s mounted. The junk’s rudder attaches to the ship such that it rotates about its leading edge. This makes it an unbalanced rudder. More modern rudders are typically mounted so that they rotate around an axis that’s partway back on the rudder. This is called a balanced rudder; I’ve illustrated both below.
The advantage of the balanced rudder is that it’s easier to turn. You can see this for yourself without adding water into the equation. Imagine holding a big rectangular sheet. If you hold it by one edge and try to rotate it, you can do it, but it’s kind of difficult. If you instead hold it about a third of the way across, you’ll find rotating it easier. Once you have a fluid moving past, it will only magnify how hard it is to turn the rudder.
So the Chinese junks had rudders that were harder to handle (by later ship-building standards) to begin with. By putting holes in the rudder, they equalized the pressure on either face of the rudder. That does make it easier to steer, since the helmsman is no longer fighting pressure differences across the rudder, but it would also reduce steering efficiency. It’s likely, however, given the slow speed of the junks, large rudder area, and their low hydrodynamic efficiency to begin with, that any drop in efficiency was negligible compared to the reduction in force necessary to steer.
Since modern designs rely on foil shapes to generate pressure differences (and therefore side force) across the rudder, adding holes to them would be a bad idea. But back in the Song dynasty, the fenestrated rudder was major advance in nautical engineering!
(Image credits: Chinese junk ship model – Premier Ship Models; Joffrey applauding – HBO; Rudder diagram – N. Sharp)
Happy 50th, Star Trek!
Today’s post is largely brought to you by the fact that I have been sick the past four days and my fiance and I have been bingeing on Star Trek Voyager. At some point, we began wondering about the sequence from 0:30-0:49 in which Voyager flies through a nebula and leaves a wake of von Karman vortices. Would a starship really leave that kind of wake in a nebula?
My first question was whether the nebula could be treated as a continuous fluid instead of a collection of particles. This is part of the continuum assumption that allows physicists to treat fluid properties like density, temperature, and velocity as well-defined quantities at all points. The continuum assumption is acceptable in flows where the Knudsen number is small. The Knudsen number is the ratio of the mean free path length to a characteristic flow length, in this case, Voyager’s size. The mean free path length is the average distance a particle travels before colliding with another particle. Nebulae are much less dense than our atmosphere, so the mean free path length is larger (~ 2 cm by my calculation) but still much smaller than Voyager’s length of 344 m. So it is reasonable to treat the nebula as a fluid.
As long as the nebula is acting like a fluid, it’s not unreasonable to see alternating vortices shed from Voyager. But are the vortices we see realistic relative to Voyager’s size and speed? Physicists use the dimensionless Strouhal number to describe oscillatory flows and vortex shedding. It’s a ratio of the vortex shedding frequency times the characteristic length to the flow’s velocity. We already know Voyager’s size, so we just need an estimate of its velocity and the number of vortices shed per second. I visually estimated these as 500 m/s and 2.5 vortices/second, respectively. That gives a Strouhal number of 0.28, very close to the value of 0.2 typically measured in the wake of a cylinder, the classical case for a von Karman vortex street.
So far Voyager’s wake is looking quite reasonable indeed. But what about its speed relative to the nebula’s speed of sound? If Voyager is moving faster than the local speed of sound, we might still see vortex shedding in the wake, but there would also be a bow shock off the ship’s leading edge. To answer this question, we need to know Voyager’s Mach number, its speed relative to the local speed of sound. After some digging through papers on nebulae, I found an equation to estimate speed of sound in a nebula (Eq 9 of Jin and Sui 2010) using the specific gas constant and temperature. Because nebulae are primarily composed of hydrogen, I approximated the nebula’s gas constant with hydrogen’s value and chose a representative temperature of 500 K (also based on Jin and Sui 2010). This gave a local speed of sound of 940 m/s, and set Voyager’s Mach number at 0.53, inside the subsonic range and well away from any shock wave formation.
Of course, these are all rough estimates and back-of-the-envelope fluid dynamics calculations, but my end conclusion is that Voyager’s vortex shedding wake through the nebula is realistic after all! (Video credit: Paramount; topic also requested by heuste11)
Happy 50th anniversary, Star Trek! Some of my earliest memories of TV are of watching TNG with my parents. Star Trek taught me that curiosity and scientific inquiry were vital and valuable, and that anyone could grow up to be a scientist, engineer, and leader. Thank you for such an inspiring and hopeful vision for humanity’s future!
And, seriously, those von Karman vortices are awesome.
Roll Cloud Over Chicago
A cold front passing through Chicago last week triggered a roll cloud, shown in the timelapse above. These clouds look like spinning horizontal tubes and form in areas where cool, sinking air displaces warmer, moist air to higher altitudes. The moist air is forced up along the cloud’s leading edge, causing it to cool and condense into cloud. Air on the trailing edge sinks downward again, warming and dissipating the cloud. The clouds are a visible form of soliton, or solitary wave, traveling through the atmosphere. They go by several other names, too, including Morning Glory clouds and arcus clouds. (Image credit: A. King; via Colossal)