Building a vortex cannon is a great way to demonstrate the power and longevity of vortex rings. As demonstrated here, it’s possible to create one with just a box with a round hole in it. Adding some smoke or stage fog helps visualize the rings. Vortex rings are found frequently in nature: volcanoes make them, some plants use them to distribute spores, and dolphins and whales use them to play. (submitted by @aggieastronaut)
Tag: science
Science Off the Sphere: Liquid Lenses
Astronaut Don Pettit delivers more “Science Off The Sphere” in his latest video. Here he demonstrates diffusion and convection in a two-dimensional water film in microgravity. He notes that the viscous damping in the water is relatively low and that, left undisturbed, mixing in the film will continue for 5-10 minutes before coming to rest, which tells us that the Reynolds numbers of the flow are reasonably large. The structures formed are also intriguing; he notes that drops mix with mushroom-like shapes that are reminiscent of Rayleigh-Taylor instabilities and cross-sectional views of vortex rings. It would be interesting to compare experiments from the International Space Station with earthbound simulations of two-dimensional mixing and turbulence, given that the latter behaves so differently in 2D.
Tornado in a Bubble
In this video, a miniature tornado-like vortex is created inside a soap bubble. Here’s how it works: after the first bubble is formed and the smoke-filled bubble is attached to the outside, he blows into the main bubble, creating a weak angular velocity, before breaking the interface between the two bubbles. As the smoke mixes in the main bubble, note how it is already spinning slowly due to the free vortex he created. Then, when the top of the bubble is popped, surface tension pulls the bubble’s surface inward. Because the bubble radius is decreasing, conservation of angular momentum causes the angular velocity of the fluid inside to increase, pulling the smoke into a tight vortex, much like a spinning ice skater who pulls her arms inward.
How Dams Affect Rivers
This video shows how the installation of a dam can affect river flow and sediment transport. Before the dam is added, the flow is shallow and the sediment transport is uniform. The installation of the dam creates deep subcritical flow upstream and supercritical flow downstream. This means that wave information–like ripples–can propagate upstream on the subcritical side; on the supercritical side, the wave velocity is lower than the flow velocity and ripples cannot propagate upstream. This is analogous to sub- and supersonic flow in air. The critical flow over the dam is analogous to a shock wave. The lower velocity upstream of the dam is unable to carry sediment downstream and transport essentially ceases until the sediment builds up to a height where the depth of the water above the dam is roughly equal to that below the dam and sediment transport resumes, scouring the downstream supercritical section. Around 0:40, a gate is closed on the downstream side (off frame), creating a hydraulic jump. In the final section of the video, after sediment has built up on both sides of the dam, the downstream gate is re-opened and the jump reforms as sediment is blown out below the dam. (Video credit: Little River Research and Design, with funding from the Missouri Department of Natural Resources)
Supersonic Flow
This video shows a sphere in a small supersonic wind tunnel at Mach 2.7. Once the tunnel starts, a curved bow shock forms in front of the sphere, close to but not touching the model’s surface. Areas of low pressure are visible behind the sphere, as is a weak shock wave caused by overexpansion in those low pressure areas. Contrast this with a sharp cone in the same tunnel at the same Mach number. In the case of the cone, the shock wave is attached at the nose of the model. The attached shock follows the body more closely, resulting in a shock that impacts the walls of the tunnel further downstream than in the sphere’s case.
Swirling Fluids
In this video, researchers investigate swirling fluids by studying the shapes of the free surface between air and the liquid. As parameters like the diameter of the glass, initial (unperturbed) height of the liquid, and angular velocity of the rotation change, the surface of the liquid displays different modal behaviors, seen in the photos on the lower left of the video. By non-dimensionalizing the physical parameters of the system (students: think Buckingham pi theorem), they are able to replicate the shape of the free surface by matching a Froude number and dimensionless depth and offset. Such similitude between fluids under different conditions is key to understanding the underlying physics. (Video credit: M. Reclari et al; submitted by co-author M. Farhat)
Surf’s Up
Diffusion of ink in water + Lego minifigs = an awesome example of fluid mechanics as art. (Photo credit: Alberto Seveso; via io9; thanks to Jennifer for the link!)
Reader Question: Creeping Flow
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David asks:
I’m taking an undergraduate fluid dynamics course, and I’m having trouble understanding what a Creeping Flow exactly is. The only thing I understand about that is that the Re should be 0 or close to 0 for the flow… Could you post an example of a creeping flow please? Thank you!
Absolutely! Creeping flow, also called Stokes flow, is, like you said, a very low Reynolds number flow. It would be hard to say that the Reynolds number is zero because that would seem to imply no flow at all. Think of it instead as a Reynolds number much, much less than one. When the Reynolds number is very low, it means that viscous forces are dominating the flow. The video above shows creeping flow around a cylinder; notice how the streamlines stay attached all the way around the surface of the cylinder. There’s no separation, no turbulent wake, no von Karman vortex street. Viscosity is so dominant here that it’s damped out all of that inertial diffusion of momentum.
We’ve posted some other great examples of creeping flow, as well, though not by that name. There are the reversible laminar flow demos and various experiments in Hele-Shaw cells, all of which qualify as creeping flow because of their highly viscous nature. If you have the time, there’s also a great instructional video from the 1960s called “Low Reynolds Number Flow” (Parts 1, 2, 3, 4) starring G. I. Taylor (a famous fluid dynamicist) that is full of one demo after another.
Reynolds Stress
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From reader jessecaps who hung it on the office door. I expect this joke will make sense to very few but as someone who once dabbled in turbulence, I could not resist.
Particle Patterning
Here a container filled with a suspension of neutrally buoyant polystyrene beads and fluid is rotated. As the container rotates, a thin layer of fluid and bunches of particles get drawn up onto the wall by capillary forces capable of holding the particles in place even if the container stops rotating. The density and patterning of the particles on the wall depends on the container’s rotation speed and the volume fraction of particles. (Video credit: J. Kao and A. Hosoi)
